Mockito is a popular Java mocking framework used for unit testing. It allows developers to create mock objects that simulate the behavior of real objects in a controlled way. The main purposes of Mockito are:

• Isolation: Test classes in isolation by mocking their dependencies
• Control: Define specific behaviors for method calls during testing
• Verification: Verify that certain methods were called with expected parameters
• Speed: Avoid expensive operations like database calls or network requests

Mockito helps create fast, reliable, and focused unit tests by eliminating external dependencies.

Maven:

<dependency>
    <groupId>org.mockito</groupId>
    <artifactId>mockito-core</artifactId>
    <version>5.7.0</version>
    <scope>test</scope>
</dependency>

Gradle:

testImplementation 'org.mockito:mockito-core:5.7.0'

JUnit 5 integration:

<dependency>
    <groupId>org.mockito</groupId>
    <artifactId>mockito-junit-jupiter</artifactId>
    <version>5.7.0</version>
    <scope>test</scope>
</dependency>

There are several ways to create mocks in Mockito:

1. Using mock() method:

List<String> mockList = mock(List.class);

2. Using @Mock annotation:

@Mock
private List<String> mockList;

3. Using @MockBean (Spring Boot):

@MockBean
private UserService userService;

4. Using MockitoAnnotations.openMocks():

@BeforeEach
void setUp() {
    MockitoAnnotations.openMocks(this);
}

The annotation approach is preferred as it makes tests cleaner and more readable.

Mockito provides the verify() method to check if methods were called:

// Verify method was called once
verify(mockList).add("item");

// Verify method was called specific number of times
verify(mockList, times(2)).add(anyString());

// Verify method was never called
verify(mockList, never()).clear();

// Verify method was called at least/at most certain times
verify(mockList, atLeast(1)).size();
verify(mockList, atMost(3)).add(anyString());

Verification should be done after the method under test has been executed.