Processing with Threadpool and executor service

Question

I have an ArrayList of 10 elements. I initiate a threadpool of size 10 and call execute with an element passed into each thread. Each thread does some processing with that element as input and outputs the result. The problem is, the output sometimes has processing results of only 7 elements sometimes and 8 sometimes with a few duplicates and 9 sometimes. I am not sure why I do not have the processing results of exactly 10 elements. Here's my code snippet.

ExecutorService exeSvc = 
                Executors.newFixedThreadPool(10)
for (Object element: arlList)//arlList is the arraylist of 
                                                           size-10
{
   exeSvc.execute({->myRunnable element});
}

What am I doing wrong?

Answer 1

groovy...

import java.util.concurrent.ExecutorService
import java.util.concurrent.Executors
import java.util.concurrent.TimeUnit

ExecutorService exeSvc = Executors.newFixedThreadPool(5)
for (int element=0;element<9;element++) {
    int elementCopy = element
    exeSvc.execute({->
        Thread.sleep(567); 
        println "${Thread.currentThread()}  element = $element elementCopy = $elementCopy"; 
    });
}
println "All Started"
exeSvc.shutdown()
exeSvc.awaitTermination(10, TimeUnit.SECONDS)
println "All Finished"

outputs

All Started
Thread[pool-12-thread-1,5,main]  element = 9 elementCopy = 0
Thread[pool-12-thread-2,5,main]  element = 9 elementCopy = 1
Thread[pool-12-thread-3,5,main]  element = 9 elementCopy = 2
Thread[pool-12-thread-4,5,main]  element = 9 elementCopy = 3
Thread[pool-12-thread-5,5,main]  element = 9 elementCopy = 4
Thread[pool-12-thread-1,5,main]  element = 9 elementCopy = 5
Thread[pool-12-thread-2,5,main]  element = 9 elementCopy = 6
Thread[pool-12-thread-3,5,main]  element = 9 elementCopy = 7
Thread[pool-12-thread-4,5,main]  element = 9 elementCopy = 8
All Finished

as you can see in my case the for loop finished before threads started and all of them has element value 9 and elementCopy is different

Answer 2

That's because main thread finishes erlier than ExecutorService .

To force Main thread to wait for ExecutorService to finish processing, use this:

exeSvc.awaitTermination(5, TimeUnit.SECONDS); //this will 5 second to finish all tasks
exeSvc.shutdown();

Answer 3

Execution time for each element is not the same. So, at the end of process, shutting down thread is required. You can use,

if ( check_whether_operation_is_done ) {
    Logger.info( "Process shutdown" );
    exeSvc.shutdownNow();
}