Mastering Spring’s HttpMessageConverter: Elegant Request/Response Handling

When working with Spring MVC, handling HttpServletRequest and HttpServletResponse directly can become cumbersome. A more elegant approach is to use Spring's HttpMessageConverter to read the request body and write the response.

HttpMessageConverter

HttpMessageConverter

is a strategy interface provided by Spring for converting HTTP requests and responses. It reads from HttpInputMessage and writes to HttpOutputMessage. All message conversion in Spring MVC is performed through implementations of this interface.

Common implementations handle Form submissions, JSON, XML, plain strings, and even Protobuf. These converters are stored in a container HttpMessageConverters within the Spring IoC context.

HttpInputMessage

HttpInputMessage

represents an HTTP input consisting of headers and a readable body, typically implemented by the server's request or the client's response.

ServletServerHttpRequest

ServletServerHttpRequest

implements HttpInputMessage and wraps an HttpServletRequest. By injecting the servlet request into this wrapper, HttpMessageConverter can process the original request data.

Practical Request Body Extraction

In a servlet filter we can use FormHttpMessageConverter (which handles application/x-www-form-urlencoded) to read the request body:

@Component
public class FormUrlencodedFilter implements Filter {
    private final FormHttpMessageConverter formHttpMessageConverter = new FormHttpMessageConverter();
    private static final Logger log = LoggerFactory.getLogger(FormUrlencodedFilter.class);

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException {
        String contentType = request.getContentType();
        MediaType type = StringUtils.hasText(contentType) ? MediaType.valueOf(contentType) : null;
        ServletServerHttpRequest serverHttpRequest = new ServletServerHttpRequest((HttpServletRequest) request);
        if (formHttpMessageConverter.canRead(MultiValueMap.class, type)) {
            MultiValueMap<String, String> read = formHttpMessageConverter.read(null, serverHttpRequest);
            log.info("Read request body: {}", read);
        }
    }
}

Sending a POST request with Content-Type: application/x-www-form-urlencoded such as:

POST /ind HTTP/1.1
Host: localhost:8080
Content-Type: application/x-www-form-urlencoded
Content-Length: 20

a=b123&c=d123&e=f123

will produce a console log like:

Read request body: {a=[b123], c=[d123], e=[f123]}

ServletServerHttpResponse

Similarly, ServletServerHttpResponse wraps HttpServletResponse. To write a JSON response we can use MappingJackson2HttpMessageConverter:

ServletServerHttpResponse servletServerHttpResponse = new ServletServerHttpResponse(response);
MappingJackson2HttpMessageConverter mappingJackson2HttpMessageConverter = new MappingJackson2HttpMessageConverter();
// authentication is the object to be serialized
mappingJackson2HttpMessageConverter.write(authentication, MediaType.APPLICATION_JSON, servletServerHttpResponse);

Summary

HttpMessageConverter

abstracts HTTP message conversion, allowing elegant handling of request and response data. Remember that the request body can be read only once, even when wrapped in ServletServerHttpRequest, and be aware of the difference between this wrapper and HttpServletRequestWrapper.