ExecutorService没有做好工作？

Question

This main giving an ExecutorService 1000 runnables (Testers) that all they do is sleep 10 millis and then add 1 to a static counter, the main was suppose to wait untill ALL executions are FINISHED, but yet the counter gets up to something around the 970 executions... why?

public class Testit {
    public static void main (String arg[]) {
        int n=1000;
        ExecutorService e1 =  Executors.newFixedThreadPool(20);
        for (int i=0 ;i <n ;i++) {
            e1.execute(new Tester());
        }
        e1.shutdown();
        try {
            e1.awaitTermination(1, TimeUnit.DAYS);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("Executed "+Tester.tester()+" Tasks.");
    }
}

and Tester Class:

public class Tester implements Runnable {
    public static long tester=0;
    @Override
    public void run() {
        try {
            Thread.sleep(10);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        finally { tester++; }
    }
    public static long tester() {
        long temp=tester;
        tester=0;
        return temp;
    }
}

EDIT

problem solved by:

finally { synchronized (lock) {tester++;} } 

Thanks JB Nizet!

Answer 1

Because you don't synchronize the access to the counter, and since writing a long is not atomic, and ++ isn't either, two concurrent threads incrmenting the counter could lead to a completely inconsistent results, or to only one increment instead of 2.

Use an AtomicLong instead, and call incrementAndGet() on this AtomicLong .