Getting a nullpointer exception when using when().thenreturn() in mockito

Question

I have a mockito junit's test using the command below. The variable serviceTask is an Interface's instancie called ServiceTask. I'm using a @Mock in the declarection

Declaraction:

@Mock 
private ServiceTask

Command Line:

Mockito.when(serviceTask.getTask(Mockito.anyLong())).thenReturn(new Task());

Stack Trace:

java.lang.NullPointerException at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:90) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:55) at java.lang.reflect.Method.invoke(Method.java:508) at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50) at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12) at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47) at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17) at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26) at org.mockito.internal.runners.DefaultInternalRunner$1$1.evaluate(DefaultInternalRunner.java:54) at org.junit.rules.ExpectedException$ExpectedExceptionStatement.evaluate(ExpectedException.java:239) at org.junit.rules.RunRules.evaluate(RunRules.java:20) at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325) at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78) at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57) at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290) at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71) at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288) at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58) at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268) at org.junit.runners.ParentRunner.run(ParentRunner.java:363) at org.mockito.internal.runners.DefaultInternalRunner$1.run(DefaultInternalRunner.java:99) at org.mockito.internal.runners.DefaultInternalRunner.run(DefaultInternalRunner.java:105) at org.mockito.internal.runners.StrictRunner.run(StrictRunner.java:40) at org.mockito.junit.MockitoJUnitRunner.run(MockitoJUnitRunner.java:163) at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:89) at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:41) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:541) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:763) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:463) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:209)

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– Progman, Commented Apr 27, 2022 at 16:59
I often do it this way: Mockito.doReturn(new Task()).when(serviceTask).getTask(org.mockito.ArgumentMatchers.anyLong())); — Vinh Truong
– Vinh Truong, Commented Apr 27, 2022 at 17:03
I tried it and the nullPointer persist. I saw there is a check inside the method serviceTask.getTask() that it tests an atributted the class Task. Then the result is null to the new Task(), this ckeck failed. How could I resolve this? — Luciana Oliveira
– Luciana Oliveira, Commented Apr 27, 2022 at 17:14
@LucianaOliveira do you use MockitoJunitRunner to run the test? — HariHaravelan
– HariHaravelan, Commented Apr 28, 2022 at 0:25
Yes, I do. @HariHaravelan, I use this command line @RunWith(MockitoJUnitRunner.class) before the public class. It's so difficult to understand it. — Luciana Oliveira
– Luciana Oliveira, Commented Apr 28, 2022 at 11:59

user1373996 · Accepted Answer · 2022-04-28 13:01:04Z

1

The only reason for NPE if it happens at the line of code you mentioned above is serviceTask is null.

If you're not using the correct test runner which can help you to initialise the mock instance, then you can actually initialise them manually in your setUp method by using MockitoAnnotations.initMocks(this);

But if you already did so or used the correct test runner, then probably you linked to a wrong line of code. Try to debug yourself again.

Your stack trace doesn't look like it caused from your linked line of code.

answered Apr 28, 2022 at 13:01

user1373996

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2 Comments

user1373996 Over a year ago

The question with only one line for declaration and one line for the test code, means you assume that you are correct for everything else, this could be a wrong assumption.

Luciana Oliveira Over a year ago

Yes, you are rigth. The problem is not in the Mockito's syntax. I found an object that it is null. So the NPE is the Java exception, not the mockito's syntax. Thanks a lot to spend your time in this simple question.

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