Multi-module Unit Test Support with Gradle

When writing tests for multi-module projects there may be some test helper classes or base test classes that you want to use in more than one module. One way to do this is to create a separate project for the re-usable test classes. The disadvantage is that the test code is separated from the source code and tests may not be executed or maintained. This post describes an easy way to create and use test classes for Android projects using Gradle and Android studio without having to create a separate project for the test classes.

It follows the approach of Maven where a test JAR can be created and used by adding some simple config. The Gradle Android plugin doesn’t package a JAR of test classes so we need to create a task to create the test JAR before using it.

Consider the following module structure. ProviderModule contains a some test helper classes that are used in the unit tests in ProviderModule/src/test/java. DependentModule uses test classes from ProviderModule in its own unit tests in DependentModule/src/test/java.

project
│   build.gradle   
│
└───ProviderModule
    │
    ├───build.gradle
        │   
        ├───src/main/java
        ├───src/test/java
        └───src/androidTest/java
        
└───DependentModule
    │
    ├───build.gradle
        │ 
        ├───src/main/java
        ├───src/test/java
        └───src/androidTest/java
           

The following steps show how to re-use test helper classes or base classes in the unit test source tree of the ProviderModule. It is also possible to re-use code in the instrumentation test source tree using a similar approach.

1. Define a task in the allProjects{} section of the top-level module build.gradle that can be used in any sub-module to create a test JAR for that package. This assumes the location of the test class output directory. A property (testJarLocation) that points to the test JAR location is created.

    task packageUnitTests(type: Jar) {
      from file("${buildDir}/intermediates/classes/test/debug/")
      archiveName "${project.name}-tests.jar"
      // dstinationDir null so use explicit property
      ext.testJarLocation="${project.buildDir}/libs/${archivePath}"
      description 'Package test JAR for sharing'
      doLast {
        println "Packaged test JAR for sharing: ${testJarLocation}"
      }
    }
   

2. Configure the packaging task to run in the ProviderModule after the test code has been compiled. This is added to the outermost scope of the build.gradle file.

     tasks.whenTaskAdded { task ->
       if (task.name == 'compileDebugUnitTestSources') {
         task.finalizedBy packageTests
       }
     }
   

3. Add the test dependencies to the DependentModule by referencing the test JAR location property of the desired project.

    dependencies {
   
      ...other dependencies...
   
      // If the test code from ProviderModule is to be used in unit tests
      testCompile files(project(':ProviderModule').packageUnitTests.testJarLocation)
         
      // If the test code from ProviderModule is to be used in instrumentation tests
      androidTestCompile files(project(':ProviderModule').packageUnitTests.testJarLocation)
    }
   

4. Ensure the ProviderModule test JAR is created before DependentModule attempts to use it

    tasks.whenTaskAdded { task ->
      if (task.name == 'compileDebugUnitTestSources') {
          task.dependsOn ":ProviderModule:packageUnitTests"
      }
    } 
   

A simple project that demonstrates this technique using trivial tests is available at https://github.com/thedrover/multi-module-test-support.git.