Android Local Unit Test - Mock FirebaseAuth with MockK

Issue

Overview

Expected Behavior - Mock FirebaseApp.class in a local unit test with JUnit5 in Android using MockK's class, relaxed, or constructor mock features.

Current Behavior - The following error is thrown.

java.lang.NoClassDefFoundError: Could not initialize class com.google.firebase.FirebaseApp

Implementation

Class/Functions to be mocked - FirebaseAuth.class

package com.google.firebase.auth;
@NonNull
    @Keep
    public static FirebaseAuth getInstance() {
        return (FirebaseAuth)FirebaseApp.getInstance().get(FirebaseAuth.class);
    }
//`getCurrentUser` is accessed via the auto generated `currentUser` attribute in Kotlin.
@Nullable
    public FirebaseUser getCurrentUser() {
        return this.zzf;
    }

Test - SomeTest.kt

// Relaxed mock attempts.
every { FirebaseApp.getInstance() } returns mockk(relaxed = true)
every { FirebaseAuth.getInstance(any())} returns mockk(relaxed = true)
every { FirebaseAuth.getInstance().currentUser } returns mockk(relaxed = true)

// Class mock attempts.
mockkClass(FirebaseApp::class)
mockkClass(FirebaseAuth::class)

// Constructor mock attempts.
mockkConstructor(FirebaseApp::class)
    every { anyConstructed<FirebaseApp>() } returns mockk(relaxed = true)
mockkConstructor(FirebaseAuth::class)
    every { anyConstructed<FirebaseAuth>() } returns mockk(relaxed = true)

Android code - HomeViewModel.kt

// Code called from the local unit test SomeTest.kt.
class HomeViewModel : ViewModel() {
    fun getCurrentUser() = FirebaseAuth.getInstance().currentUser
    ...
}

Note - The same approach has been implemented successfully in order to mock the FirebaseRemoteConfig.class with the following mock.

SomeTest.kt

mockkStatic(FirebaseRemoteConfig::class)  
every { FirebaseRemoteConfig.getInstance() } returns mockk(relaxed = true)

Constants.kt

// Code called from the local unit test SomeTest.kt.
val CONTENT_REQUEST_NETWORK_ERROR = FirebaseRemoteConfig.getInstance().getString("content_request_network_error")

