When Should I Use @Mock, @MockBean, or Mockito.mock() for Mocking in Java?

Understanding the Differences between @Mock, @MockBean, and Mockito.mock()

Background

In software testing, mocking is a technique used to create fake objects that simulate the behavior of actual dependencies. Mockito is a popular mocking framework in Java that provides multiple ways to mock objects. This guide compares the different approaches and their appropriate usage.

Mockito.mock() vs @Mock (Mockito Annotations)

Mockito.mock() and @Mock are from the standard Mockito library. Both create mock objects for classes or interfaces. @Mock provides a convenient annotation-based solution, while Mockito.mock() is more explicit. In terms of functionality, they are equivalent. However, using @Mock is preferred as it simplifies the code.

@MockBean (Spring Boot)

@MockBean is a wrapper provided by the Spring Boot test library. It combines Mockito mocking capabilities with Spring's bean management. Unlike Mockito.mock() and @Mock, @MockBean creates mock beans that are injected into the Spring ApplicationContext. It has two key characteristics:

• If a bean of the same type exists in the context, it is replaced by the mock.
• If no bean of the same type exists, the mock bean is added to the context.

Usage Guidelines

When to use plain Mockito: Use Mockito.mock() or @Mock when testing components without relying on Spring Boot dependencies or when isolation of the tested component is crucial.

When to use @MockBean: Use @MockBean in web test slices (e.g., @WebMvcTest) or other testing scenarios that require modifying Spring Boot bean definitions or interacting with beans managed by the container.

Example with @MockBean

Consider a Spring Boot web test where the FooController relies on the FooService. Using @MockBean, we can replace the real FooService bean with a mock in the test context:

<code class="java">@WebMvcTest(FooController.class)
public class FooControllerTest {

    @MockBean
    private FooService fooServiceMock;

    // ... //
}</code>

In this example, the fooServiceMock instance will be injected into the FooController, allowing us to mock its behavior and control how it interacts with the controller.

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