@PostMapping for POST requests in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Spring Boot controller method?

Consider this Spring Boot controller method annotated with @PostMapping. What will be the HTTP response body when a POST request is sent to /greet with JSON {"name":"Alice"}?

Spring Boot

import org.springframework.web.bind.annotation.*;

@RestController
public class GreetingController {

    record NameRequest(String name) {}

    @PostMapping("/greet")
    public String greet(@RequestBody NameRequest request) {
        return "Hello, " + request.name() + "!";
    }
}

A "{\"message\":\"Hello, Alice!\"}"

B HTTP 500 Internal Server Error

C HTTP 404 Not Found

D "Hello, Alice!"

💡 Hint

Look at the return type and how the method uses the request body.

The method returns a plain string greeting using the name from the JSON request body. Spring Boot sends this string as the HTTP response body.

📝 Syntax

intermediate

Which option correctly uses @PostMapping to handle POST requests with a JSON body?

Choose the correct method signature for a Spring Boot controller method that handles POST requests to /submit and accepts a JSON body mapped to a User object.

A @PostMapping("/submit") public String submit(User user) { return user.getName(); }

B @PostMapping("/submit") public String submit(@RequestBody User user) { return user.getName(); }

C @PostMapping public String submit(@RequestBody User user) { return user.getName(); }

D @PostMapping("/submit") public String submit(@RequestParam User user) { return user.getName(); }

💡 Hint

Remember how to bind JSON request bodies to Java objects.

Option B correctly uses @RequestBody to map the JSON body to the User object and specifies the path /submit.

🔧 Debug

advanced

Why does this @PostMapping method cause a 400 Bad Request error?

Given this Spring Boot controller method, why does sending a POST request with JSON {"username":"bob"} cause a 400 Bad Request error?

Spring Boot

import org.springframework.web.bind.annotation.*;

@RestController
public class UserController {

    record User(String name) {}

    @PostMapping("/user")
    public String createUser(@RequestBody User user) {
        return "User: " + user.name();
    }
}

A The @PostMapping annotation requires a consumes attribute to accept JSON.

B The method is missing @ResponseBody annotation.

C The JSON field name "username" does not match the record component "name", causing deserialization failure.

D The record User cannot be used as a request body parameter.

💡 Hint

Check if the JSON keys match the Java record components.

The JSON key "username" does not match the record component "name", so Spring cannot map the JSON to the record, causing a 400 error.

❓ state_output

advanced

What is the value of 'count' after these POST requests?

Consider this Spring Boot controller with a counter incremented on each POST request to /count. What is the value of count after two POST requests?

Spring Boot

import org.springframework.web.bind.annotation.*;

@RestController
public class CounterController {

    private int count = 0;

    @PostMapping("/count")
    public int increment() {
        count++;
        return count;
    }
}

A 2

B 1

C 0

D Throws an error due to concurrency

💡 Hint

Think about how instance variables behave in Spring controllers.

The controller instance keeps the count variable, so after two POST requests, it increments twice to 2.

🧠 Conceptual

expert

Which statement about @PostMapping and request handling is true?

Choose the correct statement about how Spring Boot handles methods annotated with @PostMapping.

A Spring Boot automatically converts JSON request bodies to Java objects if the method parameter is annotated with @RequestBody and the content type is application/json.

B Methods annotated with @PostMapping can only accept JSON request bodies and no other content types.

C @PostMapping methods must always return ResponseEntity objects to send responses.

D @PostMapping annotation disables HTTP GET requests for the same path automatically.

💡 Hint

Think about how Spring Boot handles JSON and Java object mapping.

Spring Boot uses message converters to automatically convert JSON request bodies to Java objects when @RequestBody is used and content type is application/json.

Practice

1. What is the main purpose of the @PostMapping annotation in Spring Boot?

easy

A. To map HTTP PUT requests to a specific method

B. To map HTTP POST requests to a specific method

C. To map HTTP DELETE requests to a specific method

D. To map HTTP GET requests to a specific method

Solution

1. Step 1: Understand HTTP methods

   HTTP POST is used to send data to the server, so a method handling POST requests must be mapped accordingly.

2. Step 2: Role of @PostMapping

   @PostMapping specifically maps HTTP POST requests to a method in a Spring Boot controller.

3. Final Answer:

   To map HTTP POST requests to a specific method -> Option B

4. Quick Check:

   @PostMapping = POST request handler [OK]

Hint: POST requests use @PostMapping to connect URL and method [OK]

Common Mistakes:

• Confusing @PostMapping with @GetMapping
• Using @PostMapping for GET requests
• Not understanding HTTP method differences

2. Which of the following is the correct syntax to create a POST endpoint in Spring Boot that accepts JSON data?

easy

A. @PostMapping("/submit")
public String submitData(String data) { return "Received"; }

B. @GetMapping("/submit")
public String submitData(@RequestBody String data) { return "Received"; }

C. @PostMapping("/submit")
public String submitData(@RequestBody String data) { return "Received"; }

D. @PostMapping("/submit")
public String submitData(@RequestParam String data) { return "Received"; }

Solution

1. Step 1: Identify correct annotation and parameter

   The method must use @PostMapping to handle POST requests and @RequestBody to receive JSON data from the request body.

