Android RxJava 2 JUnit test - getMainLooper in android.os.Looper not mocked RuntimeException

android unit-testing junit rx-java rx-java2

35,105

Solution 1

This error occurs because the default scheduler returned by AndroidSchedulers.mainThread() is an instance of LooperScheduler and relies on Android dependencies that are not available in JUnit tests.

We can avoid this issue by initializing RxAndroidPlugins with a different Scheduler before the tests are run. You can do this inside of a @BeforeClass method like so:

@BeforeClass
public static void setUpRxSchedulers() {
    Scheduler immediate = new Scheduler() {
        @Override
        public Disposable scheduleDirect(@NonNull Runnable run, long delay, @NonNull TimeUnit unit) {
            // this prevents StackOverflowErrors when scheduling with a delay
            return super.scheduleDirect(run, 0, unit);
        }

        @Override
        public Worker createWorker() {
            return new ExecutorScheduler.ExecutorWorker(Runnable::run);
        }
    };

    RxJavaPlugins.setInitIoSchedulerHandler(scheduler -> immediate);
    RxJavaPlugins.setInitComputationSchedulerHandler(scheduler -> immediate);
    RxJavaPlugins.setInitNewThreadSchedulerHandler(scheduler -> immediate);
    RxJavaPlugins.setInitSingleSchedulerHandler(scheduler -> immediate);
    RxAndroidPlugins.setInitMainThreadSchedulerHandler(scheduler -> immediate);
}

Or you can create a custom TestRule that will allow you to reuse the initialization logic across multiple test classes.

public class RxImmediateSchedulerRule implements TestRule {
    private Scheduler immediate = new Scheduler() {
        @Override
        public Disposable scheduleDirect(@NonNull Runnable run, long delay, @NonNull TimeUnit unit) {
            // this prevents StackOverflowErrors when scheduling with a delay
            return super.scheduleDirect(run, 0, unit);
        }

        @Override
        public Worker createWorker() {
            return new ExecutorScheduler.ExecutorWorker(Runnable::run);
        }
    };

    @Override
    public Statement apply(final Statement base, Description description) {
        return new Statement() {
            @Override
            public void evaluate() throws Throwable {
                RxJavaPlugins.setInitIoSchedulerHandler(scheduler -> immediate);
                RxJavaPlugins.setInitComputationSchedulerHandler(scheduler -> immediate);
                RxJavaPlugins.setInitNewThreadSchedulerHandler(scheduler -> immediate);
                RxJavaPlugins.setInitSingleSchedulerHandler(scheduler -> immediate);
                RxAndroidPlugins.setInitMainThreadSchedulerHandler(scheduler -> immediate);

                try {
                    base.evaluate();
                } finally {
                    RxJavaPlugins.reset();
                    RxAndroidPlugins.reset();
                }
            }
        };
    }
}

Which you can then apply to your test class

public class TestClass {
    @ClassRule public static final RxImmediateSchedulerRule schedulers = new RxImmediateSchedulerRule();

    @Test
    public void testStuff_stuffHappens() {
       ...
    }
}

Both of these methods will ensure that the default schedulers will be overridden before any of the tests execute and before AndroidSchedulers is accessed.

Overriding the RxJava schedulers with an immediate scheduler for unit testing will also make sure the RxJava usages in the code being tested gets run synchronously, which will make it much easier to write the unit tests.

Sources:
https://www.infoq.com/articles/Testing-RxJava2 https://medium.com/@peter.tackage/overriding-rxandroid-schedulers-in-rxjava-2-5561b3d14212

Solution 2

I just added

RxAndroidPlugins.setInitMainThreadSchedulerHandler(scheduler -> Schedulers.trampoline());

in @Before annoted method.

Solution 3

I was getting the same error when testing LiveData. When testing LiveData, this InstantTaskExecutorRule is needed in addition to RxImmediateSchedulerRule if the class being tested has both background thread and LiveData.

