Java - 用于固定数量任务的ExecutorService

Question

I would like to achieve the following behaviour using ExecutorService from Java. I have a large number of tasks to complete which can all be done in parallel. I would like to schedule from my main thread only N tasks at the time in the following manner:

• if the number of active tasks is < N => schedule new task (executor service does that already)
• else, block on the main thread until one of the tasks is done => pretty much the same as storing N first tasks in the queue and then dequeing the first and calling get() on it)

Is there a way to tweak the ExecutorService to do as stated above?

Answer 1

Use a ThreadPoolExecutor of N threads, constructed with a SynchronousQueue. Each time you'll submit a task to the thread pool, the main thread will be blocked by the synchronous queue until a thread from the pool takes the task from the queue.

Answer 2

You could submit N tasks to an ExecutorCompletionService, then in a loop:

Future<?> f = executor.take(); //blocks until one task completes
executor.submit(nextTask());

By doing that, you make sure that no more that N tasks are queued in the executor at the same time.