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

The Big Idea

Discover how to test your web controllers quickly without starting your whole app!

The Scenario

Imagine writing tests for your web controllers by starting the whole application and manually sending HTTP requests to check responses.

The Problem

This approach is slow, requires a full app context, and makes it hard to isolate controller logic from other parts like services or databases.

The Solution

@WebMvcTest lets you test only the web layer, loading just the controller and related components, so tests run fast and focus on controller behavior.

Before vs After

✗ Before

start full app context
send HTTP request
check response

✓ After

@WebMvcTest(MyController.class)
mockMvc.perform(get("/path"))
    .andExpect(status().isOk())

What It Enables

It enables fast, focused tests that verify your controller's behavior without the overhead of starting the entire application.

Real Life Example

When adding a new API endpoint, you can quickly write a @WebMvcTest to check if it returns the right status and JSON without waiting for the full app to start.

Key Takeaways

Manual full app tests are slow and complex.

@WebMvcTest isolates controller testing for speed and focus.

It helps catch web layer issues early and easily.

Practice

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

easy

A. To test only the web layer (controllers) without starting the full application context

B. To test the entire Spring Boot application including database and services

C. To perform integration testing with real HTTP calls

D. To test only the service layer without controllers

Solution

1. Step 1: Understand the role of @WebMvcTest

   @WebMvcTest is designed to load only the Spring MVC components like controllers, filters, and related configurations, not the full app context.

2. Step 2: Compare with other test annotations

   Unlike @SpringBootTest which loads the full context, @WebMvcTest focuses on controller layer for fast, isolated web tests.

3. Final Answer:

   To test only the web layer (controllers) without starting the full application context -> Option A

4. Quick Check:

   @WebMvcTest = controller-only test [OK]

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

Common Mistakes:

• Thinking @WebMvcTest loads full Spring context
• Confusing @WebMvcTest with @SpringBootTest
• Assuming it tests service or repository layers

2. Which of the following is the correct way to declare a test class using @WebMvcTest for a controller named UserController?

easy

A. @WebMvcTest(UserController.class)
public class UserControllerTest {}

B. @WebMvcTest
public class UserControllerTest {}

C. @WebMvcTest(services = UserController.class)
public class UserControllerTest {}

D. @WebMvcTest(controllers = UserController.class)
public class UserControllerTest {}

Solution

1. Step 1: Check @WebMvcTest syntax for specifying controllers

   The correct syntax to specify which controllers to test is using the 'controllers' attribute, e.g., @WebMvcTest(controllers = UserController.class).

2. Step 2: Eliminate incorrect options

   @WebMvcTest(UserController.class) public class UserControllerTest {} misses 'controllers =' keyword, @WebMvcTest public class UserControllerTest {} does not specify controller, @WebMvcTest(services = UserController.class) public class UserControllerTest {} wrongly uses 'services' attribute.

3. Final Answer:

   @WebMvcTest(controllers = UserController.class) public class UserControllerTest {} -> Option D

4. Quick Check:

   @WebMvcTest(controllers = ...) is correct syntax [OK]

Hint: Use 'controllers =' to specify controller class in @WebMvcTest [OK]

Common Mistakes:

• Omitting 'controllers =' keyword
• Using 'services =' instead of 'controllers ='
• Not specifying any controller class

3. Given the following test code snippet using @WebMvcTest and MockMvc, what will be the HTTP status returned by the test?

@WebMvcTest(controllers = HelloController.class)
public class HelloControllerTest {

  @Autowired
  private MockMvc mockMvc;

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

@RestController
class HelloController {
  @GetMapping("/hello")
  public String sayHello() {
    return "Hello World";
  }
}

medium

A. 404 Not Found

B. 200 OK

C. 500 Internal Server Error

D. 302 Found (Redirect)

Solution

1. Step 1: Analyze the controller mapping

   The HelloController has a GET mapping for '/hello' that returns a string "Hello World".

2. Step 2: Understand MockMvc test behavior

   The test performs a GET request to '/hello' and expects status 200 OK, which matches the controller's response.

3. Final Answer:

   200 OK -> Option B

4. Quick Check:

   GET /hello returns 200 OK [OK]

Hint: Check controller mappings match test request URL [OK]

Common Mistakes:

• Expecting 404 when mapping exists
• Confusing status codes for successful GET
• Missing controller in @WebMvcTest causing 404

4. Identify the error in the following @WebMvcTest test class that causes the test to fail with a NullPointerException.

@WebMvcTest(controllers = UserController.class)
public class UserControllerTest {

  private MockMvc mockMvc;

  @Test
  public void testGetUser() throws Exception {
    mockMvc.perform(get("/user/1"))
           .andExpect(status().isOk());
  }
}

medium

A. Missing @Autowired annotation on MockMvc field

B. Wrong controller class specified in @WebMvcTest

C. Missing @Test annotation on test method

D. MockMvc cannot perform GET requests

Solution

1. Step 1: Check MockMvc initialization

   The MockMvc field is declared but not initialized or injected, so it remains null causing NullPointerException.

2. Step 2: Fix by adding @Autowired

   Adding @Autowired on the MockMvc field lets Spring inject the mockMvc instance for testing.

3. Final Answer:

   Missing @Autowired annotation on MockMvc field -> Option A

4. Quick Check:

   MockMvc must be @Autowired to avoid null [OK]

Hint: Always @Autowired MockMvc in @WebMvcTest classes [OK]

Common Mistakes:

• Forgetting @Autowired on MockMvc
• Assuming MockMvc is auto-initialized without annotation
• Misnaming test methods without @Test

5. You want to test a controller OrderController that depends on a service OrderService. Using @WebMvcTest, how can you provide a mock OrderService to the controller during testing?

hard

A. Create a new OrderService instance manually in the test method

B. Use @Autowired on OrderService field to inject real service

C. Use @MockBean on OrderService field in the test class to inject a mock

D. Use @SpringBootTest instead of @WebMvcTest to load full context

Solution

1. Step 1: Understand @WebMvcTest scope

   @WebMvcTest loads only web layer, so service beans are not loaded automatically.

2. Step 2: Use @MockBean to provide mock dependencies

   Annotating OrderService with @MockBean in the test class creates and injects a mock into the controller.

3. Final Answer:

   Use @MockBean on OrderService field in the test class to inject a mock -> Option C

4. Quick Check:

   @MockBean mocks service dependencies in @WebMvcTest [OK]

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

Common Mistakes:

• Trying to @Autowired real service in @WebMvcTest
• Not mocking service causing NoSuchBeanDefinitionException
• Switching to @SpringBootTest unnecessarily