core-aop.adoc

Aspect Oriented Programming with Spring

Aspect-oriented Programming (AOP) complements Object-oriented Programming (OOP) by providing another way of thinking about program structure. The key unit of modularity in OOP is the class, whereas in AOP the unit of modularity is the aspect. Aspects enable the modularization of concerns (such as transaction management) that cut across multiple types and objects. (Such concerns are often termed “crosscutting” concerns in AOP literature.)

One of the key components of Spring is the AOP framework. While the Spring IoC container does not depend on AOP (meaning you do not need to use AOP if you don’t want to), AOP complements Spring IoC to provide a very capable middleware solution.

Spring AOP with AspectJ pointcuts

Spring provides simple and powerful ways of writing custom aspects by using either a schema-based approach or the @AspectJ annotation style. Both of these styles offer fully typed advice and use of the AspectJ pointcut language while still using Spring AOP for weaving.

This chapter discusses the schema- and @AspectJ-based AOP support. The lower-level AOP support is discussed in the following chapter.

AOP is used in the Spring Framework to:

• Provide declarative enterprise services. The most important such service is declarative transaction management.

• Let users implement custom aspects, complementing their use of OOP with AOP.

Note	If you are interested only in generic declarative services or other pre-packaged declarative middleware services such as pooling, you do not need to work directly with Spring AOP, and can skip most of this chapter.

AOP Concepts

Let us begin by defining some central AOP concepts and terminology. These terms are not Spring-specific. Unfortunately, AOP terminology is not particularly intuitive. However, it would be even more confusing if Spring used its own terminology.

• Aspect: A modularization of a concern that cuts across multiple classes. Transaction management is a good example of a crosscutting concern in enterprise Java applications. In Spring AOP, aspects are implemented by using regular classes (the schema-based approach) or regular classes annotated with the @Aspect annotation (the @AspectJ style).

• Join point: A point during the execution of a program, such as the execution of a method or the handling of an exception. In Spring AOP, a join point always represents a method execution.

• Advice: Action taken by an aspect at a particular join point. Different types of advice include “around”, “before” and “after” advice. (Advice types are discussed later.) Many AOP frameworks, including Spring, model an advice as an interceptor and maintain a chain of interceptors around the join point.

• Pointcut: A predicate that matches join points. Advice is associated with a pointcut expression and runs at any join point matched by the pointcut (for example, the execution of a method with a certain name). The concept of join points as matched by pointcut expressions is central to AOP, and Spring uses the AspectJ pointcut expression language by default.

• Introduction: Declaring additional methods or fields on behalf of a type. Spring AOP lets you introduce new interfaces (and a corresponding implementation) to any advised object. For example, you could use an introduction to make a bean implement an IsModified interface, to simplify caching. (An introduction is known as an inter-type declaration in the AspectJ community.)

• Target object: An object being advised by one or more aspects. Also referred to as the “advised object”. Since Spring AOP is implemented by using runtime proxies, this object is always a proxied object.

• AOP proxy: An object created by the AOP framework in order to implement the aspect contracts (advise method executions and so on). In the Spring Framework, an AOP proxy is a JDK dynamic proxy or a CGLIB proxy.

• Weaving: linking aspects with other application types or objects to create an advised object. This can be done at compile time (using the AspectJ compiler, for example), load time, or at runtime. Spring AOP, like other pure Java AOP frameworks, performs weaving at runtime.

Spring AOP includes the following types of advice:

• Before advice: Advice that runs before a join point but that does not have the ability to prevent execution flow proceeding to the join point (unless it throws an exception).

• After returning advice: Advice to be run after a join point completes normally (for example, if a method returns without throwing an exception).

• After throwing advice: Advice to be executed if a method exits by throwing an exception.

• After (finally) advice: Advice to be executed regardless of the means by which a join point exits (normal or exceptional return).

• Around advice: Advice that surrounds a join point such as a method invocation. This is the most powerful kind of advice. Around advice can perform custom behavior before and after the method invocation. It is also responsible for choosing whether to proceed to the join point or to shortcut the advised method execution by returning its own return value or throwing an exception.

Around advice is the most general kind of advice. Since Spring AOP, like AspectJ, provides a full range of advice types, we recommend that you use the least powerful advice type that can implement the required behavior. For example, if you need only to update a cache with the return value of a method, you are better off implementing an after returning advice than an around advice, although an around advice can accomplish the same thing. Using the most specific advice type provides a simpler programming model with less potential for errors. For example, you do not need to invoke the proceed() method on the JoinPoint used for around advice, and, hence, you cannot fail to invoke it.

All advice parameters are statically typed so that you work with advice parameters of the appropriate type (e.g. the type of the return value from a method execution) rather than Object arrays.

The concept of join points matched by pointcuts is the key to AOP, which distinguishes it from older technologies offering only interception. Pointcuts enable advice to be targeted independently of the object-oriented hierarchy. For example, you can apply an around advice providing declarative transaction management to a set of methods that span multiple objects (such as all business operations in the service layer).

Spring AOP Capabilities and Goals

Spring AOP is implemented in pure Java. There is no need for a special compilation process. Spring AOP does not need to control the class loader hierarchy and is thus suitable for use in a servlet container or application server.

Spring AOP currently supports only method execution join points (advising the execution of methods on Spring beans). Field interception is not implemented, although support for field interception could be added without breaking the core Spring AOP APIs. If you need to advise field access and update join points, consider a language such as AspectJ.

Spring AOP’s approach to AOP differs from that of most other AOP frameworks. The aim is not to provide the most complete AOP implementation (although Spring AOP is quite capable). Rather, the aim is to provide a close integration between AOP implementation and Spring IoC, to help solve common problems in enterprise applications.

Thus, for example, the Spring Framework’s AOP functionality is normally used in conjunction with the Spring IoC container. Aspects are configured by using normal bean definition syntax (although this allows powerful “auto-proxying” capabilities). This is a crucial difference from other AOP implementations. You cannot do some things easily or efficiently with Spring AOP, such as advise very fine-grained objects (typically, domain objects). AspectJ is the best choice in such cases. However, our experience is that Spring AOP provides an excellent solution to most problems in enterprise Java applications that are amenable to AOP.

Spring AOP never strives to compete with AspectJ to provide a comprehensive AOP solution. We believe that both proxy-based frameworks such as Spring AOP and full-blown frameworks such as AspectJ are valuable and that they are complementary, rather than in competition. Spring seamlessly integrates Spring AOP and IoC with AspectJ, to enable all uses of AOP within a consistent Spring-based application architecture. This integration does not affect the Spring AOP API or the AOP Alliance API. Spring AOP remains backward-compatible. See the following chapter for a discussion of the Spring AOP APIs.

Note

One of the central tenets of the Spring Framework is that of non-invasiveness. This is the idea that you should not be forced to introduce framework-specific classes and interfaces into your business or domain model. However, in some places, the Spring Framework does give you the option to introduce Spring Framework-specific dependencies into your codebase. The rationale in giving you such options is because, in certain scenarios, it might be just plain easier to read or code some specific piece of functionality in such a way. However, the Spring Framework (almost) always offers you the choice: You have the freedom to make an informed decision as to which option best suits your particular use case or scenario. One such choice that is relevant to this chapter is that of which AOP framework (and which AOP style) to choose. You have the choice of AspectJ, Spring AOP, or both. You also have the choice of either the @AspectJ annotation-style approach or the Spring XML configuration-style approach. The fact that this chapter chooses to introduce the @AspectJ-style approach first should not be taken as an indication that the Spring team favors the @AspectJ annotation-style approach over the Spring XML configuration-style. See Choosing which AOP Declaration Style to Use for a more complete discussion of the “whys and wherefores” of each style.

AOP Proxies

Spring AOP defaults to using standard JDK dynamic proxies for AOP proxies. This enables any interface (or set of interfaces) to be proxied.

Spring AOP can also use CGLIB proxies. This is necessary to proxy classes rather than interfaces. By default, CGLIB is used if a business object does not implement an interface. As it is good practice to program to interfaces rather than classes, business classes normally implement one or more business interfaces. It is possible to force the use of CGLIB, in those (hopefully rare) cases where you need to advise a method that is not declared on an interface or where you need to pass a proxied object to a method as a concrete type.

It is important to grasp the fact that Spring AOP is proxy-based. See Understanding AOP Proxies for a thorough examination of exactly what this implementation detail actually means.

@AspectJ support

@AspectJ refers to a style of declaring aspects as regular Java classes annotated with annotations. The @AspectJ style was introduced by the AspectJ project as part of the AspectJ 5 release. Spring interprets the same annotations as AspectJ 5, using a library supplied by AspectJ for pointcut parsing and matching. The AOP runtime is still pure Spring AOP, though, and there is no dependency on the AspectJ compiler or weaver.

Note	Using the AspectJ compiler and weaver enables use of the full AspectJ language and is discussed in Using AspectJ with Spring Applications.

Enabling @AspectJ Support

To use @AspectJ aspects in a Spring configuration, you need to enable Spring support for configuring Spring AOP based on @AspectJ aspects and auto-proxying beans based on whether or not they are advised by those aspects. By auto-proxying, we mean that, if Spring determines that a bean is advised by one or more aspects, it automatically generates a proxy for that bean to intercept method invocations and ensures that advice is executed as needed.

The @AspectJ support can be enabled with XML- or Java-style configuration. In either case, you also need to ensure that AspectJ’s aspectjweaver.jar library is on the classpath of your application (version 1.8 or later). This library is available in the lib directory of an AspectJ distribution or from the Maven Central repository.

Enabling @AspectJ Support with Java Configuration

To enable @AspectJ support with Java @Configuration, add the @EnableAspectJAutoProxy annotation, as the following example shows:

Java

@Configuration
@EnableAspectJAutoProxy
public class AppConfig {

}

Kotlin

@Configuration
@EnableAspectJAutoProxy
class AppConfig

Enabling @AspectJ Support with XML Configuration

To enable @AspectJ support with XML-based configuration, use the aop:aspectj-autoproxy element, as the following example shows:

<aop:aspectj-autoproxy/>

This assumes that you use schema support as described in XML Schema-based configuration. See the AOP schema for how to import the tags in the aop namespace.

Declaring an Aspect

With @AspectJ support enabled, any bean defined in your application context with a class that is an @AspectJ aspect (has the @Aspect annotation) is automatically detected by Spring and used to configure Spring AOP. The next two examples show the minimal definition required for a not-very-useful aspect.

The first of the two example shows a regular bean definition in the application context that points to a bean class that has the @Aspect annotation:

<bean id="myAspect" class="org.xyz.NotVeryUsefulAspect">
	<!-- configure properties of the aspect here -->
</bean>

The second of the two examples shows the NotVeryUsefulAspect class definition, which is annotated with the org.aspectj.lang.annotation.Aspect annotation;

Java

package org.xyz;
import org.aspectj.lang.annotation.Aspect;

@Aspect
public class NotVeryUsefulAspect {

}

Kotlin

package org.xyz

import org.aspectj.lang.annotation.Aspect;

@Aspect
class NotVeryUsefulAspect

Aspects (classes annotated with @Aspect) can have methods and fields, the same as any other class. They can also contain pointcut, advice, and introduction (inter-type) declarations.

Note

Autodetecting aspects through component scanning You can register aspect classes as regular beans in your Spring XML configuration or autodetect them through classpath scanning — the same as any other Spring-managed bean. However, note that the @Aspect annotation is not sufficient for autodetection in the classpath. For that purpose, you need to add a separate @Component annotation (or, alternatively, a custom stereotype annotation that qualifies, as per the rules of Spring’s component scanner).

Note	Advising aspects with other aspects? In Spring AOP, aspects themselves cannot be the targets of advice from other aspects. The @Aspect annotation on a class marks it as an aspect and, hence, excludes it from auto-proxying.

Declaring a Pointcut

Pointcuts determine join points of interest and thus enable us to control when advice executes. Spring AOP only supports method execution join points for Spring beans, so you can think of a pointcut as matching the execution of methods on Spring beans. A pointcut declaration has two parts: a signature comprising a name and any parameters and a pointcut expression that determines exactly which method executions we are interested in. In the @AspectJ annotation-style of AOP, a pointcut signature is provided by a regular method definition, and the pointcut expression is indicated by using the @Pointcut annotation (the method serving as the pointcut signature must have a void return type).

An example may help make this distinction between a pointcut signature and a pointcut expression clear. The following example defines a pointcut named anyOldTransfer that matches the execution of any method named transfer:

Java

@Pointcut("execution(* transfer(..))") // the pointcut expression
private void anyOldTransfer() {} // the pointcut signature

Kotlin

@Pointcut("execution(* transfer(..))") // the pointcut expression
private fun anyOldTransfer() {} // the pointcut signature

The pointcut expression that forms the value of the @Pointcut annotation is a regular AspectJ 5 pointcut expression. For a full discussion of AspectJ’s pointcut language, see the AspectJ Programming Guide (and, for extensions, the AspectJ 5 Developer’s Notebook) or one of the books on AspectJ (such as Eclipse AspectJ, by Colyer et. al., or AspectJ in Action, by Ramnivas Laddad).

Supported Pointcut Designators

Spring AOP supports the following AspectJ pointcut designators (PCD) for use in pointcut expressions:

• execution: For matching method execution join points. This is the primary pointcut designator to use when working with Spring AOP.

• within: Limits matching to join points within certain types (the execution of a method declared within a matching type when using Spring AOP).

• this: Limits matching to join points (the execution of methods when using Spring AOP) where the bean reference (Spring AOP proxy) is an instance of the given type.

• target: Limits matching to join points (the execution of methods when using Spring AOP) where the target object (application object being proxied) is an instance of the given type.

• args: Limits matching to join points (the execution of methods when using Spring AOP) where the arguments are instances of the given types.

• @target: Limits matching to join points (the execution of methods when using Spring AOP) where the class of the executing object has an annotation of the given type.

• @args: Limits matching to join points (the execution of methods when using Spring AOP) where the runtime type of the actual arguments passed have annotations of the given types.

• @within: Limits matching to join points within types that have the given annotation (the execution of methods declared in types with the given annotation when using Spring AOP).

• @annotation: Limits matching to join points where the subject of the join point (the method being executed in Spring AOP) has the given annotation.

Other pointcut types

The full AspectJ pointcut language supports additional pointcut designators that are not supported in Spring: call, get, set, preinitialization, staticinitialization, initialization, handler, adviceexecution, withincode, cflow, cflowbelow, if, @this, and @withincode. Use of these pointcut designators in pointcut expressions interpreted by Spring AOP results in an IllegalArgumentException being thrown.

The set of pointcut designators supported by Spring AOP may be extended in future releases to support more of the AspectJ pointcut designators.

