Http Message Converters with the Spring Framework

1. Overview

In this tutorial, we’ll learn how to Configure HttpMessageConverters in Spring.

Simply put, we can use message converters to marshall and unmarshall Java Objects to and from JSON and XML over HTTP.

2. The Basics

2.1. Enable Web MVC

To start, the Web Application needs to be configured with Spring MVC support. A convenient and very customizable way to do this is to use the @EnableWebMvc annotation:

@EnableWebMvc
@Configuration
@ComponentScan({ "com.baeldung.web" })
public class WebConfig implements WebMvcConfigurer {
    
    // ...
    
}

Note that this class implements WebMvcConfigurer, which will allow us to change the default list of Http Converters with our own.

2.2. The Default Message Converters

By default, the following HttpMessageConverters instances are pre-enabled:

• ByteArrayHttpMessageConverter – converts byte arrays
• StringHttpMessageConverter – converts Strings
• ResourceHttpMessageConverter – converts org.springframework.core.io.Resource for any type of octet stream
• SourceHttpMessageConverter – converts javax.xml.transform.Source
• FormHttpMessageConverter – converts form data to/from a MultiValueMap<String, String>
• Jaxb2RootElementHttpMessageConverter – converts Java objects to/from XML (added only if JAXB2 is present on the classpath)
• MappingJackson2HttpMessageConverter – converts JSON (added only if Jackson 2 is present on the classpath)
  
• MappingJacksonHttpMessageConverter – converts JSON (added only if Jackson is present on the classpath)
• AtomFeedHttpMessageConverter – converts Atom feeds (added only if Rome is present on the classpath)
• RssChannelHttpMessageConverter – converts RSS feeds (added only if Rome is present on the classpath)

3. Client-Server Communication – JSON Only

3.1. High-Level Content Negotiation

Each HttpMessageConverter implementation has one or several associated MIME Types.

When receiving a new request, Spring will use the “Accept” header to determine the media type that it needs to respond with.

It’ll then try to find a registered converter that’s capable of handling that specific media type. Finally, it’ll use this to convert the entity and send back the response.

The process is similar for receiving a request that contains JSON information. The framework will use the “Content-Type” header to determine the media type of the request body.

Then it’ll search for a HttpMessageConverter that can convert the body sent by the client to a Java Object.

Let’s clarify this with a quick example:

• The Client sends a GET request to /foos, with the Accept header set to application/json, to get all Foo resources as JSON.
• The Foo Spring Controller is hit, and returns the corresponding Foo Java entities.
• Then Spring uses one of the Jackson message converters to marshall the entities to JSON.

Now let’s look at the specifics of how this works, and how we can leverage the @ResponseBody and @RequestBody annotations.

3.2. @ResponseBody

@ResponseBody on a Controller method indicates to Spring that the return value of the method is serialized directly to the body of the HTTP Response. As discussed above, the “Accept” header specified by the Client will be used to choose the appropriate Http Converter to marshall the entity:

@GetMapping("/{id}")
public @ResponseBody Foo findById(@PathVariable long id) {
    return fooService.findById(id);
}

Now the client will specify the “Accept” header to application/json in the request (for example, the curl command):

curl --header "Accept: application/json" 
  http://localhost:8080/spring-boot-rest/foos/1

The Foo class:

public class Foo {
    private long id;
    private String name;
}

And the HTTP Response Body:

{
    "id": 1,
    "name": "Paul",
}

We can use the @RequestBody annotation on the argument of a Controller method to indicate that the body of the HTTP Request is deserialized to that particular Java entity. To determine the appropriate converter, Spring will use the “Content-Type” header from the client request: 

@PutMapping("/{id}")
public @ResponseBody void update(@RequestBody Foo foo, @PathVariable String id) {
    fooService.update(foo);
}

Next, we’ll consume this with a JSON object, specifying the “Content-Type“ to be application/json:

curl -i -X PUT -H "Content-Type: application/json"  
-d '{"id":"83","name":"klik"}' http://localhost:8080/spring-boot-rest/foos/1

We’ll get back a 200 OK, a successful response:

HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Content-Length: 0
Date: Fri, 10 Jan 2014 11:18:54 GMT

4. Custom Converters Configuration

We can also customize the message converters by implementing the WebMvcConfigurer interface and overriding the configureMessageConverters method:

@EnableWebMvc
@Configuration
@ComponentScan({ "com.baeldung.web" })
public class WebConfig implements WebMvcConfigurer {

    @Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> messageConverters) {
        messageConverters.add(createXmlHttpMessageConverter());
        messageConverters.add(new MappingJackson2HttpMessageConverter());
    }

    private HttpMessageConverter<Object> createXmlHttpMessageConverter() {
        MarshallingHttpMessageConverter xmlConverter = new MarshallingHttpMessageConverter();

        XStreamMarshaller xstreamMarshaller = new XStreamMarshaller();
        xmlConverter.setMarshaller(xstreamMarshaller);
        xmlConverter.setUnmarshaller(xstreamMarshaller);

        return xmlConverter;
    } 
}

In this example, we’re creating a new converter, the MarshallingHttpMessageConverter, and using the Spring XStream support to configure it. This allows a great deal of flexibility, since we’re working with the low-level APIs of the underlying marshalling framework, in this case XStream, and we can configure that however we want.

Note that this example requires adding the XStream library to the classpath.

Also be aware that by extending this support class, we’re losing the default message converters that were previously pre-registered.

We can, of course, now do the same for Jackson by defining our own MappingJackson2HttpMessageConverter. We can set a custom ObjectMapper on this converter, and have it configured as we need to.

