How to know whether there're running tasks in ExecutorService in java?

Question

I'd like to allow the program continue until all the tasks in ExecutorService is done. And I don't want to shutdown it, I use it later. How can I achieve this ? Thanks

Answer 1

Use ExecutorService#shutdown() followed by ExecutorService#awaitTermination() to make your main thread wait for all the submitted tasks to complete before it exits. From the docs for

ExecutorService#awaitTermination()

Blocks until all tasks have completed execution after a shutdown request, or the timeout occurs, or the current thread is interrupted, whichever happens first.

Answer 2

An example taken directly from the ExecutorService Javadoc:

http://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ExecutorService.html

The following method shuts down an ExecutorService in two phases, first by calling shutdown to reject incoming tasks, and then calling shutdownNow, if necessary, to cancel any lingering tasks:

void shutdownAndAwaitTermination(ExecutorService pool) {
  pool.shutdown(); // Disable new tasks from being submitted
  try {
    // Wait a while for existing tasks to terminate
    if (!pool.awaitTermination(60, TimeUnit.SECONDS)) {
      pool.shutdownNow(); // Cancel currently executing tasks
      // Wait a while for tasks to respond to being cancelled
      if (!pool.awaitTermination(60, TimeUnit.SECONDS))
          System.err.println("Pool did not terminate");
    }
  } catch (InterruptedException ie) {
    // (Re-)Cancel if current thread also interrupted
    pool.shutdownNow();
    // Preserve interrupt status
    Thread.currentThread().interrupt();
  }
}