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## Chapter 34. Asynchronous HTTP Request Processing

Asynchronous HTTP Request Processing is a relatively new technique that allows you to process a single HTTP request using non-blocking I/O and, if desired in separate threads. Some refer to it as COMET capabilities. The primary use case for Asynchronous HTTP is in the case where the client is polling the server for a delayed response. The usual example is an AJAX chat client where you want to push/pull from both the client and the server. These scenarios have the client blocking a long time on the server’s socket waiting for a new message. What happens in synchronous HTTP where the server is blocking on incoming and outgoing I/O is that you end up having a thread consumed per client connection. This eats up memory and valuable thread resources. Not such a big deal in 90% of applications (in fact using asynchronous processing may actually hurt your performance in most common scenarios), but when you start getting a lot of concurrent clients that are blocking like this, there’s a lot of wasted resources and your server does not scale that well.

The JAX-RS 2.0 specification has added asynchronous HTTP support via two classes. The @Suspended annotation, and AsyncResponse interface.

Injecting an AsynchronousResponse as a parameter to your jax-rs methods tells Resteasy that the HTTP request/response should be detached from the currently executing thread and that the current thread should not try to automatically process the response.

The AsyncResponse is the callback object. The act of calling one of the resume() methods will cause a response to be sent back to the client and will also terminate the HTTP request. Here is an example of asynchronous processing:

    import javax.ws.rs.Suspend;
    import javax.ws.rs.core.AsynchronousResponse;

    @Path("/")
    public class SimpleResource
    {

       @GET
       @Path("basic")
       @Produces("text/plain")
       public void getBasic(@Suspended final AsyncResponse response) throws Exception
       {
          Thread t = new Thread()
          {
             @Override
             public void run()
             {
                try
                {
                   Response jaxrs = Response.ok("basic").type(MediaType.TEXT_PLAIN).build();
                   response.resume(jaxrs);
                }
                catch (Exception e)
                {
                   e.printStackTrace();
                }
             }
          };
          t.start();
       }
    }

AsyncResponse also has other methods to cancel the execution. See javadoc for more details.

NOTE: The old Resteasy proprietary API for async http has been deprecated and may be removed as soon as Resteasy 3.1.

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