Because Spring AOP limits matching to only method execution join points, the preceding discussion of the pointcut designators gives a narrower definition than you can find in the AspectJ programming guide. In addition, AspectJ itself has type-based semantics and, at an execution join point, both this and target refer to the same object: the object executing the method. Spring AOP is a proxy-based system and differentiates between the proxy object itself (which is bound to this) and the target object behind the proxy (which is bound to target).

Note

Due to the proxy-based nature of Spring’s AOP framework, calls within the target object are, by definition, not intercepted. For JDK proxies, only public interface method calls on the proxy can be intercepted. With CGLIB, public and protected method calls on the proxy are intercepted (and even package-visible methods, if necessary). However, common interactions through proxies should always be designed through public signatures. Note that pointcut definitions are generally matched against any intercepted method. If a pointcut is strictly meant to be public-only, even in a CGLIB proxy scenario with potential non-public interactions through proxies, it needs to be defined accordingly. If your interception needs include method calls or even constructors within the target class, consider the use of Spring-driven native AspectJ weaving instead of Spring’s proxy-based AOP framework. This constitutes a different mode of AOP usage with different characteristics, so be sure to make yourself familiar with weaving before making a decision.

Spring AOP also supports an additional PCD named bean. This PCD lets you limit the matching of join points to a particular named Spring bean or to a set of named Spring beans (when using wildcards). The bean PCD has the following form:

Java

bean(idOrNameOfBean)

Kotlin

bean(idOrNameOfBean)

The idOrNameOfBean token can be the name of any Spring bean. Limited wildcard support that uses the * character is provided, so, if you establish some naming conventions for your Spring beans, you can write a bean PCD expression to select them. As is the case with other pointcut designators, the bean PCD can be used with the && (and), || (or), and ! (negation) operators, too.

Note

The bean PCD is supported only in Spring AOP and not in native AspectJ weaving. It is a Spring-specific extension to the standard PCDs that AspectJ defines and is, therefore, not available for aspects declared in the @Aspect model. The bean PCD operates at the instance level (building on the Spring bean name concept) rather than at the type level only (to which weaving-based AOP is limited). Instance-based pointcut designators are a special capability of Spring’s proxy-based AOP framework and its close integration with the Spring bean factory, where it is natural and straightforward to identify specific beans by name.

Combining Pointcut Expressions

You can combine pointcut expressions by using &&, || and !. You can also refer to pointcut expressions by name. The following example shows three pointcut expressions:

Java

@Pointcut("execution(public * *(..))")
private void anyPublicOperation() {} // (1)

@Pointcut("within(com.xyz.someapp.trading..*)")
private void inTrading() {} // (2)

@Pointcut("anyPublicOperation() && inTrading()")
private void tradingOperation() {} // (3)

1. anyPublicOperation matches if a method execution join point represents the execution of any public method.

2. inTrading matches if a method execution is in the trading module.

3. tradingOperation matches if a method execution represents any public method in the trading module.

Kotlin

@Pointcut("execution(public * *(..))")
private fun anyPublicOperation() {} // (1)

@Pointcut("within(com.xyz.someapp.trading..*)")
private fun inTrading() {} // (2)

@Pointcut("anyPublicOperation() && inTrading()")
private fun tradingOperation() {} // (3)

1. anyPublicOperation matches if a method execution join point represents the execution of any public method.

2. inTrading matches if a method execution is in the trading module.

3. tradingOperation matches if a method execution represents any public method in the trading module.

It is a best practice to build more complex pointcut expressions out of smaller named components, as shown earlier. When referring to pointcuts by name, normal Java visibility rules apply (you can see private pointcuts in the same type, protected pointcuts in the hierarchy, public pointcuts anywhere, and so on). Visibility does not affect pointcut matching.

Sharing Common Pointcut Definitions

When working with enterprise applications, developers often want to refer to modules of the application and particular sets of operations from within several aspects. We recommend defining a “SystemArchitecture” aspect that captures common pointcut expressions for this purpose. Such an aspect typically resembles the following example:

Java

package com.xyz.someapp;

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Pointcut;

@Aspect
public class SystemArchitecture {

	/**
	 * A join point is in the web layer if the method is defined
	 * in a type in the com.xyz.someapp.web package or any sub-package
	 * under that.
	 */
	@Pointcut("within(com.xyz.someapp.web..*)")
	public void inWebLayer() {}

	/**
	 * A join point is in the service layer if the method is defined
	 * in a type in the com.xyz.someapp.service package or any sub-package
	 * under that.
	 */
	@Pointcut("within(com.xyz.someapp.service..*)")
	public void inServiceLayer() {}

	/**
	 * A join point is in the data access layer if the method is defined
	 * in a type in the com.xyz.someapp.dao package or any sub-package
	 * under that.
	 */
	@Pointcut("within(com.xyz.someapp.dao..*)")
	public void inDataAccessLayer() {}

	/**
	 * A business service is the execution of any method defined on a service
	 * interface. This definition assumes that interfaces are placed in the
	 * "service" package, and that implementation types are in sub-packages.
	 *
	 * If you group service interfaces by functional area (for example,
	 * in packages com.xyz.someapp.abc.service and com.xyz.someapp.def.service) then
	 * the pointcut expression "execution(* com.xyz.someapp..service.*.*(..))"
	 * could be used instead.
	 *
	 * Alternatively, you can write the expression using the 'bean'
	 * PCD, like so "bean(*Service)". (This assumes that you have
	 * named your Spring service beans in a consistent fashion.)
	 */
	@Pointcut("execution(* com.xyz.someapp..service.*.*(..))")
	public void businessService() {}

	/**
	 * A data access operation is the execution of any method defined on a
	 * dao interface. This definition assumes that interfaces are placed in the
	 * "dao" package, and that implementation types are in sub-packages.
	 */
	@Pointcut("execution(* com.xyz.someapp.dao.*.*(..))")
	public void dataAccessOperation() {}

}

Kotlin

package com.xyz.someapp

import org.aspectj.lang.annotation.Aspect
import org.aspectj.lang.annotation.Pointcut

import org.springframework.aop.Pointcut

@Aspect
class SystemArchitecture {

	/**
	* A join point is in the web layer if the method is defined
	* in a type in the com.xyz.someapp.web package or any sub-package
	* under that.
	*/
	@Pointcut("within(com.xyz.someapp.web..*)")
	fun inWebLayer() {
	}

	/**
	* A join point is in the service layer if the method is defined
	* in a type in the com.xyz.someapp.service package or any sub-package
	* under that.
	*/
	@Pointcut("within(com.xyz.someapp.service..*)")
	fun inServiceLayer() {
	}

	/**
	* A join point is in the data access layer if the method is defined
	* in a type in the com.xyz.someapp.dao package or any sub-package
	* under that.
	*/
	@Pointcut("within(com.xyz.someapp.dao..*)")
	fun inDataAccessLayer() {
	}

	/**
	* A business service is the execution of any method defined on a service
	* interface. This definition assumes that interfaces are placed in the
	* "service" package, and that implementation types are in sub-packages.
	*
	* If you group service interfaces by functional area (for example,
	* in packages com.xyz.someapp.abc.service and com.xyz.someapp.def.service) then
	* the pointcut expression "execution(* com.xyz.someapp..service.*.*(..))"
	* could be used instead.
	*
	* Alternatively, you can write the expression using the 'bean'
	* PCD, like so "bean(*Service)". (This assumes that you have
	* named your Spring service beans in a consistent fashion.)
	*/
	@Pointcut("execution(* com.xyz.someapp..service.*.*(..))")
	fun businessService() {
	}

	/**
	* A data access operation is the execution of any method defined on a
	* dao interface. This definition assumes that interfaces are placed in the
	* "dao" package, and that implementation types are in sub-packages.
	*/
	@Pointcut("execution(* com.xyz.someapp.dao.*.*(..))")
	fun dataAccessOperation() {
	}

}

You can refer to the pointcuts defined in such an aspect anywhere you need a pointcut expression. For example, to make the service layer transactional, you could write the following:

<aop:config>
	<aop:advisor
		pointcut="com.xyz.someapp.SystemArchitecture.businessService()"
		advice-ref="tx-advice"/>
</aop:config>

<tx:advice id="tx-advice">
	<tx:attributes>
		<tx:method name="*" propagation="REQUIRED"/>
	</tx:attributes>
</tx:advice>

The <aop:config> and <aop:advisor> elements are discussed in Schema-based AOP Support. The transaction elements are discussed in Transaction Management.

Examples

Spring AOP users are likely to use the execution pointcut designator the most often. The format of an execution expression follows:

	execution(modifiers-pattern? ret-type-pattern declaring-type-pattern?name-pattern(param-pattern)
				throws-pattern?)

All parts except the returning type pattern (ret-type-pattern in the preceding snippet), the name pattern, and the parameters pattern are optional. The returning type pattern determines what the return type of the method must be in order for a join point to be matched. * is most frequently used as the returning type pattern. It matches any return type. A fully-qualified type name matches only when the method returns the given type. The name pattern matches the method name. You can use the * wildcard as all or part of a name pattern. If you specify a declaring type pattern, include a trailing . to join it to the name pattern component. The parameters pattern is slightly more complex: () matches a method that takes no parameters, whereas (..) matches any number (zero or more) of parameters. The (*) pattern matches a method that takes one parameter of any type. (*,String) matches a method that takes two parameters. The first can be of any type, while the second must be a String. Consult the Language Semantics section of the AspectJ Programming Guide for more information.

The following examples show some common pointcut expressions:

• The execution of any public method:

  	execution(public * *(..))

• The execution of any method with a name that begins with set:

  	execution(* set*(..))

• The execution of any method defined by the AccountService interface:

  	execution(* com.xyz.service.AccountService.*(..))

• The execution of any method defined in the service package:

  	execution(* com.xyz.service.*.*(..))

• The execution of any method defined in the service package or one of its sub-packages:

  	execution(* com.xyz.service..*.*(..))

• Any join point (method execution only in Spring AOP) within the service package:

  	within(com.xyz.service.*)

• Any join point (method execution only in Spring AOP) within the service package or one of its sub-packages:

  	within(com.xyz.service..*)

• Any join point (method execution only in Spring AOP) where the proxy implements the AccountService interface:

  	this(com.xyz.service.AccountService)

  Note	'this' is more commonly used in a binding form. See the section on Declaring Advice for how to make the proxy object available in the advice body.

• Any join point (method execution only in Spring AOP) where the target object implements the AccountService interface:

  	target(com.xyz.service.AccountService)

  Note	'target' is more commonly used in a binding form. See the Declaring Advice section for how to make the target object available in the advice body.

• Any join point (method execution only in Spring AOP) that takes a single parameter and where the argument passed at runtime is Serializable:

  	args(java.io.Serializable)

  Note	'args' is more commonly used in a binding form. See the Declaring Advice section for how to make the method arguments available in the advice body.

  Note that the pointcut given in this example is different from execution(* *(java.io.Serializable)). The args version matches if the argument passed at runtime is Serializable, and the execution version matches if the method signature declares a single parameter of type Serializable.

• Any join point (method execution only in Spring AOP) where the target object has a @Transactional annotation:

  	@target(org.springframework.transaction.annotation.Transactional)

  Note	You can also use '@target' in a binding form. See the Declaring Advice section for how to make the annotation object available in the advice body.

• Any join point (method execution only in Spring AOP) where the declared type of the target object has an @Transactional annotation:

  	@within(org.springframework.transaction.annotation.Transactional)

  Note	You can also use '@within' in a binding form. See the Declaring Advice section for how to make the annotation object available in the advice body.

• Any join point (method execution only in Spring AOP) where the executing method has an @Transactional annotation:

  	@annotation(org.springframework.transaction.annotation.Transactional)

  Note	You can also use '@annotation' in a binding form. See the Declaring Advice section for how to make the annotation object available in the advice body.

• Any join point (method execution only in Spring AOP) which takes a single parameter, and where the runtime type of the argument passed has the @Classified annotation:

  	@args(com.xyz.security.Classified)

  Note	You can also use '@args' in a binding form. See the Declaring Advice section how to make the annotation object(s) available in the advice body.

• Any join point (method execution only in Spring AOP) on a Spring bean named tradeService:

  	bean(tradeService)

• Any join point (method execution only in Spring AOP) on Spring beans having names that match the wildcard expression *Service:

  	bean(*Service)

Writing Good Pointcuts

During compilation, AspectJ processes pointcuts in order to optimize matching performance. Examining code and determining if each join point matches (statically or dynamically) a given pointcut is a costly process. (A dynamic match means the match cannot be fully determined from static analysis and that a test is placed in the code to determine if there is an actual match when the code is running). On first encountering a pointcut declaration, AspectJ rewrites it into an optimal form for the matching process. What does this mean? Basically, pointcuts are rewritten in DNF (Disjunctive Normal Form) and the components of the pointcut are sorted such that those components that are cheaper to evaluate are checked first. This means you do not have to worry about understanding the performance of various pointcut designators and may supply them in any order in a pointcut declaration.

However, AspectJ can work only with what it is told. For optimal performance of matching, you should think about what they are trying to achieve and narrow the search space for matches as much as possible in the definition. The existing designators naturally fall into one of three groups: kinded, scoping, and contextual:

• Kinded designators select a particular kind of join point: execution, get, set, call, and handler.

• Scoping designators select a group of join points of interest (probably of many kinds): within and withincode

• Contextual designators match (and optionally bind) based on context: this, target, and @annotation

A well written pointcut should include at least the first two types (kinded and scoping). You can include the contextual designators to match based on join point context or bind that context for use in the advice. Supplying only a kinded designator or only a contextual designator works but could affect weaving performance (time and memory used), due to extra processing and analysis. Scoping designators are very fast to match, and using them usage means AspectJ can very quickly dismiss groups of join points that should not be further processed. A good pointcut should always include one if possible.

Declaring Advice

Advice is associated with a pointcut expression and runs before, after, or around method executions matched by the pointcut. The pointcut expression may be either a simple reference to a named pointcut or a pointcut expression declared in place.

