InjectMocks

InjectMocks is a feature of the Mockito testing framework used in unit tests for Java. It marks the class under test so that Mockito will create an instance of that class and automatically inject its dependencies with mock objects defined in the test.

In a typical setup, fields in the test are annotated with @Mock or @Spy to create mock or spy instances of dependencies, and the field representing the class under test is annotated with @InjectMocks. Mockito then constructs the class under test and injects the mocks into it. Injection can occur through the constructor, through setter methods, or directly into fields, with constructor injection preferred when a matching constructor is available.

Initialization of the annotated fields requires enabling Mockito’s annotation processing. This can be done with the MockitoJUnitRunner (JUnit 4), the MockitoExtension (JUnit 5), or by calling MockitoAnnotations.initMocks(this) in setup code.

Limitations and considerations include that injection relies on available mocks that match the dependencies of the class under test. If multiple candidates exist for a dependency type, disambiguation may be needed, potentially using naming or qualifiers. Final fields, private injection points, or certain complex initialization scenarios may complicate or prevent automatic injection. InjectMocks does not create real instances of dependencies; it uses mocks defined in the test instead, helping to isolate the unit under test from its collaborators.