Migrating Junit4 tests to androidx: What causes 'delegate runner could not be loaded'?

Question

I am migrating my app to androidx, I can't seem to get my unit tests working. I took example from Google's AndroidJunitRunnerSample, which has been updated to use the new androidx api. I get the following error when trying to run my tests:

java.lang.Exception: Delegate runner 'androidx.test.internal.runner.junit4.AndroidJUnit4ClassRunner' for AndroidJUnit4 could not be loaded. Check your build configuration. 

Here is my module build.gradle:

android {     defaultConfig {         testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"     } } dependencies {     // Test dependencies     androidTestImplementation 'androidx.test:core:1.0.0-beta02'     androidTestImplementation 'androidx.test.ext:junit:1.0.0-beta02'     androidTestImplementation 'androidx.test:runner:1.1.0-beta02'     androidTestImplementation 'androidx.test.espresso:espresso-core:3.1.0-beta02'     androidTestImplementation "androidx.arch.core:core-testing:2.0.0"     androidTestImplementation 'androidx.room:room-testing:2.1.0-alpha01'     androidTestImplementation 'androidx.test.espresso:espresso-core:3.1.0-beta02'     androidTestImplementation 'org.hamcrest:hamcrest-library:1.3' } 

And here is how my tests are structured:

import org.junit.BeforeClass; import org.junit.Test; import org.junit.runner.RunWith;  import androidx.test.ext.junit.runners.AndroidJUnit4;  @RunWith(AndroidJUnit4.class) public class EntityParcelTest {      @BeforeClass     public void createEntities() {         // Setup...     }      @Test     void someTest() {         // Testing here     } 

What am I doing wrong?

Answer 1

Removing @RunWith(AndroidJUnit4.class) annotations from the test classes fixed the issue, although I can't really say why or how it fixed it.

Edit: Allright I did some more testing. I migrated my app to Kotlin, and suddenly I noticed the tests began to work with the @RunWith annotation, too. Here's what I found out:

import org.junit.BeforeClass; import org.junit.Test; import org.junit.runner.RunWith;  import androidx.test.ext.junit.runners.AndroidJUnit4;  @RunWith(AndroidJUnit4.class) // <-- @RunWith + @BeforeClass = Error public class AndroidXJunitTestJava {      @BeforeClass     public static void setup() {         // Setting up once before all tests     }      @Test     public void testing() {         // Testing....     } } 

This java test fails with the Delegate runner for AndroidJunit4 could not be loaded error. But If I remove the @RunWith annotation, it works. Also, if I replace the @BeforeClass setup with just a @Before, like this:

import org.junit.Before; import org.junit.Test; import org.junit.runner.RunWith;  import androidx.test.ext.junit.runners.AndroidJUnit4;  @RunWith(AndroidJUnit4.class) // <-- @RunWith + @Before = works? public class AndroidXJunitTestJava {      @Before     public void setup() {         // Setting up before every test     }      @Test     public void testing() {         // Testing....     } } 

The tests will run without errors. I needed to use the @BeforeClass annotation, so I just removed @RunWith.

But now that I am using Kotlin, the following (which should be equal to the first java example) works:

import androidx.test.ext.junit.runners.AndroidJUnit4 import org.junit.BeforeClass import org.junit.Test import org.junit.runner.RunWith  @RunWith(AndroidJUnit4::class) class AndroidXJunitTest {      companion object {         @BeforeClass fun setup() {             // Setting up         }     }      @Test     fun testing() {         // Testing...     }  } 

Also, as Alessandro Biessek said in an answer and @Ioane Sharvadze in the comments, the same error can happen with the @Rule annotation. If I add a line

 @Rule val instantTaskExecutorRule = InstantTaskExecutorRule() 

To the Kotlin example, the same delegate runner error happens. This must be replaced with

@get:Rule val instantTaskExecutorRule = InstantTaskExecutorRule() 

Explanation here.