RuntimeException: Delegate runnerfor AndroidJUnit4 could not be loaded

Question

I am trying to do an instrumentation test using Android Studio for my MainActivity.java class using Junit4. I don't know if my code is correct or not but this is my test class code:

import android.widget.EditText;
import android.widget.TextView;

import androidx.test.espresso.Espresso;
import androidx.test.ext.junit.runners.AndroidJUnit4;
import androidx.test.rule.ActivityTestRule;

import static org.junit.Assert.*;

import org.junit.Before;
import org.junit.Rule;
import org.junit.Test;
import org.junit.runner.RunWith;

@RunWith(AndroidJUnit4.class)
public class MainActivityTest{

    private EditText edt;
    private TextView txt;

    @Rule
    ActivityTestRule<MainActivity> activityTestRule = new ActivityTestRule<>(MainActivity.class);

    @Before
    public void setup(){
        edt = activityTestRule.getActivity().findViewById(R.id.edt);
        txt = activityTestRule.getActivity().findViewById(R.id.txt);
    }

    @Test
    public void edt_text_change() {
        //i want this test to be passed
        assertEquals("147", "147");
    }
}

Build.gradle

apply plugin: 'com.android.application'

android {
    compileSdkVersion 29
    buildToolsVersion "29.0.1"
    defaultConfig {
        applicationId "package"
        minSdkVersion 15
        targetSdkVersion 29
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'androidx.appcompat:appcompat:1.0.2'
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test:runner:1.2.0'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'
    androidTestImplementation 'androidx.test.ext:junit:1.1.1'
    androidTestImplementation 'com.android.support.test:rules:1.0.2'
}

But when i run that class, Android Studio shows me this error:

java.lang.RuntimeException: Delegate runner 'androidx.test.internal.runner.junit4.AndroidJUnit4ClassRunner' for AndroidJUnit4 could not be loaded.
at androidx.test.ext.junit.runners.AndroidJUnit4.throwInitializationError(AndroidJUnit4.java:92)
at androidx.test.ext.junit.runners.AndroidJUnit4.loadRunner(AndroidJUnit4.java:82)
at androidx.test.ext.junit.runners.AndroidJUnit4.loadRunner(AndroidJUnit4.java:51)
at androidx.test.ext.junit.runners.AndroidJUnit4.<init>(AndroidJUnit4.java:46)
at java.lang.reflect.Constructor.newInstance(Native Method)
at org.junit.internal.builders.AnnotatedBuilder.buildRunner(AnnotatedBuilder.java:104)
at org.junit.internal.builders.AnnotatedBuilder.runnerForClass(AnnotatedBuilder.java:86)
at androidx.test.internal.runner.junit4.AndroidAnnotatedBuilder.runnerForClass(AndroidAnnotatedBuilder.java:63)
at org.junit.runners.model.RunnerBuilder.safeRunnerForClass(RunnerBuilder.java:59)
at org.junit.internal.builders.AllDefaultPossibilitiesBuilder.runnerForClass(AllDefaultPossibilitiesBuilder.java:26)
at androidx.test.internal.runner.AndroidRunnerBuilder.runnerForClass(AndroidRunnerBuilder.java:153)
at org.junit.runners.model.RunnerBuilder.safeRunnerForClass(RunnerBuilder.java:59)
at androidx.test.internal.runner.TestLoader.doCreateRunner(TestLoader.java:73)
at androidx.test.internal.runner.TestLoader.getRunnersFor(TestLoader.java:104)
at androidx.test.internal.runner.TestRequestBuilder.build(TestRequestBuilder.java:793)
at androidx.test.runner.AndroidJUnitRunner.buildRequest(AndroidJUnitRunner.java:547)
at androidx.test.runner.AndroidJUnitRunner.onStart(AndroidJUnitRunner.java:390)
at android.app.Instrumentation$InstrumentationThread.run(Instrumentation.java:1890)

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. This is how i fixed my issue

Answer 2

One more possible reason for this error is that you could accidentally miss that method with @Before annotation isn't void (Unit). It's super easy when you use Kotlin:

@Before
fun setupSearch() = runBlocking {
    dao.saveSearchResults(listOf(testResult1, testRestult2), testSearchId))
}

If the last line in runBlocking block returns something, setupSearch won't be void. To fix that specify generic parameter explicitly

@Before
fun setupSearch() = runBlocking<Unit> {
    dao.saveSearchResults(listOf(testResult1, testRestult2), testSearchId))
}

I know that it doesn't directly answer Hadi's question, this answer is for people who googled java.lang.RuntimeException: Delegate runner 'androidx.test.internal.runner.junit4.AndroidJUnit4ClassRunner' for AndroidJUnit4 could not be loaded. and found this question, hope it will be useful for somebody.