What\'s the Difference Between Mockito\'s @Mock and @InjectMocks Annotations?

Understanding the Distinction Between @Mock and @InjectMocks in Mockito

Mockito, a popular mocking framework for Java, provides two important annotations: @Mock and @InjectMocks. While both are essential for mocking tests, they serve distinct purposes.

@Mock: Creating Mocks

@Mock is used to create a mock object. It represents a fake or simulated implementation of a real object. Mocks allow you to control their behavior and assert their interactions within your tests. For example:

@Mock
private SomeDependency someDependency;

This code creates a mock of the SomeDependency interface. You can then use this mock in your tests.

@InjectMocks: Injecting Mocks

In contrast, @InjectMocks plays a different role. It is used to create an instance of a class and inject the mocks created with @Mock (or @Spy) annotations into that instance. This allows you to test interactions between multiple classes and their dependencies without creating real instances. For example:

@InjectMocks
private SomeManager someManager;

In this case, someManager is an instance of the SomeManager class. The @InjectMocks annotation tells Mockito to inject the someDependency mock into the someManager instance.

Initialization and Usage Notes

To utilize @InjectMocks, you must initialize the mocks using @RunWith(MockitoJUnitRunner.class) or Mockito.initMocks(this) for JUnit 4 and @ExtendWith(MockitoExtension.class) for JUnit 5. Furthermore, remember that @InjectMocks must be used only once in a test class, and it should be placed on the field representing the class under test.

Example Usage

The following example illustrates how to use @Mock and @InjectMocks:

@RunWith(MockitoJUnitRunner.class)
public class SomeManagerTest {

    @InjectMocks
    private SomeManager someManager;

    @Mock
    private SomeDependency someDependency; // this will be injected into someManager
 
    // tests...
}

In this test, the someDependency mock will be automatically injected into the someManager instance, allowing you to test their interactions effectively.

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