Http Message Converters with the Spring Framework--转载

原文：http://www.baeldung.com/spring-httpmessageconverter-rest

1. Overview

This article describes how to Configure HttpMessageConverter in Spring.

Simply put, message converters are used to marshall and unmarshall Java Objects to and from JSON, XML, etc – over HTTP.

2. The Basics

2.1. Enable Web MVC

The Web Application needs to be configured with Spring MVC support – one convenient and very customizable way to do this is to use the @EnableWebMvc annotation:

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@EnableWebMvc @Configuration @ComponentScan({ "org.baeldung.web" }) public class WebConfig extends WebMvcConfigurerAdapter {     ... }

Note that this class extends WebMvcConfigurerAdapter – 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 of the “Accept” header to determine the media type that it needs to respond with.

It will then try to find a registered converter that is capable of handling that specific media type – and it will use it to convert the entity and send back the response.

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

It will then 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
• Spring then uses one of the Jackson message converters to marshall the entities to json

Let’s now look at the specifics of how this works – and how we should 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.

Let’s look at a simple example:

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@RequestMapping(method=RequestMethod.GET, value="/foos/{id}") public @ResponseBody Foo findById(@PathVariable long id) {     return fooService.get(id); }

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

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

The Foo class:

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

And the Http Response Body:

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{     "id": 1,     "name": "Paul", }

3.3. @RequestBody

@RequestBodyis used on the argument of a Controller method – it indicates to Spring that the body of the HTTP Request is deserialized to that particular Java entity. As discussed previously, the “Content-Type” header specified by the Client will be used to determine the appropriate converter for this.

Let’s look at an example:

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@RequestMapping(method=RequestMethod.PUT, value="/foos/{id}") public @ResponseBody void updateFoo(@RequestBody Foo foo, @PathVariable String id) {     fooService.update(foo); }

Now, let’s consume this with a JSON object – we’re specifying “Content-Type” to beapplication/json:

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curl -i -X PUT -H "Content-Type: application/json" 

-d '{"id":"83","name":"klik"}' http://localhost:8080/spring-rest/foos/1

We get back a 200 OK – a successful response:

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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 – adding XML Support

We can customize the message converters by extending the WebMvcConfigurerAdapter class and overriding the configureMessageConvertersmethod:

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@EnableWebMvc @Configuration @ComponentScan({ "org.baeldung.web" }) public class WebConfig extends WebMvcConfigurerAdapter {       @Override     public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {         messageConverters.add(createXmlHttpMessageConverter());         messageConverters.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;     } }

Note that the XStream library now needs to be present on the classpath.

Also be aware that by extending this support class, we are losing the default message converters which were previously pre-registered – we only have what we define.

Let’s go over this example – we are creating a new converter – theMarshallingHttpMessageConverter – and we’re 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.

We can of course now do the same for Jackson – by defining our ownMappingJackson2HttpMessageConverter we can now 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 CastorMarshaller can be used to – refer to Spring api documentation for full list of available marshallers.

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

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

5. Using Spring’s RestTemplate with Http Message Converters

As well as with the server side, Http Message Conversion can be configured in the client side on the Spring RestTemplate.

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

5.1. Retrieving the Resource with no Accept Header

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@Test public void testGetFoo() {       String URI = “http://localhost:8080/spring-rest/foos/{id}";       RestTemplate restTemplate = new RestTemplate();       Foo foo = restTemplate.getForObject(URI, Foo.class, "1");       Assert.assertEquals(new Integer(1), foo.getId()); }

5.2. Retrieving a Resource with application/xml Accept header

Let’s now explicitly retrieve the Resource as an XML Representation – we’re going to define a set of Converters – same way we did previously – and we’re going to set these on the RestTemplate.

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

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@Test public void givenConsumingXml_whenReadingTheFoo_thenCorrect() {     String URI = BASE_URI + "foos/{id}";     RestTemplate restTemplate = new RestTemplate();     restTemplate.setMessageConverters(getMessageConverters());       HttpHeaders headers = new HttpHeaders();     headers.setAccept(Arrays.asList(MediaType.APPLICATION_XML));     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<?>> getMessageConverters() {     XStreamMarshaller marshaller = new XStreamMarshaller();     MarshallingHttpMessageConverter marshallingConverter =       new MarshallingHttpMessageConverter(marshaller);     converters.add(marshallingConverter);

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

5.3. Retrieving a Resource with application/json Accept header

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

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@Test public void givenConsumingJson_whenReadingTheFoo_thenCorrect() {     String URI = BASE_URI + "foos/{id}";       RestTemplate restTemplate = new RestTemplate();     restTemplate.setMessageConverters(getMessageConverters());       HttpHeaders headers = new HttpHeaders();     headers.setAccept(Arrays.asList(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<?>> getMessageConverters() {     List<HttpMessageConverter<?>> converters = new ArrayList<HttpMessageConverter<?>>();     converters.add(new MappingJackson2HttpMessageConverter());     return converters; }

5.4. Update a Resource with XML Content-Type

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

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@Test public void givenConsumingXml_whenWritingTheFoo_thenCorrect() {     String URI = BASE_URI + "foos/{id}";     RestTemplate restTemplate = new RestTemplate();     restTemplate.setMessageConverters(getMessageConverters());       Foo resource = new Foo(4, "jason");     HttpHeaders headers = new HttpHeaders();     headers.setAccept(Arrays.asList(MediaType.APPLICATION_JSON));     headers.setContentType((MediaType.APPLICATION_XML));     HttpEntity<Foo> entity = new HttpEntity<Foo>(resource, headers);       ResponseEntity<Foo> response =       restTemplate.exchange(URI, HttpMethod.PUT, entity, Foo.class, resource.getId());     Foo fooResponse = response.getBody();       Assert.assertEquals(resource.getId(), fooResponse.getId()); }

What’s interesting here is that we’re able to mix the media types – we are 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 tutorial, we looked at 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 is so much more that the message conversion mechanism can do – as we can see from the last test example.

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