Mock in Bean

@MockInBean and @SpyInBean are alternatives to @MockBean and @SpyBean for Spring Boot tests (>= 2.6.15 including >= 3.X.X).

They surgically replace a field value in a Spring Bean by a Mock/Spy for the duration of a test and set back the original value afterwards, leaving the Spring Context clean.

@MockInBean 'mocks a bean in a bean' whereas @MockBean 'mocks a bean in the whole context'.

But why ?

The problem:

Spring Context pollution was a fairly common problem before the introduction of @MockBean and @SpyBean. Many developers would inject mocks in beans during tests through @InjectMock or using manual setters and often forget to set back the original field values which would leave the Spring context polluted and cause test failures in unrelated tests.

@MockBean and @SpyBean solved this issue by providing Mockito injection directly in the Spring Context but introduced an undesirable side-effect: their usage dirties the context and may lead to the re-creation of new Spring contexts for any unique combination, which can be incredibly time-consuming. See The Problems with @MockBean

The solution:

Assuming you really need to run the test in the Spring Context, the most straight-forward solution is still to inject your mock/spy in your bean and reset it afterwards.

@MockInBean and @SpyInBean brings the convenience of @MockBean and @SpyBean and do just that:

1. Set the mock/spy in the bean.
2. Replace the mock/spy by the original value afterwards.

Example:

Assuming that you want to test the following service:

@Service
public class MyService {

    @Autowired
    protected ThirdPartyApiService thirdPartyService;

    @Autowired
    protected ExpensiveProcessor expensiveProcessor;

    public void doSomething() {
        final Object somethingExpensive = expensiveProcessor.returnSomethingExpensive();
        thirdPartyService.doSomethingOnThirdPartyApi(somethingExpensive);
    }

}

You can write your test this way:

@SpringBootTest
public class MyServiceTest {

    @MockInBean(MyService.class)
    private ThirdPartyApiService thirdPartyApiService;

    @SpyInBean(MyService.class)
    private ExpensiveProcessor expensiveProcessor;

    @Autowired
    private MyService myService;

    @Test
    public void test() {
        final Object somethingExpensive = new Object();
        Mockito.when(expensiveProcessor.returnSomethingExpensive()).thenReturn(somethingExpensive);
        myService.doSomething();
        Mockito.verify(thirdPartyApiService).doSomethingOnThirdPartyApi(somethingExpensive);
    }

}

What happens:

Before each test:

• MyServiceTest.thirdPartyService will be created as a mock and injected in the target of @MockInBean: MyService.
• MyServiceTest.expensiveProcessor will be created as a spy of the bean expensiveProcessor and injected in the target of @SpyInBean: MyService.

After the tests of MyServiceTest are done:

• MyService.thirdPartyService will be reset to the original Spring bean thirdPartyService
• MyService.expensiveProcessor will be reset to the original Spring bean expensiveProcessor

Usage:

Simply include the maven dependency (from central maven) to start using @MockInBean and @SpyInBean in your tests.

<dependency>
  <groupId>com.teketik</groupId>
  <artifactId>mock-in-bean</artifactId>
  <version>boot2-v1.8</version>
  <scope>test</scope>
</dependency>

@MockInBean and @SpyInBean also support:

• Injection in multiple Spring beans: Repeat the annotation on your field.
• Injection in bean identified by name if multiple instances exist in the context: Specify a name in your annotation.

Checkout the javadoc for more information.

Limitations:

This approach has some limitations compared to the @MockBean/@SpyBean equivalent:

• There is currently no isolation per thread. It is not advised to use this library in parallel test suites. (Parallel test execution is supported experimentally in https://github.com/antoinemeyer/mock-in-bean/tree/v2.1-RC).
• Operations on beans happening within another bean's constructor will not be performed against the mocks since the mocks are injected directly into the fields. Do not use @MockInBean/@SpyInBean for beans that are manipulated in constructors.