Here is a working example of DocumentRepositoryTest class for reference:

@RunWith(MockitoJUnitRunner.class)
public final class DocumentRepositoryTest {

    @Mock
    private DocumentRepository documentRepository;

    @Test
    public void testConvertToNewDocument() {
        String paperColor = new String("white");
        Mockito.when(this.documentRepository.getPaperColor()).thenReturn(paperColor);
        String color = this.documentRepository.getDocumentColor();
        Assert.assertThat(color, is(new String("white"));
    }
}

DocumentRepositoryTest class is mocking documentRepository object.

So you are running your test and suddenly you see NullPointerException:

java.lang.NullPointerException at com.your.custom.clazz.Method.insertTest(CustomServiceTest.java:50)

You looked it up, and it seems your test is written correctly. So what might be a problem if you know how the mocking works?

If you are sure by your mocking skills, the issue will be probably somewhere else. And most likely very trivial. Here is a list of 3 things you should check out.

1. Return something for your Mock.

Most likely, you mistyped returning function. You probably wanted to return the value for the mocked object. So instead of when-thenReturn , you might type just when-then. Maybe it was IntelliSense. Maybe you did it accidentally. But for sure, NullPointerException happened because you want something which is not there. Debug and check if you are returning something.

2. Specify Mockito running class

Don’t forget to annotate your Testing class with @RunWith(MockitoJUnitRunner.class). Most of the people just forget to specify the test runner, running class for mocking.

3. You need to annotate the mocking object with the @Mock annotation

If you want to mock an object, you need to annotate the object with @Mock annotation.