Before Advice

You can declare before advice in an aspect by using the @Before annotation:

Java

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Before;

@Aspect
public class BeforeExample {

	@Before("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
	public void doAccessCheck() {
		// ...
	}

}

Kotlin

import org.aspectj.lang.annotation.Aspect
import org.aspectj.lang.annotation.Before

@Aspect
class BeforeExample {

	@Before("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
	fun doAccessCheck() {
		// ...
	}

}

If we use an in-place pointcut expression, we could rewrite the preceding example as the following example:

Java

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Before;

@Aspect
public class BeforeExample {

	@Before("execution(* com.xyz.myapp.dao.*.*(..))")
	public void doAccessCheck() {
		// ...
	}

}

Kotlin

import org.aspectj.lang.annotation.Aspect
import org.aspectj.lang.annotation.Before

@Aspect
class BeforeExample {

	@Before("execution(* com.xyz.myapp.dao.*.*(..))")
	fun doAccessCheck() {
		// ...
	}

}

After Returning Advice

After returning advice runs when a matched method execution returns normally. You can declare it by using the @AfterReturning annotation:

Java

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.AfterReturning;

@Aspect
public class AfterReturningExample {

	@AfterReturning("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
	public void doAccessCheck() {
		// ...
	}

}

Kotlin

import org.aspectj.lang.annotation.Aspect
import org.aspectj.lang.annotation.AfterReturning

@Aspect
class AfterReturningExample {

	@AfterReturning("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
	fun doAccessCheck() {
		// ...
	}

Note	You can have multiple advice declarations (and other members as well), all inside the same aspect. We show only a single advice declaration in these examples to focus the effect of each one.

Sometimes, you need access in the advice body to the actual value that was returned. You can use the form of @AfterReturning that binds the return value to get that access, as the following example shows:

Java

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.AfterReturning;

@Aspect
public class AfterReturningExample {

	@AfterReturning(
		pointcut="com.xyz.myapp.SystemArchitecture.dataAccessOperation()",
		returning="retVal")
	public void doAccessCheck(Object retVal) {
		// ...
	}

}

Kotlin

import org.aspectj.lang.annotation.Aspect
import org.aspectj.lang.annotation.AfterReturning

@Aspect
class AfterReturningExample {

	@AfterReturning(pointcut = "com.xyz.myapp.SystemArchitecture.dataAccessOperation()", returning = "retVal")
	fun doAccessCheck(retVal: Any) {
		// ...
	}

}

The name used in the returning attribute must correspond to the name of a parameter in the advice method. When a method execution returns, the return value is passed to the advice method as the corresponding argument value. A returning clause also restricts matching to only those method executions that return a value of the specified type (in this case, Object, which matches any return value).

Please note that it is not possible to return a totally different reference when using after returning advice.

After Throwing Advice

After throwing advice runs when a matched method execution exits by throwing an exception. You can declare it by using the @AfterThrowing annotation, as the following example shows:

Java

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.AfterThrowing;

@Aspect
public class AfterThrowingExample {

	@AfterThrowing("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
	public void doRecoveryActions() {
		// ...
	}

}

Kotlin

import org.aspectj.lang.annotation.Aspect
import org.aspectj.lang.annotation.AfterThrowing

@Aspect
class AfterThrowingExample {

	@AfterThrowing("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
	fun doRecoveryActions() {
		// ...
	}

}

Often, you want the advice to run only when exceptions of a given type are thrown, and you also often need access to the thrown exception in the advice body. You can use the throwing attribute to both restrict matching (if desired — use Throwable as the exception type otherwise) and bind the thrown exception to an advice parameter. The following example shows how to do so:

Java

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.AfterThrowing;

@Aspect
public class AfterThrowingExample {

	@AfterThrowing(
		pointcut="com.xyz.myapp.SystemArchitecture.dataAccessOperation()",
		throwing="ex")
	public void doRecoveryActions(DataAccessException ex) {
		// ...
	}

}

Kotlin

import org.aspectj.lang.annotation.Aspect
import org.aspectj.lang.annotation.AfterThrowing

@Aspect
class AfterThrowingExample {

	@AfterThrowing(pointcut = "com.xyz.myapp.SystemArchitecture.dataAccessOperation()", throwing = "ex")
	fun doRecoveryActions(ex: DataAccessException) {
		// ...
	}

}

The name used in the throwing attribute must correspond to the name of a parameter in the advice method. When a method execution exits by throwing an exception, the exception is passed to the advice method as the corresponding argument value. A throwing clause also restricts matching to only those method executions that throw an exception of the specified type ( DataAccessException, in this case).

After (Finally) Advice

After (finally) advice runs when a matched method execution exits. It is declared by using the @After annotation. After advice must be prepared to handle both normal and exception return conditions. It is typically used for releasing resources and similar purposes. The following example shows how to use after finally advice:

Java

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.After;

@Aspect
public class AfterFinallyExample {

	@After("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
	public void doReleaseLock() {
		// ...
	}

}

Kotlin

import org.aspectj.lang.annotation.Aspect
import org.aspectj.lang.annotation.After

@Aspect
class AfterFinallyExample {

	@After("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
	fun doReleaseLock() {
		// ...
	}

}

Around Advice

The last kind of advice is around advice. Around advice runs “around” a matched method’s execution. It has the opportunity to do work both before and after the method executes and to determine when, how, and even if the method actually gets to execute at all. Around advice is often used if you need to share state before and after a method execution in a thread-safe manner (starting and stopping a timer, for example). Always use the least powerful form of advice that meets your requirements (that is, do not use around advice if before advice would do).

Around advice is declared by using the @Around annotation. The first parameter of the advice method must be of type ProceedingJoinPoint. Within the body of the advice, calling proceed() on the ProceedingJoinPoint causes the underlying method to execute. The proceed method can also pass in an Object[]. The values in the array are used as the arguments to the method execution when it proceeds.

Note

The behavior of proceed when called with an Object[] is a little different than the behavior of proceed for around advice compiled by the AspectJ compiler. For around advice written using the traditional AspectJ language, the number of arguments passed to proceed must match the number of arguments passed to the around advice (not the number of arguments taken by the underlying join point), and the value passed to proceed in a given argument position supplants the original value at the join point for the entity the value was bound to (do not worry if this does not make sense right now). The approach taken by Spring is simpler and a better match to its proxy-based, execution-only semantics. You only need to be aware of this difference if you compile @AspectJ aspects written for Spring and use proceed with arguments with the AspectJ compiler and weaver. There is a way to write such aspects that is 100% compatible across both Spring AOP and AspectJ, and this is discussed in the following section on advice parameters.

The following example shows how to use around advice:

Java

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Around;
import org.aspectj.lang.ProceedingJoinPoint;

@Aspect
public class AroundExample {

	@Around("com.xyz.myapp.SystemArchitecture.businessService()")
	public Object doBasicProfiling(ProceedingJoinPoint pjp) throws Throwable {
		// start stopwatch
		Object retVal = pjp.proceed();
		// stop stopwatch
		return retVal;
	}

}

Kotlin

import org.aspectj.lang.annotation.Aspect
import org.aspectj.lang.annotation.Around
import org.aspectj.lang.ProceedingJoinPoint

@Aspect
class AroundExample {

	@Around("com.xyz.myapp.SystemArchitecture.businessService()")
	fun doBasicProfiling(pjp: ProceedingJoinPoint): Any {
		// start stopwatch
		val retVal = pjp.proceed()
		// stop stopwatch
		return retVal
	}

}

The value returned by the around advice is the return value seen by the caller of the method. For example, a simple caching aspect could return a value from a cache if it has one and invoke proceed() if it does not. Note that proceed may be invoked once, many times, or not at all within the body of the around advice. All of these are legal.

Advice Parameters

Spring offers fully typed advice, meaning that you declare the parameters you need in the advice signature (as we saw earlier for the returning and throwing examples) rather than work with Object[] arrays all the time. We see how to make argument and other contextual values available to the advice body later in this section. First, we take a look at how to write generic advice that can find out about the method the advice is currently advising.

Access to the Current JoinPoint

Any advice method may declare, as its first parameter, a parameter of type org.aspectj.lang.JoinPoint (note that around advice is required to declare a first parameter of type ProceedingJoinPoint, which is a subclass of JoinPoint. The JoinPoint interface provides a number of useful methods:

• getArgs(): Returns the method arguments.

• getThis(): Returns the proxy object.

• getTarget(): Returns the target object.

• getSignature(): Returns a description of the method that is being advised.

• toString(): Prints a useful description of the method being advised.

See the javadoc for more detail.

Passing Parameters to Advice

We have already seen how to bind the returned value or exception value (using after returning and after throwing advice). To make argument values available to the advice body, you can use the binding form of args. If you use a parameter name in place of a type name in an args expression, the value of the corresponding argument is passed as the parameter value when the advice is invoked. An example should make this clearer. Suppose you want to advise the execution of DAO operations that take an Account object as the first parameter, and you need access to the account in the advice body. You could write the following:

Java

@Before("com.xyz.myapp.SystemArchitecture.dataAccessOperation() && args(account,..)")
public void validateAccount(Account account) {
	// ...
}

Kotlin

@Before("com.xyz.myapp.SystemArchitecture.dataAccessOperation() && args(account,..)")
fun validateAccount(account: Account) {
	// ...
}

The args(account,..) part of the pointcut expression serves two purposes. First, it restricts matching to only those method executions where the method takes at least one parameter, and the argument passed to that parameter is an instance of Account. Second, it makes the actual Account object available to the advice through the account parameter.

Another way of writing this is to declare a pointcut that “provides” the Account object value when it matches a join point, and then refer to the named pointcut from the advice. This would look as follows:

Java

@Pointcut("com.xyz.myapp.SystemArchitecture.dataAccessOperation() && args(account,..)")
private void accountDataAccessOperation(Account account) {}

@Before("accountDataAccessOperation(account)")
public void validateAccount(Account account) {
	// ...
}

Kotlin

@Pointcut("com.xyz.myapp.SystemArchitecture.dataAccessOperation() && args(account,..)")
private fun accountDataAccessOperation(account: Account) {
}

@Before("accountDataAccessOperation(account)")
fun validateAccount(account: Account) {
	// ...
}

See the AspectJ programming guide for more details.

The proxy object ( this), target object ( target), and annotations ( @within, @target, @annotation, and @args) can all be bound in a similar fashion. The next two examples show how to match the execution of methods annotated with an @Auditable annotation and extract the audit code:

The first of the two examples shows the definition of the @Auditable annotation:

Java

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Auditable {
	AuditCode value();
}

Kotlin

@Retention(AnnotationRetention.RUNTIME)
@Target(AnnotationTarget.FUNCTION)
annotation class Auditable(val value: AuditCode)

The second of the two examples shows the advice that matches the execution of @Auditable methods:

Java

@Before("com.xyz.lib.Pointcuts.anyPublicMethod() && @annotation(auditable)")
public void audit(Auditable auditable) {
	AuditCode code = auditable.value();
	// ...
}

Kotlin

@Before("com.xyz.lib.Pointcuts.anyPublicMethod() && @annotation(auditable)")
fun audit(auditable: Auditable) {
	val code = auditable.value()
	// ...
}

Advice Parameters and Generics

Spring AOP can handle generics used in class declarations and method parameters. Suppose you have a generic type like the following:

Java

public interface Sample<T> {
	void sampleGenericMethod(T param);
	void sampleGenericCollectionMethod(Collection<T> param);
}

Kotlin

interface Sample<T> {
	fun sampleGenericMethod(param: T)
	fun sampleGenericCollectionMethod(param: Collection<T>)
}

You can restrict interception of method types to certain parameter types by typing the advice parameter to the parameter type for which you want to intercept the method:

Java

@Before("execution(* ..Sample+.sampleGenericMethod(*)) && args(param)")
public void beforeSampleMethod(MyType param) {
	// Advice implementation
}

Kotlin

@Before("execution(* ..Sample+.sampleGenericMethod(*)) && args(param)")
fun beforeSampleMethod(param: MyType) {
	// Advice implementation
}

This approach does not work for generic collections. So you cannot define a pointcut as follows:

Java

@Before("execution(* ..Sample+.sampleGenericCollectionMethod(*)) && args(param)")
public void beforeSampleMethod(Collection<MyType> param) {
	// Advice implementation
}

Kotlin

@Before("execution(* ..Sample+.sampleGenericCollectionMethod(*)) && args(param)")
fun beforeSampleMethod(param: Collection<MyType>) {
	// Advice implementation
}

To make this work, we would have to inspect every element of the collection, which is not reasonable, as we also cannot decide how to treat null values in general. To achieve something similar to this, you have to type the parameter to Collection<?> and manually check the type of the elements.

Determining Argument Names

The parameter binding in advice invocations relies on matching names used in pointcut expressions to declared parameter names in advice and pointcut method signatures. Parameter names are not available through Java reflection, so Spring AOP uses the following strategy to determine parameter names:

• If the parameter names have been explicitly specified by the user, the specified parameter names are used. Both the advice and the pointcut annotations have an optional argNames attribute that you can use to specify the argument names of the annotated method. These argument names are available at runtime. The following example shows how to use the argNames attribute:

Java

@Before(value="com.xyz.lib.Pointcuts.anyPublicMethod() && target(bean) && @annotation(auditable)",
		argNames="bean,auditable")
public void audit(Object bean, Auditable auditable) {
	AuditCode code = auditable.value();
	// ... use code and bean
}

Kotlin

@Before(value = "com.xyz.lib.Pointcuts.anyPublicMethod() && target(bean) && @annotation(auditable)", argNames = "bean,auditable")
fun audit(bean: Any, auditable: Auditable) {
	val code = auditable.value()
	// ... use code and bean
}

If the first parameter is of the JoinPoint, ProceedingJoinPoint, or JoinPoint.StaticPart type, you can leave out the name of the parameter from the value of the argNames attribute. For example, if you modify the preceding advice to receive the join point object, the argNames attribute need not include it:

Java

@Before(value="com.xyz.lib.Pointcuts.anyPublicMethod() && target(bean) && @annotation(auditable)",
		argNames="bean,auditable")
public void audit(JoinPoint jp, Object bean, Auditable auditable) {
	AuditCode code = auditable.value();
	// ... use code, bean, and jp
}

Kotlin

@Before(value = "com.xyz.lib.Pointcuts.anyPublicMethod() && target(bean) && @annotation(auditable)", argNames = "bean,auditable")
fun audit(jp: JoinPoint, bean: Any, auditable: Auditable) {
	val code = auditable.value()
	// ... use code, bean, and jp
}

The special treatment given to the first parameter of the JoinPoint, ProceedingJoinPoint, and JoinPoint.StaticPart types is particularly convenient for advice instances that do not collect any other join point context. In such situations, you may omit the argNames attribute. For example, the following advice need not declare the argNames attribute:

Java

@Before("com.xyz.lib.Pointcuts.anyPublicMethod()")
public void audit(JoinPoint jp) {
	// ... use jp
}

Kotlin

@Before("com.xyz.lib.Pointcuts.anyPublicMethod()")
fun audit(jp: JoinPoint) {
	// ... use jp
}

• Using the 'argNames' attribute is a little clumsy, so if the 'argNames' attribute has not been specified, Spring AOP looks at the debug information for the class and tries to determine the parameter names from the local variable table. This information is present as long as the classes have been compiled with debug information ( '-g:vars' at a minimum). The consequences of compiling with this flag on are: (1) your code is slightly easier to understand (reverse engineer), (2) the class file sizes are very slightly bigger (typically inconsequential), (3) the optimization to remove unused local variables is not applied by your compiler. In other words, you should encounter no difficulties by building with this flag on.

