Question

I ran into this concurrency problem which I’ve been scratching my head for several days.

Basically, I want my ThreadPoolExecutor to wait for all tasks (number of tasks unknown) to finish before shutting down.

public class AutoShutdownThreadPoolExecutor extends ThreadPoolExecutor{
    private static final Logger logger = Logger.getLogger(AutoShutdownThreadPoolExecutor.class);
    private int executing = 0;
    private ReentrantLock lock = new ReentrantLock();
    private final Condition newTaskCondition = lock.newCondition(); 
    private final int WAIT_FOR_NEW_TASK = 120000;

    public AutoShutdownThreadPoolExecutor(int coorPoolSize, int maxPoolSize, long keepAliveTime,
        TimeUnit seconds, BlockingQueue<Runnable> queue) {
        super(coorPoolSize, maxPoolSize, keepAliveTime, seconds, queue);
    }


    @Override
    public void execute(Runnable command) {
        lock.lock();
        executing++;
        lock.unlock();
        super.execute(command);
    }


    @Override
    protected void afterExecute(Runnable r, Throwable t) {
        super.afterExecute(r, t);
        try{
            lock.lock();
            int count = executing--;
            if(count == 0) {
                newTaskCondition.await(WAIT_FOR_NEW_TASK, TimeUnit.MILLISECONDS); 
                if(count == 0){
                    this.shutdown();
                    logger.info("Shutting down Executor...");
                }
            }
        }
        catch (InterruptedException e) {
            logger.error("Sleeping task interrupted", e);
        }
        finally{
            lock.unlock();
        }
    }
}

The intention is that the task checks for the task counter (executing), and if it equals 0, it blocks for a while, and later releases its lock so that other tasks may have the chance to execute and not shutting down the executor too soon.

However, it is not happening. All 4 threads in the executor go into the waiting state:

"pool-1-thread-4" prio=6 tid=0x034a1000 nid=0x2d0 waiting on condition [0x039cf000]
"pool-1-thread-3" prio=6 tid=0x034d0400 nid=0x1328 waiting on condition [0x0397f000]
"pool-1-thread-2" prio=6 tid=0x03493400 nid=0x15ec waiting on condition [0x0392f000]
"pool-1-thread-1" prio=6 tid=0x034c3800 nid=0x1fe4 waiting on condition [0x038df000]

If I put a log statement(supposed to slow the thread down) in the Runnable class, the problem seems to disappear.

public void run() {
    //  logger.info("Starting task" + taskName);
        try {
            //doTask();
        }
        catch (Exception e){
            logger.error("task " + taskName + " failed", e);
        }
}

The question is similar to this post
Java ExecutorService: awaitTermination of all recursively created tasks

I have adopted the original poster solution and attempted to address the race condition in afterExecute() but it doesn’t work.

Please help shed some light to this.
Thank you.

Answer 1

You have your tasks waiting for this newTaskCondition, but nothing ever signals this condition. So your threads all pile up, all waiting on newTaskCondition, until it times out. Additionally, blocking in afterExecute will delay the completion of the task, so it’s probably not what you want to do.

If you want to keep it waiting around a little while to see if more work comes in, this functionality already exists. Simply call setKeepAliveTime to set how long to wait, and set allowCoreThreadTimeOut to ensure that all threads (including the core threads) can be terminated.