Java Multi thread messaging

Question

I have an app with two threads, 1 that writes to a queue and the second one that read async from it. I need to create a third one that generate 20 more. the newly created threads will run till explicitly stopped. those 20 threads should get "live" data in order to analyze it. each of the 20 has a unique ID/name. I need to send the relevant data (that the READ thread collect) to the correct thread (of the 20 threads). e.g. if the data include a string with id (in it) of 2 --> I need to send it to thread with the ID =2. my question is: how should I hold a "pointer" to each of the 20 threads and send it the relevant data? (I can search the id in a runnable list (that will hold the threads)--> but then I need to call to a method "NewData(string)" in order to send the data to the running thread). How should I do it? TIA Paz

Answer 1

You would probably be better to use a Queue to communicate with your threads. You could then put all of the queues in a map for easy access. I would recommend a BlockingQueue.

public class Test {
  // Special stop message to tell the worker to stop.
  public static final Message Stop = new Message("Stop!");

  static class Message {
    final String msg;

    // A message to a worker.
    public Message(String msg) {
      this.msg = msg;
    }

    public String toString() {
      return msg;
    }

  }

  class Worker implements Runnable {
    private volatile boolean stop = false;
    private final BlockingQueue<Message> workQueue;

    public Worker(BlockingQueue<Message> workQueue) {
      this.workQueue = workQueue;
    }

    @Override
    public void run() {
      while (!stop) {
        try {
          Message msg = workQueue.poll(10, TimeUnit.SECONDS);
          // Handle the message ...

          System.out.println("Worker " + Thread.currentThread().getName() + " got message " + msg);
          // Is it my special stop message.
          if (msg == Stop) {
            stop = true;
          }
        } catch (InterruptedException ex) {
          // Just stop on interrupt.
          stop = true;
        }
      }
    }
  }

  Map<Integer, BlockingQueue<Message>> queues = new HashMap<>();

  public void test() throws InterruptedException {
    // Keep track of my threads.
    List<Thread> threads = new ArrayList<>();
    for (int i = 0; i < 20; i++) {
      // Make the queue for it.
      BlockingQueue<Message> queue = new ArrayBlockingQueue(10);
      // Build its thread, handing it the queue to use.
      Thread thread = new Thread(new Worker(queue), "Worker-" + i);
      threads.add(thread);
      // Store the queue in the map.
      queues.put(i, queue);
      // Start the process.
      thread.start();
    }

    // Test one.
    queues.get(5).put(new Message("Hello"));

    // Close down.
    for (BlockingQueue<Message> q : queues.values()) {
      // Stop each queue.
      q.put(Stop);
    }

    // Join all threads to wait for them to finish.
    for (Thread t : threads) {
      t.join();
    }
  }

  public static void main(String args[]) {
    try {
      new Test().test();
    } catch (Throwable t) {
      t.printStackTrace(System.err);
    }
  }

}