Using Mockito in Java Tests | Generated by AI

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To use Mockito in Java tests, follow these steps to create mock objects, stub methods, and verify interactions:

1. Add Dependencies

Include Mockito and JUnit in your pom.xml (Maven):

<dependency>
    <groupId>org.mockito</groupId>
    <artifactId>mockito-core</artifactId>
    <version>5.3.1</version>
    <scope>test</scope>
</dependency>
<dependency>
    <groupId>org.junit.jupiter</groupId>
    <artifactId>junit-jupiter</artifactId>
    <version>5.9.2</version>
    <scope>test</scope>
</dependency>

2. Set Up the Test Class

Use JUnit 5 with Mockito’s extension:

import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.junit.jupiter.MockitoExtension;

@ExtendWith(MockitoExtension.class)
public class UserServiceTest {
    // ...
}

3. Create Mocks and Inject Dependencies

• Use @Mock for dependencies you want to mock.
• Use @InjectMocks to inject mocks into the class under test.

@Mock
private UserRepository userRepository;

@InjectMocks
private UserService userService; // UserService depends on UserRepository

4. Stub Method Calls

Use when().thenReturn() to define mock behavior:

@Test
void getUserById_ReturnsUser_WhenUserExists() {
    User mockUser = new User(1L, "Alice");
    // Stub the repository's method
    when(userRepository.findById(1L)).thenReturn(mockUser);

    User result = userService.getUserById(1L);
    
    assertEquals("Alice", result.getName());
}

5. Verify Interactions

Check if a mock method was called as expected:

@Test
void getUserById_CallsRepository() {
    userService.getUserById(1L);
    // Verify the repository method was called once with ID 1
    verify(userRepository, times(1)).findById(1L);
}

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Common Use Cases

Stub Exceptions

@Test
void getUserById_ThrowsException_WhenRepositoryFails() {
    when(userRepository.findById(anyLong())).thenThrow(new RuntimeException("DB Error"));
    
    assertThrows(RuntimeException.class, () -> userService.getUserById(1L));
}

Argument Matchers

Use any(), eq(), etc., to match arguments flexibly:

when(userRepository.findById(anyLong())).thenReturn(new User(1L, "Bob"));

BDD (Behavior-Driven Development) Style

Use given().willReturn() for readability:

import static org.mockito.BDDMockito.*;

@Test
void bddStyleExample() {
    given(userRepository.findById(1L)).willReturn(new User(1L, "Alice"));
    
    User result = userService.getUserById(1L);
    
    then(userRepository).should().findById(1L);
    assertEquals("Alice", result.getName());
}

Argument Captor

Capture arguments to perform detailed assertions:

@Test
void saveUser_CapturesArgument() {
    ArgumentCaptor<User> userCaptor = ArgumentCaptor.forClass(User.class);
    
    userService.saveUser("Charlie");
    
    verify(userRepository).save(userCaptor.capture());
    assertEquals("Charlie", userCaptor.getValue().getName());
}

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Key Notes

• Initialization: Use @ExtendWith(MockitoExtension.class) (JUnit 5) or MockitoAnnotations.openMocks(this) in @BeforeEach.
• Static Mocks: Use mockito-inline dependency for mocking static methods/constructors.
• Spies: Use @Spy to wrap real objects (partial mocks).

Troubleshooting

• Null Mocks: Ensure mocks are initialized (use @ExtendWith or openMocks()).
• Stubbing Errors: Stub methods before they’re called in the test.

For advanced usage, refer to the Mockito Documentation.

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