In this case, XStream was the selected marshaller/unmarshaller implementation, but others, like JibxMarshaller, can be used as well.

At this point, with XML enabled on the back end, we can consume the API with XML Representations:

curl --header "Accept: application/xml" 
  http://localhost:8080/spring-boot-rest/foos/1

4.1. Spring Boot Support

If we’re using Spring Boot, we can avoid implementing the WebMvcConfigurer and adding all the Message Converters manually, as we did above.

We can just define different HttpMessageConverter beans in the context, and Spring Boot will add them automatically to the autoconfiguration that it creates:

@Bean
public HttpMessageConverter<Object> createXmlHttpMessageConverter() {
    MarshallingHttpMessageConverter xmlConverter = new MarshallingHttpMessageConverter();

    // ...

    return xmlConverter;
}

5. Using Spring’s RestTemplate With HTTP Message Converters

As well as on the server-side, HTTP Message Conversion can be configured on the client-side of the Spring RestTemplate.

We’ll configure the template with the “Accept” and “Content-Type” headers when appropriate. Then we’ll try to consume the REST API with full marshalling and unmarshalling of the Foo Resource, both with JSON and XML.

5.1. Retrieving the Resource With No Accept Header

@Test
public void whenRetrievingAFoo_thenCorrect() {
    String URI = BASE_URI + "foos/{id}";

    RestTemplate restTemplate = new RestTemplate();
    Foo resource = restTemplate.getForObject(URI, Foo.class, "1");

    assertThat(resource, notNullValue());
}

5.2. Retrieving a Resource With application/xml Accept Header

Now let’s explicitly retrieve the Resource as an XML Representation. We’ll define a set of Converters and set these on the RestTemplate.

Because we’re consuming XML, we’ll use the same XStream marshaller as before:

@Test
public void givenConsumingXml_whenReadingTheFoo_thenCorrect() {
    String URI = BASE_URI + "foos/{id}";

    RestTemplate restTemplate = new RestTemplate();
    restTemplate.setMessageConverters(getXmlMessageConverters());

    HttpHeaders headers = new HttpHeaders();
    headers.setAccept(Collections.singletonList(MediaType.APPLICATION_XML));
    HttpEntity<String> entity = new HttpEntity<>(headers);

    ResponseEntity<Foo> response = 
      restTemplate.exchange(URI, HttpMethod.GET, entity, Foo.class, "1");
    Foo resource = response.getBody();

    assertThat(resource, notNullValue());
}

private List<HttpMessageConverter<?>> getXmlMessageConverters() {
    XStreamMarshaller marshaller = new XStreamMarshaller();
    marshaller.setAnnotatedClasses(Foo.class);
    MarshallingHttpMessageConverter marshallingConverter = 
      new MarshallingHttpMessageConverter(marshaller);

    List<HttpMessageConverter<?>> converters = new ArrayList<>();
    converters.add(marshallingConverter);
    return converters;
}

5.3. Retrieving a Resource With application/json Accept Header

Similarly, let’s now consume the REST API by asking for JSON:

@Test
public void givenConsumingJson_whenReadingTheFoo_thenCorrect() {
    String URI = BASE_URI + "foos/{id}";

    RestTemplate restTemplate = new RestTemplate();
    restTemplate.setMessageConverters(getJsonMessageConverters());

    HttpHeaders headers = new HttpHeaders();
    headers.setAccept(Collections.singletonList(MediaType.APPLICATION_JSON));
    HttpEntity<String> entity = new HttpEntity<String>(headers);

    ResponseEntity<Foo> response = 
      restTemplate.exchange(URI, HttpMethod.GET, entity, Foo.class, "1");
    Foo resource = response.getBody();

    assertThat(resource, notNullValue());
}

private List<HttpMessageConverter<?>> getJsonMessageConverters() {
    List<HttpMessageConverter<?>> converters = new ArrayList<>();
    converters.add(new MappingJackson2HttpMessageConverter());
    return converters;
}

5.4. Update a Resource With XML Content-Type

Finally, we’ll send JSON data to the REST API, and specify the media type of that data via the Content-Type header:

@Test
public void givenConsumingXml_whenWritingTheFoo_thenCorrect() {
    String URI = BASE_URI + "foos";
    RestTemplate restTemplate = new RestTemplate();
    restTemplate.setMessageConverters(getJsonAndXmlMessageConverters());

    Foo resource = new Foo("jason");
    HttpHeaders headers = new HttpHeaders();
    headers.setAccept(Collections.singletonList(MediaType.APPLICATION_JSON));
    headers.setContentType((MediaType.APPLICATION_XML));
    HttpEntity<Foo> entity = new HttpEntity<>(resource, headers);

    ResponseEntity<Foo> response = 
      restTemplate.exchange(URI, HttpMethod.POST, entity, Foo.class);
    Foo fooResponse = response.getBody();

    assertThat(fooResponse, notNullValue());
    assertEquals(resource.getName(), fooResponse.getName());
}

private List<HttpMessageConverter<?>> getJsonAndXmlMessageConverters() {
    List<HttpMessageConverter<?>> converters = getJsonMessageConverters();
    converters.addAll(getXmlMessageConverters());
    return converters;
}

What’s interesting here is that we’re able to mix the media types. We’re sending XML data, but we’re waiting for JSON data back from the server. This shows just how powerful the Spring conversion mechanism really is.

6. Conclusion

In this article, we learned 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. This is, of course, a simplistic definition, and there’s so much more that the message conversion mechanism can do, as we can see from the last test example.

We also looked at how to leverage the same powerful mechanism with the RestTemplate client, leading to a fully type-safe way of consuming the API.

As always, the code presented in this article is available over on GitHub.