unit-test-controller-layer

Unit Testing REST Controllers with MockMvc

Overview

Provides patterns for unit testing @RestController and @Controller classes using MockMvc. Covers request/response handling, HTTP status codes, request parameter binding, validation, content negotiation, response headers, and exception handling with mocked service dependencies.

When to Use

Use for: controller tests, API endpoint testing, Spring MVC tests, mock HTTP requests, unit testing web layer endpoints, verifying REST controllers in isolation.

Instructions

1. Setup standalone MockMvc: MockMvcBuilders.standaloneSetup(controller) for isolated testing
2. Mock service dependencies: Use @Mock for all services, @InjectMocks for the controller
3. Test HTTP methods: GET, POST, PUT, PATCH, DELETE with correct status codes
4. Validate responses: JsonPath assertions for JSON, content matchers for body
5. Test validation: Send invalid input, verify 400 status with error details
6. Test errors: Verify 404, 400, 401, 403, 500 for appropriate conditions
7. Validate headers: Both request (Authorization) and response headers
8. Test content negotiation: Different Accept and Content-Type headers

Validation Workflow

Run test → If fails: add .andDo(print()) → Check actual vs expected → Fix assertion

Examples

Maven / Gradle Dependencies

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

Basic Pattern: GET Endpoint

import static org.mockito.Mockito.*;
import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.*;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*;

@ExtendWith(MockitoExtension.class)
class UserControllerTest {

  @Mock
  private UserService userService;

  @InjectMocks
  private UserController userController;

  private MockMvc mockMvc;

  @BeforeEach
  void setUp() {
    mockMvc = MockMvcBuilders.standaloneSetup(userController).build();
  }

  @Test
  void shouldReturnAllUsers() throws Exception {
    List<UserDto> users = List.of(new UserDto(1L, "Alice"), new UserDto(2L, "Bob"));
    when(userService.getAllUsers()).thenReturn(users);

    mockMvc.perform(get("/api/users"))
      .andExpect(status().isOk())
      .andExpect(jsonPath("$[0].id").value(1))
      .andExpect(jsonPath("$[0].name").value("Alice"));

    verify(userService, times(1)).getAllUsers();
  }

  @Test
  void shouldReturn404WhenUserNotFound() throws Exception {
    when(userService.getUserById(999L))
      .thenThrow(new UserNotFoundException("User not found"));

    mockMvc.perform(get("/api/users/999"))
      .andExpect(status().isNotFound());

    verify(userService).getUserById(999L);
  }
}