MockitoJUnitRunner

MockitoJUnitRunner is a JUnit 4 test runner provided by the Mockito framework. It allows the use of Mockito's annotation-based feature set in test classes by wiring Mockito annotations automatically during test execution.

To use it, annotate the test class with @RunWith(MockitoJUnitRunner.class). The runner will instantiate and initialize fields annotated with @Mock, @Spy, @Captor, and it will perform dependency injection into fields annotated with @InjectMocks. This removes the need to call MockitoAnnotations.initMocks manually.

The runner also performs validation of mock usage after each test, helping to detect issues such as unused stubs or incorrect stubbing patterns. If such issues are found, the test will fail with a descriptive error message.

For projects using JUnit 4, MockitoJUnitRunner is one of the common ways to enable mock annotations. Alternatives include using MockitoAnnotations.initMocks(this) in a @Before method or using a MockitoRule via @Rule MockitoRule rule = MockitoJUnit.rule().

For newer projects or those using JUnit 5, MockitoJUnitRunner is not used. JUnit 5 supports the MockitoExtension (MockitoExtension.class) to provide similar annotation-based initialization and integration.