Earthly Powers

More philosophical content negotiation

In a previous blog entry i showed a way to support platonic and distinct URIs (with a redirection twist). An alternative way to do this (motivated by a comment from Patrick Mueller) is to use a URI template variable that captures the suffix corresponding to the media type, do some initialization work in a constructor and then use one HTTP method, as follows:

@UriTemplate("/helloworld{media}")
public class HelloWorldResource {
    static final MediaType XML = new MediaType("application/xml");
    static final MediaType JSON = new MediaType("application/json");
    static final List<MediaType> acceptableList = Arrays.asList(XML, JSON);
    
    @HttpContext
    HttpContextAccess context;

    @HttpContext
    UriInfo uriInfo;
    
    MediaType contentType;
    
    public HelloWorldResource(@UriParam("media") String media) {
        if (media.equals(".xml")) {
            contentType = XML;
        } else if (media.equals(".json")) {
            contentType = JSON;
        } else if (media.length() == 0) {
            contentType = null;
        } else {
            throw new NotFoundException();            
        }
    }
    
    @HttpMethod
    @ProduceMime({"application/xml", "application/json"})
    public Response get() {
        if (contentType == XML) {
            contentType = context.getHttpRequestContext().
                getAcceptableMediaType(Arrays.asList(XML));            
        } else if (contentType == JSON) {
            contentType = context.getHttpRequestContext().
                getAcceptableMediaType(Arrays.asList(JSON));            
        } else if (contentType == null) {
            contentType = context.getHttpRequestContext().
                getAcceptableMediaType(acceptableList);            
        }

        if (contentType == null) {
            throw new WebApplicationException(Response.Builder.
                    noContent().status(406).build());
        }
        
        return Response.Builder.representation(contentType.toString(), 
                contentType).build();
    }

(I am cheating a bit by not returning actual XML or JSON content as i am too lazy to put some JAXB code together.)  

In this example the platonic URI returns a representation instead of redirecting. To be correct it is necessary to check what is acceptable for distinct URIs and return a Not Acceptable (406) response, for example, if the distinct URI is "/helloworld.xml" but the requested acceptable content is "application/json".

There is still a lot of boiler plate in the application code but the way this code is structured indicates that it should be possible to push such things into the runtime.

Posted at 04:17PM Sep 20, 2007 by Paul Sandoz in REST  |  Comments[0]