@RunWith(SpringRunner.class) vs @RunWith(MockitoJUnitRunner.class)

The SpringRunner provides support for loading a Spring ApplicationContext and having beans @Autowired into your test instance. It actually does a whole lot more than that (covered in the Spring Reference Manual), but that's the basic idea.

Whereas, the MockitoJUnitRunner provides support for creating mocks and spies with Mockito.

However, with JUnit 4, you can only use one Runner at a time.

Thus, if you want to use support from Spring and Mockito simultaneously, you can only pick one of those runners.

But you're in luck since both Spring and Mockito provide rules in addition to runners.

For example, you can use the Spring runner with the Mockito rule as follows.

@RunWith(SpringRunner.class)
@SpringBootTest
public class MyTests {

    @Rule
    public MockitoRule rule = MockitoJUnit.rule();

    @Mock
    MyService myService;

    // ...
}

Though, typically, if you're using Spring Boot and need to mock a bean from the Spring ApplicationContext you would then use Spring Boot's @MockBean support instead of simply @Mock.

When SpringRunner.class is used, Spring provides corresponding annotations:

• @MockBean
• @SpyBean

Mocks are injected to objects under tests via @Autowired annotation. To enable this functionality tests must be annotated with

• @SpringBootTest

or

• @TestExecutionListeners(MockitoTestExecutionListener.class)

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More details and examples can be found in the official documentation: Mocking and Spying Beans

As per the JavaDoc:

SpringRunner is an alias for the SpringJUnit4ClassRunner. To use this class, simply annotate a JUnit 4 based test class with @RunWith(SpringRunner.class). If you would like to use the Spring TestContext Framework with a runner other than this one, use org.springframework.test.context.junit4.rules.SpringClassRule and org.springframework.test.context.junit4.rules.SpringMethodRule.

And the JavaDoc of TestContext:

TestContext encapsulates the context in which a test is executed, agnostic of the actual testing framework in use.

That of method getApplicationContext():

Get the application context for this test context, possibly cached. Implementations of this method are responsible for loading the application context if the corresponding context has not already been loaded, potentially caching the context as well.

So, SpringRunner does load the context and is responsible for maintaining it. For example, if you want to persist data into an embedded database, like H2 in-memory database, you have to use SpringRunner.class; and, to clean the tables to get rid of the records you inserted after every test, you annotate the test with @DirtiesContext to tell Spring to clean it.

But, this is already an integration or component test. If your test is pure unit test, you don't have to load DB, or you just want to verify some method of a dependency is called, MockitoJUnit4Runner suffices. You just use @Mock as you like and Mockito.verify(...) and the test will pass. And it is a lot quicker.

Test should be fast. As fast as possible. So whenever possible, use MockitoJUnit4Runner to speed it up.