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 say why or how it fixed it.

Edit: All right 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.