Chapter 6. Representations and Responses
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# Chapter 6. Representations and Responses

## 6.1. Representations and Java Types

Previous sections on @Produces and @Consumes annotations referred to media type of an entity representation. Examples above depicted resource methods that could consume and/or produce String Java type for a number of different media types. This approach is easy to understand and relatively straightforward when applied to simple use cases.

To cover also other cases, handling non-textual data for example or handling data stored in the file system, etc., JAX-RS implementations are required to support also other kinds of media type conversions where additional, non-String, Java types are being utilized. Following is a short listing of the Java types that are supported out of the box with respect to supported media type:

- All media types (`*/*`)
  - `byte[]`
  - `java.lang.String`
  - `java.io.Reader` (inbound only)
  - `java.io.File`
  - `javax.activation.DataSource`
  - `javax.ws.rs.core.StreamingOutput` (outbound only)
- XML media types (`text/xml`, `application/xml` and `application/...+xml`)
  - `javax.xml.transform.Source`
  - `javax.xml.bind.JAXBElement`
  - Application supplied JAXB classes (types annotated with @XmlRootElement or@XmlType)
- Form content (`application/x-www-form-urlencoded`)
  - `MultivaluedMap<String,String>`
- Plain text (`text/plain`)
  - `java.lang.Boolean`
  - `java.lang.Character`
  - `java.lang.Number`

Unlike method parameters that are associated with the extraction of request parameters, the method parameter associated with the representation being consumed does not require annotating. In other words the representation (entity) parameter does not require a specific 'entity' annotation. A method parameter without a annotation is an entity. A maximum of one such unannotated method parameter may exist since there may only be a maximum of one such representation sent in a request.

The representation being produced corresponds to what is returned by the resource method. For example JAX-RS makes it simple to produce images that are instance of `File` as follows:

Example 6.1. Using `File` with a specific media type to produce a response

    @GET
    @Path("/images/{image}")
    @Produces("image/*")
    public Response getImage(@PathParam("image") String image) {
      File f = new File(image);

      if (!f.exists()) {
        throw new WebApplicationException(404);
      }

      String mt = new MimetypesFileTypeMap().getContentType(f);
      return Response.ok(f, mt).build();
    }

The `File` type can also be used when consuming a representation (request entity). In that case a temporary file will be created from the incoming request entity and passed as a parameter to the resource method.

The `Content-Type` response header (if not set programmatically as described in the next section) will be automatically set based on the media types declared by @Produces annotation. Given the following method, the most acceptable media type is used when multiple output media types are allowed:

    @GET
    @Produces({"application/xml", "application/json"})
    public String doGetAsXmlOrJson() {
      ...
    }

If `application/xml` is the most acceptable media type defined by the request (e.g. by header `Accept: application/xml`), then the `Content-Type` response header will be set to `application/xml`.

## 6.2. Building Responses

Sometimes it is necessary to return additional information in response to a HTTP request. Such information may be built and returned using Response and Response.ResponseBuilder. For example, a common RESTful pattern for the creation of a new resource is to support a POST request that returns a 201 (Created) status code and a `Location` header whose value is the URI to the newly created resource. This may be achieved as follows:

Example 6.2. Returning 201 status code and adding `Location` header in response to POST request

    @POST
    @Consumes("application/xml")
    public Response post(String content) {
      URI createdUri = ...
      create(content);
      return Response.created(createdUri).build();
    }

In the above no representation produced is returned, this can be achieved by building an entity as part of the response as follows:

Example 6.3. Adding an entity body to a custom response

    @POST
    @Consumes("application/xml")
    public Response post(String content) {
      URI createdUri = ...
      String createdContent = create(content);
      return Response.created(createdUri).entity(Entity.text(createdContent)).build();
    }

Response building provides other functionality such as setting the entity tag and last modified date of the representation.

## 6.3. WebApplicationException and Mapping Exceptions to Responses

Previous section shows how to return HTTP responses, that are built up programmatically. It is possible to use the very same mechanism to return HTTP errors directly, e.g. when handling exceptions in a try-catch block. However, to better align with the Java programming model, JAX-RS allows to define direct mapping of Java exceptions to HTTP error responses.

