Question

How do I implement @SessionScoped in a Netty based TCP server? Creating Custom Scopes is documented in Guice manual, but it seems that the solution only works for thread based and not asynchronous IO servers.

Is it enough to create the Channel Pipeline between scope.enter() and scope.exit()?

Answer 1

Disclaimer : this answer is for Netty 3. I’ve not had the opportunity to try Netty 4 yet, so I don’t know if what follows can be applied to the newer version.

Netty is asynchronous on the network side, but unless you explicity submit tasks to Executors, or change threads by any other means, the handling of ChannelEvents by the ChannelHandlers on a pipeline is synchronous and sequential. For instance, if you use Netty 3 and have an ExecutionHandler on the pipeline, the scope handler should be upstream of the ExecutionHandler; for Netty 4, see Trustin Lee’s comment.

Thus, you can put a handler near the beginning of your pipeline that manages the session scope, for example:

public class ScopeHandler implements ChannelUpstreamHandler {

    @Override
    public void handleUpstream(ChannelHandlerContext ctx, ChannelEvent e) {

        if (e instanceof WriteCompletionEvent || e instanceof ExceptionEvent)
            ctx.sendUpstream(e);

        Session session = ...; // get session, presumably using e.getChannel()

        scope.enter();
        try {
            scope.seed(Key.get(Session.class), session);
            ctx.sendUpstream(e);
        }
        finally {
            scope.exit();
        }
    }

    private SessionScope scope;

}

A couple of quick remarks:

• You will want to filter some event types out, especially WriteCompletionEvent and ExceptionEvent which the framework will put at the downstream end of the pipeline during event processing and wil cause reentrancy issues if not excluded. In our application, we use this kind of handler but actually only consider UpstreamMessageEvents.
• The try/finally construct is not actually necessary as Netty will catch any Throwables and fire an ExceptionEvent, but it feels more idiomatic this way.

HTH