  Note	If an @AspectJ aspect has been compiled by the AspectJ compiler (ajc) even without the debug information, you need not add the argNames attribute, as the compiler retain the needed information.

• If the code has been compiled without the necessary debug information, Spring AOP tries to deduce the pairing of binding variables to parameters (for example, if only one variable is bound in the pointcut expression, and the advice method takes only one parameter, the pairing is obvious). If the binding of variables is ambiguous given the available information, an AmbiguousBindingException is thrown.

• If all of the above strategies fail, an IllegalArgumentException is thrown.

Proceeding with Arguments

We remarked earlier that we would describe how to write a proceed call with arguments that works consistently across Spring AOP and AspectJ. The solution is to ensure that the advice signature binds each of the method parameters in order. The following example shows how to do so:

Java

@Around("execution(List<Account> find*(..)) && " +
		"com.xyz.myapp.SystemArchitecture.inDataAccessLayer() && " +
		"args(accountHolderNamePattern)")
public Object preProcessQueryPattern(ProceedingJoinPoint pjp,
		String accountHolderNamePattern) throws Throwable {
	String newPattern = preProcess(accountHolderNamePattern);
	return pjp.proceed(new Object[] {newPattern});
}

Kotlin

@Around("execution(List<Account> find*(..)) && " +
		"com.xyz.myapp.SystemArchitecture.inDataAccessLayer() && " +
		"args(accountHolderNamePattern)")
fun preProcessQueryPattern(pjp: ProceedingJoinPoint,
						accountHolderNamePattern: String): Any {
	val newPattern = preProcess(accountHolderNamePattern)
	return pjp.proceed(arrayOf<Any>(newPattern))
}

In many cases, you do this binding anyway (as in the preceding example).

Advice Ordering

What happens when multiple pieces of advice all want to run at the same join point? Spring AOP follows the same precedence rules as AspectJ to determine the order of advice execution. The highest precedence advice runs first "on the way in" (so, given two pieces of before advice, the one with highest precedence runs first). "On the way out" from a join point, the highest precedence advice runs last (so, given two pieces of after advice, the one with the highest precedence will run second).

When two pieces of advice defined in different aspects both need to run at the same join point, unless you specify otherwise, the order of execution is undefined. You can control the order of execution by specifying precedence. This is done in the normal Spring way by either implementing the org.springframework.core.Ordered interface in the aspect class or annotating it with the @Order annotation. Given two aspects, the aspect returning the lower value from Ordered.getValue() (or the annotation value) has the higher precedence.

Note

As of Spring Framework 5.2.7, advice methods defined in the same @Aspect class that need to run at the same join point are assigned precedence based on their advice type in the following order, from highest to lowest precedence: @Around, @Before, @After, @AfterReturning, @AfterThrowing. Note, however, that due to the implementation style in Spring’s AspectJAfterAdvice, an @After advice method will effectively be invoked after any @AfterReturning or @AfterThrowing advice methods in the same aspect. When two pieces of the same type of advice (for example, two @After advice methods) defined in the same @Aspect class both need to run at the same join point, the ordering is undefined (since there is no way to retrieve the source code declaration order through reflection for javac-compiled classes). Consider collapsing such advice methods into one advice method per join point in each @Aspect class or refactor the pieces of advice into separate @Aspect classes that you can order at the aspect level via Ordered or @Order.

Introductions

Introductions (known as inter-type declarations in AspectJ) enable an aspect to declare that advised objects implement a given interface, and to provide an implementation of that interface on behalf of those objects.

You can make an introduction by using the @DeclareParents annotation. This annotation is used to declare that matching types have a new parent (hence the name). For example, given an interface named UsageTracked and an implementation of that interface named DefaultUsageTracked, the following aspect declares that all implementors of service interfaces also implement the UsageTracked interface (to expose statistics via JMX for example):

Java

@Aspect
public class UsageTracking {

	@DeclareParents(value="com.xzy.myapp.service.*+", defaultImpl=DefaultUsageTracked.class)
	public static UsageTracked mixin;

	@Before("com.xyz.myapp.SystemArchitecture.businessService() && this(usageTracked)")
	public void recordUsage(UsageTracked usageTracked) {
		usageTracked.incrementUseCount();
	}

}

Kotlin

@Aspect
class UsageTracking {

	companion object {
		@DeclareParents(value = "com.xzy.myapp.service.*+", defaultImpl = DefaultUsageTracked::class)
		lateinit var mixin: UsageTracked
	}

	@Before("com.xyz.myapp.SystemArchitecture.businessService() && this(usageTracked)")
	fun recordUsage(usageTracked: UsageTracked) {
		usageTracked.incrementUseCount()
	}
}

The interface to be implemented is determined by the type of the annotated field. The value attribute of the @DeclareParents annotation is an AspectJ type pattern. Any bean of a matching type implements the UsageTracked interface. Note that, in the before advice of the preceding example, service beans can be directly used as implementations of the UsageTracked interface. If accessing a bean programmatically, you would write the following:

Java

UsageTracked usageTracked = (UsageTracked) context.getBean("myService");

Kotlin

val usageTracked = context.getBean("myService") as UsageTracked

Aspect Instantiation Models

Note	This is an advanced topic. If you are just starting out with AOP, you can safely skip it until later.

By default, there is a single instance of each aspect within the application context. AspectJ calls this the singleton instantiation model. It is possible to define aspects with alternate lifecycles. Spring supports AspectJ’s perthis and pertarget instantiation models ( percflow, percflowbelow, and pertypewithin are not currently supported).

You can declare a perthis aspect by specifying a perthis clause in the @Aspect annotation. Consider the following example:

Java

@Aspect("perthis(com.xyz.myapp.SystemArchitecture.businessService())")
public class MyAspect {

	private int someState;

	@Before(com.xyz.myapp.SystemArchitecture.businessService())
	public void recordServiceUsage() {
		// ...
	}

}

Kotlin

@Aspect("perthis(com.xyz.myapp.SystemArchitecture.businessService())")
class MyAspect {

	private val someState: Int = 0

	@Before(com.xyz.myapp.SystemArchitecture.businessService())
	fun recordServiceUsage() {
		// ...
	}

}

In the preceding example, the effect of the 'perthis' clause is that one aspect instance is created for each unique service object that executes a business service (each unique object bound to 'this' at join points matched by the pointcut expression). The aspect instance is created the first time that a method is invoked on the service object. The aspect goes out of scope when the service object goes out of scope. Before the aspect instance is created, none of the advice within it executes. As soon as the aspect instance has been created, the advice declared within it executes at matched join points, but only when the service object is the one with which this aspect is associated. See the AspectJ Programming Guide for more information on per clauses.

The pertarget instantiation model works in exactly the same way as perthis, but it creates one aspect instance for each unique target object at matched join points.

An AOP Example

Now that you have seen how all the constituent parts work, we can put them together to do something useful.

The execution of business services can sometimes fail due to concurrency issues (for example, a deadlock loser). If the operation is retried, it is likely to succeed on the next try. For business services where it is appropriate to retry in such conditions (idempotent operations that do not need to go back to the user for conflict resolution), we want to transparently retry the operation to avoid the client seeing a PessimisticLockingFailureException. This is a requirement that clearly cuts across multiple services in the service layer and, hence, is ideal for implementing through an aspect.

Because we want to retry the operation, we need to use around advice so that we can call proceed multiple times. The following listing shows the basic aspect implementation:

Java

@Aspect
public class ConcurrentOperationExecutor implements Ordered {

	private static final int DEFAULT_MAX_RETRIES = 2;

	private int maxRetries = DEFAULT_MAX_RETRIES;
	private int order = 1;

	public void setMaxRetries(int maxRetries) {
		this.maxRetries = maxRetries;
	}

	public int getOrder() {
		return this.order;
	}

	public void setOrder(int order) {
		this.order = order;
	}

	@Around("com.xyz.myapp.SystemArchitecture.businessService()")
	public Object doConcurrentOperation(ProceedingJoinPoint pjp) throws Throwable {
		int numAttempts = 0;
		PessimisticLockingFailureException lockFailureException;
		do {
			numAttempts++;
			try {
				return pjp.proceed();
			}
			catch(PessimisticLockingFailureException ex) {
				lockFailureException = ex;
			}
		} while(numAttempts <= this.maxRetries);
		throw lockFailureException;
	}

}

Kotlin

@Aspect
class ConcurrentOperationExecutor : Ordered {

	private val DEFAULT_MAX_RETRIES = 2
	private var maxRetries = DEFAULT_MAX_RETRIES
	private var order = 1

	fun setMaxRetries(maxRetries: Int) {
		this.maxRetries = maxRetries
	}

	override fun getOrder(): Int {
		return this.order
	}

	fun setOrder(order: Int) {
		this.order = order
	}

	@Around("com.xyz.myapp.SystemArchitecture.businessService()")
	fun doConcurrentOperation(pjp: ProceedingJoinPoint): Any {
		var numAttempts = 0
		var lockFailureException: PessimisticLockingFailureException
		do {
			numAttempts++
			try {
				return pjp.proceed()
			} catch (ex: PessimisticLockingFailureException) {
				lockFailureException = ex
			}

		} while (numAttempts <= this.maxRetries)
		throw lockFailureException
	}
}

Note that the aspect implements the Ordered interface so that we can set the precedence of the aspect higher than the transaction advice (we want a fresh transaction each time we retry). The maxRetries and order properties are both configured by Spring. The main action happens in the doConcurrentOperation around advice. Notice that, for the moment, we apply the retry logic to each businessService(). We try to proceed, and if we fail with a PessimisticLockingFailureException, we try again, unless we have exhausted all of our retry attempts.

The corresponding Spring configuration follows:

<aop:aspectj-autoproxy/>

<bean id="concurrentOperationExecutor" class="com.xyz.myapp.service.impl.ConcurrentOperationExecutor">
	<property name="maxRetries" value="3"/>
	<property name="order" value="100"/>
</bean>

To refine the aspect so that it retries only idempotent operations, we might define the following Idempotent annotation:

Java

@Retention(RetentionPolicy.RUNTIME)
public @interface Idempotent {
	// marker annotation
}

Kotlin

@Retention(AnnotationRetention.RUNTIME)
annotation class Idempotent// marker annotation

We can then use the annotation to annotate the implementation of service operations. The change to the aspect to retry only idempotent operations involves refining the pointcut expression so that only @Idempotent operations match, as follows:

Java

@Around("com.xyz.myapp.SystemArchitecture.businessService() && " +
		"@annotation(com.xyz.myapp.service.Idempotent)")
public Object doConcurrentOperation(ProceedingJoinPoint pjp) throws Throwable {
	// ...
}

Kotlin

@Around("com.xyz.myapp.SystemArchitecture.businessService() && " + "@annotation(com.xyz.myapp.service.Idempotent)")
fun doConcurrentOperation(pjp: ProceedingJoinPoint): Any {
	// ...
}

Schema-based AOP Support

If you prefer an XML-based format, Spring also offers support for defining aspects using the aop namespace tags. The exact same pointcut expressions and advice kinds as when using the @AspectJ style are supported. Hence, in this section we focus on that syntax and refer the reader to the discussion in the previous section (@AspectJ support) for an understanding of writing pointcut expressions and the binding of advice parameters.

To use the aop namespace tags described in this section, you need to import the spring-aop schema, as described in XML Schema-based configuration. See the AOP schema for how to import the tags in the aop namespace.

Within your Spring configurations, all aspect and advisor elements must be placed within an <aop:config> element (you can have more than one <aop:config> element in an application context configuration). An <aop:config> element can contain pointcut, advisor, and aspect elements (note that these must be declared in that order).

Warning

The <aop:config> style of configuration makes heavy use of Spring’s auto-proxying mechanism. This can cause issues (such as advice not being woven) if you already use explicit auto-proxying through the use of BeanNameAutoProxyCreator or something similar. The recommended usage pattern is to use either only the <aop:config> style or only the AutoProxyCreator style and never mix them.

Declaring an Aspect

When you use the schema support, an aspect is a regular Java object defined as a bean in your Spring application context. The state and behavior are captured in the fields and methods of the object, and the pointcut and advice information are captured in the XML.

You can declare an aspect by using the <aop:aspect> element, and reference the backing bean by using the ref attribute, as the following example shows:

<aop:config>
	<aop:aspect id="myAspect" ref="aBean">
		...
	</aop:aspect>
</aop:config>

<bean id="aBean" class="...">
	...
</bean>

The bean that backs the aspect (aBean in this case) can of course be configured and dependency injected just like any other Spring bean.

Declaring a Pointcut

You can declare a named pointcut inside an <aop:config> element, letting the pointcut definition be shared across several aspects and advisors.

A pointcut that represents the execution of any business service in the service layer can be defined as follows:

<aop:config>

	<aop:pointcut id="businessService"
		expression="execution(* com.xyz.myapp.service.*.*(..))"/>

</aop:config>

Note that the pointcut expression itself is using the same AspectJ pointcut expression language as described in @AspectJ support. If you use the schema based declaration style, you can refer to named pointcuts defined in types (@Aspects) within the pointcut expression. Another way of defining the above pointcut would be as follows:

<aop:config>

	<aop:pointcut id="businessService"
		expression="com.xyz.myapp.SystemArchitecture.businessService()"/>

</aop:config>

Assume that you have a SystemArchitecture aspect as described in Sharing Common Pointcut Definitions.

Then declaring a pointcut inside an aspect is very similar to declaring a top-level pointcut, as the following example shows:

<aop:config>

	<aop:aspect id="myAspect" ref="aBean">

		<aop:pointcut id="businessService"
			expression="execution(* com.xyz.myapp.service.*.*(..))"/>

		...

	</aop:aspect>

</aop:config>

In much the same way as an @AspectJ aspect, pointcuts declared by using the schema based definition style can collect join point context. For example, the following pointcut collects the this object as the join point context and passes it to the advice:

<aop:config>

	<aop:aspect id="myAspect" ref="aBean">

		<aop:pointcut id="businessService"
			expression="execution(* com.xyz.myapp.service.*.*(..)) &amp;&amp; this(service)"/>

		<aop:before pointcut-ref="businessService" method="monitor"/>

		...

