How to notify another thread

Question

I want to know the best way how to notify another thread. For example, I have a background thread:

public void StartBackgroundThread(){
new Thread(new Runnable() {         
        @Override
        public void run() {
            //Do something big...
            //THEN HOW TO NOTIFY MAIN THREAD?
        }
    }).start();
}

When it finished it has to notify main thread? If somebody knows the best way how to do this I'll appreciate it!

Answer 1

The typical answer is a BlockingQueue. Both BackgroundThread (often called the Producer) and MainThread (often called the Consumer) share a single instance of the queue (perhaps they get it when they are instantiated). BackgroundThread calls queue.put(message) each time it has a new message and MainThread calls 'queue.take()which will block until there's a message to receive. You can get fancy with timeouts and peeking but typically people want aBlockingQueueinstance such asArrayBlockingQueue`.

Answer 2

Purely based on your question you could do this:

public class test
{
  Object syncObj = new Object();

  public static void main(String args[])
  {
    new test();
  }

  public test()
  {
    startBackgroundThread();

    System.out.println("Main thread waiting...");
    try
    {
      synchronized(syncObj)
      {
        syncObj.wait();
      }
    }
    catch(InterruptedException ie) { }
    System.out.println("Main thread exiting...");
  }

  public void startBackgroundThread()
  {
    (new Thread(new Runnable()
    {
      @Override
      public void run()
      {
        //Do something big...
        System.out.println("Background Thread doing something big...");
        //THEN HOW TO NOTIFY MAIN THREAD?
        synchronized(syncObj)
        {
          System.out.println("Background Thread notifing...");
      syncObj.notify();
        }
        System.out.println("Background Thread exiting...");
      }
    })).start();
  }
}

and see this output

PS C:\Users\java> javac test.java
PS C:\Users\java> java test
Main thread waiting...
Background Thread doing something big...
Background Thread notifing...
Background Thread exiting...
Main thread exiting...