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| # Robolectric architecture | ||
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| Robolectric is a unit testing framework that allows Android code to be tested on | ||
| the JVM without the need for an emulator or device. This allows tests to run | ||
| very quickly in a more hermetic environment. Robolectric has a complex | ||
| architecture and makes use of many advanced features of the JVM such as bytecode | ||
| instrumentation and custom ClassLoaders. This document provides a high level | ||
| overview of the architecture of Robolectric. | ||
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| # Android framework Jars and instrumentation | ||
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| At the heart of Robolectric are the Android framework Jars and the bytecode | ||
| instrumentation. The Android framework Jars are a collection of Jar files that | ||
| are built directly from Android platform sources. There is a single Jar file for | ||
| each version of Android. These Jar files can be built by checking out an AOSP | ||
| repo and building the | ||
| [robolectric-host-android\_all](https://cs.android.com/android/platform/superproject/main/+/main:external/robolectric/Android.bp;l=99) | ||
| target. Unlike the android.jar (stubs jar) files managed by Android Studio, | ||
| which only contain public method signatures, the Robolectric android-all Jars | ||
| contain the implementation of the Android Java framework. This gives Robolectric | ||
| the ability to use as much real Android code as possible. A new android-all jar | ||
| is uploaded to MavenCentral for each Android release. You can see the current | ||
| android-all jars | ||
| [here](https://repo1.maven.org/maven2/org/robolectric/android-all/). | ||
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| However, the pristine android-all jars are not the ones used during tests. | ||
| Instead, Robolectric modifies the pristine android-all jars using bytecode | ||
| instrumentation (see | ||
| [ClassInstrumentor](https://github.com/robolectric/robolectric/blob/master/sandbox/src/main/java/org/robolectric/internal/bytecode/ClassInstrumentor.java)). | ||
| It performs several modifications: | ||
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| 1. All Android methods, including constructors and static initializers, are | ||
| modified to support `shadowing`. This allows any method call to the Android | ||
| framework to be intercepted by Robolectric and delegated to a shadow method. | ||
| At a high level, this is done by iterating over each Android method and | ||
| converting it into two methods: the original method (but renamed), and the | ||
| `invokedynamic delegator` which can optionally invoke shadow methods if they | ||
| are available. | ||
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| 1. Android constructors are specially modified to create shadow objects, if a | ||
| shadow class is bound to the Android class being instantiated. | ||
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| 1. Because the Android version of Java core classes (libcore) contain subtle | ||
| differences to the JDKs, certain problematic method calls have to be | ||
| intercepted and rewritten. See | ||
| [AndroidInterceptors](https://github.com/robolectric/robolectric/blob/master/sandbox/src/main/java/org/robolectric/interceptors/AndroidInterceptors.java). | ||
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| 1. Native methods undergo special instrumentation. Currently native methods are | ||
| converted to no-op non-native methods that are shadowable by default. | ||
| However, there is now a native variant of each method also created. There is | ||
| more details about native code in a section below. | ||
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| 1. The `final` keyword is stripped from classes and methods. | ||
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| 1. Some bespoke pieces of instrumentation, such as supporting | ||
| [SparseArray.set](https://github.com/robolectric/robolectric/blob/master/sandbox/src/main/java/org/robolectric/internal/bytecode/ClassInstrumentor.java#L201). | ||
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| This instrumentation is typically performed when a new release of Robolectric is | ||
| made. These pre-instrumented Android-all jars are published on MavenCentral. See | ||
| the | ||
| [android-all-instrumented](https://repo1.maven.org/maven2/org/robolectric/android-all-instrumented/) | ||
| path. They are lazily downloaded and during tests runtime using | ||
| [MavenArtifactFetcher](https://github.com/robolectric/robolectric/blob/master/plugins/maven-dependency-resolver/src/main/java/org/robolectric/internal/dependency/MavenArtifactFetcher.java). | ||
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| Although Robolectric supports shadowing for Android framework classes, it is | ||
| also possible for users to perform Robolectric instrumentation for any package | ||
| (with the exception of built in Java packages). This enables shadowing of | ||
| arbitrary third-party code. | ||
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| # Shadows | ||
