Responding to HTTP Resources

The behavior of a resource is determined by the HTTP methods (typically, GET, POST, PUT, DELETE) to which the resource is responding.

The Request Method Designator Annotations

Request method designator annotations are runtime annotations, defined by JAX-RS, that correspond to the similarly named HTTP methods. Within a resource class file, HTTP methods are mapped to Java programming language methods by using the request method designator annotations. The behavior of a resource is determined by which HTTP method the resource is responding to. JAX-RS defines a set of request method designators for the common HTTP methods @GET, @POST, @PUT, @DELETE, and @HEAD; you can also create your own custom request method designators. Creating custom request method designators is outside the scope of this document.

The following example, an extract from the storage service sample, shows the use of the PUT method to create or update a storage container:

@PUT
public Response putContainer() {
    System.out.println("PUT CONTAINER " + container);

    URI uri =  uriInfo.getAbsolutePath();
    Container c = new Container(container, uri.toString());

    Response r;
    if (!MemoryStore.MS.hasContainer(c)) {
        r = Response.created(uri).build();
    } else {
        r = Response.noContent().build();
    }

    MemoryStore.MS.createContainer(c);
    return r;
}

By default, the JAX-RS runtime will automatically support the methods HEAD and OPTIONS if not explicitly implemented. For HEAD, the runtime will invoke the implemented GET method, if present, and ignore the response entity, if set. For OPTIONS, the Allow response header will be set to the set of HTTP methods supported by the resource. In addition, the JAX-RS runtime will return a Web Application Definition Language (WADL) document describing the resource; see https://wadl.dev.java.net/ for more information.

Methods decorated with request method designators must return void, a Java programming language type, or a javax.ws.rs.core.Response object. Multiple parameters may be extracted from the URI by using the PathParam or QueryParam annotations as described in Extracting Request Parameters. Conversion between Java types and an entity body is the responsibility of an entity provider, such as MessageBodyReader or MessageBodyWriter. Methods that need to provide additional metadata with a response should return an instance of the Response class. The ResponseBuilder class provides a convenient way to create a Response instance using a builder pattern. The HTTP PUT and POST methods expect an HTTP request body, so you should use a MessageBodyReader for methods that respond to PUT and POST requests.

Both @PUT and @POST can be used to create or update a resource. POST can mean anything, so when using POST, it is up to the application to define the semantics. PUT has well-defined semantics. When using PUT for creation, the client declares the URI for the newly created resource.

PUT has very clear semantics for creating and updating a resource. The representation the client sends must be the same representation that is received using a GET, given the same media type. PUT does not allow a resource to be partially updated, a common mistake when attempting to use the PUT method. A common application pattern is to use POST to create a resource and return a 201 response with a location header whose value is the URI to the newly created resource. In this pattern, the web service declares the URI for the newly created resource.

Using Entity Providers to Map HTTP Response and Request Entity Bodies

Entity providers supply mapping services between representations and their associated Java types. The two types of entity providers are MessageBodyReader and MessageBodyWriter. For HTTP requests, the MessageBodyReader is used to map an HTTP request entity body to method parameters. On the response side, a return value is mapped to an HTTP response entity body by using a MessageBodyWriter. If the application needs to supply additional metadata, such as HTTP headers or a different status code, a method can return a Response that wraps the entity and that can be built by using Response.ResponseBuilder.

Table 13–3 shows the standard types that are supported automatically for entities. You need to write an entity provider only if you are not choosing one of these standard types.

Table 13–3 Types Supported for Entities

Java Type	Supported Media Types
byte[]	All media types (*/*)
java.lang.String	All text media types (text/*)
java.io.InputStream	All media types (*/*)
java.io.Reader	All media types (*/*)
java.io.File	All media types (*/*)
javax.activation.DataSource	All media types (*/*)
javax.xml.transform.Source	XML media types (text/xml, application/xml, and application/*+xml)
javax.xml.bind.JAXBElement and application-supplied JAXB classes	XML media types (text/xml, application/xml, and application/*+xml)
MultivaluedMap<String, String>	Form content (application/x-www-form-urlencoded)
StreamingOutput	All media types (*/*), MessageBodyWriter only

The following example shows how to use MessageBodyReader with the @Consumes and @Provider annotations:

@Consumes("application/x-www-form-urlencoded")
@Provider
public class FormReader implements MessageBodyReader<NameValuePair> {

The following example shows how to use MessageBodyWriter with the @Produces and @Provider annotations:

@Produces("text/html")
@Provider
public class FormWriter implements 
        MessageBodyWriter<Hashtable<String, String>> {

The following example shows how to use ResponseBuilder:

@GET
public Response getItem() {
    System.out.println("GET ITEM " + container + " " + item);
    
    Item i = MemoryStore.MS.getItem(container, item);
    if (i == null)
        throw new NotFoundException("Item not found");
    Date lastModified = i.getLastModified().getTime();
    EntityTag et = new EntityTag(i.getDigest());
    ResponseBuilder rb = request.evaluatePreconditions(lastModified, et);
    if (rb != null)
        return rb.build();
        
    byte[] b = MemoryStore.MS.getItemData(container, item);
    return Response.ok(b, i.getMimeType()).
            lastModified(lastModified).tag(et).build();
}