JSR315 servlet 3.0 specifies a new async api that allows the
suspension of a request. A servlet's doGet or doPost method can
establish an async context and then return. The request is not
completed at that time (no response is sent to the client/browser).
Later a server event or a timeout can cause the doGet/doPost to be
invoked again, at which time it can deliver a response to the original
request.
In Jetty 7.2, using servlet spec 2.5, this model is followed; but the
API is slightly different, and specific to Jetty. The key point is
that the servlet's doPost returns immediately after calling suspend,
as in JSR315. The usage is something like this:
doPost(...) {
Continuation c = ContinuationSupport.getContinuation(request);
if (need to suspend) {
c.suspend();
return;
}
... process the request ...
}
After the return, the request is in a suspended state. Unfortunately,
suspending and then returning doesn't work directly in gwt-rpc service
implementation methods, because when such a method returns, the
returned value is always sent back to the client, regardless of the
request's continuation state.
I found a way to use continuation in a gwt-rpc service method: by
throwing an exception (MyContinuationException) immediately after the
call to continuation.suspend. This exception is caught in
RPC.invokeAndEncodeResponse, and recorded as the cause of an
UnexpectedException, which is caught in
AbstractRemoteServiceServlet.doPost, and passed to the method
doUnexpectedFailure in that class. My servlet overrides
doUnexpectedFailure. If the passed exception has
MyContinuationException as its cause, then doUnexpectedFailure simply
returns, causing doGet to return without sending a response, and thus
to behave properly wrt suspension. Otherwise it calls
super.doUnexpectedFailure.
There is some obsolete and confusing information on the web about
jetty continuations and GWT, because both systems have evolved over
time. In earlier versions, the throw was done in the suspend method;
this was part of Jetty's original design for continuations, but it
appears that negotiations in the JSR315 committee resulted in its
elimination (that's just speculation, I didn't actually research it).
Essentially, I just re-used Jetty's earlier approach to work around
the problem of using continuation with gwt-rpc. It all seems so
straightforward now, but I spent half a day figuring it out!
It's also necessary still to override
AbstractRemoteServiceServlet.readContent, as mentioned in
http://docs.codehaus.org/display/JETTY/GWT.
I'd like to hear if anyone notices a problem with this solution, or
just knows something I don't know about the subject.
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