@RunWith(MockitoJUnitRunner::class)
class MainViewModelTest {

    companion object {
        @ClassRule @JvmField
        val schedulers = RxImmediateSchedulerRule()
    }

    @Rule
    @JvmField
    val rule = InstantTaskExecutorRule()

    @Mock
    lateinit var dataRepository: DataRepository

    lateinit var model: MainViewModel

    @Before
    fun setUp() {
      model = MainViewModel(dataRepository)
    }

    @Test
    fun fetchData() {
      //given    
      val returnedItem = createDummyItem()    
      val observer = mock<Observer<List<Post>>>()    
      model.getPosts().observeForever(observer)    
      //when    
      liveData.value = listOf(returnedItem)    
      //than    
      verify(observer).onChanged(listOf(Post(returnedItem.id, returnedItem.title, returnedItem.url)))
    }

}

Reference: https://pbochenski.pl/blog/07-12-2017-testing_livedata.html

Solution 4

Based on @starkej2 answer, with some changes, the correct answer for Kotlin developers would be:

Create RxImmediateSchedulerRule.kt class:

import io.reactivex.Scheduler
import io.reactivex.android.plugins.RxAndroidPlugins
import io.reactivex.internal.schedulers.ExecutorScheduler
import io.reactivex.plugins.RxJavaPlugins
import org.junit.rules.TestRule
import org.junit.runner.Description
import org.junit.runners.model.Statement
import java.util.concurrent.Executor

class RxImmediateSchedulerRule : TestRule {
    private val immediate = object : Scheduler() {
        override fun createWorker(): Worker {
            return ExecutorScheduler.ExecutorWorker(Executor { it.run() })
        }
    }

    override fun apply(base: Statement, description: Description): Statement {
        return object : Statement() {
            @Throws(Throwable::class)
            override fun evaluate() {
                RxJavaPlugins.setInitIoSchedulerHandler { immediate }
                RxJavaPlugins.setInitComputationSchedulerHandler { immediate }
                RxJavaPlugins.setInitNewThreadSchedulerHandler { immediate }
                RxJavaPlugins.setInitSingleSchedulerHandler { immediate }
                RxAndroidPlugins.setInitMainThreadSchedulerHandler { immediate }

                try {
                    base.evaluate()
                } finally {
                    RxJavaPlugins.reset()
                    RxAndroidPlugins.reset()
                }
            }
        }
    }
}

On your test class, create schedulers ClassRule:

class TestViewModelTest {

companion object {
   @ClassRule
   @JvmField
   val schedulers = RxImmediateSchedulerRule()
}

@Before
fun setUp() {
    //your setup code here
}

@Test
fun yourTestMethodHere{}
}

Solution 5

As in the advice in this Medium article by Peter Tackage you can inject the Schedulers yourself.

We all know that directly calling static methods can make for classes that are hard to test and if you use a dependency injection framework like Dagger 2 injecting the Schedulers can be especially easy. The example is as follows:

Define an interface in your project:

public interface SchedulerProvider {
    Scheduler ui();
    Scheduler computation();
    Scheduler io();
    Scheduler special();
    // Other schedulers as required…
}

Define an implementation:

final class AppSchedulerProvider implements SchedulerProvider {
    @Override 
    public Scheduler ui() {
        return AndroidSchedulers.mainThread();
    }
    @Override 
    public Scheduler computation() {
        return Schedulers.computation();
    }
    @Override 
    public Scheduler io() {
        return Schedulers.io();
    }
    @Override 
    public Scheduler special() {
        return MyOwnSchedulers.special();
    }
}

Now instead of using direct references to the Schedulers like this:

 bookstoreModel.getFavoriteBook()
               .map(Book::getTitle)
               .delay(5, TimeUnit.SECONDS)
               .observeOn(AndroidSchedulers.mainThread())
               .subscribe(view::setBookTitle));

You use references to your interface:

bookstoreModel.getFavoriteBook()
          .map(Book::getTitle)
          .delay(5, TimeUnit.SECONDS, 
                 this.schedulerProvider.computation())
          .observeOn(this.schedulerProvider.ui())
          .subscribe(view::setBookTitle));

Now for your tests, you could define a TestSchedulersProvider like this:

public final class TestSchedulersProvider implements SchedulerProvider {

      @Override
      public Scheduler ui() {
          return new TestScheduler();
      }

      @Override
      public Scheduler io() {
          return Schedulers.trampoline(); //or test scheduler if you want
      }

