MockMvc for HTTP assertions in Spring Boot

Introduction

MockMvc helps you test your web controllers without starting a real server. It lets you check HTTP requests and responses quickly and easily.

You want to test your Spring Boot controller methods without running the full application.

You need to verify that your API endpoints return the correct status and data.

You want to simulate HTTP requests like GET, POST, PUT, DELETE in tests.

You want fast feedback on your web layer logic during development.

You want to check JSON responses or headers from your controllers.

Syntax

Spring Boot

mockMvc.perform(requestBuilder)
       .andExpect(status().isOk())
       .andExpect(content().json(expectedJson))
       .andReturn();

mockMvc.perform() sends a fake HTTP request to your controller.

andExpect() checks the response status, content, or headers.

Examples

Test a GET request to '/api/hello' expecting status 200 and response body 'Hello World'.

Spring Boot

mockMvc.perform(get("/api/hello"))
       .andExpect(status().isOk())
       .andExpect(content().string("Hello World"));

Test a POST request with JSON body to create a user, expecting status 201 Created.

Spring Boot

mockMvc.perform(post("/api/users")
       .contentType(MediaType.APPLICATION_JSON)
       .content("{\"name\":\"Alice\"}"))
       .andExpect(status().isCreated());

Test GET request for item with id 1, checking JSON fields using jsonPath.

Spring Boot

mockMvc.perform(get("/api/items/1"))
       .andExpect(status().isOk())
       .andExpect(jsonPath("$.id").value(1))
       .andExpect(jsonPath("$.name").value("Item One"));

Sample Program

This test sends a GET request to '/hello' and checks that the response is 200 OK with the text 'Hello World'.

Spring Boot

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.servlet.WebMvcTest;
import org.springframework.test.web.servlet.MockMvc;
import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*;

@WebMvcTest(HelloController.class)
public class HelloControllerTest {

    @Autowired
    private MockMvc mockMvc;

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

// Controller for reference:
// @RestController
// public class HelloController {
//     @GetMapping("/hello")
//     public String hello() {
//         return "Hello World";
//     }
// }

Important Notes

MockMvc tests only the web layer, so other parts like database are not started unless mocked.

Use @WebMvcTest to load only controller-related beans for faster tests.

You can chain multiple andExpect() calls to check different parts of the response.

Summary

MockMvc lets you test Spring Boot controllers without a real server.

It simulates HTTP requests and checks responses easily.

Use it to verify status codes, response bodies, and headers in your tests.

Practice

1. What is the main purpose of using MockMvc in Spring Boot tests?

easy

A. To test HTTP requests and responses without starting a real server

B. To deploy the application to a production server

C. To generate HTML pages automatically

D. To manage database connections

Solution

1. Step 1: Understand MockMvc role

   MockMvc simulates HTTP requests and responses in tests without needing a real server.

2. Step 2: Compare options

   Options A, B, and D describe unrelated tasks like HTML generation, deployment, or database management.

3. Final Answer:

   To test HTTP requests and responses without starting a real server -> Option A

4. Quick Check:

   MockMvc = HTTP testing without server [OK]

Hint: MockMvc tests HTTP calls without running the server [OK]

Common Mistakes:

• Thinking MockMvc starts a real server
• Confusing MockMvc with deployment tools
• Assuming MockMvc manages databases

2. Which of the following is the correct way to perform a GET request using MockMvc?

easy

A. mockMvc.perform(get("/api/data"))

B. mockMvc.get("/api/data")

C. mockMvc.request("GET", "/api/data")

D. mockMvc.callGet("/api/data")

Solution

1. Step 1: Recall MockMvc syntax

   The correct syntax uses mockMvc.perform(get("/url")) with static import of get.

2. Step 2: Check other options

   Options B, C, and D use invalid method names not provided by MockMvc API.

3. Final Answer:

   mockMvc.perform(get("/api/data")) -> Option A

4. Quick Check:

   Use perform(get()) for GET requests [OK]

Hint: Use perform(get("url")) to send GET requests [OK]

Common Mistakes:

• Using mockMvc.get() directly
• Confusing method names like request() or callGet()
• Not importing static get() method

3. Given the following test snippet, what HTTP status code will be asserted?

mockMvc.perform(get("/hello"))
       .andExpect(status().isOk());

medium

A. 404 Not Found

B. 500 Internal Server Error

C. 200 OK

D. 302 Redirect

Solution

1. Step 1: Understand status().isOk()

   The method status().isOk() checks for HTTP status code 200.

2. Step 2: Match status codes

   200 means success, 404 means not found, 500 means server error, 302 means redirect.

3. Final Answer:

   200 OK -> Option C

4. Quick Check:

   status().isOk() = 200 OK [OK]

Hint: isOk() always means HTTP 200 success [OK]

Common Mistakes:

• Confusing isOk() with error codes
• Assuming isOk() means redirect
• Mixing status codes meanings

4. Identify the error in this MockMvc test code snippet:

mockMvc.perform(post("/submit"))
       .andExpect(status().isOk)
       .andExpect(content().string("Success"));

medium

A. Using post instead of get for /submit

B. Missing parentheses after isOk method call

C. Incorrect content matcher method name

D. No error, code is correct

Solution

1. Step 1: Check method calls

   The method isOk must be called with parentheses: isOk().

2. Step 2: Verify other parts

   Using post for /submit is valid; content().string() is correct; so no other errors.

3. Final Answer:

   Missing parentheses after isOk method call -> Option B

4. Quick Check:

   Method calls need parentheses like isOk() [OK]

Hint: Always add () after status check methods like isOk() [OK]

Common Mistakes:

• Forgetting parentheses on isOk()
• Confusing post and get methods
• Using wrong content matcher method

5. You want to test a JSON response from /user/1 endpoint that returns {"name":"Alice","age":30}. Which MockMvc assertion correctly checks the name field in the JSON response?

hard

A. andExpect(content().string("Alice"))

B. andExpect(content().json("name: Alice"))

C. andExpect(jsonPath("name").isEqualTo("Alice"))

D. andExpect(jsonPath("$.name").value("Alice"))

Solution

1. Step 1: Understand JSON path syntax

   To check JSON fields, use jsonPath("$.fieldName") with value() for expected value.

2. Step 2: Evaluate options

   andExpect(jsonPath("$.name").value("Alice")) uses correct syntax. andExpect(content().json("name: Alice")) has invalid JSON format. andExpect(jsonPath("name").isEqualTo("Alice")) uses wrong jsonPath syntax. andExpect(content().string("Alice")) checks whole string, not JSON field.

3. Final Answer:

   andExpect(jsonPath("$.name").value("Alice")) -> Option D

4. Quick Check:

   Use jsonPath("$.field").value() for JSON checks [OK]

Hint: Use jsonPath("$.field").value(expected) to check JSON fields [OK]

Common Mistakes:

• Wrong jsonPath syntax without $
• Trying to match JSON with plain string
• Using incorrect matcher methods