	</aop:aspect>

</aop:config>

The advice must be declared to receive the collected join point context by including parameters of the matching names, as follows:

Java

public void monitor(Object service) {
	// ...
}

Kotlin

fun monitor(service: Any) {
	// ...
}

When combining pointcut sub-expressions, && is awkward within an XML document, so you can use the and, or, and not keywords in place of &&, ||, and !, respectively. For example, the previous pointcut can be better written as follows:

<aop:config>

	<aop:aspect id="myAspect" ref="aBean">

		<aop:pointcut id="businessService"
			expression="execution(* com.xyz.myapp.service.*.*(..)) and this(service)"/>

		<aop:before pointcut-ref="businessService" method="monitor"/>

		...
	</aop:aspect>
</aop:config>

Note that pointcuts defined in this way are referred to by their XML id and cannot be used as named pointcuts to form composite pointcuts. The named pointcut support in the schema-based definition style is thus more limited than that offered by the @AspectJ style.

Declaring Advice

The schema-based AOP support uses the same five kinds of advice as the @AspectJ style, and they have exactly the same semantics.

Before Advice

Before advice runs before a matched method execution. It is declared inside an <aop:aspect> by using the <aop:before> element, as the following example shows:

<aop:aspect id="beforeExample" ref="aBean">

	<aop:before
		pointcut-ref="dataAccessOperation"
		method="doAccessCheck"/>

	...

</aop:aspect>

Here, dataAccessOperation is the id of a pointcut defined at the top (<aop:config>) level. To define the pointcut inline instead, replace the pointcut-ref attribute with a pointcut attribute, as follows:

<aop:aspect id="beforeExample" ref="aBean">

	<aop:before
		pointcut="execution(* com.xyz.myapp.dao.*.*(..))"
		method="doAccessCheck"/>

	...

</aop:aspect>

As we noted in the discussion of the @AspectJ style, using named pointcuts can significantly improve the readability of your code.

The method attribute identifies a method (doAccessCheck) that provides the body of the advice. This method must be defined for the bean referenced by the aspect element that contains the advice. Before a data access operation is executed (a method execution join point matched by the pointcut expression), the doAccessCheck method on the aspect bean is invoked.

After Returning Advice

After returning advice runs when a matched method execution completes normally. It is declared inside an <aop:aspect> in the same way as before advice. The following example shows how to declare it:

<aop:aspect id="afterReturningExample" ref="aBean">

	<aop:after-returning
		pointcut-ref="dataAccessOperation"
		method="doAccessCheck"/>

	...

</aop:aspect>

As in the @AspectJ style, you can get the return value within the advice body. To do so, use the returning attribute to specify the name of the parameter to which the return value should be passed, as the following example shows:

<aop:aspect id="afterReturningExample" ref="aBean">

	<aop:after-returning
		pointcut-ref="dataAccessOperation"
		returning="retVal"
		method="doAccessCheck"/>

	...

</aop:aspect>

The doAccessCheck method must declare a parameter named retVal. The type of this parameter constrains matching in the same way as described for @AfterReturning. For example, you can declare the method signature as follows:

Java

public void doAccessCheck(Object retVal) {...

Kotlin

fun doAccessCheck(retVal: Any) {...

After Throwing Advice

After throwing advice executes when a matched method execution exits by throwing an exception. It is declared inside an <aop:aspect> by using the after-throwing element, as the following example shows:

<aop:aspect id="afterThrowingExample" ref="aBean">

	<aop:after-throwing
		pointcut-ref="dataAccessOperation"
		method="doRecoveryActions"/>

	...

</aop:aspect>

As in the @AspectJ style, you can get the thrown exception within the advice body. To do so, use the throwing attribute to specify the name of the parameter to which the exception should be passed as the following example shows:

<aop:aspect id="afterThrowingExample" ref="aBean">

	<aop:after-throwing
		pointcut-ref="dataAccessOperation"
		throwing="dataAccessEx"
		method="doRecoveryActions"/>

	...

</aop:aspect>

The doRecoveryActions method must declare a parameter named dataAccessEx. The type of this parameter constrains matching in the same way as described for @AfterThrowing. For example, the method signature may be declared as follows:

Java

public void doRecoveryActions(DataAccessException dataAccessEx) {...

Kotlin

fun doRecoveryActions(dataAccessEx: DataAccessException) {...

After (Finally) Advice

After (finally) advice runs no matter how a matched method execution exits. You can declare it by using the after element, as the following example shows:

<aop:aspect id="afterFinallyExample" ref="aBean">

	<aop:after
		pointcut-ref="dataAccessOperation"
		method="doReleaseLock"/>

	...

</aop:aspect>

Around Advice

The last kind of advice is around advice. Around advice runs "around" a matched method execution. It has the opportunity to do work both before and after the method executes and to determine when, how, and even if the method actually gets to execute at all. Around advice is often used to share state before and after a method execution in a thread-safe manner (starting and stopping a timer, for example). Always use the least powerful form of advice that meets your requirements. Do not use around advice if before advice can do the job.

You can declare around advice by using the aop:around element. The first parameter of the advice method must be of type ProceedingJoinPoint. Within the body of the advice, calling proceed() on the ProceedingJoinPoint causes the underlying method to execute. The proceed method may also be called with an Object[]. The values in the array are used as the arguments to the method execution when it proceeds. See Around Advice for notes on calling proceed with an Object[]. The following example shows how to declare around advice in XML:

<aop:aspect id="aroundExample" ref="aBean">

	<aop:around
		pointcut-ref="businessService"
		method="doBasicProfiling"/>

	...

</aop:aspect>

The implementation of the doBasicProfiling advice can be exactly the same as in the @AspectJ example (minus the annotation, of course), as the following example shows:

Java

public Object doBasicProfiling(ProceedingJoinPoint pjp) throws Throwable {
	// start stopwatch
	Object retVal = pjp.proceed();
	// stop stopwatch
	return retVal;
}

Kotlin

fun doBasicProfiling(pjp: ProceedingJoinPoint): Any {
	// start stopwatch
	val retVal = pjp.proceed()
	// stop stopwatch
	return pjp.proceed()
}

Advice Parameters

The schema-based declaration style supports fully typed advice in the same way as described for the @AspectJ support — by matching pointcut parameters by name against advice method parameters. See Advice Parameters for details. If you wish to explicitly specify argument names for the advice methods (not relying on the detection strategies previously described), you can do so by using the arg-names attribute of the advice element, which is treated in the same manner as the argNames attribute in an advice annotation (as described in Determining Argument Names). The following example shows how to specify an argument name in XML:

<aop:before
	pointcut="com.xyz.lib.Pointcuts.anyPublicMethod() and @annotation(auditable)"
	method="audit"
	arg-names="auditable"/>

The arg-names attribute accepts a comma-delimited list of parameter names.

The following slightly more involved example of the XSD-based approach shows some around advice used in conjunction with a number of strongly typed parameters:

Java

package x.y.service;

public interface PersonService {

	Person getPerson(String personName, int age);
}

public class DefaultPersonService implements PersonService {

	public Person getPerson(String name, int age) {
		return new Person(name, age);
	}
}

Kotlin

package x.y.service

interface PersonService {

	fun getPerson(personName: String, age: Int): Person
}

class DefaultPersonService : PersonService {

	fun getPerson(name: String, age: Int): Person {
		return Person(name, age)
	}
}

Next up is the aspect. Notice the fact that the profile(..) method accepts a number of strongly-typed parameters, the first of which happens to be the join point used to proceed with the method call. The presence of this parameter is an indication that the profile(..) is to be used as around advice, as the following example shows:

Java

package x.y;

import org.aspectj.lang.ProceedingJoinPoint;
import org.springframework.util.StopWatch;

public class SimpleProfiler {

	public Object profile(ProceedingJoinPoint call, String name, int age) throws Throwable {
		StopWatch clock = new StopWatch("Profiling for '" + name + "' and '" + age + "'");
		try {
			clock.start(call.toShortString());
			return call.proceed();
		} finally {
			clock.stop();
			System.out.println(clock.prettyPrint());
		}
	}
}

Kotlin

import org.aspectj.lang.ProceedingJoinPoint
import org.springframework.util.StopWatch

class SimpleProfiler {

	fun profile(call: ProceedingJoinPoint, name: String, age: Int): Any {
		val clock = StopWatch("Profiling for '$name' and '$age'")
		try {
			clock.start(call.toShortString())
			return call.proceed()
		} finally {
			clock.stop()
			println(clock.prettyPrint())
		}
	}
}

Finally, the following example XML configuration effects the execution of the preceding advice for a particular join point:

<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns:aop="http://www.springframework.org/schema/aop"
	xsi:schemaLocation="
		http://www.springframework.org/schema/beans https://www.springframework.org/schema/beans/spring-beans.xsd
		http://www.springframework.org/schema/aop https://www.springframework.org/schema/aop/spring-aop.xsd">

	<!-- this is the object that will be proxied by Spring's AOP infrastructure -->
	<bean id="personService" class="x.y.service.DefaultPersonService"/>

	<!-- this is the actual advice itself -->
	<bean id="profiler" class="x.y.SimpleProfiler"/>

	<aop:config>
		<aop:aspect ref="profiler">

			<aop:pointcut id="theExecutionOfSomePersonServiceMethod"
				expression="execution(* x.y.service.PersonService.getPerson(String,int))
				and args(name, age)"/>

			<aop:around pointcut-ref="theExecutionOfSomePersonServiceMethod"
				method="profile"/>

		</aop:aspect>
	</aop:config>

</beans>

Consider the following driver script:

Java

import org.springframework.beans.factory.BeanFactory;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import x.y.service.PersonService;

public final class Boot {

	public static void main(final String[] args) throws Exception {
		BeanFactory ctx = new ClassPathXmlApplicationContext("x/y/plain.xml");
		PersonService person = (PersonService) ctx.getBean("personService");
		person.getPerson("Pengo", 12);
	}
}

Kotlin

fun main() {
	val ctx = ClassPathXmlApplicationContext("x/y/plain.xml")
	val person = ctx.getBean("personService") as PersonService
	person.getPerson("Pengo", 12)
}

With such a Boot class, we would get output similar to the following on standard output:

StopWatch 'Profiling for 'Pengo' and '12'': running time (millis) = 0
-----------------------------------------
ms     %     Task name
-----------------------------------------
00000  ?  execution(getFoo)

Advice Ordering

When multiple pieces of advice need to execute at the same join point (executing method) the ordering rules are as described in Advice Ordering. The precedence between aspects is determined via the order attribute in the <aop:aspect> element or by either adding the @Order annotation to the bean that backs the aspect or by having the bean implement the Ordered interface.

Note

In contrast to the precedence rules for advice methods defined in the same @Aspect class, when two pieces of advice defined in the same <aop:aspect> element both need to run at the same join point, the precedence is determined by the order in which the advice elements are declared within the enclosing <aop:aspect> element, from highest to lowest precedence. For example, given an around advice and a before advice defined in the same <aop:aspect> element that apply to the same join point, to ensure that the around advice has higher precedence than the before advice, the <aop:around> element must be declared before the <aop:before> element. As a general rule of thumb, if you find that you have multiple pieces of advice defined in the same <aop:aspect> element that apply to the same join point, consider collapsing such advice methods into one advice method per join point in each <aop:aspect> element or refactor the pieces of advice into separate <aop:aspect> elements that you can order at the aspect level.

Introductions

Introductions (known as inter-type declarations in AspectJ) let an aspect declare that advised objects implement a given interface and provide an implementation of that interface on behalf of those objects.

You can make an introduction by using the aop:declare-parents element inside an aop:aspect. You can use the aop:declare-parents element to declare that matching types have a new parent (hence the name). For example, given an interface named UsageTracked and an implementation of that interface named DefaultUsageTracked, the following aspect declares that all implementors of service interfaces also implement the UsageTracked interface. (In order to expose statistics through JMX for example.)

<aop:aspect id="usageTrackerAspect" ref="usageTracking">

	<aop:declare-parents
		types-matching="com.xzy.myapp.service.*+"
		implement-interface="com.xyz.myapp.service.tracking.UsageTracked"
		default-impl="com.xyz.myapp.service.tracking.DefaultUsageTracked"/>

	<aop:before
		pointcut="com.xyz.myapp.SystemArchitecture.businessService()
			and this(usageTracked)"
			method="recordUsage"/>

</aop:aspect>

The class that backs the usageTracking bean would then contain the following method:

Java

public void recordUsage(UsageTracked usageTracked) {
	usageTracked.incrementUseCount();
}

Kotlin

fun recordUsage(usageTracked: UsageTracked) {
	usageTracked.incrementUseCount()
}

The interface to be implemented is determined by the implement-interface attribute. The value of the types-matching attribute is an AspectJ type pattern. Any bean of a matching type implements the UsageTracked interface. Note that, in the before advice of the preceding example, service beans can be directly used as implementations of the UsageTracked interface. To access a bean programmatically, you could write the following:

Java

UsageTracked usageTracked = (UsageTracked) context.getBean("myService");

Kotlin

val usageTracked = context.getBean("myService") as UsageTracked

Aspect Instantiation Models

The only supported instantiation model for schema-defined aspects is the singleton model. Other instantiation models may be supported in future releases.

Advisors

The concept of “advisors” comes from the AOP support defined in Spring and does not have a direct equivalent in AspectJ. An advisor is like a small self-contained aspect that has a single piece of advice. The advice itself is represented by a bean and must implement one of the advice interfaces described in [aop-api-advice-types]. Advisors can take advantage of AspectJ pointcut expressions.

Spring supports the advisor concept with the <aop:advisor> element. You most commonly see it used in conjunction with transactional advice, which also has its own namespace support in Spring. The following example shows an advisor:

<aop:config>

	<aop:pointcut id="businessService"
		expression="execution(* com.xyz.myapp.service.*.*(..))"/>

	<aop:advisor
		pointcut-ref="businessService"
		advice-ref="tx-advice"/>

</aop:config>

<tx:advice id="tx-advice">
	<tx:attributes>
		<tx:method name="*" propagation="REQUIRED"/>
	</tx:attributes>
</tx:advice>

As well as the pointcut-ref attribute used in the preceding example, you can also use the pointcut attribute to define a pointcut expression inline.

To define the precedence of an advisor so that the advice can participate in ordering, use the order attribute to define the Ordered value of the advisor.

An AOP Schema Example

This section shows how the concurrent locking failure retry example from An AOP Example looks when rewritten with the schema support.

The execution of business services can sometimes fail due to concurrency issues (for example, a deadlock loser). If the operation is retried, it is likely to succeed on the next try. For business services where it is appropriate to retry in such conditions (idempotent operations that do not need to go back to the user for conflict resolution), we want to transparently retry the operation to avoid the client seeing a PessimisticLockingFailureException. This is a requirement that clearly cuts across multiple services in the service layer and, hence, is ideal for implementing through an aspect.