      //etc
}

You now have all of the advantages of using TestScheduler when you want to in your unit tests. This comes in handy for situations where you might want to test a delay:

@Test
public void testIntegerOneIsEmittedAt20Seconds() {
    //arrange
    TestObserver<Integer> o = delayedRepository.delayedInt()
            .test();

    //act
    testScheduler.advanceTimeTo(20, TimeUnit.SECONDS);

    //assert
    o.assertValue(1);
}

Otherwise, if you don't want to use injected Schedulers the static hooks mentioned in the other methods can be done using lambdas:

@Before
public void setUp() {
    RxAndroidPlugins.setInitMainThreadSchedulerHandler(h -> Schedulers.trampoline());
    RxJavaPlugins.setIoSchedulerHandler(h -> Schedulers.trampoline());
//etc
}

View more solutions

35,105

Author by

starkej2

Updated on July 05, 2022

Comments

starkej2 almost 2 years

I am encountering a RuntimeException when attempting to run JUnit tests for a presenter that is using observeOn(AndroidSchedulers.mainThread()).

Since they are pure JUnit tests and not Android instrumentation tests, they don't have access to Android dependencies, causing me to encounter the following error when executing the tests:

java.lang.ExceptionInInitializerError
    at io.reactivex.android.schedulers.AndroidSchedulers$1.call(AndroidSchedulers.java:35)
    at io.reactivex.android.schedulers.AndroidSchedulers$1.call(AndroidSchedulers.java:33)
    at io.reactivex.android.plugins.RxAndroidPlugins.callRequireNonNull(RxAndroidPlugins.java:70)
    at io.reactivex.android.plugins.RxAndroidPlugins.initMainThreadScheduler(RxAndroidPlugins.java:40)
    at io.reactivex.android.schedulers.AndroidSchedulers.<clinit>(AndroidSchedulers.java:32)
    …
Caused by: java.lang.RuntimeException: Method getMainLooper in android.os.Looper not mocked. See http://g.co/androidstudio/not-mocked for details.
    at android.os.Looper.getMainLooper(Looper.java)
    at io.reactivex.android.schedulers.AndroidSchedulers$MainHolder.<clinit>(AndroidSchedulers.java:29)
    ...


java.lang.NoClassDefFoundError: Could not initialize class io.reactivex.android.schedulers.AndroidSchedulers
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    …

AA_PV about 7 years

pls also check my answer for a small change which might be required if you run into StackOverflowError
starkej2 about 7 years

Thanks! I actually ran into that same issue shortly after posting my answer. Had a similar solution as you...I'll update the answer since this info is probably useful for people.
Nelson Ramirez almost 7 years

Update: You can use the RxJavaHooks methods to set the schedulers. Also you have the TestScheduler available, as well as simply using Schedulers.immediate()
starkej2 almost 7 years

@NelsonRamirez I believe RxJavaHooks was removed in RxJava 2 and it's functionality is now incorporated into RxJavaPlugins
Ahmed Mostafa almost 6 years

kotlin and @JvmField, you save me :)
Etienne Lawlor over 5 years

So i did something similar in a Kotlin project and it works but for whatever reason in a Java project the unit tests will only fail if you run it through ./gradlew test but it passes when you click run in the gutter next to the test class name in Android Studio. I thought it runs the test the same way. Does anyone know about this nuance ?
JDenais over 5 years

Don't forget to check that your dependencies are all either androidx or all android.arch / com.android. You can't mix and match and you will lose a lot of time wondering why it's not working =)
ericn over 5 years

This is the best answer, static hooks look like hack to me. Dependency injection is your friend
Subhrajyoti Sen over 5 years

This worked for me when I had only 1 test that used Schedulers but it failed if I had multiple schedulers. The accepted answer works though
Sam over 4 years

this worked for me after adding the below rule. @get:Rule var rule: TestRule = InstantTaskExecutorRule()
A. Steenbergen over 4 years

This worked for me instantly without anything else required. Thank you!
Mario Codes about 4 years

This did it for me. None of the above solutions were good for me, as they don't include the imports (as always), and as someone new to Android I don't know the frameworks they reference to.
fahrizal89 over 2 years

just using @Rule @JvmField val rule = InstantTaskExecutorRule() it saved my day. thank you!