Mockito NullPointerException while using any

Question

I am trying to use Mockito like this :

    Mockito.when(Mockito.any(ObjectMapper.class).readValue(Mockito.any(BufferedReader.class),Mockito.any(Class.class))).thenReturn(new Person("1","abc"));

This is from Jackson library .

public <T> T readValue(Reader src, Class<T> valueType)

The reason I am doing it is because the time I reach this point of the code there are a ton of objects which were created. Mocking on every step would take time.

Any reason why I am getting NPE when code reaches this mockito statement?

Stack Trace :

java.lang.NullPointerException
    at com.prashant.flax.ShellTest.givenDirectoryHasFiles(ShellTest.java:139)
    at com.prashant.flax.ShellTest.testExecute(ShellTest.java:61)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.rules.ExternalResource$1.evaluate(ExternalResource.java:48)
    at org.junit.rules.RunRules.evaluate(RunRules.java:20)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:160)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:78)
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:212)
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:68)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)

As you can see it is in a given method(this method only has this piece of code) , so I can see while debugging, it reaches over there and crashes.

Answer 1

As Oliver mentioned in the comments, you can't apply when to happen to all objects. Mockito works via subclassing, so you have to create a mock instance using mock, spy, or a @Mock or @Spy annotation; customize the behavior; and then install the mock using dependency injection or other similar tricks.

---

As to why this happens, the return value for any() actually is null; matchers like any are only supposed to be used as arguments to when and verify, and Mockito can't produce a specialized instance of Class that represents "any Class", so Mockito returns a dummy value (null) and stores data on a specialized stack of argument matchers. Though Mockito has better error messages to alert you to this situation, your code NPEs before Mockito can give you a proper exception with usage examples.

For more about matcher return values and the stack, see my other SO answer on "How do Mockito matchers work?".