WebSocket endpoint as Singleton EJB

By default …

… a WebSocket implementation creates a new (server) endpoint instance per client. In case you need a single instance, you can implement this using a custom ServerEndpointConfig.Configurator (by overriding the getEndpointInstance method)

The catch: you might have to sacrifice some of the (Java EE) platform related services like dependency injection and interceptors

More details here

Alternate solution ?

A similar behavior can be achieved by decorating the WebSocket endpoint with @Singleton

This approach has the caveat of not being a standard feature outlined by the spec (although injection of EJBs as well as interceptors support is clearly mentioned in the Java WebSocket spec Sec 7.1)

	@Singleton
	@ServerEndpoint("/singleton/")
	public class SingletonEndpoint {
	@OnOpen
	public void onopen(Session s) throws IOException {
	s.getBasicRemote().sendText(String.valueOf(hashCode()));
	}
	@PreDestroy
	public void onDestroy() {
	System.out.println("Singleton bean " + hashCode() + " will be destroyed");
	}
	@OnClose
	public void onClose(Session session, CloseReason closeReason) {
	System.out.println("Closed " + session.getId() + " due to " + closeReason.getCloseCode());
	}
	}

view raw

SingletonEndpoint.java

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Concurrency semantics ?

In case of a @Singleton, all the clients will interact with the one-and-only server endpoint instance. Here is a quick summary of how the EJB as well as WebSocket threading semantics are applied

• The Singleton bean default approach WRITE lock ensures single threaded access across all connected clients
• If thread-safety is not a concern (e.g. in case where you do not deal with client specific data/state in your logic) and in case the single-threaded access model proves to be a bottleneck, override the default behavior by switching to a READ lock which allows concurrent threads to access the methods (unless of course a WRITE lock is not already in effect)

Note: The above mentioned semantics are with respect to ALL the WebSocket clients. From the point of view of a single client, the default strategy of one thread at a time, per endpoint instance per client continues to apply (more details here)

Further reading..

• eBook: Java WebSocket API Handbook
• WebSocket specification
• JSR 356 RI: Tyrus

Cheers!