Full Error Log

java.lang.ExceptionInInitializerError
    at com.google.firebase.FirebaseApp.<clinit>(com.google.firebase:firebase-common@@19.1.0:96)
    at app.coinverse.contentviewmodel.PlayContentTests$mockComponents$1.invoke(PlayContentTests.kt:170)
    at app.coinverse.contentviewmodel.PlayContentTests$mockComponents$1.invoke(PlayContentTests.kt:43)
    at io.mockk.impl.eval.RecordedBlockEvaluator$record$block$1.invoke(RecordedBlockEvaluator.kt:24)
    at io.mockk.impl.eval.RecordedBlockEvaluator$enhanceWithNPERethrow$1.invoke(RecordedBlockEvaluator.kt:74)
    at io.mockk.impl.recording.JvmAutoHinter.autoHint(JvmAutoHinter.kt:23)
    at io.mockk.impl.eval.RecordedBlockEvaluator.record(RecordedBlockEvaluator.kt:36)
    at io.mockk.impl.eval.EveryBlockEvaluator.every(EveryBlockEvaluator.kt:30)
    at io.mockk.MockKDsl.internalEvery(API.kt:92)
    at io.mockk.MockKKt.every(MockK.kt:104)
    at app.coinverse.contentviewmodel.PlayContentTests.mockComponents(PlayContentTests.kt:170)
    at app.coinverse.contentviewmodel.PlayContentTests.access$mockComponents(PlayContentTests.kt:43)
    at app.coinverse.contentviewmodel.PlayContentTests$Play Content$1.invokeSuspend(PlayContentTests.kt:124)
    at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
    at kotlinx.coroutines.DispatchedTask.run(Dispatched.kt:241)
    at kotlinx.coroutines.EventLoopImplBase.processNextEvent(EventLoop.common.kt:270)
    at kotlinx.coroutines.BlockingCoroutine.joinBlocking(Builders.kt:79)
    at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking(Builders.kt:54)
    at kotlinx.coroutines.BuildersKt.runBlocking(Unknown Source)
    at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking$default(Builders.kt:36)
    at kotlinx.coroutines.BuildersKt.runBlocking$default(Unknown Source)
    at app.coinverse.contentviewmodel.PlayContentTests.Play Content(PlayContentTests.kt:123)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:675)
    at org.junit.jupiter.engine.execution.MethodInvocation.proceed(MethodInvocation.java:60)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain$ValidatingInvocation.proceed(InvocationInterceptorChain.java:125)
    at org.junit.jupiter.engine.extension.TimeoutExtension.intercept(TimeoutExtension.java:132)
    at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestableMethod(TimeoutExtension.java:124)
    at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestTemplateMethod(TimeoutExtension.java:81)
    at org.junit.jupiter.engine.execution.ExecutableInvoker$ReflectiveInterceptorCall.lambda$ofVoidMethod$0(ExecutableInvoker.java:115)
    at org.junit.jupiter.engine.execution.ExecutableInvoker.lambda$invoke$0(ExecutableInvoker.java:105)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain$InterceptedInvocation.proceed(InvocationInterceptorChain.java:104)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain.proceed(InvocationInterceptorChain.java:62)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain.chainAndInvoke(InvocationInterceptorChain.java:43)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain.invoke(InvocationInterceptorChain.java:35)
    at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:104)
    at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:98)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.lambda$invokeTestMethod$6(TestMethodTestDescriptor.java:202)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.invokeTestMethod(TestMethodTestDescriptor.java:198)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:135)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:69)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:135)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:125)
    at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:135)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:123)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:122)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:80)
    at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.submit(SameThreadHierarchicalTestExecutorService.java:32)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask$DefaultDynamicTestExecutor.execute(NodeTestTask.java:198)
    at org.junit.jupiter.engine.descriptor.TestTemplateTestDescriptor.execute(TestTemplateTestDescriptor.java:138)
    at org.junit.jupiter.engine.descriptor.TestTemplateTestDescriptor.lambda$execute$2(TestTemplateTestDescriptor.java:106)
    at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:184)
    at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
    at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:175)
    at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
    at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:184)
    at java.util.stream.ReferencePipeline$11$1.accept(ReferencePipeline.java:373)
    at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
    at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
    at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
    at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:184)
    at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
    at java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:948)
    at java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:580)
    at java.util.stream.ReferencePipeline$7$1.accept(ReferencePipeline.java:270)
    at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
    at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
    at java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:948)
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
    at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
    at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:151)
    at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:174)
    at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
    at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:418)
    at java.util.stream.ReferencePipeline$7$1.accept(ReferencePipeline.java:270)
    at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
    at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
    at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
    at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382)
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
    at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
    at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:151)
    at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:174)
    at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
    at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:418)
    at java.util.stream.ReferencePipeline$7$1.accept(ReferencePipeline.java:270)
    at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382)
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
    at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
    at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:151)
    at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:174)
    at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
    at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:418)
    at org.junit.jupiter.engine.descriptor.TestTemplateTestDescriptor.execute(TestTemplateTestDescriptor.java:106)
    at org.junit.jupiter.engine.descriptor.TestTemplateTestDescriptor.execute(TestTemplateTestDescriptor.java:41)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:135)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:125)
    at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:135)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:123)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:122)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:80)
    at java.util.ArrayList.forEach(ArrayList.java:1257)
    at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:139)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:125)
    at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:135)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:123)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:122)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:80)
    at java.util.ArrayList.forEach(ArrayList.java:1257)
    at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:139)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:125)
    at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:135)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:123)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:122)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:80)
    at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.submit(SameThreadHierarchicalTestExecutorService.java:32)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:57)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestEngine.execute(HierarchicalTestEngine.java:51)
    at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:229)
    at org.junit.platform.launcher.core.DefaultLauncher.lambda$execute$6(DefaultLauncher.java:197)
    at org.junit.platform.launcher.core.DefaultLauncher.withInterceptedStreams(DefaultLauncher.java:211)
    at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:191)
    at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:128)
    at com.intellij.junit5.JUnit5IdeaTestRunner.startRunnerWithArgs(JUnit5IdeaTestRunner.java:69)
    at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
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 com.google.firebase.FirebaseApp$UiExecutor.<clinit>(com.google.firebase:firebase-common@@19.1.0:670)
    ... 141 more

Libraries

• MockK version: 1.9.3
• OS: Mac 10.14.6
• Kotlin version: 1.3.50
• JDK version: 12.0.1
• JUnit version: 5.5.1
• Type of test: unit test

Solution

Use MockK's Extension & top-level functions implementation strategy to mock Firebase's FirebaseAuth.kt class. This will result in the FirebaseAuth class being essentially ignored when called in the unit test code, thus not crashing the test due to external Firebase dependencies.

The source of the mocking issue is either due to the static method getInstance, how the currentUser variable is auto-generated, or both.

Test - SomeLocalJUnitTest.kt

mockkStatic(FirebaseAuth::class)
every { FirebaseAuth.getInstance() } returns mockk(relaxed = true)

Class/functions to be mocked - FirebaseAuth.class

package com.google.firebase.auth;
@NonNull
    @Keep
    public static FirebaseAuth getInstance() {
        return (FirebaseAuth)FirebaseApp.getInstance().get(FirebaseAuth.class);
    }
//`getCurrentUser` is accessed via the auto generated `currentUser` attribute in Kotlin.
@Nullable
    public FirebaseUser getCurrentUser() {
        return this.zzf;
    }

Android code - HomeViewModel.kt

// Code called from the local unit test SomeTest.kt.
class HomeViewModel : ViewModel() {
    fun getCurrentUser() = FirebaseAuth.getInstance().currentUser
    ...
}

Answered By - Adam Hurwitz