synchronizing thread in java

Question

I am implementing java Runnable interface for multithreading. I have some n number of threads. Each of the thread has its own life. I would like to wait till life of all the threads get expire. Lets say following is the case

for(int i = 0; i< k;i++){
 Thread thread1 = new Thread(new xyz())
 Thread thread2 = new Thread(new abc())
 Thread thread3 = new Thread(new mno())
 thread1.start();
 thread2.start();
 thread3.start();
}

I am doing following to synchronize it. I don't know if it is correct. Please let me know how would I do it? And is there any way to check, if my threaded program is working correctly?

          if(thread2.isAlive())
                try {
                    thread2.join();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            if(thread1.isAlive())
                    try {
                        thread1.join();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
            if(thread3.isAlive())
                try {
                        thread3.join();
                } catch (InterruptedException e) {
                e.printStackTrace();
                    }

assylias · Accepted Answer · 2012-06-28 16:03:00Z

8

You could add your Runnables to an ExecutorService and call shutdown / awaitTermination which will return once all tasks have completed. There are a few examples in the javadoc - in summary you would write something like:

ExecutorService executor = Executors.newFixedThreadPool(3);

executor.submit(runnable1);
executor.submit(runnable2);
executor.submit(runnable3);

executor.shutdown();
boolean allRunnableAreDone = executor.awaitTermination(60, TimeUnit.SECONDS);

// This line is reached once all runnables have finished their job
// or the 60 second timeout has expired

edited Jun 28, 2012 at 16:03

answered Jun 28, 2012 at 15:48

assylias

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2 Comments

OldCurmudgeon Over a year ago

It's a good idea to inspect the return value of awaitTermination and loop on it, just in case it takes longer than your time specified to complete.

Gray Over a year ago

Or to call awaitTermination(Long.MAX_VALUE, ...) as opposed to just 60 seconds.

Gray · Accepted Answer · 2012-06-28 15:52:49Z

4

Although the ExecutorService answer from @assylias is a good one, here's more information about join().

You do not need to test the isAlive() before you join(). The Thread.join() code already does that. Here's a snippet from the code there:

while (isAlive()) {
    wait(0);
}

So all you need to do is join with your threads:

   try {
       thread2.join();
       thread1.join();
       thread3.join();
   } catch (InterruptedException e) {
       // always a good pattern
       Thread.currentThread().interrupt();
       e.printStackTrace();
   }

answered Jun 28, 2012 at 15:52

Gray

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1 Comment

Denys Séguret Over a year ago

I didn't knew this one. This seems to be the simplest.

Hunter McMillen · Accepted Answer · 2012-06-28 15:51:12Z

2

As a simple solution you could put all of your threads in a List then call join on them after they have been started:

List<Thread> threads = new ArrayList<>();
for(int i = 0; i< k;i++)
{
  //create threads
  Thread thread1 = new Thread(new xyz());
  Thread thread2 = new Thread(new abc());
  Thread thread3 = new Thread(new mno());

  //store threads
  threads.add(thread1);
  threads.add(thread2);
  threads.add(thread3);

  //start threads
  thread1.start();
  thread2.start();
  thread3.start();
}

//join all threads
for(Thread t : threads)
   t.join();

//You are here after all threads have terminated

answered Jun 28, 2012 at 15:51

Hunter McMillen

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Comments

Denys Séguret · Accepted Answer · 2012-06-28 15:51:10Z

1

Java contains mechanism to do this kind of things in java.util.concurrent.

In your case, you probably need CountDownLatch or ExecutorService

edited Jun 28, 2012 at 15:51

answered Jun 28, 2012 at 15:45

Denys Séguret

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Comments

Stephan · Accepted Answer · 2012-06-28 15:51:27Z

1

The join() method does not join on a thread, but on a lock object. The waiting thread must call lockObject.join() and the working thread must call lockObject.notify() when it is done. The waiting thread will be notified and can continue its work. You also need synchronize blocks around these calls.

I also recommend the Executor like assylias mentioned, it is much easier than implementing this behaviour by yourself.

answered Jun 28, 2012 at 15:51

Stephan

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