AbstractInterfaceDrivenDependencyInjectionAspect.aj

/*
 * Copyright 2002-2020 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
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package org.springframework.beans.factory.aspectj;

import java.io.ObjectStreamException;
import java.io.Serializable;

/**
 * An aspect that injects dependency into any object whose type implements the
 * {@link ConfigurableObject} interface.
 *
 * <p>This aspect supports injecting into domain objects when they are created
 * for the first time as well as upon deserialization. Subaspects need to simply
 * provide definition for the configureBean() method. This method may be
 * implemented without relying on Spring container if so desired.
 *
 * <p>There are two cases that needs to be handled:
 * <ol>
 * <li>Normal object creation via the '{@code new}' operator: this is
 * taken care of by advising {@code initialization()} join points.</li>
 * <li>Object creation through deserialization: since no constructor is
 * invoked during deserialization, the aspect needs to advise a method that a
 * deserialization mechanism is going to invoke. Ideally, we should not
 * require user classes to implement any specific method. This implies that
 * we need to <i>introduce</i> the chosen method. We should also handle the cases
 * where the chosen method is already implemented in classes (in which case,
 * the user's implementation for that method should take precedence over the
 * introduced implementation). There are a few choices for the chosen method:
 * <ul>
 * <li>readObject(ObjectOutputStream): Java requires that the method must be
 * {@code private}</p>. Since aspects cannot introduce a private member,
 * while preserving its name, this option is ruled out.</li>
 * <li>readResolve(): Java doesn't pose any restriction on an access specifier.
 * Problem solved! There is one (minor) limitation of this approach in
 * that if a user class already has this method, that method must be
 * {@code public}. However, this shouldn't be a big burden, since
 * use cases that need classes to implement readResolve() (custom enums,
 * for example) are unlikely to be marked as &#64;Configurable, and
 * in any case asking to make that method {@code public} should not
 * pose any undue burden.</li>
 * </ul>
 * The minor collaboration needed by user classes (i.e., that the implementation of
 * {@code readResolve()}, if any, must be {@code public}) can be lifted as well if we
 * were to use an experimental feature in AspectJ - the {@code hasmethod()} PCD.</li>
 * </ol>
 *
 * <p>While having type implement the {@link ConfigurableObject} interface is certainly
 * a valid choice, an alternative is to use a 'declare parents' statement another aspect
 * (a subaspect of this aspect would be a logical choice) that declares the classes that
 * need to be configured by supplying the {@link ConfigurableObject} interface.
 *
 * @author Ramnivas Laddad
 * @since 2.5.2
 */
public abstract aspect AbstractInterfaceDrivenDependencyInjectionAspect extends AbstractDependencyInjectionAspect {

	/**
	 * Select initialization join point as object construction
	 */
	public pointcut beanConstruction(Object bean) :
			initialization(ConfigurableObject+.new(..)) && this(bean);

	/**
	 * Select deserialization join point made available through ITDs for ConfigurableDeserializationSupport
	 */
	public pointcut beanDeserialization(Object bean) :
			execution(Object ConfigurableDeserializationSupport+.readResolve()) && this(bean);

	public pointcut leastSpecificSuperTypeConstruction() : initialization(ConfigurableObject.new(..));



	// Implementation to support re-injecting dependencies once an object is deserialized

	/**
	 * Declare any class implementing Serializable and ConfigurableObject as also implementing
	 * ConfigurableDeserializationSupport. This allows us to introduce the {@code readResolve()}
	 * method and select it with the beanDeserialization() pointcut.
	 * <p>Here is an improved version that uses the hasmethod() pointcut and lifts
	 * even the minor requirement on user classes:
	 * <pre class="code">
	 * declare parents: ConfigurableObject+ Serializable+
	 * && !hasmethod(Object readResolve() throws ObjectStreamException)
	 * implements ConfigurableDeserializationSupport;
	 * </pre>
	 */
	declare parents: ConfigurableObject+ && Serializable+ implements ConfigurableDeserializationSupport;

	/**
	 * A marker interface to which the {@code readResolve()} is introduced.
	 */
	static interface ConfigurableDeserializationSupport extends Serializable {
	}

	/**
	 * Introduce the {@code readResolve()} method so that we can advise its
	 * execution to configure the object.
	 * <p>Note if a method with the same signature already exists in a
	 * {@code Serializable} class of ConfigurableObject type,
	 * that implementation will take precedence (a good thing, since we are
	 * merely interested in an opportunity to detect deserialization.)
	 */
	public Object ConfigurableDeserializationSupport.readResolve() throws ObjectStreamException {
		return this;
	}

}