# JAX-RS 2.1 Additions

JAX-RS 2.1 adds more asynchronous processing support in both the Client and the Server API. The specification adds a Reactive programming style to the Client side and Server-Sent Events (SSE) protocol support to both client and server.

## `CompletionStage` support

The specification adds support for declaring asynchronous resource methods by returning a `CompletionStage` instead of using the `@Suspended` annotation.

RESTEasy supports more reactive types than the specification.

## Reactive Clients API

The specification defines a new type of invoker named RxInvoker, and a default implementation of this type named CompletionStageRxInvoker. CompletionStageRxInvoker implements Java 8's interface CompletionStage. This interface declares a large number of methods dedicated to managing asynchronous computations.

There is also a new rx method which is used in a similar manner to async.

## Server-Sent Events (SSE)

SSE is part of HTML standard, currently supported by many browsers. It is a server push technology, which provides a way to establish a one-way channel to continuously send data to clients. SSE events are pushed to the client via a long-running HTTP connection. In case of lost connection, clients can retrieve missed events by setting a "Last-Event-ID" HTTP header in a new request.

SSE stream has text/event-stream media type and contains multiple SSE events. SSE event is a data structure encoded with UTF-8 and contains fields and comment. The field can be event, data, id, retry and other kinds of field will be ignored.

From JAX-RS 2.1, Server-sent Events APIs are introduced to support sending, receiving and broadcasting SSE events.

### SSE Server

As shown in the following example, a SSE resource method has the text/event-stream produce media type and an injected context parameter SseEventSink. The injected SseEventSink is the connected SSE stream where events can be sent. Another injected context Sse is an entry point for creating and broadcasting SSE events. Here is an example to demonstrate how to send SSE events every 200ms and close the stream after a "done" event.

       @GET
       @Path("domains/{id}")
       @Produces(MediaType.SERVER_SENT_EVENTS)
       public void startDomain(@PathParam("id") final String id, @Context SseEventSink sink @Context Sse sse)
       {
          ExecutorService service = (ExecutorService) servletContext
                .getAttribute(ExecutorServletContextListener.TEST_EXECUTOR);
          service.execute(new Thread()
          {
             public void run()
             {
                try
                {
                   sink.send(sse.newEventBuilder().name("domain-progress")
                         .data(String.class, "starting domain " + id + " ...").build());
                   Thread.sleep(200);
                   sink.send(sse.newEvent("domain-progress", "50%"));
                   Thread.sleep(200);
                   sink.send(sse.newEvent("domain-progress", "60%"));
                   Thread.sleep(200);
                   sink.send(sse.newEvent("domain-progress", "70%"));
                   Thread.sleep(200);
                   sink.send(sse.newEvent("domain-progress", "99%"));
                   Thread.sleep(200);
                   sink.send(sse.newEvent("domain-progress", "Done.")).thenAccept((Object obj) -> {
                      sink.close();
                   });
                }
                catch (final InterruptedException e)
                {
                   logger.error(e.getMessage(), e);
                }
             }
          });
       }

RESTEasy supports sending SSE events via reactive types.

### SSE Broadcasting

With SseBroadcaster, SSE events can be broadcasted to multiple clients simultaneously. It will iterate over all registered SseEventSinks and send events to all requested SSE Stream. An application can create a SseBroadcaster from an injected context Sse. The broadcast method on a SseBroadcaster is used to send SSE events to all registered clients. The following code snippet is an example on how to create SseBroadcaster, subscribe and broadcast events to all subscribed consumers.

       @GET
       @Path("/subscribe")
       @Produces(MediaType.SERVER_SENT_EVENTS)
       public void subscribe(@Context SseEventSink sink) throws IOException
       {
          if (sink == null)
          {
             throw new IllegalStateException("No client connected.");
          }
          if (sseBroadcaster == null)
          {
             sseBroadcaster = sse.newBroadcaster();
          }
          sseBroadcaster.register(sink);
       }

       @POST
       @Path("/broadcast")
       public void broadcast(String message) throws IOException
       {
          if (sseBroadcaster == null)
          {
             sseBroadcaster = sse.newBroadcaster();
          }
          sseBroadcaster.broadcast(sse.newEvent(message));

       }

### SSE Client

SseEventSource is the entry point to read and process incoming SSE events. A SseEventSource instance can be initialized with a WebTarget. Once SseEventSource is created and connected to a server, registered event consumer will be invoked when an inbound event arrives. In case of errors, an exception will be passed to a registered consumer so that it can be processed. SseEventSource can automatically reconnect the server and continuously receive pushed events after the connection has been lost. SseEventSource can send lastEventId to the server by default when it is reconnected, and server may use this id to replay all missed events. But reply event is really upon on SSE resource method implementation. If the server responds HTTP 503 with a RETRY_AFTER header, SseEventSource will automatically schedule a reconnect task with this RETRY_AFTER value. The following code snippet is to create a SseEventSource and print the inbound event data value and error if it happens.

        public void printEvent() throws Exception
        {
          WebTarget target = client.target("http://localhost:8080/service/server-sent-events"));
          SseEventSource msgEventSource = SseEventSource.target(target).build();
          try (SseEventSource eventSource = msgEventSource)
          {
             eventSource.register(event -> {
                System.out.println(event.readData(String.class));
             }, ex -> {
                ex.printStackTrace();
             });
             eventSource.open();
          }
        }

## Java API for JSON Binding

RESTEasy supports both JSON-B and JSON-P. In accordance with the specification, entity providers for JSON-B take precedence over those for JSON-P for all types except JsonValue and its sub-types.

The support for JSON-B is provided by the `JsonBindingProvider` from `resteasy-json-binding-provider` module. To satisfy JAX-RS 2.1 requirements, JsonBindingProvider takes precedence over the other providers for dealing with JSON payloads, in particular the Jackson one. The JSON outputs (for the same input) from Jackson and JSON-B reference implementation can be slightly different. As a consequence, in order to allow retaining backward compatibility, RESTEasy offers a `resteasy.preferJacksonOverJsonB` context property that can be set to `true` to disable JsonBindingProvider for the current deloyment.

WildFly 14 supports specifying the default value for the `resteasy.preferJacksonOverJsonB` context property by setting a system property with the same name. Moreover, if no value is set for the context and system properties, it scans JAX-RS deployments for Jackson annotations and sets the property to `true` if any of those annotations is found.