Because we want to retry the operation, we need to use around advice so that we can call proceed multiple times. The following listing shows the basic aspect implementation (which is a regular Java class that uses the schema support):

Java

public class ConcurrentOperationExecutor implements Ordered {

	private static final int DEFAULT_MAX_RETRIES = 2;

	private int maxRetries = DEFAULT_MAX_RETRIES;
	private int order = 1;

	public void setMaxRetries(int maxRetries) {
		this.maxRetries = maxRetries;
	}

	public int getOrder() {
		return this.order;
	}

	public void setOrder(int order) {
		this.order = order;
	}

	public Object doConcurrentOperation(ProceedingJoinPoint pjp) throws Throwable {
		int numAttempts = 0;
		PessimisticLockingFailureException lockFailureException;
		do {
			numAttempts++;
			try {
				return pjp.proceed();
			}
			catch(PessimisticLockingFailureException ex) {
				lockFailureException = ex;
			}
		} while(numAttempts <= this.maxRetries);
		throw lockFailureException;
	}

}

Kotlin

class ConcurrentOperationExecutor : Ordered {

	private val DEFAULT_MAX_RETRIES = 2

	private var maxRetries = DEFAULT_MAX_RETRIES
	private var order = 1

	fun setMaxRetries(maxRetries: Int) {
		this.maxRetries = maxRetries
	}

	override fun getOrder(): Int {
		return this.order
	}

	fun setOrder(order: Int) {
		this.order = order
	}

	fun doConcurrentOperation(pjp: ProceedingJoinPoint): Any {
		var numAttempts = 0
		var lockFailureException: PessimisticLockingFailureException
		do {
			numAttempts++
			try {
				return pjp.proceed()
			} catch (ex: PessimisticLockingFailureException) {
				lockFailureException = ex
			}

		} while (numAttempts <= this.maxRetries)
		throw lockFailureException
	}
}

Note that the aspect implements the Ordered interface so that we can set the precedence of the aspect higher than the transaction advice (we want a fresh transaction each time we retry). The maxRetries and order properties are both configured by Spring. The main action happens in the doConcurrentOperation around advice method. We try to proceed. If we fail with a PessimisticLockingFailureException, we try again, unless we have exhausted all of our retry attempts.

Note	This class is identical to the one used in the @AspectJ example, but with the annotations removed.

The corresponding Spring configuration is as follows:

<aop:config>

	<aop:aspect id="concurrentOperationRetry" ref="concurrentOperationExecutor">

		<aop:pointcut id="idempotentOperation"
			expression="execution(* com.xyz.myapp.service.*.*(..))"/>

		<aop:around
			pointcut-ref="idempotentOperation"
			method="doConcurrentOperation"/>

	</aop:aspect>

</aop:config>

<bean id="concurrentOperationExecutor"
	class="com.xyz.myapp.service.impl.ConcurrentOperationExecutor">
		<property name="maxRetries" value="3"/>
		<property name="order" value="100"/>
</bean>

Notice that, for the time, being we assume that all business services are idempotent. If this is not the case, we can refine the aspect so that it retries only genuinely idempotent operations, by introducing an Idempotent annotation and using the annotation to annotate the implementation of service operations, as the following example shows:

Java

@Retention(RetentionPolicy.RUNTIME)
public @interface Idempotent {
	// marker annotation
}

Kotlin

@Retention(AnnotationRetention.RUNTIME)
annotation class Idempotent {
	// marker annotation
}

The change to the aspect to retry only idempotent operations involves refining the pointcut expression so that only @Idempotent operations match, as follows:

<aop:pointcut id="idempotentOperation"
		expression="execution(* com.xyz.myapp.service.*.*(..)) and
		@annotation(com.xyz.myapp.service.Idempotent)"/>

Choosing which AOP Declaration Style to Use

Once you have decided that an aspect is the best approach for implementing a given requirement, how do you decide between using Spring AOP or AspectJ and between the Aspect language (code) style, the @AspectJ annotation style, or the Spring XML style? These decisions are influenced by a number of factors including application requirements, development tools, and team familiarity with AOP.

Spring AOP or Full AspectJ?

Use the simplest thing that can work. Spring AOP is simpler than using full AspectJ, as there is no requirement to introduce the AspectJ compiler / weaver into your development and build processes. If you only need to advise the execution of operations on Spring beans, Spring AOP is the right choice. If you need to advise objects not managed by the Spring container (such as domain objects, typically), you need to use AspectJ. You also need to use AspectJ if you wish to advise join points other than simple method executions (for example, field get or set join points and so on).

When you use AspectJ, you have the choice of the AspectJ language syntax (also known as the “code style”) or the @AspectJ annotation style. Clearly, if you do not use Java 5+, the choice has been made for you: Use the code style. If aspects play a large role in your design, and you are able to use the AspectJ Development Tools (AJDT) plugin for Eclipse, the AspectJ language syntax is the preferred option. It is cleaner and simpler because the language was purposefully designed for writing aspects. If you do not use Eclipse or have only a few aspects that do not play a major role in your application, you may want to consider using the @AspectJ style, sticking with regular Java compilation in your IDE, and adding an aspect weaving phase to your build script.

@AspectJ or XML for Spring AOP?

If you have chosen to use Spring AOP, you have a choice of @AspectJ or XML style. There are various tradeoffs to consider.

The XML style may most familiar to existing Spring users, and it is backed by genuine POJOs. When using AOP as a tool to configure enterprise services, XML can be a good choice (a good test is whether you consider the pointcut expression to be a part of your configuration that you might want to change independently). With the XML style, it is arguably clearer from your configuration which aspects are present in the system.

The XML style has two disadvantages. First, it does not fully encapsulate the implementation of the requirement it addresses in a single place. The DRY principle says that there should be a single, unambiguous, authoritative representation of any piece of knowledge within a system. When using the XML style, the knowledge of how a requirement is implemented is split across the declaration of the backing bean class and the XML in the configuration file. When you use the @AspectJ style, this information is encapsulated in a single module: the aspect. Secondly, the XML style is slightly more limited in what it can express than the @AspectJ style: Only the “singleton” aspect instantiation model is supported, and it is not possible to combine named pointcuts declared in XML. For example, in the @AspectJ style you can write something like the following:

Java

@Pointcut("execution(* get*())")
public void propertyAccess() {}

@Pointcut("execution(org.xyz.Account+ *(..))")
public void operationReturningAnAccount() {}

@Pointcut("propertyAccess() && operationReturningAnAccount()")
public void accountPropertyAccess() {}

Kotlin

@Pointcut("execution(* get*())")
fun propertyAccess() {}

@Pointcut("execution(org.xyz.Account+ *(..))")
fun operationReturningAnAccount() {}

@Pointcut("propertyAccess() && operationReturningAnAccount()")
fun accountPropertyAccess() {}

In the XML style you can declare the first two pointcuts:

<aop:pointcut id="propertyAccess"
		expression="execution(* get*())"/>

<aop:pointcut id="operationReturningAnAccount"
		expression="execution(org.xyz.Account+ *(..))"/>

The downside of the XML approach is that you cannot define the accountPropertyAccess pointcut by combining these definitions.

The @AspectJ style supports additional instantiation models and richer pointcut composition. It has the advantage of keeping the aspect as a modular unit. It also has the advantage that the @AspectJ aspects can be understood (and thus consumed) both by Spring AOP and by AspectJ. So, if you later decide you need the capabilities of AspectJ to implement additional requirements, you can easily migrate to a classic AspectJ setup. On balance, the Spring team prefers the @AspectJ style for custom aspects beyond simple configuration of enterprise services.

Mixing Aspect Types

It is perfectly possible to mix @AspectJ style aspects by using the auto-proxying support, schema-defined <aop:aspect> aspects, <aop:advisor> declared advisors, and even proxies and interceptors in other styles in the same configuration. All of these are implemented by using the same underlying support mechanism and can co-exist without any difficulty.

Proxying Mechanisms

Spring AOP uses either JDK dynamic proxies or CGLIB to create the proxy for a given target object. JDK dynamic proxies are built into the JDK, whereas CGLIB is a common open-source class definition library (repackaged into spring-core).

If the target object to be proxied implements at least one interface, a JDK dynamic proxy is used. All of the interfaces implemented by the target type are proxied. If the target object does not implement any interfaces, a CGLIB proxy is created.

If you want to force the use of CGLIB proxying (for example, to proxy every method defined for the target object, not only those implemented by its interfaces), you can do so. However, you should consider the following issues:

• With CGLIB, final methods cannot be advised, as they cannot be overridden in runtime-generated subclasses.

• As of Spring 4.0, the constructor of your proxied object is NOT called twice anymore, since the CGLIB proxy instance is created through Objenesis. Only if your JVM does not allow for constructor bypassing, you might see double invocations and corresponding debug log entries from Spring’s AOP support.

To force the use of CGLIB proxies, set the value of the proxy-target-class attribute of the <aop:config> element to true, as follows:

<aop:config proxy-target-class="true">
	<!-- other beans defined here... -->
</aop:config>

To force CGLIB proxying when you use the @AspectJ auto-proxy support, set the proxy-target-class attribute of the <aop:aspectj-autoproxy> element to true, as follows:

<aop:aspectj-autoproxy proxy-target-class="true"/>

Note

Multiple <aop:config/> sections are collapsed into a single unified auto-proxy creator at runtime, which applies the strongest proxy settings that any of the <aop:config/> sections (typically from different XML bean definition files) specified. This also applies to the <tx:annotation-driven/> and <aop:aspectj-autoproxy/> elements. To be clear, using proxy-target-class="true" on <tx:annotation-driven/>, <aop:aspectj-autoproxy/>, or <aop:config/> elements forces the use of CGLIB proxies for all three of them.

Understanding AOP Proxies

Spring AOP is proxy-based. It is vitally important that you grasp the semantics of what that last statement actually means before you write your own aspects or use any of the Spring AOP-based aspects supplied with the Spring Framework.

Consider first the scenario where you have a plain-vanilla, un-proxied, nothing-special-about-it, straight object reference, as the following code snippet shows:

Java

public class SimplePojo implements Pojo {

	public void foo() {
		// this next method invocation is a direct call on the 'this' reference
		this.bar();
	}

	public void bar() {
		// some logic...
	}
}

Kotlin

class SimplePojo : Pojo {

	fun foo() {
		// this next method invocation is a direct call on the 'this' reference
		this.bar()
	}

	fun bar() {
		// some logic...
	}
}

If you invoke a method on an object reference, the method is invoked directly on that object reference, as the following image and listing show:

Java

public class Main {

	public static void main(String[] args) {
		Pojo pojo = new SimplePojo();
		// this is a direct method call on the 'pojo' reference
		pojo.foo();
	}
}

Kotlin

fun main() {
	val pojo = SimplePojo()
	// this is a direct method call on the 'pojo' reference
	pojo.foo()
}

Things change slightly when the reference that client code has is a proxy. Consider the following diagram and code snippet:

Java

public class Main {

	public static void main(String[] args) {
		ProxyFactory factory = new ProxyFactory(new SimplePojo());
		factory.addInterface(Pojo.class);
		factory.addAdvice(new RetryAdvice());

		Pojo pojo = (Pojo) factory.getProxy();
		// this is a method call on the proxy!
		pojo.foo();
	}
}

Kotlin

fun main() {
	val factory = ProxyFactory(SimplePojo())
	factory.addInterface(Pojo::class.java)
	factory.addAdvice(RetryAdvice())

	val pojo = factory.proxy as Pojo
	// this is a method call on the proxy!
	pojo.foo()
}

The key thing to understand here is that the client code inside the main(..) method of the Main class has a reference to the proxy. This means that method calls on that object reference are calls on the proxy. As a result, the proxy can delegate to all of the interceptors (advice) that are relevant to that particular method call. However, once the call has finally reached the target object (the SimplePojo, reference in this case), any method calls that it may make on itself, such as this.bar() or this.foo(), are going to be invoked against the this reference, and not the proxy. This has important implications. It means that self-invocation is not going to result in the advice associated with a method invocation getting a chance to execute.

Okay, so what is to be done about this? The best approach (the term, “best,” is used loosely here) is to refactor your code such that the self-invocation does not happen. This does entail some work on your part, but it is the best, least-invasive approach. The next approach is absolutely horrendous, and we hesitate to point it out, precisely because it is so horrendous. You can (painful as it is to us) totally tie the logic within your class to Spring AOP, as the following example shows:

Java

public class SimplePojo implements Pojo {

	public void foo() {
		// this works, but... gah!
		((Pojo) AopContext.currentProxy()).bar();
	}

	public void bar() {
		// some logic...
	}
}

Kotlin

class SimplePojo : Pojo {

	fun foo() {
		// this works, but... gah!
		(AopContext.currentProxy() as Pojo).bar()
	}

	fun bar() {
		// some logic...
	}
}

This totally couples your code to Spring AOP, and it makes the class itself aware of the fact that it is being used in an AOP context, which flies in the face of AOP. It also requires some additional configuration when the proxy is being created, as the following example shows:

Java

public class Main {

	public static void main(String[] args) {
		ProxyFactory factory = new ProxyFactory(new SimplePojo());
		factory.addInterface(Pojo.class);
		factory.addAdvice(new RetryAdvice());
		factory.setExposeProxy(true);

		Pojo pojo = (Pojo) factory.getProxy();
		// this is a method call on the proxy!
		pojo.foo();
	}
}

Kotlin

fun main() {
	val factory = ProxyFactory(SimplePojo())
	factory.addInterface(Pojo::class.java)
	factory.addAdvice(RetryAdvice())
	factory.isExposeProxy = true

	val pojo = factory.proxy as Pojo
	// this is a method call on the proxy!
	pojo.foo()
}

Finally, it must be noted that AspectJ does not have this self-invocation issue because it is not a proxy-based AOP framework.

Programmatic Creation of @AspectJ Proxies

In addition to declaring aspects in your configuration by using either <aop:config> or <aop:aspectj-autoproxy>, it is also possible to programmatically create proxies that advise target objects. For the full details of Spring’s AOP API, see the next chapter. Here, we want to focus on the ability to automatically create proxies by using @AspectJ aspects.

You can use the org.springframework.aop.aspectj.annotation.AspectJProxyFactory class to create a proxy for a target object that is advised by one or more @AspectJ aspects. The basic usage for this class is very simple, as the following example shows:

Java

// create a factory that can generate a proxy for the given target object
AspectJProxyFactory factory = new AspectJProxyFactory(targetObject);

// add an aspect, the class must be an @AspectJ aspect
// you can call this as many times as you need with different aspects
factory.addAspect(SecurityManager.class);

// you can also add existing aspect instances, the type of the object supplied must be an @AspectJ aspect
factory.addAspect(usageTracker);

// now get the proxy object...
MyInterfaceType proxy = factory.getProxy();

Kotlin

// create a factory that can generate a proxy for the given target object
val factory = AspectJProxyFactory(targetObject)

// add an aspect, the class must be an @AspectJ aspect
// you can call this as many times as you need with different aspects
factory.addAspect(SecurityManager::class.java)

// you can also add existing aspect instances, the type of the object supplied must be an @AspectJ aspect
factory.addAspect(usageTracker)

// now get the proxy object...
val proxy = factory.getProxy<Any>()

See the {api-spring-framework}/aop/aspectj/annotation/AspectJProxyFactory.html[javadoc] for more information.

