MockMvc for HTTP assertions in Spring Boot - Mini Project: Build & Apply

MockMvc for HTTP assertions

📖 Scenario: You are building a simple Spring Boot web application that has a REST endpoint to greet users by name.To ensure your endpoint works correctly, you want to write tests that simulate HTTP requests and check the responses without starting the full server.

🎯 Goal: Create a test class using MockMvc to send a GET request to the greeting endpoint and assert the HTTP status and response content.

📋 What You'll Learn

Create a Spring Boot REST controller with a GET endpoint /greet that accepts a name query parameter and returns a greeting message.

Set up a MockMvc instance in the test class.

Write a test method that performs a GET request to /greet?name=Alice using MockMvc.

Assert that the HTTP status is 200 (OK) and the response body contains the greeting message Hello, Alice!.

💡 Why This Matters

🌍 Real World

MockMvc is used in Spring Boot projects to test web controllers quickly and reliably without deploying the full application server.

💼 Career

Understanding MockMvc is essential for backend developers working with Spring Boot to write automated tests that ensure web endpoints behave as expected.

1

Create the GreetingController REST endpoint

Create a Spring Boot REST controller class called GreetingController with a GET mapping for /greet that accepts a name query parameter and returns the string "Hello, " + name + "!".

Spring Boot

// Your code here

Hint

Use @RestController and @GetMapping annotations. The method should take a @RequestParam String name and return the greeting string.

2

Set up MockMvc in the test class

Create a test class called GreetingControllerTest annotated with @WebMvcTest(GreetingController.class). Inject a MockMvc field using @Autowired.

Spring Boot

import org.springframework.boot.test.autoconfigure.web.servlet.WebMvcTest;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.test.web.servlet.MockMvc;
import org.junit.jupiter.api.Test;

@WebMvcTest(GreetingController.class)
public class GreetingControllerTest {

    // Your code here
}

Hint

Use @WebMvcTest on the test class and @Autowired to inject MockMvc.

3

Write a test method to perform GET request and assert response

In GreetingControllerTest, write a test method called testGreetReturnsHelloMessage annotated with @Test. Use mockMvc.perform to send a GET request to /greet?name=Alice. Assert that the status is 200 and the response content is exactly Hello, Alice!.

Spring Boot

import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.content;

@Test
public void testGreetReturnsHelloMessage() throws Exception {
    // Your code here
}

Hint

Use mockMvc.perform(get(...)) and chain andExpect calls for status and content.

4

Add @AutoConfigureMockMvc annotation to enable MockMvc auto-configuration

In the GreetingControllerTest class, add the annotation @AutoConfigureMockMvc above the class declaration to enable automatic MockMvc configuration.

Spring Boot

// Add @AutoConfigureMockMvc annotation above the class
@WebMvcTest(GreetingController.class)
public class GreetingControllerTest {

    @Autowired
    private MockMvc mockMvc;

    @Test
    public void testGreetReturnsHelloMessage() throws Exception {
        mockMvc.perform(get("/greet?name=Alice"))
            .andExpect(status().isOk())
            .andExpect(content().string("Hello, Alice!"));
    }
}

Hint

Add @AutoConfigureMockMvc above the test class to enable MockMvc auto-configuration.

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