Thread code not work properly without Thread.sleep(...) call

Question

I have the following code:

public class ThreadTest implements Runnable {
    public int ThrCount    = 0;

    public void ThrCountIncr() {
        while (true) {
            ThrCount++;
            System.out.println(ThrCount);
            try {
                Thread.currentThread().sleep(500);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }

    public void run() {
        while (true) {
            if (ThrCount > 10) {
                System.out.println(ThrCount + "\n Thread finished");
                System.exit(1);
            }
            try {
                Thread.currentThread().sleep(100);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }
}

But when I remove this line from run, it stops working:

Thread.currentThread().sleep(100);

First, I start the thread, then I use ThrCountIncr.

ThreadTest Thrtest = new ThreadTest();
Thread thr = new Thread(Thrtest);
thr.start();
Thrtest.ThrCountIncr();

The thread checks the ThrCount variable value, and if it is greater than 10, it stops the program. Without sleep(100), the thread doesn't stop the program, and I think it doesn't check the variable value. Why does the call to sleep make this code work?

Answer 1

Even with the Thread.sleep() it might not work. This is because you don't properly synchronize accesses to the shared ThrCount variable.

If you make that variable volatile, you should not see any issues any longer. However it might not loop exactly 10 times as the ++ operation is not atomic.

Ideally, you should use an AtomicInteger and use its incrementAndGet() method.

Also note:

• Java naming conventions: variables and method names should start in lower case (thrCount, thrCountIncr()
• sleep is a static method so you can simply call Thread.sleep(...); and it will sleep in the current thread.

Answer 2

If your thread doesn't sleep, other threads may not be able to work, so the the loop inside ThrCountIncr may get stuck at any time (probably at its first sleep or at the println).

Never have a thread looping without any kind of sleep or wait.

Note also that ThrCount++; may fail if you don't protect it using a synchronization, as it's not an atomic operation.