unit-test-controller-layer

---
name: unit-test-controller-layer
description: Provides patterns for unit testing REST controllers using MockMvc and @WebMvcTest. Generates controller tests that validates request/response mapping, validation, exception handling, and HTTP status codes. Use when testing web layer endpoints in isolation for API endpoint testing, Spring MVC tests, mock HTTP requests, or controller layer unit tests.
license: MIT
---

# 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

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

### Basic Pattern: GET Endpoint

```java
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);
  }
}
```

### POST: Create Resource

```java
@Test
void shouldCreateUserAndReturn201() throws Exception {
  UserDto createdUser = new UserDto(1L, "Alice", "alice@example.com");
  when(userService.createUser(any())).thenReturn(createdUser);

  mockMvc.perform(post("/api/users")
      .contentType("application/json")
      .content("{\"name\":\"Alice\",\"email\":\"alice@example.com\"}"))
    .andExpect(status().isCreated())
    .andExpect(jsonPath("$.id").value(1))
    .andExpect(jsonPath("$.name").value("Alice"));

  verify(userService).createUser(any(UserCreateRequest.class));
}
```

### PUT: Update Resource

```java
@Test
void shouldUpdateUserAndReturn200() throws Exception {
  UserDto updatedUser = new UserDto(1L, "Updated");
  when(userService.updateUser(eq(1L), any())).thenReturn(updatedUser);

  mockMvc.perform(put("/api/users/1")
      .contentType("application/json")
      .content("{\"name\":\"Updated\"}"))
    .andExpect(status().isOk())
    .andExpect(jsonPath("$.name").value("Updated"));

  verify(userService).updateUser(eq(1L), any());
}
```

### DELETE: Remove Resource

```java
@Test
void shouldDeleteUserAndReturn204() throws Exception {
  doNothing().when(userService).deleteUser(1L);

  mockMvc.perform(delete("/api/users/1"))
    .andExpect(status().isNoContent());

  verify(userService).deleteUser(1L);
}
```

### Query Parameters

```java
@Test
void shouldFilterUsersByName() throws Exception {
  when(userService.searchUsers("Alice")).thenReturn(List.of(new UserDto(1L, "Alice")));

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

  verify(userService).searchUsers("Alice");
}
```

### Path Variables

```java
@Test
void shouldGetUserByIdFromPath() throws Exception {
  when(userService.getUserById(123L)).thenReturn(new UserDto(123L, "Alice"));

  mockMvc.perform(get("/api/users/{id}", 123L))
    .andExpect(status().isOk())
    .andExpect(jsonPath("$.id").value(123));
}
```

### Validation Errors (400)

```java
@Test
void shouldReturn400WhenRequestBodyInvalid() throws Exception {
  mockMvc.perform(post("/api/users")
      .contentType("application/json")
      .content("{\"name\":\"\"}"))
    .andExpect(status().isBadRequest())
    .andExpect(jsonPath("$.errors").isArray());
}
```

### Response Headers

```java
@Test
void shouldReturnCustomHeaders() throws Exception {
  when(userService.getAllUsers()).thenReturn(List.of());

  mockMvc.perform(get("/api/users"))
    .andExpect(status().isOk())
    .andExpect(header().exists("X-Total-Count"))
    .andExpect(header().string("X-Total-Count", "0"));
}
```

### Authorization Header

```java
@Test
void shouldRequireAuthorizationHeader() throws Exception {
  mockMvc.perform(get("/api/users"))
    .andExpect(status().isUnauthorized());

  mockMvc.perform(get("/api/users").header("Authorization", "Bearer token"))
    .andExpect(status().isOk());
}
```

### Content Negotiation

```java
@Test
void shouldReturnJsonWhenAcceptHeaderIsJson() throws Exception {
  when(userService.getUserById(1L)).thenReturn(new UserDto(1L, "Alice"));

  mockMvc.perform(get("/api/users/1").accept("application/json"))
    .andExpect(status().isOk())
    .andExpect(content().contentType("application/json"));
}
```

## Best Practices

- Use `standaloneSetup()` for isolated controller testing
- Mock service layer — controllers handle HTTP, services handle business logic
- Verify mock interactions: `verify(service).method(args)`
- Test happy path AND error scenarios (404, 400, 500)
- Use `jsonPath()` for fluent JSON assertions
- One focused assertion per test method

## Constraints and Warnings

- Controller tests verify HTTP handling only — not full request flow
- `standaloneSetup()` may not support `@Validated` without full context
- JsonPath requires valid JSON in response body
- `@PreAuthorize`/`@Secured` need additional setup — consider separate security tests
- File uploads require `MockMultipartFile`

## References

- [Spring MockMvc Documentation](https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/test/web/servlet/MockMvc.html)
- [Spring Testing Best Practices](https://docs.spring.io/spring-boot/docs/current/reference/html/features.html#features.testing)