Objects annotated with Mockito annotations @Mock@Spy@Captor, or @InjectMocks need to be initialized explicitly.

There are several ways to do this:

Test using uninitialized mocks will fail.

Note that this only applies to annotated Mockito objects. It is not necessary to initialize objects instantiated via Mockito.mock() or Mockito.spy().

This rule raises an issue when a test class uses uninitialized mocks.

Noncompliant Code Example

public class FooTest { // Noncompliant: Mockito initialization missing
  @Mock private Bar bar;

  @Spy private Baz baz;

  @InjectMocks private Foo fooUnderTest;

  @Test
  void someTest() {
    // test something ...
  }

  @Nested
  public class Nested {
    @Mock
    private Bar bar;
  }

Compliant Solution

@RunWith(MockitoJUnitRunner.class)
public class FooTest {
  @Mock private Bar bar;
  // ...
}
@ExtendWith(MockitoExtension.class)
public class FooTest {
  @Mock private Bar bar;
  // ...
}
public class FooTest {
  @Rule
  public MockitoRule rule = MockitoJUnit.rule();

  @Mock private Bar bar;
  // ...
}
public class FooTest {
  @Mock private Bar bar;
  // ...

  @BeforeEach
  void setUp() {
    MockitoAnnotations.openMocks(this);
  }
  // ...
}
public class FooTest {
  @Mock private Bar bar;
  // ...

  @Before
  void setUp() {
    MockitoAnnotations.initMocks(this);
  }
  // ...
}
@ExtendWith(MockitoExtension.class)
public class FooTest {
  @Nested
  public class Nested {
    @Mock
    private Bar bar;
  }
}

See