2. Step 2: Check each option

   @PostMapping("/submit") public String submitData(@RequestBody String data) { return "Received"; } uses @PostMapping and @RequestBody correctly. @GetMapping("/submit") public String submitData(@RequestBody String data) { return "Received"; } uses @GetMapping which is wrong. @PostMapping("/submit") public String submitData(String data) { return "Received"; } misses @RequestBody, so data won't be parsed from JSON. @PostMapping("/submit") public String submitData(@RequestParam String data) { return "Received"; } uses @RequestParam which reads query parameters, not JSON body.

3. Final Answer:

   @PostMapping("/submit") public String submitData(@RequestBody String data) { return "Received"; } -> Option C

4. Quick Check:

   @PostMapping + @RequestBody = correct syntax [OK]

Hint: Use @RequestBody to get JSON data in POST method [OK]

Common Mistakes:

• Using @GetMapping instead of @PostMapping
• Forgetting @RequestBody for JSON input
• Using @RequestParam for JSON body data

3. Given the following Spring Boot controller method, what will be the output when a POST request with body "Hello" is sent to /greet? @PostMapping("/greet") public String greet(@RequestBody String name) { return "Hi, " + name + "!"; }

medium

A. "Hi, Hello!"

B. "Hello"

C. "Hi, name!"

D. HTTP 404 Not Found

Solution

1. Step 1: Understand method behavior

   The method takes the request body as a String parameter named 'name' and returns "Hi, " plus that name plus "!".

2. Step 2: Apply input to output

   Input body is "Hello", so the output will be "Hi, Hello!".

3. Final Answer:

   "Hi, Hello!" -> Option A

4. Quick Check:

   Input "Hello" returns "Hi, Hello!" [OK]

Hint: Return greeting by concatenating input string [OK]

Common Mistakes:

• Ignoring @RequestBody and expecting query param
• Confusing output with input
• Assuming endpoint is missing (404)

4. Identify the error in this Spring Boot POST method: @PostMapping("/addUser") public String addUser(String user) { return "User " + user + " added"; }

medium

A. Method return type should be void

B. Incorrect HTTP method annotation, should be @GetMapping

C. Missing @PathVariable annotation

D. Missing @RequestBody annotation for 'user' parameter

Solution

1. Step 1: Check parameter binding

   The method expects to receive data in the POST request body, so the parameter must be annotated with @RequestBody to bind JSON or raw data.

2. Step 2: Verify annotations

   Without @RequestBody, Spring won't map the request body to the 'user' parameter, causing it to be null or empty.

3. Final Answer:

   Missing @RequestBody annotation for 'user' parameter -> Option D

4. Quick Check:

   POST body needs @RequestBody on parameter [OK]

Hint: Add @RequestBody to method parameter for POST data [OK]

Common Mistakes:

• Forgetting @RequestBody on POST method parameters
• Using @GetMapping for POST endpoints
• Confusing @PathVariable with request body

5. You want to create a Spring Boot POST endpoint /register that accepts a JSON object with fields username and email, maps it to a Java record UserData(String username, String email), and returns a confirmation message. Which method signature correctly implements this?

hard

A. @PostMapping("/register")
public String registerUser(@RequestBody UserData user) {
return "User " + user.username() + " registered with email " + user.email();
}

B. @PostMapping("/register")
public String registerUser(UserData user) {
return "User " + user.username() + " registered";
}

C. @PostMapping("/register")
public String registerUser(@RequestParam UserData user) {
return "User " + user.username() + " registered";
}

D. @PostMapping("/register")
public String registerUser(@PathVariable UserData user) {
return "User " + user.username() + " registered";
}

Solution

1. Step 1: Understand data binding for POST JSON

   To map JSON request body to a Java record, the parameter must be annotated with @RequestBody.

2. Step 2: Check each option

   @PostMapping("/register") public String registerUser(@RequestBody UserData user) { return "User " + user.username() + " registered with email " + user.email(); } correctly uses @RequestBody with UserData record and accesses its fields. @PostMapping("/register") public String registerUser(UserData user) { return "User " + user.username() + " registered"; } misses @RequestBody, so mapping fails. Options C and D use @RequestParam and @PathVariable which are for query/path parameters, not JSON body.

3. Final Answer:

   @PostMapping("/register") public String registerUser(@RequestBody UserData user) { return "User " + user.username() + " registered with email " + user.email(); } -> Option A

4. Quick Check:

   @RequestBody + record parameter = correct POST JSON mapping [OK]

Hint: Use @RequestBody with record to map JSON POST data [OK]

Common Mistakes:

• Omitting @RequestBody on complex objects
• Using @RequestParam or @PathVariable for JSON body
• Not accessing record fields correctly