Unit Testing with JUnit and Mockito

Imagine you're building a car. Before putting it on the road, you test each component individually – engine, brakes, lights. Unit testing is exactly that: testing individual components (methods) of your application in isolation.

JUnit is the most popular framework for writing unit tests in Java. Mockito helps you create mock objects to isolate the class you're testing.

Add Dependencies:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-test</artifactId>
    <scope>test</scope>
</dependency>

Basic JUnit Test:

import static org.junit.jupiter.api.Assertions.*;

public class CalculatorTest {
    
    private Calculator calculator = new Calculator();
    
    @Test
    public void testAdd() {
        int result = calculator.add(5, 3);
        assertEquals(8, result, "5 + 3 should be 8");
    }
    
    @Test
    public void testDivideByZero() {
        assertThrows(ArithmeticException.class, () -> {
            calculator.divide(10, 0);
        });
    }
}

Testing Service Layer with Mockito:

@ExtendWith(MockitoExtension.class)
public class UserServiceTest {
    
    @Mock
    private UserRepository userRepository;
    
    @InjectMocks
    private UserService userService;
    
    @Test
    public void testGetUserById() {
        // Arrange
        Long userId = 1L;
        User mockUser = new User(userId, "John", "john@example.com");
        when(userRepository.findById(userId)).thenReturn(Optional.of(mockUser));
        
        // Act
        User result = userService.getUserById(userId);
        
        // Assert
        assertNotNull(result);
        assertEquals("John", result.getName());
        verify(userRepository, times(1)).findById(userId);
    }
    
    @Test
    public void testGetUserByIdNotFound() {
        // Arrange
        Long userId = 99L;
        when(userRepository.findById(userId)).thenReturn(Optional.empty());
        
        // Act & Assert
        assertThrows(RuntimeException.class, () -> {
            userService.getUserById(userId);
        });
    }
}

Testing Controllers with MockMvc:

@WebMvcTest(UserController.class)
public class UserControllerTest {
    
    @Autowired
    private MockMvc mockMvc;
    
    @MockBean
    private UserService userService;
    
    @Test
    public void testGetUser() throws Exception {
        User user = new User(1L, "John", "john@example.com");
        when(userService.getUserById(1L)).thenReturn(user);
        
        mockMvc.perform(get("/users/1"))
            .andExpect(status().isOk())
            .andExpect(jsonPath("$.name", is("John")))
            .andExpect(jsonPath("$.email", is("john@example.com")));
    }
}

Two Minute Drill

Unit tests test individual components in isolation.
JUnit provides annotations: @Test, @BeforeEach, @AfterEach.
Assertions: assertEquals, assertTrue, assertThrows.
Mockito creates mocks: @Mock, @InjectMocks.
when().thenReturn() defines mock behavior.
verify() checks if methods were called.

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Unit Testing with JUnit and Mockito

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