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## Chapter 20. JAX-RS Content Negotiation

The HTTP protocol has built in content negotiation headers that allow the client and server to specify what content they are transferring and what content they would prefer to get. The server declares content preferences via the @Produces and @Consumes headers.

@Consumes is an array of media types that a particular resource or resource method consumes. For example:

    @Consumes("text/*")
    @Path("/library")
    public class Library {

        @POST
        public String stringBook(String book) {...}

        @Consumes("text/xml")
        @POST
        public String jaxbBook(Book book) {...}
    }

When a client makes a request, JAX-RS first finds all methods that match the path, then, it sorts things based on the content-type header sent by the client. So, if a client sent:

    POST /library
    Content-Type: text/plain

    This is a nice book

The stringBook() method would be invoked because it matches to the default "text/*" media type. Now, if the client instead sends XML:

    POST /library
    Content-Type: text/xml

    <book name="EJB 3.0" author="Bill Burke"/>

The jaxbBook() method would be invoked.

The @Produces is used to map a client request and match it up to the client's Accept header. The Accept HTTP header is sent by the client and defines the media types the client prefers to receive from the server.

    @Produces("text/*")
    @Path("/library")
    public class Library {

    @GET
    @Produces("application/json")
    public String getJSON() {...}

    @GET
    public String get() {...}

So, if the client sends:

    GET /library
    Accept: application/json

The getJSON() method would be invoked.

@Consumes and @Produces can list multiple media types that they support. The client's Accept header can also send multiple types it might like to receive. More specific media types are chosen first. The client Accept header or @Produces @Consumes can also specify weighted preferences that are used to match up requests with resource methods. This is best explained by RFC 2616 section 14.1 . RESTEasy supports this complex way of doing content negotiation.

A variant in JAX-RS is a combination of media type, content-language, and content encoding as well as etags, last modified headers, and other preconditions. This is a more complex form of content negotiation that is done programmatically by the application developer using the javax.ws.rs.Variant, VarianListBuilder, and Request objects. Request is injected via @Context. Read the javadoc for more info on these.

## 20.1. URL-based negotiation

Some clients, like browsers, cannot use the Accept and Accept-Language headers to negotiation the representation's media type or language. RESTEasy allows you to map file name suffixes like (.xml, .txt, .en, .fr) to media types and languages. These file name suffixes take the place and override any Accept header sent by the client. You configure this using the resteasy.media.type.mappings and resteasy.language.mappings parameters. If configured within your web.xml, it would look like:

    <web-app>
        <display-name>Archetype Created Web Application</display-name>
        <context-param>
            <param-name>resteasy.media.type.mappings</param-name>
            <param-value>html : text/html, json : application/json, xml : application/xml</param-value>
        </context-param>

       <context-param>
            <param-name>resteasy.language.mappings</param-name>
            <param-value>en : en-US, es : es, fr : fr</param-value>
       </context-param>

       ...
    </web-app>

See Section 3.4, “Configuration” for more information about application configuration.

Mappings are a comma delimited list of suffix/mediatype or suffix/language mappings. Each mapping is delimited by a ':'. So, if you invoked GET /foo/bar.xml.en, this would be equivalent to invoking the following request:

    GET /foo/bar
    Accept: application/xml
    Accept-Language: en-US

The mapped file suffixes are stripped from the target URL path before the request is dispatched to a corresponding JAX-RS resource.

## 20.2. Query String Parameter-based negotiation

RESTEasy can do content negotiation based in a parameter in query string. To enable this, the parameter resteasy.media.type.param.mapping can be configured. In web.xml, it would look like the following:

    <web-app>
        <display-name>Archetype Created Web Application</display-name>
        <context-param>
            <param-name>resteasy.media.type.param.mapping</param-name>
            <param-value>someName</param-value>
        </context-param>

        ...
    </web-app>

See Section 3.4, “Configuration” for more information about application configuration.

The param-value is the name of the query string parameter that RESTEasy will use in the place of the Accept header.

Invoking http://service.foo.com/resouce?someName=application/xml, will give the application/xml media type the highest priority in the content negotiation.

In cases where the request contains both the parameter and the Accept header, the parameter will be more relevant.

It is possible to left the param-value empty, what will cause the processor to look for a parameter named 'accept'.

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