Module 3: Mocking Review: TDD with Mockito

Learning Objectives

• Understand what a mocked object is and its purpose in testing
• Know how to work with Mockito to create and configure mocks
  • Setting up mock objects
  • Configuring return values and behaviors
  • Capturing and validating arguments
• Master techniques for working with mocks and controlling mock object behavior
• Know how to verify that specific methods were called on a mocked object
• Learn how to mock exceptions to test error handling
• Understand the concept of a unit test and how mocking allows us to write unit tests that otherwise couldn't be written
• Practice applying mocks in Test Driven Development workflows

Code Example: Mocking with Mockito

// Basic mocking example
@Test
void shouldFetchUserAndSendNotification() {
    // Arrange
    UserRepository mockRepository = mock(UserRepository.class);
    NotificationService mockNotificationService = mock(NotificationService.class);
    
    User testUser = new User("user123", "user@example.com");
    when(mockRepository.findById("user123")).thenReturn(testUser);
    
    UserService userService = new UserService(mockRepository, mockNotificationService);
    
    // Act
    userService.notifyUser("user123", "Hello!");
    
    // Assert
    verify(mockRepository).findById("user123");
    verify(mockNotificationService).sendEmail(testUser.getEmail(), "Hello!");
}

// Mocking exceptions
@Test
void shouldHandleUserNotFound() {
    // Arrange
    UserRepository mockRepository = mock(UserRepository.class);
    when(mockRepository.findById("missing")).thenThrow(new UserNotFoundException());
    
    UserService userService = new UserService(mockRepository, mock(NotificationService.class));
    
    // Act & Assert
    assertThrows(UserNotFoundException.class, () -> {
        userService.notifyUser("missing", "Hello!");
    });
}

// Using argument captors
@Test
void shouldCreateProperNotificationContent() {
    // Arrange
    NotificationService mockNotificationService = mock(NotificationService.class);
    UserService userService = new UserService(mock(UserRepository.class), mockNotificationService);
    
    // Capture what gets passed to the notification service
    ArgumentCaptor<String> contentCaptor = ArgumentCaptor.forClass(String.class);
    
    // Act
    userService.sendSystemNotification("System maintenance");
    
    // Assert
    verify(mockNotificationService).broadcast(contentCaptor.capture());
    String capturedContent = contentCaptor.getValue();
    assertTrue(capturedContent.contains("System maintenance"));
    assertTrue(capturedContent.startsWith("[SYSTEM NOTIFICATION]"));
}