In this example we have Android app module that depends Android library feature module. feature-robots Android library module only contains UI instrumentation helpers for UI testing feature, specifically it references one of the resources from feature. app has UI test that depends on feature-robots in order to test feature. Note that feature-robots depends on feature in compile time only using compileOnly Gradle configuration. That's because target and instrumentation APKs run in the same JVM so they have the same classpath which makes it unnecessary to ship the app code into the instrumentation apk. This however doesn't work because compileOnly does not seem to provide Android resources, only jar with classes, whereas implementation does both + packages them. Ultimately, we want to keep our robot modules (UI helpers) as thin as possible.

AGP bug: compileOnly configuration doesn't propagate R class

java.lang.NoClassDefFoundError: Failed resolution of: Lcom/kirillzh/featurerobots/R$string;
at com.kirillzh.featurerobots.FeatureRobot.clickSomething(FeatureRobot.java:5)
at com.kirillzh.ExampleTest.testSomething(ExampleTest.java:11)
at java.lang.reflect.Method.invoke(Native Method)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runners.Suite.runChild(Suite.java:128)
at org.junit.runners.Suite.runChild(Suite.java:27)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at org.junit.runner.JUnitCore.run(JUnitCore.java:115)
at androidx.test.internal.runner.TestExecutor.execute(TestExecutor.java:56)
at androidx.test.runner.AndroidJUnitRunner.onStart(AndroidJUnitRunner.java:388)
at android.app.Instrumentation$InstrumentationThread.run(Instrumentation.java:2145)
Caused by: java.lang.ClassNotFoundException: Didn't find class "com.kirillzh.featurerobots.R$string" on path: DexPathList[[zip file "/system/framework/android.test.runner.jar", zip file "/system/framework/android.test.mock.jar", zip file "/data/app/com.kirillzh.test-hCYsJlL3_Ly8iceHF9eEFw==/base.apk", zip file "/data/app/com.kirillzh-APjSsSOmbffB2fCDHiOstQ==/base.apk"],nativeLibraryDirectories=[/data/app/com.kirillzh.test-hCYsJlL3_Ly8iceHF9eEFw==/lib/x86, /data/app/com.kirillzh-APjSsSOmbffB2fCDHiOstQ==/lib/x86, /system/lib]]
at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:134)
at java.lang.ClassLoader.loadClass(ClassLoader.java:379)
at java.lang.ClassLoader.loadClass(ClassLoader.java:312)
... 29 more

E/TestRunner: java.lang.NoClassDefFoundError: Failed resolution of: Lcom/kirillzh/featurerobots/R$string;
        at com.kirillzh.featurerobots.FeatureRobot.clickSomething(FeatureRobot.java:5)
        at com.kirillzh.ExampleTest.testSomething(ExampleTest.java:11)
        at java.lang.reflect.Method.invoke(Native Method)
        at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
        at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
        at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
        at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
        at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
        at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
        at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
        at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
        at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
        at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
        at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
        at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
        at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
        at org.junit.runners.Suite.runChild(Suite.java:128)
        at org.junit.runners.Suite.runChild(Suite.java:27)
        at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
        at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
        at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
        at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
        at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
        at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
        at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
        at org.junit.runner.JUnitCore.run(JUnitCore.java:115)
        at androidx.test.internal.runner.TestExecutor.execute(TestExecutor.java:56)
        at androidx.test.runner.AndroidJUnitRunner.onStart(AndroidJUnitRunner.java:388)
        at android.app.Instrumentation$InstrumentationThread.run(Instrumentation.java:2145)
     Caused by: java.lang.ClassNotFoundException: Didn't find class "com.kirillzh.featurerobots.R$string" on path: DexPathList[[zip file "/system/framework/android.test.runner.jar", zip file "/system/framework/android.test.mock.jar", zip file "/data/app/com.kirillzh.test-hCYsJlL3_Ly8iceHF9eEFw==/base.apk", zip file "/data/app/com.kirillzh-APjSsSOmbffB2fCDHiOstQ==/base.apk"],nativeLibraryDirectories=[/data/app/com.kirillzh.test-hCYsJlL3_Ly8iceHF9eEFw==/lib/x86, /data/app/com.kirillzh-APjSsSOmbffB2fCDHiOstQ==/lib/x86, /system/lib]]