Why @WebMvcTest for controller testing in JUnit? - Purpose & Use Cases

The Big Idea

What if you could test your web pages without opening a browser or clicking buttons manually?

The Scenario

Imagine you have a web app with many pages and buttons. You want to check if clicking a button shows the right page. Doing this by opening the browser and clicking every button manually takes hours.

The Problem

Manual testing is slow and tiring. You might miss some buttons or forget steps. It's easy to make mistakes or skip tests. Also, you can't quickly check if a small change broke something else.

The Solution

@WebMvcTest lets you test your web controllers automatically without starting the whole app. It focuses only on the web layer, so tests run fast and catch errors early.

Before vs After

✗ Before

Start app, open browser, click buttons, check pages manually.

✓ After

@WebMvcTest
public class MyControllerTest {
  @Autowired
  private MockMvc mockMvc;

  @Test
  void testPage() throws Exception {
    mockMvc.perform(get("/page"))
           .andExpect(status().isOk())
           .andExpect(view().name("pageView"));
  }
}

What It Enables

You can quickly and reliably check your web controllers work as expected without the hassle of full app startup or manual clicks.

Real Life Example

A developer changes a button's URL. With @WebMvcTest, they run tests that catch the broken link immediately before release.

Key Takeaways

Manual web testing is slow and error-prone.

@WebMvcTest tests controllers fast and focused.

It helps catch web layer bugs early and reliably.

Practice

1. What is the main purpose of using @WebMvcTest in Spring Boot testing?

easy

A. To test only the Spring MVC controllers without starting the full application context

B. To test the entire application including database and services

C. To perform integration testing of all layers

D. To mock external APIs automatically

Solution

1. Step 1: Understand the role of @WebMvcTest

   @WebMvcTest is designed to load only the Spring MVC components, mainly controllers, for focused testing.

2. Step 2: Compare with other test types

   Unlike full integration tests, it does not load services or repositories, making tests faster and isolated.

3. Final Answer:

   To test only the Spring MVC controllers without starting the full application context -> Option A

4. Quick Check:

   @WebMvcTest = Controller-only test [OK]

Hint: Remember: @WebMvcTest loads controllers only, not full app [OK]

Common Mistakes:

• Thinking @WebMvcTest loads full application context
• Confusing @WebMvcTest with @SpringBootTest
• Assuming it mocks external APIs automatically

2. Which annotation is used to mock a service dependency inside a @WebMvcTest annotated test class?

easy

A. @InjectMocks

B. @MockMvc

C. @Autowired

D. @MockBean

Solution

1. Step 1: Identify mocking annotation for Spring beans

   @MockBean is used to add mock implementations of Spring beans into the application context during tests.

2. Step 2: Differentiate from other annotations

   @MockMvc is not an annotation, @Autowired injects real beans, and @InjectMocks is from Mockito but not for Spring context injection.

3. Final Answer:

   @MockBean -> Option D

4. Quick Check:

   Mock dependencies in @WebMvcTest = @MockBean [OK]

Hint: Use @MockBean to mock Spring beans in @WebMvcTest [OK]

Common Mistakes:

• Using @Autowired instead of @MockBean for mocks
• Confusing @MockBean with Mockito's @InjectMocks
• Thinking @MockMvc is an annotation

3. Given the following test snippet using @WebMvcTest and MockMvc:

@WebMvcTest(MyController.class)
class MyControllerTest {
  @Autowired
  private MockMvc mockMvc;

  @Test
  void testHello() throws Exception {
    mockMvc.perform(get("/hello"))
      .andExpect(status().isOk())
      .andExpect(content().string("Hello World"));
  }
}

What will happen if the controller method mapped to /hello returns "Hello World" as response?

medium

A. The test will fail because MockMvc is not autowired correctly

B. The test will pass because the response matches the expected string

C. The test will fail due to status code mismatch

D. The test will throw a NullPointerException

Solution

1. Step 1: Analyze the test expectations

   The test expects HTTP status 200 OK and response body "Hello World".

2. Step 2: Match with controller behavior

   If the controller returns "Hello World" with status 200, both expectations are met, so the test passes.

3. Final Answer:

   The test will pass because the response matches the expected string -> Option B

4. Quick Check:

   Matching response and status = test pass [OK]

Hint: Match expected status and content to pass MockMvc tests [OK]

Common Mistakes:

• Assuming test fails if no explicit status set in controller
• Confusing autowiring issues with test failures
• Ignoring response content in assertions

4. Consider this test class snippet:

@WebMvcTest(MyController.class)
class MyControllerTest {
  @Autowired
  private MockMvc mockMvc;

  @MockBean
  private MyService myService;

  @Test
  void testEndpoint() throws Exception {
    when(myService.getData()).thenReturn("data");
    mockMvc.perform(get("/data"))
      .andExpect(status().isOk())
      .andExpect(content().string("data"));
  }
}

If the test fails with a 404 Not Found error, what is the most likely cause?

medium

A. The test class is missing @SpringBootTest annotation

B. The mock service is not returning the expected value

C. The controller method for /data is missing or not mapped correctly

D. MockMvc is not autowired properly

Solution

1. Step 1: Understand 404 error in MockMvc test

   A 404 means the requested URL is not found in the controller mappings.

2. Step 2: Check test setup

   @WebMvcTest loads only controllers; if /data mapping is missing or incorrect, 404 occurs despite mocks.

3. Final Answer:

   The controller method for /data is missing or not mapped correctly -> Option C

4. Quick Check:

   404 error = missing controller mapping [OK]

Hint: 404 in @WebMvcTest usually means missing controller mapping [OK]

Common Mistakes:

• Blaming mock service return value for 404
• Assuming @SpringBootTest is needed for controller tests
• Ignoring URL mapping correctness

5. You want to test a controller method that depends on a service returning a list of users. Using @WebMvcTest, how should you mock the service and verify the controller returns JSON with the user list correctly?

hard

A. Use @MockBean to mock the service, stub the method to return a list, then use MockMvc to perform GET and expect JSON content

B. Use @Autowired to inject the real service and call the controller method directly

C. Use @SpringBootTest to load full context and test the controller with real service

D. Use Mockito's @InjectMocks on the controller and call methods without MockMvc

Solution

1. Step 1: Mock the service dependency with @MockBean

   In @WebMvcTest, use @MockBean to replace the service bean with a mock and stub its method to return the user list.

2. Step 2: Use MockMvc to perform HTTP GET and verify JSON response

   Perform a GET request on the controller endpoint and assert the JSON response matches the expected user list.

3. Final Answer:

   Use @MockBean to mock the service, stub the method to return a list, then use MockMvc to perform GET and expect JSON content -> Option A

4. Quick Check:

   Mock service + MockMvc GET + JSON assert = correct test [OK]

Hint: Mock service with @MockBean and verify JSON via MockMvc [OK]

Common Mistakes:

• Injecting real service instead of mocking
• Using full context test instead of @WebMvcTest
• Calling controller methods directly without MockMvc