主类完成后，执行器线程继续运行

Question

I have the following code which runs on a simple main activity:

 Executor executor = Executors.newFixedThreadPool(1);
    executor.execute(new Runnable() {
      public void run() {
        try {

            System.out.println("sleep");
            Thread.sleep(5000); 



        } catch (InterruptedException e) {
          System.out.println("Interrupted, so exiting.");
        }
      }
    });

It looks like that when i run this code application doesnt terminate and nothing gets printed either (except the first sleep).

On the other hand when i run this:

 Thread t = new Thread(new Runnable() {
        public void run() {
             try {
                    Thread.sleep(5000);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
        }
    });

application terminates just fine. Why?

Answer 1

Executor interface would not allow you to shutdown the service. Preferred way is to use ExecutorService instead of Executor

ExecutorService executorService =  Executors.newSingleThreadExecutor(factory);
  executorService.execute(new Runnable() {
   @Override
   public void run() {
    try {
     doTask();
    } catch (Exception e) {
     logger.error("indexing failed", e);
    }
   }
  });
  executorService.shutdown();

Answer 2

Thread t = new Thread(new Runnable() {
    public void run() {
         try {
                Thread.sleep(5000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
    }
});

With this, the thread will terminate after executing the run method.

Runnbale r = ...
Executor executor = Executors.newFixedThreadPool(1);
executor.execute(r);

With this, the executor will create a thread which works like below:

new Thread(new Runnable() {
    @Override
    public void run() {
        while(notShutdown()) {
            waitingForTask(); // may get blocked
            runTask(); // execute Runnable user submits
        }
    }
});

This thread will not terminate after the first task, it will keep waiting for new tasks. You need call executor.shutdown() explicitly.

Answer 3

Executor thread keeps running after main class is finished

Yep, it's designed to do that. The whole point of an ExecutorService is that there is a pool of threads. Even though the Runnable that you submitted has finished, the threads in the pool are waiting for other jobs to come along. You need to shutdown the pool to get your application to terminate.

It looks like that when i run this code the application doesn't terminate and nothing gets printed either (except the first sleep).

The right way to use the ExecutorService is something like the following:

ExecutorService threadPool = Executors.newFixedThreadPool(1);
threadPool.submit(new Runnable() ...);
// submit any other jobs to the pool here
...
// after last submit, you shutdown the pool, submitted jobs will continue to run
threadPool.shutdown();
// optionally wait for all jobs to finish in pool, similar to thread.join()
threadPool.awaitTermination(Long.MAX_VALUE, TimeUnit.MILLISECONDS);

Once you shutdown the pool and the jobs that have submitted finish, the threads in the pool will terminate and, if there are no more non-daemon threads, your application will stop.

Answer 4

• Cause

: You did not shutdown the executor .And When you do the shutdown be aware to do it after it's termination

• Solution

  : shut it down only after its termination using a simple code.

For example :

executor.shutdown(); while (!executor.isTerminated()) {}

• Another solution is to use ExecutorCompletionService if you want to take tasks as they complete you need an ExecutorCompletionService. This acts as a BlockingQueue that will allow you to poll for tasks as and when they finish.