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| By default when an Android method is invoked during a Robolectric test, the real | ||
| Android framework code is invoked. This is because a lot of Android framework | ||
| classes are pure Java code (e.g the | ||
| [Intent](https://cs.android.com/android/platform/superproject/main/+/main:frameworks/base/core/java/android/content/Intent.java) | ||
| class or the | ||
| [org.json](https://cs.android.com/android/platform/superproject/main/+/main:libcore/json/src/main/java/org/json/) | ||
| package) and that code can run on the JVM without any modifications needed. | ||
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| However, there are cases where Robolectric needs to intercept and replace | ||
| Android method calls. This most commonly occurs when Android system service or | ||
| native methods are invoked. To do this, Robolectric uses a system called Shadow | ||
| classes. | ||
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| Shadow classes are Java classes that contain the replacement code of Android | ||
| methods when they are invoked. Each shadow class is bound to specific Android | ||
| classes and methods through annotations. There are currently hundreds of shadow | ||
| classes that can be found | ||
| [here](https://github.com/robolectric/robolectric/tree/master/shadows/framework/src/main/java/org/robolectric/shadows). | ||
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| Shadow classes may optionally contain public apis APIs that can customize the | ||
| behavior of the methods they are shadowing. | ||
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| Robolectric allows tests to specify custom shadows as well to provide user | ||
| defined implementation for Android classes. | ||
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| ## Shadow Packages and the Robolectric Annotation Processor | ||
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| There are two categories of shadows: Robolectric’s built-in shadows that are | ||
| aggregated using the [Robolectric Annotation Processor | ||
| (RAP)](https://github.com/robolectric/robolectric/blob/master/processor/src/main/java/org/robolectric/annotation/processing/RobolectricProcessor.java), | ||
| and custom shadows that are commonly specified using `@Config(shadows = …)`. RAP | ||
| is configured to process all of the shadow files that exist in Robolectric’s | ||
| code. The main shadow package is [framework | ||
| shadows](https://github.com/robolectric/robolectric/tree/master/shadows/framework), | ||
| which contain shadows for the Android framework. There are other shadow packages | ||
| in Robolectric's code, such as [httpclient | ||
| shadows](https://github.com/robolectric/robolectric/tree/master/shadows/httpclient), | ||
| but all of them outside of framework shadows are deprecated. When Robolectric is | ||
| built, each shadow package is processed by RAP and a | ||
| [ShadowProvider](https://github.com/robolectric/robolectric/blob/master/shadowapi/src/main/java/org/robolectric/internal/ShadowProvider.java) | ||
| file is generated. For example, to see the ShadowProvider for the framework | ||
| shadows, you can run: | ||
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| ```sh | ||
| ./gradlew :shadows:framework:assemble | ||
| cat ./shadows/framework/build/generated/src/apt/main/org/robolectric/Shadows.java | ||
| ``` | ||
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| In this file you will see the class `public class Shadows implements | ||
| ShadowProvider`. | ||
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| During runtime, Robolectric will use ServiceLoader to detect all shadow packages | ||
| that implement ShadowProvider and the shadow classes contained in them. | ||
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| # Sandbox and ClassLoader | ||
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| Before a Robolectric test is executed, a | ||
| [Sandbox](https://github.com/robolectric/robolectric/blob/master/sandbox/src/main/java/org/robolectric/internal/bytecode/Sandbox.java) | ||
| must be initialized. A Sandbox consists of some high-level structures that are | ||
| necessary to run a Robolectric test. It primarily contains a | ||
| [SandboxClassLoader](https://github.com/robolectric/robolectric/blob/master/sandbox/src/main/java/org/robolectric/internal/bytecode/SandboxClassLoader.java), | ||
| which is a custom ClassLoader that is bound to a specific instrumented | ||
| Android-all jar. Sandboxes also contain the ExecutorService that serves as the | ||
| main thread (UI thread) as well as high-level instrumentation configuration. The | ||
| SandboxClassLoader is installed as the default ClassLoader for the test method. | ||
| When any Android class is requested, SandboxClassLoader will attempt to load the | ||
| Android class from the instrumented Android-all Jar first. The primary goal of | ||
| SandboxClassLoader is to ensure that classes from the android.jar stubs jar are | ||
| not inadvertently loaded. When classes from the android.jar stubs jar are | ||
| loaded, attempting to invoke any method on them will result in a | ||
