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<p><b>HttpMessageConverter </b>is a strategy interface that specifies a converter that can convert from and to HTTP requests and responses in Spring REST Restful web services. Internally Spring MVC uses it to convert the Http request to an object representation and back. Let&#8217;s see following code for REST endpoint service.</p>
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<pre class="highlight">@RestController 
public class AccountController { 
 @PostMapping("/account") 
 public Account create (@RequestBody Account account){ 
 return accountService.create(account); 
 } 
} 
</pre>
<p><b>Sending Request</b><br />
Let&#8217;s see API call for this endpoint. REST client consumes this endpoint with JSON object and we can specify &#8220;Content-Type&#8221; to be &#8220;application/json&#8221;</p>
<pre class="highlight">POST /restapi/account/ HTTP/1.1 
Host: localhost:8080 
Content-Type: application/json 
Cache-Control: no-cache 
 
{ 
"name":"Arnav Rajput", 
"city":"Noida", 
"balance":4000202.33 
} 
</pre>
<p><b>Getting Response</b><br />
We get back a 200 OK – a successful response as below:</p>
<pre class="highlight">HTTP/1.1 200 OK 
Content-Type ?application/json;charset=UTF-8 
Date ?Thu, 30 Mar 2017 12:46:49 GMT 
Server ?Apache-Coyote/1.1 
Transfer-Encoding ?chunked 
{ 
 "accountId": 1, 
 "name": "Arnav Rajput", 
 "city": "Noida", 
 "balance": 4000202.33 
} 
</pre>
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<p><b>Client-Server Communication for this API Endpoint:</b></p>
<ul style="text-align: left;">
<li>REST Client sends a POST request to <b>/restapi/account/</b> with the Accept header set to <b>application/json</b> to get all <b>Account </b>resources as <b>Json</b></li>
<li><b>AccountController </b>is hit and returns the corresponding <b>Account </b>Java object as response</li>
<li>Now Spring Framework then uses one of the Jackson message converters to <b>marshall </b>and <b>unmarshall </b>Java Objects to and from <b>JSON </b>over HTTP.</li>
</ul>
<p>In this above code we are using <b><i>@RestController</i></b>, it is actually combination of <b><i>@ResponseBody</i></b> and <b><i>@Controller </i></b>as of Spring 4.0, before it we have to specify with handler method as below:</p>
<pre class="highlight">@RequestMapping(value="/account", method=RequestMethod.POST) 
public @ResponseBody Account create (@RequestBody Account account){ 
 return accountService.create(account); 
} 
</pre>
<p><b>@ResponseBody</b><br />
In Spring MVC, @ResponseBody annotation on a AccoutnController method indicates to Spring that the return Account object of the method is serialized to JSON directly to the body of the HTTP Response. It uses &#8220;Accept&#8221; header to choose the appropriate Http Converter to marshall the object.</p>
<p><b>@RequestBody</b><br />
In Spring MVC, @RequestBody annotation on the argument of a AccountController method <b><i>create (@RequestBody Account account)</i></b> – it indicates to Spring that the body of the HTTP Request (either in JSON or XML) is deserialized to that Account Java Object. It uses &#8220;Content-Type&#8221; header specified by the REST Client will be used to determine the appropriate converter for this.</p>
<p><b>Default Message Converters in Spring MVC</b><br />
In Spring MVC Framework, there is a set of default converters automatically registered which supports a whole range of different resource representation formats &#8211; json, xml for object. In RestTemplate in Spring REST, objects passed to and returned from the methods of RestTemplate class like getForObject(), postForLocation(), and put() are converted to HTTP requests and from HTTP responses by registered HttpMessageConverters.</p>
<ul style="text-align: left;">
<li><b>StringHttpMessageConverter</b>: it converts Strings from the HTTP request and response.</li>
<li><b>FormHttpMessageConverter</b>: it converts form data to/from a MultiValueMap<;String, String>;.</li>
<li><b>ByteArrayHttpMessageConverter</b>: it converts byte arrays from the HTTP request and response.</li>
<li><b>MappingJackson2HttpMessageConverter</b>: it converts JSON from the HTTP request and response.</li>
<li><b>Jaxb2RootElementHttpMessageConverter</b>: it converts Java objects to/from XML.</li>
<li><b>SourceHttpMessageConverter</b>: it converts javax.xml.transform.Source from the HTTP request and response.</li>
<li><b>AtomFeedHttpMessageConverter</b>: it converts Atom feeds.</li>
<li><b>RssChannelHttpMessageConverter</b>: it converts RSS feeds.</li>
</ul>
<p><b>Customizing HttpMessageConverters with Spring MVC</b><br />
Let&#8217;s see Spring MVC configuration file.</p>
<pre class="highlight">@Configuration 
@EnableWebMvc 
@ComponentScan("com.doj.restapi.web.controller") 
public class WebConfiguration{ 
 
} 
</pre>
<p>In the above MVC configuration file, we are using @EnableWebMvc annotation, it automatically registered default Http message converters with application as listed above according to available library in the class path. Now, if there is a need to customize the default message converters in some way. Spring provides a WebMvcConfigurerAdapter class, which allow us to change the default list of Http Converters with our own. Now let&#8217;s below changed configuration class for Spring MVC.</p>
<pre class="highlight">/** 
 * 
 */ 
package com.doj.restapi.web.config; 
 
import java.util.List; 
 
import org.springframework.context.annotation.ComponentScan; 
import org.springframework.context.annotation.Configuration; 
import org.springframework.http.converter.HttpMessageConverter; 
import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter; 
import org.springframework.http.converter.xml.MarshallingHttpMessageConverter; 
import org.springframework.web.servlet.config.annotation.EnableWebMvc; 
import org.springframework.web.servlet.config.annotation.WebMvcConfigurerAdapter; 
 
/** 
 * @author Dinesh.Rajput 
 * 
 */ 
@Configuration 
@EnableWebMvc 
@ComponentScan("com.doj.restapi.web.controller") 
public class WebConfiguration extends WebMvcConfigurerAdapter{ 
 @Override 
 public void configureMessageConverters(List<;HttpMessageConverter<;?>;>; converters) { 
 converters.add(createXmlHttpMessageConverter()); 
 converters.add(new MappingJackson2HttpMessageConverter()); 
 super.configureMessageConverters(converters); 
 } 
 private HttpMessageConverter<;Object>; createXmlHttpMessageConverter() { 
 MarshallingHttpMessageConverter xmlConverter = new MarshallingHttpMessageConverter(); 
 XStreamMarshaller xstreamMarshaller = new XStreamMarshaller(); 
 xmlConverter.setMarshaller(xstreamMarshaller); 
 xmlConverter.setUnmarshaller(xstreamMarshaller); 
 return xmlConverter; 
 } 
} 
 
</pre>
<p>Make sure that XStream library should be in the classpath of application. Here we are creating a new converter, the MarshallingHttpMessageConverter and we are using the Spring XStream support to configure it.</p>
<p><b>Summary</b><br />
In this tutorial, we have seen how Spring MVC allows us to specify and fully customize Http Message Converters to automatically marshall/unmarshall Java Entities to and from XML or JSON.</p>
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