Communication between Servlets

I would like to write a method which handles the flow of communication on XMPP. The sequence of things I'd like to do is:

1. Send message.
2. Wait for response.
3. Process the response.

Since 开发者_高级运维we could be waiting longer than 30s for the response (step 2) I'll be teeing up a task to take care of this. This task will need to send the message and then wait for a response on the XMPP servlet handling the incoming message. My question is: How do I wait in the task servlet thread for the response to arrive in the XMPP Servlet?

I'd normally use a listener pattern where the listener would store the message in a field in the Task object and then trigger a Semaphore to signal that a message has arrived. Like this:

1. Install listener in XMPP servlet in a static field.
2. Send message.
3. Wait for semaphore. ........ Meanwhile, in the XMPP servlet thread, a response will arrive and it will call the listener's callback method which stores the message and releases the semaphore.
4. Get message from field and process.

I tried this and it worked fine on the development server. However, when I uploaded to the cloud I found that I'd install the listener on the XMPP servlet (step 1) but then a new instance of the servlet would be instantiated when the message came in and there would no longer be a reference to the listener to call, event through the listener is a static field. My conclusion is XMPPServlet is run in a completely different VM meaning the static field is not shared between that servlet and the task one. Is this correct?

In general what is the best practice for communication between these servlets? How to I share data (normally I would've stored it in an object's field) and how do I signal from one to the other when events occur (normally I would've used a semaphore)?

Sorry about the long winded question. Tell me if it's not clear and I'll refine it a bit.

Reposting my answer to the same question you asked on the mailing list:

You can't [wait for a response in the sending process]. Instead, you should use an asynchronous pattern: Send the message, and register a handler for incoming XMPP messages. That handler should match up the response to the corresponding request (stored in the datastore if necessary) and perform appropriate processing on it.

An App Engine app can be run on any number of machines; synchronization primitives designed for communication between threads will not work.