Chapter 12. URIs and Links

A very important aspect of REST is hyperlinks, URIs, in representations that clients can use to transition the Web service to new application states (this is otherwise known as "hypermedia as the engine of application state"). HTML forms present a good example of this in practice.

Building URIs and building them safely is not easy with URI, which is why JAX-RS has the UriBuilder class that makes it simple and easy to build URIs safely. UriBuilder can be used to build new URIs or build from existing URIs. For resource classes it is more than likely that URIs will be built from the base URI the web service is deployed at or from the request URI. The class UriInfo provides such information (in addition to further information, see next section).

The following example shows URI building with UriInfo and UriBuilder from the bookmark example:

@Path("/users/")
public class UsersResource {

    @Context
    UriInfo uriInfo;

    ...

    @GET
    @Produces("application/json")
    public JSONArray getUsersAsJsonArray() {
        JSONArray uriArray = new JSONArray();
        for (UserEntity userEntity : getUsers()) {
            UriBuilder ub = uriInfo.getAbsolutePathBuilder();
            URI userUri = ub.
                    path(userEntity.getUserId()).
                    build();
            uriArray.put(userUri.toASCIIString());
        }
        return uriArray;
    }
}

UriInfo is obtained using the @Context annotation, and in this particular example injection onto the field of the root resource class is performed, previous examples showed the use of @Context on resource method parameters.

UriInfo can be used to obtain URIs and associated UriBuilder instances for the following URIs: the base URI the application is deployed at; the request URI; and the absolute path URI, which is the request URI minus any query components.

The getUsersAsJsonArray method constructs a JSONArrray, where each element is a URI identifying a specific user resource. The URI is built from the absolute path of the request URI by calling UriInfo.getAbsolutePathBuilder(). A new path segment is added, which is the user ID, and then the URI is built. Notice that it is not necessary to worry about the inclusion of '/' characters or that the user ID may contain characters that need to be percent encoded. UriBuilder takes care of such details.

UriBuilder can be used to build/replace query or matrix parameters. URI templates can also be declared, for example the following will build the URI "http://localhost/segment?name=value":

UriBuilder.fromUri("http://localhost/")
    .path("{a}")
    .queryParam("name", "{value}")
    .build("segment", "value");

JAX-RS 2.0 introduced additional URI resolution and relativization methods in the UriBuilder:

Resolve and relativize methods in UriInfo are essentially counterparts to the methods listed above - UriInfo.resolve(java.net.URI) resolves given relative URI to an absolute URI using application context URI as the base URI; UriInfo.relativize(java.net.URI) then transforms an absolute URI to a relative one, using again the applications context URI as the base URI.

UriBuilder also introduces a set of methods that provide ways of resolving URI templates by replacing individual templates with a provided value(s). A short example:

final URI uri = UriBuilder.fromUri("http://{host}/{path}?q={param}")
    .resolveTemplate("host", "localhost")
    .resolveTemplate("path", "myApp")
    .resolveTemplate("param", "value").build();

uri.toString(); // returns "http://localhost/myApp?q=value"

See the UriBuilder javadoc for more details.

JAX-RS 2.0 introduces Link class, which serves as a representation of Web Link defined in RFC 5988. The JAX-RS Link class adds API support for providing additional metadata in HTTP messages, for example, if you are consuming a REST interface of a public library, you might have a resource returning description of a single book. Then you can include links to related resources, such as a book category, author, etc. to make the produced response concise but complete at the same time. Clients are then able to query all the additional information they are interested in and are not forced to consume details they are not interested in. At the same time, this approach relieves the server resources as only the information that is truly requested is being served to the clients.

A Link can be serialized to an HTTP message (tyically a response) as additional HTTP header (there might be multiple Link headers provided, thus multiple links can be served in a single message). Such HTTP header may look like:

Link: <http://example.com/TheBook/chapter2>; rel="prev"; title="previous chapter"

Producing and consuming Links with JAX-RS API is demonstrated in the following example:

// server side - adding links to a response:
Response r = Response.ok().
    link("http://oracle.com", "parent").
    link(new URI("http://jersey.java.net"), "framework").
    build();

...

// client-side processing:
final Response response = target.request().get();

URI u = response.getLink("parent").getUri();
URI u = response.getLink("framework").getUri();

Instances of Link can be also created directly by invoking one of the factory methods on the Link API that returns a Link.Builder that can be used to configure and produce new links.