Question

The setup:

I am in the process of changing the way a program works under the hood. In the current version works like this:

public void threadWork( List<MyCallable> workQueue )
{
    ExecutorService pool = Executors.newFixedThreadPool(someConst);
    List<Future<myOutput>> returnValues = new ArrayList<Future<myOutput>>();
    List<myOutput> finishedStuff = new ArrayList<myOutput>();

    for( int i = 0; i < workQueue.size(); i++ )
    {
        returnValues.add( pool.submit( workQueue.get(i) ) );
    }

    while( !returnValues.isEmpty() )
    {
        try
        {
            // Future.get() waits for a value from the callable
            finishedStuff.add( returnValues.remove(0).get(0) );
        }
        catch(Throwable iknowthisisbaditisjustanexample){}
    }

    doLotsOfThings(finsihedStuff);
}

But the new system is going to use a private inner Runnable to call a synchronized method that writes the data into a global variable. My basic setup is:

public void threadReports( List<String> workQueue )
{
    ExecutorService pool = Executors.newFixedThreadPool(someConst); 
    List<MyRunnable> runnables = new ArrayList<MyRunnable>()

    for ( int i = 0; i < modules.size(); i++ )
    {
        runnables.add( new MyRunnable( workQueue.get(i) );
        pool.submit(threads.get(i));
    }

    while( !runnables.isEmpty() )
    {
        try 
        {
            runnables.remove(0).wait(); // I realized that this wouldn't work
        } 
        catch(Throwable iknowthisisbaditisjustanexample){}
    }
    doLotsOfThings(finsihedStuff); // finishedStuff is the global the Runnables write to
}

If you read my comment in the try of the second piece of code you will notice that I don’t know how to use wait(). I had thought it was basically like thread.join() but after reading the documentation I see it is not.

I’m okay with changing some structure as needed, but the basic system of taking work, using runnables, having the runnables write to a global variable, and using a threadpool are requirements.

---

The Question

How can I wait for the threadpool to be completely finished before I doLotsOfThings()?

Answer 1

You should call ExecutorService.shutdown() and then ExecutorService.awaitTermination.

...
pool.shutdown();
if (pool.awaitTermination(<long>,<TimeUnit>)) {
    // finished before timeout
    doLotsOfThings(finsihedStuff);
} else {
    // Timeout occured.
}