@RequestBody for JSON input in Spring Boot

Introduction

@RequestBody helps your Spring Boot app read JSON data sent by a client and turn it into Java objects automatically.

When a client sends JSON data in an HTTP POST request to create a new record.

When you want to update data by receiving JSON in an HTTP PUT request.

When building REST APIs that accept JSON input from web or mobile apps.

When you want to easily convert JSON request bodies into Java objects without manual parsing.

Syntax

Spring Boot

@PostMapping("/path")
public ResponseEntity<Type> methodName(@RequestBody Type variableName) {
    // use variableName here
}

The @RequestBody annotation tells Spring to convert JSON from the request body into the Java object.

The Java class used must have getters/setters or be a record for Spring to map JSON fields properly.

Examples

Receives a JSON user object and converts it to a User Java object.

Spring Boot

@PostMapping("/users")
public ResponseEntity<User> addUser(@RequestBody User user) {
    // save user
    return ResponseEntity.ok(user);
}

Updates a product by reading JSON data from the request body.

Spring Boot

@PutMapping("/products/{id}")
public ResponseEntity<Product> updateProduct(@PathVariable int id, @RequestBody Product product) {
    // update product with id
    return ResponseEntity.ok(product);
}

Sample Program

This Spring Boot app has a POST endpoint at /users that accepts JSON like {"name":"Alice","age":30}.

Spring converts the JSON into a User record automatically using @RequestBody.

The endpoint returns the same user data as confirmation.

Spring Boot

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RestController;

@SpringBootApplication
public class DemoApplication {
    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
}

record User(String name, int age) {}

@RestController
class UserController {
    @PostMapping("/users")
    public ResponseEntity<User> addUser(@RequestBody User user) {
        // Just return the user received
        return ResponseEntity.ok(user);
    }
}

Important Notes

Make sure your JSON keys match the Java object's field names exactly for mapping to work.

If JSON is invalid or missing required fields, Spring will return a 400 Bad Request error automatically.

Use records or classes with public getters/setters for smooth JSON to Java conversion.

Summary

@RequestBody reads JSON from HTTP requests and converts it to Java objects automatically.

It is useful for REST APIs that accept JSON input from clients.

Spring Boot handles errors if JSON is missing or malformed.