Using AspectJ with Spring Applications

Everything we have covered so far in this chapter is pure Spring AOP. In this section, we look at how you can use the AspectJ compiler or weaver instead of or in addition to Spring AOP if your needs go beyond the facilities offered by Spring AOP alone.

Spring ships with a small AspectJ aspect library, which is available stand-alone in your distribution as spring-aspects.jar. You need to add this to your classpath in order to use the aspects in it. Using AspectJ to Dependency Inject Domain Objects with Spring and Other Spring aspects for AspectJ discuss the content of this library and how you can use it. Configuring AspectJ Aspects by Using Spring IoC discusses how to dependency inject AspectJ aspects that are woven using the AspectJ compiler. Finally, Load-time Weaving with AspectJ in the Spring Framework provides an introduction to load-time weaving for Spring applications that use AspectJ.

Using AspectJ to Dependency Inject Domain Objects with Spring

The Spring container instantiates and configures beans defined in your application context. It is also possible to ask a bean factory to configure a pre-existing object, given the name of a bean definition that contains the configuration to be applied. spring-aspects.jar contains an annotation-driven aspect that exploits this capability to allow dependency injection of any object. The support is intended to be used for objects created outside of the control of any container. Domain objects often fall into this category because they are often created programmatically with the new operator or by an ORM tool as a result of a database query.

The @Configurable annotation marks a class as being eligible for Spring-driven configuration. In the simplest case, you can use purely it as a marker annotation, as the following example shows:

Java

package com.xyz.myapp.domain;

import org.springframework.beans.factory.annotation.Configurable;

@Configurable
public class Account {
	// ...
}

Kotlin

package com.xyz.myapp.domain

import org.springframework.beans.factory.annotation.Configurable

@Configurable
class Account {
	// ...
}

When used as a marker interface in this way, Spring configures new instances of the annotated type (Account, in this case) by using a bean definition (typically prototype-scoped) with the same name as the fully-qualified type name (com.xyz.myapp.domain.Account). Since the default name for a bean is the fully-qualified name of its type, a convenient way to declare the prototype definition is to omit the id attribute, as the following example shows:

<bean class="com.xyz.myapp.domain.Account" scope="prototype">
	<property name="fundsTransferService" ref="fundsTransferService"/>
</bean>

If you want to explicitly specify the name of the prototype bean definition to use, you can do so directly in the annotation, as the following example shows:

Java

package com.xyz.myapp.domain;

import org.springframework.beans.factory.annotation.Configurable;

@Configurable("account")
public class Account {
	// ...
}

Kotlin

package com.xyz.myapp.domain

import org.springframework.beans.factory.annotation.Configurable

@Configurable("account")
class Account {
	// ...
}

Spring now looks for a bean definition named account and uses that as the definition to configure new Account instances.

You can also use autowiring to avoid having to specify a dedicated bean definition at all. To have Spring apply autowiring, use the autowire property of the @Configurable annotation. You can specify either @Configurable(autowire=Autowire.BY_TYPE) or @Configurable(autowire=Autowire.BY_NAME for autowiring by type or by name, respectively. As an alternative, it is preferable to specify explicit, annotation-driven dependency injection for your @Configurable beans through @Autowired or @Inject at the field or method level (see [beans-annotation-config] for further details).

Finally, you can enable Spring dependency checking for the object references in the newly created and configured object by using the dependencyCheck attribute (for example, @Configurable(autowire=Autowire.BY_NAME,dependencyCheck=true)). If this attribute is set to true, Spring validates after configuration that all properties (which are not primitives or collections) have been set.

Note that using the annotation on its own does nothing. It is the AnnotationBeanConfigurerAspect in spring-aspects.jar that acts on the presence of the annotation. In essence, the aspect says, “after returning from the initialization of a new object of a type annotated with @Configurable, configure the newly created object using Spring in accordance with the properties of the annotation”. In this context, “initialization” refers to newly instantiated objects (for example, objects instantiated with the new operator) as well as to Serializable objects that are undergoing deserialization (for example, through readResolve()).

Note

One of the key phrases in the above paragraph is “in essence”. For most cases, the exact semantics of “after returning from the initialization of a new object” are fine. In this context, “after initialization” means that the dependencies are injected after the object has been constructed. This means that the dependencies are not available for use in the constructor bodies of the class. If you want the dependencies to be injected before the constructor bodies execute and thus be available for use in the body of the constructors, you need to define this on the @Configurable declaration, as follows: Java @Configurable(preConstruction = true) Kotlin @Configurable(preConstruction = true) You can find more information about the language semantics of the various pointcut types in AspectJ in this appendix of the AspectJ Programming Guide.

For this to work, the annotated types must be woven with the AspectJ weaver. You can either use a build-time Ant or Maven task to do this (see, for example, the AspectJ Development Environment Guide) or load-time weaving (see Load-time Weaving with AspectJ in the Spring Framework). The AnnotationBeanConfigurerAspect itself needs to be configured by Spring (in order to obtain a reference to the bean factory that is to be used to configure new objects). If you use Java-based configuration, you can add @EnableSpringConfigured to any @Configuration class, as follows:

Java

@Configuration
@EnableSpringConfigured
public class AppConfig {
}

Kotlin

@Configuration
@EnableSpringConfigured
class AppConfig {
}

If you prefer XML based configuration, the Spring context namespace defines a convenient context:spring-configured element, which you can use as follows:

<context:spring-configured/>

Instances of @Configurable objects created before the aspect has been configured result in a message being issued to the debug log and no configuration of the object taking place. An example might be a bean in the Spring configuration that creates domain objects when it is initialized by Spring. In this case, you can use the depends-on bean attribute to manually specify that the bean depends on the configuration aspect. The following example shows how to use the depends-on attribute:

<bean id="myService"
		class="com.xzy.myapp.service.MyService"
		depends-on="org.springframework.beans.factory.aspectj.AnnotationBeanConfigurerAspect">

	<!-- ... -->

</bean>

Note

Do not activate @Configurable processing through the bean configurer aspect unless you really mean to rely on its semantics at runtime. In particular, make sure that you do not use @Configurable on bean classes that are registered as regular Spring beans with the container. Doing so results in double initialization, once through the container and once through the aspect.

Unit Testing @Configurable Objects

One of the goals of the @Configurable support is to enable independent unit testing of domain objects without the difficulties associated with hard-coded lookups. If @Configurable types have not been woven by AspectJ, the annotation has no affect during unit testing. You can set mock or stub property references in the object under test and proceed as normal. If @Configurable types have been woven by AspectJ, you can still unit test outside of the container as normal, but you see a warning message each time that you construct a @Configurable object indicating that it has not been configured by Spring.

Working with Multiple Application Contexts

The AnnotationBeanConfigurerAspect that is used to implement the @Configurable support is an AspectJ singleton aspect. The scope of a singleton aspect is the same as the scope of static members: There is one aspect instance per classloader that defines the type. This means that, if you define multiple application contexts within the same classloader hierarchy, you need to consider where to define the @EnableSpringConfigured bean and where to place spring-aspects.jar on the classpath.

Consider a typical Spring web application configuration that has a shared parent application context that defines common business services, everything needed to support those services, and one child application context for each servlet (which contains definitions particular to that servlet). All of these contexts co-exist within the same classloader hierarchy, and so the AnnotationBeanConfigurerAspect can hold a reference to only one of them. In this case, we recommend defining the @EnableSpringConfigured bean in the shared (parent) application context. This defines the services that you are likely to want to inject into domain objects. A consequence is that you cannot configure domain objects with references to beans defined in the child (servlet-specific) contexts by using the @Configurable mechanism (which is probably not something you want to do anyway).

When deploying multiple web applications within the same container, ensure that each web application loads the types in spring-aspects.jar by using its own classloader (for example, by placing spring-aspects.jar in 'WEB-INF/lib'). If spring-aspects.jar is added only to the container-wide classpath (and hence loaded by the shared parent classloader), all web applications share the same aspect instance (which is probably not what you want).

Other Spring aspects for AspectJ

In addition to the @Configurable aspect, spring-aspects.jar contains an AspectJ aspect that you can use to drive Spring’s transaction management for types and methods annotated with the @Transactional annotation. This is primarily intended for users who want to use the Spring Framework’s transaction support outside of the Spring container.

The aspect that interprets @Transactional annotations is the AnnotationTransactionAspect. When you use this aspect, you must annotate the implementation class (or methods within that class or both), not the interface (if any) that the class implements. AspectJ follows Java’s rule that annotations on interfaces are not inherited.

A @Transactional annotation on a class specifies the default transaction semantics for the execution of any public operation in the class.

A @Transactional annotation on a method within the class overrides the default transaction semantics given by the class annotation (if present). Methods of any visibility may be annotated, including private methods. Annotating non-public methods directly is the only way to get transaction demarcation for the execution of such methods.

Tip	Since Spring Framework 4.2, spring-aspects provides a similar aspect that offers the exact same features for the standard javax.transaction.Transactional annotation. Check JtaAnnotationTransactionAspect for more details.

For AspectJ programmers who want to use the Spring configuration and transaction management support but do not want to (or cannot) use annotations, spring-aspects.jar also contains abstract aspects you can extend to provide your own pointcut definitions. See the sources for the AbstractBeanConfigurerAspect and AbstractTransactionAspect aspects for more information. As an example, the following excerpt shows how you could write an aspect to configure all instances of objects defined in the domain model by using prototype bean definitions that match the fully qualified class names:

public aspect DomainObjectConfiguration extends AbstractBeanConfigurerAspect {

	public DomainObjectConfiguration() {
		setBeanWiringInfoResolver(new ClassNameBeanWiringInfoResolver());
	}

	// the creation of a new bean (any object in the domain model)
	protected pointcut beanCreation(Object beanInstance) :
		initialization(new(..)) &&
		SystemArchitecture.inDomainModel() &&
		this(beanInstance);
}

Configuring AspectJ Aspects by Using Spring IoC

When you use AspectJ aspects with Spring applications, it is natural to both want and expect to be able to configure such aspects with Spring. The AspectJ runtime itself is responsible for aspect creation, and the means of configuring the AspectJ-created aspects through Spring depends on the AspectJ instantiation model (the per-xxx clause) used by the aspect.

The majority of AspectJ aspects are singleton aspects. Configuration of these aspects is easy. You can create a bean definition that references the aspect type as normal and include the factory-method="aspectOf" bean attribute. This ensures that Spring obtains the aspect instance by asking AspectJ for it rather than trying to create an instance itself. The following example shows how to use the factory-method="aspectOf" attribute:

<bean id="profiler" class="com.xyz.profiler.Profiler"
		factory-method="aspectOf"> (1)

	<property name="profilingStrategy" ref="jamonProfilingStrategy"/>
</bean>

1. Note the factory-method="aspectOf" attribute

Non-singleton aspects are harder to configure. However, it is possible to do so by creating prototype bean definitions and using the @Configurable support from spring-aspects.jar to configure the aspect instances once they have bean created by the AspectJ runtime.

If you have some @AspectJ aspects that you want to weave with AspectJ (for example, using load-time weaving for domain model types) and other @AspectJ aspects that you want to use with Spring AOP, and these aspects are all configured in Spring, you need to tell the Spring AOP @AspectJ auto-proxying support which exact subset of the @AspectJ aspects defined in the configuration should be used for auto-proxying. You can do this by using one or more <include/> elements inside the <aop:aspectj-autoproxy/> declaration. Each <include/> element specifies a name pattern, and only beans with names matched by at least one of the patterns are used for Spring AOP auto-proxy configuration. The following example shows how to use <include/> elements:

<aop:aspectj-autoproxy>
	<aop:include name="thisBean"/>
	<aop:include name="thatBean"/>
</aop:aspectj-autoproxy>

Note	Do not be misled by the name of the <aop:aspectj-autoproxy/> element. Using it results in the creation of Spring AOP proxies. The @AspectJ style of aspect declaration is being used here, but the AspectJ runtime is not involved.

Load-time Weaving with AspectJ in the Spring Framework

Load-time weaving (LTW) refers to the process of weaving AspectJ aspects into an application’s class files as they are being loaded into the Java virtual machine (JVM). The focus of this section is on configuring and using LTW in the specific context of the Spring Framework. This section is not a general introduction to LTW. For full details on the specifics of LTW and configuring LTW with only AspectJ (with Spring not being involved at all), see the LTW section of the AspectJ Development Environment Guide.

The value that the Spring Framework brings to AspectJ LTW is in enabling much finer-grained control over the weaving process. 'Vanilla' AspectJ LTW is effected by using a Java (5+) agent, which is switched on by specifying a VM argument when starting up a JVM. It is, thus, a JVM-wide setting, which may be fine in some situations but is often a little too coarse. Spring-enabled LTW lets you switch on LTW on a per-ClassLoader basis, which is more fine-grained and which can make more sense in a 'single-JVM-multiple-application' environment (such as is found in a typical application server environment).

Further, in certain environments, this support enables load-time weaving without making any modifications to the application server’s launch script that is needed to add -javaagent:path/to/aspectjweaver.jar or (as we describe later in this section) -javaagent:path/to/spring-instrument.jar. Developers configure the application context to enable load-time weaving instead of relying on administrators who typically are in charge of the deployment configuration, such as the launch script.

Now that the sales pitch is over, let us first walk through a quick example of AspectJ LTW that uses Spring, followed by detailed specifics about elements introduced in the example. For a complete example, see the Petclinic sample application.

A First Example

Assume that you are an application developer who has been tasked with diagnosing the cause of some performance problems in a system. Rather than break out a profiling tool, we are going to switch on a simple profiling aspect that lets us quickly get some performance metrics. We can then apply a finer-grained profiling tool to that specific area immediately afterwards.

Note	The example presented here uses XML configuration. You can also configure and use @AspectJ with Java configuration. Specifically, you can use the @EnableLoadTimeWeaving annotation as an alternative to <context:load-time-weaver/> (see below for details).

