等待线程池中所有工作程序完成的最佳方法是什么？

Question

Imagine I have following code:

final ExecutorService threadPool = Executors.newFixedThreadPool(
    NUMBER_OF_WORKERS);

for (int i=0; i < NUMBER_OF_WORKERS; i++)
{
  final Worker worker = new BirthWorker(...);
  threadPool.execute(worker);
}

Now I need a piece of code, which waits, until all workers have completed their work.

Options I'm aware of:

1. while (!threadPool.isTerminated()) {}

2. Modify the code like that:

   final List futures = new ArrayList(NUMBER_OF_WORKERS); final ExecutorService threadPool = Executors.newFixedThreadPool(NUMBER_OF_WORKERS);

   for (int i=0; i < NUMBER_OF_WORKERS; i++) { final Worker worker = new Worker(...); futures.add(threadPool.submit(worker)); }

   for (final Future future : futures) { future.get(); }

   // When we arrive here, all workers are guaranteed to have completed their work.

What is the best practice to wait for the completion of all workers?

Answer 1

I would suggest you use CountDownLatch (assuming this is one time activity) where in your constructor, you can specify how many threads you want to wait for and you share that instance accross the threads and you wait on all the threads to complete using await api (using timeout or complete blocking) and your thread's calling countdown api when they are done.

Another option would be, to call join method in thread to wait for their completion if you have access to each and every thread that you wish to complete.

Answer 2

I would use ThreadPoolExecutor.invokeAll(Collection<? extends Callable<T>> tasks)

API: Executes the given tasks, returning a list of Futures holding their status and results when all complete

Answer 3

CountDownLatch,as stated above, would do the work well, just keep in mind that you want to shut down the executur after your done:

final ExecutorService threadPool = Executors.newFixedThreadPool(
NUMBER_OF_WORKERS);

for (int i=0; i < NUMBER_OF_WORKERS; i++)
{
  final Worker worker = new BirthWorker(...);
  threadPool.execute(worker);
}
threadPool.shutdown();

unless you shut it down, threadPool.isTerminated will stay false, even when all the workers are done.