Thread Jobs in Java

Question

Hi,

I want to spawn 200 threads simultaneously in Java. What I'm doing right now is running into a loop and creating 200 threads and starting them. After these 200 gets completed, I want to spawn another 200 set of threads and so on.

The gist here is that the first 200 threads I spawned need to be FINISHED before spawning the next set. I tried the code below, but its not working

for(int i=0;i<200;i++){
    Thread myThread = new Thread(runnableInstance);
    myThread.start();
}
for(int i=0;i<200;i++){
    Thread myThread = new Thread(runnableInstance);
    myThread.start();
}

Note: I have intentionally put the for loop Twice, but the desired effect I intend is not happening simply because the second for loop is executed before the first set of threads end their execution.

Please advise

Answer 1

You need to wait on the threads to finish by using join():

// run first set of threads
List<Thread> threads = threadsList; // save reference to all threads
for(Thread t : threads){
    t.join();
}
// all threads finished

// run other set of threads

Answer 2

You should keep a list of the threads you have created. Then once you have started all of them, you can loop over the list and perform a join on each one. When the join loop finishes, all of your threads will have run to completion.

List<Thread> threads = new List<Thread>();
for(int i=0;i<200;i++){
    Thread myThread = new Thread(runnableInstance);
    myThread.start();
    threads.add(myThread);
}
//All threads are running
for(Thread t : threads) {
    t.join();
}
//All threads are done

Answer 3

Feels homeworky, but ...

spawnAndWait() {
 Thread T[] = new Thread[n];
 for(int i=0;i<n;i++){
     T[i] = new Thread(runnableInstance);
     T[i].start();
 }
 for (Thread t : T) {
  t.join();
 }
}

Answer 4

If you are using Java 1.5, use the concurrent package, if you are using Java 1.4.2, the package is still available as a backport, I am pretty sure.

That being said, I had a similar tasks to do lately; you can achieve it very easily by using ExecutorService from which you wait on the Future object to return to know whether your tasks are complete. Very clean pattern - maybe not exactly what you are trying to achieve, but in real life it works really well :-).

Some code:

BlockingQueue<Runnable> workQueue = new LinkedBlockingQueue<Runnable>();
taskExecutor = new ThreadPoolExecutor(200, 200, 30, TimeUnit.SECONDS, workQueue);

futureList.add(taskExecutor.submit(new MyCallable()));

for (Future<Object> future : futureList) {
    future.get(); // wait for the task to complete
}

//... instantiate your second group of executors

Answer 5

Sort of pseudocode, you acnt instanciate callable ;-)

while( moreThreads ) {
    List<Callable<Integer>> threads = new ArrayList<Callable<Integer>>();
    for( int i = 0; i < 200; ++i ) {
        threads.add(new Callable<Integer>() );
    }

    FixedThreadPool pool = Executers.newFixedThreadPool(200);
    pool.invokeAll(threads);
    pool.shutdown();
}

Answer 6

Another solution is using Latch

final CountDownLatch latch = new CountDownLatch(200);

  for(int i=0; i<10; i++)
{

 Thread thread = new Thread()
{
    public void run()
{
     try{
         //do your work  

     }finally{
      latch.countDown();
     }
    }
   };

     }  
            //will block here util your threads all finished
  latch.await();