Properly shut down ThreadPoolExecutors

Question

I'm new to java concurrency an would like to ask the following basic question. I'm creating a ThreadPoolExecutor for imporving performance as follows:

int n = Runtime.getRuntime().availableProcessors()
ExecutorService executor = Executors.newFixedThreadPool(n);
for( int i = 0; i < n; i++)
            executor.execute(new Work());

After all thread in the thread pool have finished their tasks I need to shutdown the pool properly. I would tried this:

while(true){
    if(executor.isTerminated()){
        executor.shutdownNow();
        break;
    }
}

But I'm not sure about that because I think we waste a lot of processors resources to queriyng the executor for termination.

What is the right solution for that?

UPD: Runnable task:

public class Work implements Runnable{

    private String sql;

    public Work() {
        //init sql
    }
    @Override
    public void run() {
        JdbcTemplate template = new JdbcTemplate(dataSource);
        @SuppressWarnings("unchecked")
        List<Integer> ints = template.queryForList(sql, Integer.class);
        //Storing the list into a global cache
    }

}

Could you show the Runnable class too? Your thread implementation makes your threads safe to shut down and you should write them according to a safe shutdown — Neron, Jul 29 '15 at 07:54
Please see [this answer](http://stackoverflow.com/a/10935734/2196460) to a simillar question. — hotkey, Jul 29 '15 at 07:54
possible duplicate of [How to wait for a ThreadPoolExecutor to finish](http://stackoverflow.com/questions/10934187/how-to-wait-for-a-threadpoolexecutor-to-finish) — hotkey, Jul 29 '15 at 07:55
From [the documentation](http://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ThreadPoolExecutor.html#isTerminated--): “*Note that `isTerminated` is never `true` unless either `shutdown` or `shutdownNow` was called first*”. So your code makes no sense at all. — Holger, Jul 29 '15 at 07:57

score 2 · Accepted Answer · answered Jul 29 '15 at 08:04

There seems to be something mystical around shutting down an ExecutorService.

From the documentation of shutdown():

Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted.

So all you have to do is to invoke shutdown() after you have submitted all your tasks, the exact time doesn’t matter. They don’t have to be completed at that time. The ThreadPoolExecutor will finish all tasks and then clean up all resources.

And it will do so regardless of whether you wait for it or not. So you don’t need to wait, just invoke shutdown() when you are confident that you will not submit new tasks, the rest will happen as soon as possible.

score 1 · Answer 2 · answered Jul 29 '15 at 08:00

It says:

There are no guarantees beyond best-effort attempts to stop processing actively executing tasks. For example, typical implementations will cancel via Thread.interrupt(), so any task that fails to respond to interrupts may never terminate.

http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ExecutorService.html#shutdownNow()

So use awaitTermination instead. And for threads that take time, use a boolean variable as volatile and check it if it is set outside.If set then exit etc. something like that

rajadilipkolli · Answer 3 · 2016-12-07T07:38:56.420

  try {
            executor = Executors.newSingleThreadExecutor();
            future = executor.submit(task);
            executor.shutdown();
            executor.awaitTermination(5, TimeUnit.SECONDS);
        }
        finally {
            if (Objects.nonNull(executor) && !executor.isTerminated()) {
                LOGGER.error("cancelling all non-finished tasks");
            }
            if (Objects.nonNull(executor)) {
                executor.shutdownNow();
                LOGGER.info("shutdown finished");
            }
        }

This way you shutdown executor and waiting for 5 seconds to complete all tasks and then finally calling executor.shutdownNow() to completely kill the executor.

This is the best way to shutdown executor.

Properly shut down ThreadPoolExecutors

3 Answers3