The following example shows throwing `CustomNotFoundException` from a resource method in order to return an error HTTP response to the client:

Example 6.4. Throwing exceptions to control response

    @Path("items/{itemid}/")
    public Item getItem(@PathParam("itemid") String itemid) {
      Item i = getItems().get(itemid);
      if (i == null) {
        throw new CustomNotFoundException("Item, " + itemid + ", is not found");
      }

      return i;
    }

This exception is an application specific exception that extends WebApplicationException and builds a HTTP response with the 404 status code and an optional message as the body of the response:

Example 6.5. Application specific exception implementation

    public class CustomNotFoundException extends WebApplicationException {

      /**
      * Create a HTTP 404 (Not Found) exception.
      */
      public CustomNotFoundException() {
        super(Responses.notFound().build());
      }

      /**
      * Create a HTTP 404 (Not Found) exception.
      * @param message the String that is the entity of the 404 response.
      */
      public CustomNotFoundException(String message) {
        super(Response.status(Responses.NOT_FOUND).
        entity(message).type("text/plain").build());
      }
    }

In other cases it may not be appropriate to throw instances of WebApplicationException, or classes that extend WebApplicationException, and instead it may be preferable to map an existing exception to a response. For such cases it is possible to use a custom exception mapping provider. The provider must implement the ExceptionMapper<E extends Throwable> interface. For example, the following maps the EntityNotFoundException to a HTTP 404 (Not Found) response:

Example 6.6. Mapping generic exceptions to responses

    @Provider
    public class EntityNotFoundMapper implements ExceptionMapper<javax.persistence.EntityNotFoundException> {
      public Response toResponse(javax.persistence.EntityNotFoundException ex) {
        return Response.status(404).
          entity(ex.getMessage()).
          type("text/plain").
          build();
      }
    }

The above class is annotated with @Provider, this declares that the class is of interest to the JAX-RS runtime. Such a class may be added to the set of classes of the Application instance that is configured. When an application throws an EntityNotFoundException the `toResponse` method of the `EntityNotFoundMapper` instance will be invoked.

Jersey supports extension of the exception mappers. These extended mappers must implement the `org.glassfish.jersey.spi.ExtendedExceptionMapper` interface. This interface additionally defines method `isMappable(Throwable)` which will be invoked by the Jersey runtime when exception is thrown and this provider is considered as mappable based on the exception type. Using this method the provider can reject mapping of the exception before the method `toResponse` is invoked. The provider can for example check the exception parameters and based on them return false and let other provider to be chosen for the exception mapping.

## 6.4. Conditional GETs and Returning 304 (Not Modified) Responses

Conditional GETs are a great way to reduce bandwidth, and potentially improve on the server-side performance, depending on how the information used to determine conditions is calculated. A well-designed web site may for example return 304 (Not Modified) responses for many of static images it serves.

JAX-RS provides support for conditional GETs using the contextual interface Request.

The following example shows conditional GET support:

Example 6.7. Conditional GET support

    public SparklinesResource(
      @QueryParam("d") IntegerList data,
      @DefaultValue("0,100") @QueryParam("limits") Interval limits,
      @Context Request request,
      @Context UriInfo ui) {
      if (data == null) {
        throw new WebApplicationException(400);
      }

      this.data = data;
      this.limits = limits;

      if (!limits.contains(data)) {
        throw new WebApplicationException(400);
      }

      this.tag = computeEntityTag(ui.getRequestUri());

      if (request.getMethod().equals("GET")) {
        Response.ResponseBuilder rb = request.evaluatePreconditions(tag);
        if (rb != null) {
          throw new WebApplicationException(rb.build());
        }
      }
    }

The constructor of the `SparklinesResouce` root resource class computes an entity tag from the request URI and then calls the request.evaluatePreconditions with that entity tag. If a client request contains an `If-None-Match` header with a value that contains the same entity tag that was calculated then the evaluatePreconditions returns a pre-filled out response, with the 304 status code and entity tag set, that may be built and returned. Otherwise, evaluatePreconditions returns `null` and the normal response can be returned.

Notice that in this example the constructor of a resource class is used to perform actions that may otherwise have to be duplicated to invoked for each resource method. The life cycle of resource classes is per-request which means that the resource instance is created for each request and therefore can work with request parameters and for example make changes to the request processing by throwing an exception as it is shown in this example.

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