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)

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..



About Abhishek

Java EE & distributed systems junkie who frequently blogs at abhirockzz.wordpress.com as well as simplydistributed.wordpress.com. Oh, I have also authored a few (mini) books, articles, Refcards etc. :-)
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3 Responses to WebSocket endpoint as Singleton EJB

  1. Dimitris says:

    Dear Abhishek,
    I have to deploy a Websocket @ServerEndPoint in the EJB level (jar), so I have used the Singleton’s approach. The problem is that, the singleton websocket endpoint is not discoverable by the clients (Response code was not 101: 404). Should the Websocket being deployed in a war file or is there any alternative? Please Advice. Thanks In Advance.


    • Abhishek says:

      Yes. The WebSocket endpoint needs to be deployed in a WAR file (as it is a web component). In fact you can deploy EJB in WAR files too. You are not forced to have a structure with an EAR, EJB JAR and different WAR files (although you can do that as well)


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