Java Web应用程序与Web服务之间的线程间通信

Question

some premise changed on my original question posted in inter-thread communication between java application and jax-ws web service ( i am also simplifying the question to make it simpler to understand (and hence get answered)... hope that's ok with the moderators.)

a web service thread (THREAD-1) places a request on a socket and goes to sleep waiting for the response. another listener thread (THREAD-2) (running as a separate web application, once it receives the response) has to wake up THREAD-1 out of its sleep.

how do i do that ( in a push notification way)?

They both have access to a database table. The THREAD-1 can put its unique id in the table before it goes to sleep. THREAD-2, once it receives the response and determines it belongs to THREAD-1, it updates the appropriate row in the database table. Now THREAD-1 can do polling (at regular intervals) on the database table to check if the response has arrived.

But I am looking to do it in a push-notification way. The THREAD-1 should be notified right away when the response has arrived without it having to poll every few seconds.

Answer 1

If your backend request completes quickly and you won't have a torrent of client requests to handle, you can have the web service wait for a response over the same socket it just opened. It can block waiting to read the response.

However, if you have access to Servlet 3.0 (eg Tomcat 7), you can use asynchronous HTTP requests . It allows you to release the thread handling the web service client back into the pool without responding to the client's request. When a response message arrives from the backend service, the grabs the appropriate web service client request from the servlet container and sends the final response back to the web service client.

Answer 2

Well, as the comment suggest: better don't try to implement this yourself. However, basically you could use standard Java thread-synchronization with wait()/notify()

• Thread-1 sets of remote call to Thread-2 together with a unique call-ID.
• Thread-1 does now wait() on a synchronization object (instead of sleep() )
• Thread-2 does the work and to return the result it calls some remote callback method in the JVM Thread-1 resides in - call-ID is passed together with the result
• The callback method on the Thread-1 side makes the result available via call-ID and wakes up all waiting threads by means of notifyAll()
• Threads incl. Thread-1 check if their result arrived, if yes they continue to work if no the wait() again.

Pseudo code for the Thread-1 side:

HashMap<String, Object> results;

// Called by Thread-1 to access Thread-2
public void doIt() {
    String callId = "aUniqueCallId";
    Object result = null;
    remoteCallToThread2(callId);
    synchronized(results) {
        while((result = results.remove(callId)) == null) {
            results.wait();
        }
    }
    doSomethingWith(result);
}

// Called remotely by Thread-2 when result is available
public void callback(String callId, Object result) {    
    synchronized(results) {
        results.put(callId, result);
        results.notifyAll();
    } 
}

Of course this is just the basic idea and can not be used as such, there's a lot of stuff to be considered here.