@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.