How it works...

This recipe is made up of three parts:

• The server, represented by the ServerMock class
• The SSE engine, represented by the SseResource class
• The client, represented by the ClientConsumer class

So, once ServerMock is instantiated, it registers the SseResource class:

final ResourceConfig resourceConfig = new ResourceConfig(SseResource.class);
final HttpServer server = GrizzlyHttpServerFactory.createHttpServer(CONTEXT, resourceConfig, false);
server.start();

Then, two key methods from SseResource take place. The first one adds messages to the server queue:

addMessage(final String message, @Context Sse sse)

The second one consumes this queue and sends the messages to the clients:

@GET
@Produces(MediaType.SERVER_SENT_EVENTS)
public void getMessageQueue(@Context SseEventSink sink)

Note that this one has a media type, SERVER_SENT_EVENTS, introduced in this version for this very purpose. And finally, we have our client. In this recipe, it is both posting and consuming messages. 

It consumes here:

sseSource.register(System.out::println);
sseSource.open();

It posts here:

ServerMock.WEB_TARGET.request().post(Entity.json("event " + innerCounter));