For most modern webapps push notifications have become a standard requirement. WebSockets would be my first choice, however it has only become available with JEE 7. It will take some time until JEE 7 compliant servers reach main stream production environments.
In the mean time we can achieve similar behavior with Servlet 3.0, which has become part of the JEE 6 specification, either by polling or long polling.
Polling relies on the client repeatedly sending requests to the server, polling for new messages. This will often result in empty responses, to avoid this unnecessary traffic we can hold on to the request until a new message is available or time-out is reached (long polling).
Effectively long polling requires the client to send a request, which is kept alive by the server until a response is available or a time out occurs. It is up to the client to initiate another request in both cases.
To do this we have to accept incoming GET requests and set asyncSupported to true. This stops the response object from being committed on method exit.
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