Simultaneously processing in one Servlet

问题

I have a Servlet which recieves a request, has to process 5 tasks (Grab data Form external Servers) and send all the data back to the client ordered.

How to process the 5 Task simultaneously and continue with the servlet code after all 5 tasks are completed?

回答1:

You can use CoundDownLatch

A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.

sample code:

    CountDownLatch startSignal = new CountDownLatch(1);
    CountDownLatch doneSignal = new CountDownLatch(5); // 5 tasks

    class Worker implements Runnable {
        private final CountDownLatch startSignal;
        private final CountDownLatch doneSignal;
        private final int threadNumber;

        // you can pass additional arguments as well
        Worker(CountDownLatch startSignal, CountDownLatch doneSignal,
                                                   int threadNumber) {
            this.startSignal = startSignal;
            this.doneSignal = doneSignal;
            this.threadNumber = threadNumber;
        }

        public void run() {
            try {
                startSignal.await();
                doWork(); // actual work to be performed here    
                doneSignal.countDown();
            } catch (InterruptedException ex) {
                LOGGER.error(ex);
            }
        }
    }

    // 5 new threads are started
    for(int i=1;i<=5;i++){
        new Thread(new Worker(startSignal, doneSignal, i)).start();
    }

    startSignal.countDown(); // let all threads proceed
    try {
        doneSignal.await(); // wait for all to finish
        // all 5 tasks are finished and do whatever you want to do next
    } catch (InterruptedException interruptedException) {
        LOGGER.error(interruptedException);
    }

回答2:

Another option is the ExecutorService. There are a variety of examples available including the following:

How to wait for all threads to finish, using ExecutorService?
ExecutorService, how to wait for all tasks to finish

Here is some example code taken from the first link found above:

ExecutorService taskExecutor = Executors.newFixedThreadPool(4);
while(...) {
  taskExecutor.execute(new MyTask());
}
taskExecutor.shutdown();
try {
  taskExecutor.awaitTermination(Long.MAX_VALUE, TimeUnit.NANOSECONDS);
} catch (InterruptedException e) {
  ...
}

回答3:

5 seconds is too long, it might take up the web server resource, you can let the client to send a request A, which trigger your 5 seconds task, but it don't wait, return immediately.Since you know it will take 5 seconds, you can send another request B, 5 seconds later, to get the data. If data not ready yet, you can request after another 5 seconds, until MAX_RETRY_COUNT (defined by yourself) reached.

来源：https://stackoverflow.com/questions/24981726/simultaneously-processing-in-one-servlet

标签

java

multithreading

servlets

asynchronous