HttpMessageConverter in Spring — Serialization and Deserialization

Direct Answer

HttpMessageConverter reads a supported HTTP body into a controller argument and writes a return value to a response body. Spring selects a converter from the Java type, request Content-Type, response Accept choices, declared consumes/produces conditions, and registered converter capabilities. @RequestBody and @ResponseBody participate; methods with no body do not necessarily use a converter.

Code Example

@RestController
@RequestMapping("/api/orders")
public class OrderController {
    @Autowired
    private OrderService orderService;

    @PostMapping
    public OrderResponse createOrder(@RequestBody OrderRequest request) {
        // MappingJackson2HttpMessageConverter deserializes JSON → OrderRequest
        var order = orderService.create(request.userId(), request.items());
        // Same converter serializes OrderResponse → JSON response body
        return new OrderResponse(order.id(), order.total(), "CREATED");
    }
}

record OrderRequest(Long userId, List<String> items) {}
record OrderResponse(Long orderId, Double total, String status) {}

// When client sends: POST /api/orders with Content-Type: application/json
// Converter reads JSON bytes → OrderRequest object
// When method returns OrderResponse, converter writes object → JSON bytes

Common Mistakes & Pitfalls

• Forgetting Jackson dependency in non-Boot projects: Freshers expect JSON to work without adding Jackson to pom.xml. Spring Boot includes it by default, but plain Spring MVC doesn't. Without a JSON converter, you get 406 Not Acceptable errors.
• Not understanding Content-Type vs Accept headers: They send JSON but forget Content-Type: application/json header, so Spring can't find a converter to read the request body. Or they expect JSON response but the Accept header says text/html, so Spring picks StringHttpMessageConverter instead.
• Returning raw List from @RestController: Returning List<User> works because MappingJackson2HttpMessageConverter handles collections, but freshers think they need to manually convert it to JSON string first. Never call ObjectMapper yourself in a controller — the converter does it.
• Assuming XML support exists by default: Jackson handles JSON automatically in Boot, but XML requires adding jackson-dataformat-xml dependency. Without it, clients requesting application/xml get 406 errors.