| `RuntimeException(“Stub!”)` error. Typically the Android stubs jar is on the | ||
| class path during a Robolectric test, but it is important not to load classes | ||
| from the stubs jar. | ||
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| # Invokedynamic Delegators and ShadowWrangler | ||
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| This section provides more detail for `invokedynamic delegators` that were | ||
| referenced in the instrumentation section. For an overview of the | ||
| `invokedynamic` JVM instructions, you can search for articles or watch [YouTube | ||
| videos such as this](https://www.youtube.com/watch?v=KhiECfzyVt0). | ||
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| To reiterate, for any Android method, Robolectric’s instrumentation adds an | ||
| `invokedynamic delegator` that is responsible for determining at runtime to | ||
| either invoke the real Android framework code or a shadow method. The first time | ||
| an Android method is invoked in a Sandbox, it will result in a call to one of | ||
| the bootstrap methods in | ||
| [InvokeDynamicSupport](https://github.com/robolectric/robolectric/blob/master/sandbox/src/main/java/org/robolectric/internal/bytecode/InvokeDynamicSupport.java). | ||
| This will subsequently invoke the | ||
| [ShadowWrangler.findShadowMethodHandle](https://github.com/robolectric/robolectric/blob/master/sandbox/src/main/java/org/robolectric/internal/bytecode/ShadowWrangler.java#L197) | ||
| to determine if a shadow method exists for the method that is being invoked. If | ||
| a shadow method is available a MethodHandle to it will be returned. Otherwise a | ||
| MethodHandle for the original framework code will be returned. | ||
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| # Test lifecycle | ||
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| There is a lot of work done by Robolectric before and after a test is run. | ||
| Besides the Sandbox and ClassLoader initialization mentioned above, there is | ||
| also extensive Android environment initialization that occurs before each test. | ||
| The high-level class for this is | ||
| [AndroidTestEnvironment](https://github.com/robolectric/robolectric/blob/master/robolectric/src/main/java/org/robolectric/android/internal/AndroidTestEnvironment.java). | ||
| This involves: | ||
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| * Initializing up the Looper mode (i.e. the scheduler) | ||
| * Initializing system and app resources | ||
| * Initializing the application context and system context | ||
| * Loading the Android manifest for the test | ||
| * Creating the Application object used for the test | ||
| * Initializing the [display configuration](https://robolectric.org/device-configuration/) | ||
| * Setting up the ActivityThread | ||
| * Creating app directories | ||
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| It is possible for users to extend the test environment setup using | ||
| [TestEnvironmentLifecyclePlugin](https://github.com/robolectric/robolectric/blob/master/pluginapi/src/main/java/org/robolectric/pluginapi/TestEnvironmentLifecyclePlugin.java). | ||
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| Similarly, after each test, many Android classes are reset during | ||
| [RobolectricTestRunner.finallyAfterTest](https://github.com/robolectric/robolectric/blob/master/robolectric/src/main/java/org/robolectric/RobolectricTestRunner.java#L301). | ||
| This will iterate over all shadows and invoke their static `@Resetter` methods. | ||
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| # Plugin System | ||
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| Many parts of Robolectric can be customized using a plugin system based on | ||
| Java’s | ||
| [ServiceLoader](https://docs.oracle.com/javase/8/docs/api/java/util/ServiceLoader.html). | ||
| This extensibility is useful when running Robolectric in more constrained | ||
| environments. For example, by default, most of the Robolectric classes are | ||
| designed to work in a Gradle/Android Studio environment. However, there are | ||
| companies (such as Google) that use alternate build systems (such as Bazel), and | ||
| it can be helpful to have the ability to customize the behavior of some core | ||
| modules. | ||
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| The | ||
| [pluginapi](https://github.com/robolectric/robolectric/tree/master/pluginapi/src) | ||
| subproject contains many extension points of Robolectric. However, virtually any | ||
| class that is loaded by Robolectric’s | ||
| [Injector](https://github.com/robolectric/robolectric/blob/master/utils/src/main/java/org/robolectric/util/inject/Injector.java) | ||
| has the ability to use | ||
| [PluginFinder](https://github.com/robolectric/robolectric/blob/master/utils/src/main/java/org/robolectric/util/inject/PluginFinder.java), | ||
| which means it can be extended at runtime. | ||
|
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| Typically ServiceLoaders plugins can be easily written using the | ||
| [AutoService](https://github.com/google/auto/tree/main/service) project. | ||
|
|
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