How to use multithreading/concurrency in my Java code

Question

This is a simplification of my code:

for(int i = 0; i < 500; i++) {
    someMethod(i);
}

someMethod() takes a long time to execute, so I want to use multithreading to break up the for-loop into 5 intervals of 100:

for(int i = 0; i < 100; i++) {
    someMethod(i);
}

for(int i = 100; i < 200; i++) {
    someMethod(i);
}

...

for(int i = 400; i < 500; i++) {
    someMethod(i);
}

so I can execute someMethod() concurrently for different i.

How do I use multithreading to accomplish this?

Please help! Thanks!

Use ExecutorServices and threadpool

kosa
– kosa

2012-07-26 20:52:43 +00:00
Commented Jul 26, 2012 at 20:52 — kosa
– kosa, Commented Jul 26, 2012 at 20:52
Small and really basic sample here

Francisco Spaeth
– Francisco Spaeth

2012-07-26 20:53:05 +00:00
Commented Jul 26, 2012 at 20:53 — Francisco Spaeth
– Francisco Spaeth, Commented Jul 26, 2012 at 20:53

Eng.Fouad · Accepted Answer · 2012-07-26 20:56:23Z

9

It is recommended to use the great ExecutorService code for situations like this. What you do is submit all of your tasks (0 to 499) into the thread pool and they will be run concurrently by the 5 threads in the pool.

Something like the following:

// create a thread pool with 5 workers
ExecutorService threadPool = Executors.newFixedThreadPool(5);
// submit all of your jobs here
for (int i = 0; i < 500; i++) {
    threadPool.submit(new MyJob(i));
}
// once we have submitted all jobs to the thread pool, it should be shutdown
threadPool.shutdown();
// if you need to wait for the pool you can do
threadPool.awaitTerminatation(Long.MAX_VALUE, TimeUnit.MILLISECONDS);

private static class MyJob implements Runnable {
   private int i;
   public MyJob(int i) {
       this.i = i;
   }
   public void run() {
       // do the thread stuff here
   }
}

edited Jul 26, 2012 at 20:56

Eng.Fouad

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answered Jul 26, 2012 at 20:52

Gray

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6 Comments

Jessica Myers Over a year ago

By // do the thread stuff here, what do you mean? Also, where does someMethod() factor into this?

Gray Over a year ago

So @Jessica, you can copy what is in someMethod() and put that code into run(). You could also pass in the class that forks the thread into the MyJob constructor and then inside of run do: caller.someMethod(i). Your choice.

Eng.Fouad Over a year ago

just replace // do the thread stuff here with someMethod(i); :) .. +1 for Gray

Jessica Myers Over a year ago

One more question, if anyone is still around: I have a boolean done = false at the beginning of my code. One of those i values will toggle done. I want to stop the for-loop when that happens. Right now I added if(done) return; to run() but all the someMethod()s that have already started need to return; so there's a short time delay. What's a better way to do this?

Gray Over a year ago

That's harder because the jobs have already been submitted to the thread-pool. You could make done be volatile and just put a if (done) return; the first thing in the run() method @Jessica.

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