@RequestBody in Spring REST — JSON to Java Object Deserialization

Direct Answer

@RequestBody tells Spring MVC to read the HTTP request body into a method parameter using an HttpMessageConverter. JSON with Jackson is common, but the annotation is not JSON-only: text, XML, bytes, forms, and custom media types can use other converters. Content-Type identifies the representation sent by the client.

Code Example

@RestController
@RequestMapping("/api/users")
public class UserController {

    @PostMapping
    public ResponseEntity<User> createUser(@RequestBody User user) {
        // user object is already populated from JSON request body
        var saved = userService.save(user);
        return ResponseEntity.status(HttpStatus.CREATED).body(saved);
    }
}

record User(String name, String email, int age) {}

// Client sends: POST /api/users with body {"name":"Alice","email":"alice@example.com","age":25}
// Spring deserializes JSON into User record automatically
// If Content-Type header is missing, you get 415 Unsupported Media Type

Common Mistakes & Pitfalls

• Missing Content-Type header: Freshers test with Postman or curl but forget to set Content-Type: application/json. Spring returns 415 Unsupported Media Type because it doesn't know the body is JSON. Always set this header when sending JSON.
• Using @RequestBody with GET: GET requests have no body by design. Freshers sometimes add @RequestBody to GET methods, which doesn't work — use @RequestParam or @PathVariable instead.
• Not handling deserialization failures: If the JSON structure doesn't match the Java class (wrong field names, type mismatches), Jackson throws an exception. Freshers don't catch this — the client sees a 400 error with no useful message. Use @ControllerAdvice to handle HttpMessageNotReadableException and return a clear error response.