@ResponseBody in Spring MVC — Java Object to JSON Serialization

Direct Answer

@ResponseBody tells Spring MVC to write a handler return value to the HTTP response through an HttpMessageConverter instead of resolving it as a view name. JSON with Jackson is one possible representation, not the definition. @RestController applies @ResponseBody semantics to its methods automatically.

Code Example

@Controller
@RequestMapping("/api/products")
public class ProductController {

    @GetMapping("/{id}")
    @ResponseBody // explicitly needed because this is @Controller, not @RestController
    public Product getProduct(@PathVariable Long id) {
        return productService.findById(id);
        // Spring serializes Product object to JSON automatically
    }
}

// Better: use @RestController so @ResponseBody is implicit
@RestController
@RequestMapping("/api/orders")
public class OrderController {

    @GetMapping
    public List<Order> getAllOrders() {
        return orderService.findAll(); // no @ResponseBody needed, it's implicit
    }
}

record Product(Long id, String name, double price) {}
record Order(Long id, String customer, List<String> items) {}

Common Mistakes & Pitfalls

• Adding @ResponseBody to @RestController methods: Freshers see @ResponseBody in tutorials and add it explicitly even when using @RestController. It's redundant — @RestController already includes @ResponseBody on every method.
• Returning String thinking it will be JSON: If you return a plain String like "success" from a @ResponseBody method, Spring sends exactly that string (not JSON-encoded). Freshers expect {"message":"success"} but get the raw text. Return an object or record instead.
• Mixing view names with @ResponseBody: In a @Controller with @ResponseBody on some methods, freshers forget which mode they're in. A method without @ResponseBody tries to find a template named after the returned String, leading to template not found errors. Keep REST endpoints in @RestController classes separate from view controllers.