How do I print summary message after all threads (in a pool) complete? [duplicate]

Question

I have a command line application. It runs a loop say 100 times and in the loop schedules a task using a thread. I am using ExecutorService so there are 4 threads running at any time.

After the loop ends, I want to print a summary message. E.g. time taken to complete all 100 tasks. When I stepped through the code the debugger went straight to the summary part, but the other tasks are still running. I understand this is because each thread runs on its own. So how do I print messages only after all threads complete?

ExecutorService exec = Executors.newFixedThreadPool(4); long startTime = System.currentTimeMillis(); for (int i = 0; i < 100; i++) { Runnable requestHandler = new Runnable() { @Override public void run() { try { // call task function in here } catch (Exception ex) { } } }; exec.execute(requestHandler); } exec.shutdown(); long endTime = System.currentTimeMillis(); LOGGER.info("******************SUMMARY******************"); LOGGER.info("Time taken : " + ((endTime - startTime)/1000) + " seconds, " + ((endTime - startTime)/1000/60) + " minutes");

After some more googling. I ended up using CountDownLatch. An example is here javamex.com/tutorials/threads/CountDownLatch.shtml — user373201
– user373201, Commented Nov 17, 2011 at 14:55

Dragos · Accepted Answer · 2011-11-17 14:44:41Z

From the main-thread, you could create another thread that does everything from declaring exec to exec.shutdown();. After creating this thread, you put the main-thread to wait. At the end of the new thread's actions(after exec.shutdown();) you should notify it.

Prashant Bhate · Accepted Answer · 2011-11-17 14:39:19Z

See http://download.oracle.com/javase/6/docs/api/java/util/concurrent/ExecutorService.html example copied for brevity

 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(); } }

pushy · Accepted Answer · 2011-11-17 14:37:38Z

Basically you need to wait until the ExecutorService isTerminated() method returns true. You can use awaitTermination() to that end.

Vijay Palaniappan · Accepted Answer · 2017-10-12 09:57:58Z

The solution for you based on your code:

ExecutorService exec = Executors.newFixedThreadPool(4); long start = System.currentTimeMillis(); //Your code exec.shutdown(); while(true) { if(exec.isTerminated()) { long end = System.currentTimeMillis(); System.out.println("Time : " + (end - start)); break; }

Check this out! It works!

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