The following example shows the profiling aspect, which is not fancy. It is a time-based profiler that uses the @AspectJ-style of aspect declaration:

Java

package foo;

import org.aspectj.lang.ProceedingJoinPoint;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Around;
import org.aspectj.lang.annotation.Pointcut;
import org.springframework.util.StopWatch;
import org.springframework.core.annotation.Order;

@Aspect
public class ProfilingAspect {

	@Around("methodsToBeProfiled()")
	public Object profile(ProceedingJoinPoint pjp) throws Throwable {
		StopWatch sw = new StopWatch(getClass().getSimpleName());
		try {
			sw.start(pjp.getSignature().getName());
			return pjp.proceed();
		} finally {
			sw.stop();
			System.out.println(sw.prettyPrint());
		}
	}

	@Pointcut("execution(public * foo..*.*(..))")
	public void methodsToBeProfiled(){}
}

Kotlin

package foo

import org.aspectj.lang.ProceedingJoinPoint
import org.aspectj.lang.annotation.Aspect
import org.aspectj.lang.annotation.Around
import org.aspectj.lang.annotation.Pointcut
import org.springframework.util.StopWatch
import org.springframework.core.annotation.Order

@Aspect
class ProfilingAspect {

	@Around("methodsToBeProfiled()")
	fun profile(pjp: ProceedingJoinPoint): Any {
		val sw = StopWatch(javaClass.simpleName)
		try {
			sw.start(pjp.getSignature().getName())
			return pjp.proceed()
		} finally {
			sw.stop()
			println(sw.prettyPrint())
		}
	}

	@Pointcut("execution(public * foo..*.*(..))")
	fun methodsToBeProfiled() {
	}
}

We also need to create an META-INF/aop.xml file, to inform the AspectJ weaver that we want to weave our ProfilingAspect into our classes. This file convention, namely the presence of a file (or files) on the Java classpath called META-INF/aop.xml is standard AspectJ. The following example shows the aop.xml file:

<!DOCTYPE aspectj PUBLIC "-//AspectJ//DTD//EN" "https://www.eclipse.org/aspectj/dtd/aspectj.dtd">
<aspectj>

	<weaver>
		<!-- only weave classes in our application-specific packages -->
		<include within="foo.*"/>
	</weaver>

	<aspects>
		<!-- weave in just this aspect -->
		<aspect name="foo.ProfilingAspect"/>
	</aspects>

</aspectj>

Now we can move on to the Spring-specific portion of the configuration. We need to configure a LoadTimeWeaver (explained later). This load-time weaver is the essential component responsible for weaving the aspect configuration in one or more META-INF/aop.xml files into the classes in your application. The good thing is that it does not require a lot of configuration (there are some more options that you can specify, but these are detailed later), as can be seen in the following example:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns:context="http://www.springframework.org/schema/context"
	xsi:schemaLocation="
		http://www.springframework.org/schema/beans
		https://www.springframework.org/schema/beans/spring-beans.xsd
		http://www.springframework.org/schema/context
		https://www.springframework.org/schema/context/spring-context.xsd">

	<!-- a service object; we will be profiling its methods -->
	<bean id="entitlementCalculationService"
			class="foo.StubEntitlementCalculationService"/>

	<!-- this switches on the load-time weaving -->
	<context:load-time-weaver/>
</beans>

Now that all the required artifacts (the aspect, the META-INF/aop.xml file, and the Spring configuration) are in place, we can create the following driver class with a main(..) method to demonstrate the LTW in action:

Java

package foo;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public final class Main {

	public static void main(String[] args) {
		ApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml", Main.class);

		EntitlementCalculationService entitlementCalculationService =
				(EntitlementCalculationService) ctx.getBean("entitlementCalculationService");

		// the profiling aspect is 'woven' around this method execution
		entitlementCalculationService.calculateEntitlement();
	}
}

Kotlin

package foo

import org.springframework.context.support.ClassPathXmlApplicationContext

fun main() {
	val ctx = ClassPathXmlApplicationContext("beans.xml")

	val entitlementCalculationService = ctx.getBean("entitlementCalculationService") as EntitlementCalculationService

	// the profiling aspect is 'woven' around this method execution
	entitlementCalculationService.calculateEntitlement()
}

We have one last thing to do. The introduction to this section did say that one could switch on LTW selectively on a per-ClassLoader basis with Spring, and this is true. However, for this example, we use a Java agent (supplied with Spring) to switch on LTW. We use the following command to run the Main class shown earlier:

java -javaagent:C:/projects/foo/lib/global/spring-instrument.jar foo.Main

The -javaagent is a flag for specifying and enabling agents to instrument programs that run on the JVM. The Spring Framework ships with such an agent, the InstrumentationSavingAgent, which is packaged in the spring-instrument.jar that was supplied as the value of the -javaagent argument in the preceding example.

The output from the execution of the Main program looks something like the next example. (I have introduced a Thread.sleep(..) statement into the calculateEntitlement() implementation so that the profiler actually captures something other than 0 milliseconds (the 01234 milliseconds is not an overhead introduced by the AOP). The following listing shows the output we got when we ran our profiler:

Calculating entitlement

StopWatch 'ProfilingAspect': running time (millis) = 1234
------ ----- ----------------------------
ms     %     Task name
------ ----- ----------------------------
01234  100%  calculateEntitlement

Since this LTW is effected by using full-blown AspectJ, we are not limited only to advising Spring beans. The following slight variation on the Main program yields the same result:

Java

package foo;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public final class Main {

	public static void main(String[] args) {
		new ClassPathXmlApplicationContext("beans.xml", Main.class);

		EntitlementCalculationService entitlementCalculationService =
				new StubEntitlementCalculationService();

		// the profiling aspect will be 'woven' around this method execution
		entitlementCalculationService.calculateEntitlement();
	}
}

Kotlin

package foo

import org.springframework.context.support.ClassPathXmlApplicationContext

fun main(args: Array<String>) {
	ClassPathXmlApplicationContext("beans.xml")

	val entitlementCalculationService = StubEntitlementCalculationService()

	// the profiling aspect will be 'woven' around this method execution
	entitlementCalculationService.calculateEntitlement()
}

Notice how, in the preceding program, we bootstrap the Spring container and then create a new instance of the StubEntitlementCalculationService totally outside the context of Spring. The profiling advice still gets woven in.

Admittedly, the example is simplistic. However, the basics of the LTW support in Spring have all been introduced in the earlier example, and the rest of this section explains the “why” behind each bit of configuration and usage in detail.

Note	The ProfilingAspect used in this example may be basic, but it is quite useful. It is a nice example of a development-time aspect that developers can use during development and then easily exclude from builds of the application being deployed into UAT or production.

Aspects

The aspects that you use in LTW have to be AspectJ aspects. You can write them in either the AspectJ language itself, or you can write your aspects in the @AspectJ-style. Your aspects are then both valid AspectJ and Spring AOP aspects. Furthermore, the compiled aspect classes need to be available on the classpath.

'META-INF/aop.xml'

The AspectJ LTW infrastructure is configured by using one or more META-INF/aop.xml files that are on the Java classpath (either directly or, more typically, in jar files).

The structure and contents of this file is detailed in the LTW part of the AspectJ reference documentation. Because the aop.xml file is 100% AspectJ, we do not describe it further here.

Required libraries (JARS)

At minimum, you need the following libraries to use the Spring Framework’s support for AspectJ LTW:

• spring-aop.jar

• aspectjweaver.jar

If you use the Spring-provided agent to enable instrumentation, you also need:

• spring-instrument.jar

Spring Configuration

The key component in Spring’s LTW support is the LoadTimeWeaver interface (in the org.springframework.instrument.classloading package), and the numerous implementations of it that ship with the Spring distribution. A LoadTimeWeaver is responsible for adding one or more java.lang.instrument.ClassFileTransformers to a ClassLoader at runtime, which opens the door to all manner of interesting applications, one of which happens to be the LTW of aspects.

Tip

If you are unfamiliar with the idea of runtime class file transformation, see the javadoc API documentation for the java.lang.instrument package before continuing. While that documentation is not comprehensive, at least you can see the key interfaces and classes (for reference as you read through this section).

Configuring a LoadTimeWeaver for a particular ApplicationContext can be as easy as adding one line. (Note that you almost certainly need to use an ApplicationContext as your Spring container — typically, a BeanFactory is not enough because the LTW support uses BeanFactoryPostProcessors.)

To enable the Spring Framework’s LTW support, you need to configure a LoadTimeWeaver, which typically is done by using the @EnableLoadTimeWeaving annotation, as follows:

Java

@Configuration
@EnableLoadTimeWeaving
public class AppConfig {
}

Kotlin

@Configuration
@EnableLoadTimeWeaving
class AppConfig {
}

Alternatively, if you prefer XML-based configuration, use the <context:load-time-weaver/> element. Note that the element is defined in the context namespace. The following example shows how to use <context:load-time-weaver/>:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns:context="http://www.springframework.org/schema/context"
	xsi:schemaLocation="
		http://www.springframework.org/schema/beans
		https://www.springframework.org/schema/beans/spring-beans.xsd
		http://www.springframework.org/schema/context
		https://www.springframework.org/schema/context/spring-context.xsd">

	<context:load-time-weaver/>

</beans>

The preceding configuration automatically defines and registers a number of LTW-specific infrastructure beans, such as a LoadTimeWeaver and an AspectJWeavingEnabler, for you. The default LoadTimeWeaver is the DefaultContextLoadTimeWeaver class, which attempts to decorate an automatically detected LoadTimeWeaver. The exact type of LoadTimeWeaver that is “automatically detected” is dependent upon your runtime environment. The following table summarizes various LoadTimeWeaver implementations:

Table 1. DefaultContextLoadTimeWeaver LoadTimeWeavers

Runtime Environment	LoadTimeWeaver implementation
Running in Apache Tomcat	TomcatLoadTimeWeaver
Running in GlassFish (limited to EAR deployments)	GlassFishLoadTimeWeaver
Running in Red Hat’s JBoss AS or WildFly	JBossLoadTimeWeaver
Running in IBM’s WebSphere	WebSphereLoadTimeWeaver
Running in Oracle’s WebLogic	WebLogicLoadTimeWeaver
JVM started with Spring InstrumentationSavingAgent (java -javaagent:path/to/spring-instrument.jar)	InstrumentationLoadTimeWeaver
Fallback, expecting the underlying ClassLoader to follow common conventions (namely addTransformer and optionally a getThrowawayClassLoader method)	ReflectiveLoadTimeWeaver

Note that the table lists only the LoadTimeWeavers that are autodetected when you use the DefaultContextLoadTimeWeaver. You can specify exactly which LoadTimeWeaver implementation to use.

To specify a specific LoadTimeWeaver with Java configuration, implement the LoadTimeWeavingConfigurer interface and override the getLoadTimeWeaver() method. The following example specifies a ReflectiveLoadTimeWeaver:

Java

@Configuration
@EnableLoadTimeWeaving
public class AppConfig implements LoadTimeWeavingConfigurer {

	@Override
	public LoadTimeWeaver getLoadTimeWeaver() {
		return new ReflectiveLoadTimeWeaver();
	}
}

Kotlin

@Configuration
@EnableLoadTimeWeaving
class AppConfig : LoadTimeWeavingConfigurer {

	override fun getLoadTimeWeaver(): LoadTimeWeaver {
		return ReflectiveLoadTimeWeaver()
	}
}

If you use XML-based configuration, you can specify the fully qualified classname as the value of the weaver-class attribute on the <context:load-time-weaver/> element. Again, the following example specifies a ReflectiveLoadTimeWeaver:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns:context="http://www.springframework.org/schema/context"
	xsi:schemaLocation="
		http://www.springframework.org/schema/beans
		https://www.springframework.org/schema/beans/spring-beans.xsd
		http://www.springframework.org/schema/context
		https://www.springframework.org/schema/context/spring-context.xsd">

	<context:load-time-weaver
			weaver-class="org.springframework.instrument.classloading.ReflectiveLoadTimeWeaver"/>

</beans>

The LoadTimeWeaver that is defined and registered by the configuration can be later retrieved from the Spring container by using the well known name, loadTimeWeaver. Remember that the LoadTimeWeaver exists only as a mechanism for Spring’s LTW infrastructure to add one or more ClassFileTransformers. The actual ClassFileTransformer that does the LTW is the ClassPreProcessorAgentAdapter (from the org.aspectj.weaver.loadtime package) class. See the class-level javadoc of the ClassPreProcessorAgentAdapter class for further details, because the specifics of how the weaving is actually effected is beyond the scope of this document.

There is one final attribute of the configuration left to discuss: the aspectjWeaving attribute (or aspectj-weaving if you use XML). This attribute controls whether LTW is enabled or not. It accepts one of three possible values, with the default value being autodetect if the attribute is not present. The following table summarizes the three possible values:

Table 2. AspectJ weaving attribute values

Annotation Value	XML Value	Explanation
ENABLED	on	AspectJ weaving is on, and aspects are woven at load-time as appropriate.
DISABLED	off	LTW is off. No aspect is woven at load-time.
AUTODETECT	autodetect	If the Spring LTW infrastructure can find at least one META-INF/aop.xml file, then AspectJ weaving is on. Otherwise, it is off. This is the default value.

Environment-specific Configuration

This last section contains any additional settings and configuration that you need when you use Spring’s LTW support in environments such as application servers and web containers.

Tomcat, JBoss, WebSphere, WebLogic

Tomcat, JBoss/WildFly, IBM WebSphere Application Server and Oracle WebLogic Server all provide a general app ClassLoader that is capable of local instrumentation. Spring’s native LTW may leverage those ClassLoader implementations to provide AspectJ weaving. You can simply enable load-time weaving, as described earlier. Specifically, you do not need to modify the JVM launch script to add -javaagent:path/to/spring-instrument.jar.

Note that on JBoss, you may need to disable the app server scanning to prevent it from loading the classes before the application actually starts. A quick workaround is to add to your artifact a file named WEB-INF/jboss-scanning.xml with the following content:

<scanning xmlns="urn:jboss:scanning:1.0"/>

Generic Java Applications

When class instrumentation is required in environments that are not supported by specific LoadTimeWeaver implementations, a JVM agent is the general solution. For such cases, Spring provides InstrumentationLoadTimeWeaver which requires a Spring-specific (but very general) JVM agent, spring-instrument.jar, autodetected by common @EnableLoadTimeWeaving and <context:load-time-weaver/> setups.

To use it, you must start the virtual machine with the Spring agent by supplying the following JVM options:

-javaagent:/path/to/spring-instrument.jar

Note that this requires modification of the JVM launch script, which may prevent you from using this in application server environments (depending on your server and your operation policies). That said, for one-app-per-JVM deployments such as standalone Spring Boot applications, you typically control the entire JVM setup in any case.

Further Resources

More information on AspectJ can be found on the AspectJ website.

Eclipse AspectJ by Adrian Colyer et. al. (Addison-Wesley, 2005) provides a comprehensive introduction and reference for the AspectJ language.

AspectJ in Action, Second Edition by Ramnivas Laddad (Manning, 2009) comes highly recommended. The focus of the book is on AspectJ, but a lot of general AOP themes are explored (in some depth).