mockito

Mockito is an open-source Java-based mocking framework used in unit testing to create and configure mock objects. Mocks stand in for collaborators and dependencies, enabling tests to focus on the behavior of the code under test rather than its real collaborators. The framework provides a fluent API for creating mocks and spies, stubbing method behavior, and verifying interactions.

Core concepts include mocks, spies, stubbing, and verification. Mocks are created for interfaces or classes, while spies wrap real objects to allow selective overriding. Stubbing is done with patterns such as when(...).thenReturn(...) or when(...).thenThrow(...), enabling tests to specify expected outcomes. Verification uses methods like verify(...) to ensure that certain interactions occurred, with support for argument matchers and verification modes (for example, times or inOrder). ArgumentCaptor can capture values passed to mocks for assertions.

Mockito supports both annotation-based configuration and programmatic creation. Common patterns include @Mock and @InjectMocks annotations, with initialization provided by MockitoAnnotations.initMocks(this) or through integrations for JUnit 4 or JUnit 5 (such as MockitoJUnitRunner or MockitoExtension). It integrates with build tools like Maven and Gradle and with test frameworks like JUnit and TestNG.

The ecosystem includes additional modules and extensions, such as Mockito-Kotlin for Kotlin projects and mockito-inline for inline mocking of final or static methods in suitable environments. Mockito is widely adopted for its simplicity, performance, and strong community support, making it a staple in many Java testing strategies.