使用AtomicInteger进行多线程处理不起作用

Question

public class SynchroExample {

   public final AtomicInteger integer = new AtomicInteger(0);

   private ExecutorService service = Executors.newFixedThreadPool(100);

   public void syn() {
      for (int i = 0; i < 1000; i++) {
          service.submit(() -> {
              integer.incrementAndGet();
          });
       }
       service.shutdown();
       System.out.println(integer.get());
   }

   public static void main(String [] args) {
      SynchroExample ex = new SynchroExample();
       ex.syn();
    }
}

Can someone please explain why this code doesn't work? I was under the impression that AtomicInteger is threadsafe. And this code is not returning 1000.

Answer 1

AtomicInteger is thread safe, but you have called AtomicInteger#get before all tasks finished. ExecutorService#shutdown is not waiting for tasks to finish.

See ExecutorService#shutdown docs :

This method does not wait for previously submitted tasks to complete execution. Use awaitTermination to do that.

Use

service.awaitTermination(10, TimeUnit.SECONDS)

to wait for all tasks finished.