Question

Are the primitive data types like int & short thread-safe in Java? I have executed the following code and couldn’t see expected result 500 some times.

public class SampleThree extends Thread
{
    static long wakeUpTime = System.currentTimeMillis() + (1000*20);
    static int inT;
    public static void main(String args[])
    {
        System.out.println("initial:" + inT);
        for(int i=0; i<500; i++)
            new SampleThree().start();
        try {
            Thread.sleep(wakeUpTime - System.currentTimeMillis() + (1000*30));
            System.out.println("o/p:" + inT);
        }
        catch(Exception e){
            e.printStackTrace();
        }
    }

    public void run()
    {
        try {
            long s = wakeUpTime - System.currentTimeMillis();
            System.out.println("will sleep ms: " + s);
            Thread.sleep(s);
            inT++; // System.out.println(inT);
        }
        catch(Exception e) {
            e.printStackTrace();
        }
    }
}

Here concurrently 500 thread will update the int variable inT. Main thread after waiting for concurrent update to be completed, prints inT value.

Find similar example here

Answer 1

There are three ways in which they’re not safe:

• long and double aren’t even guaranteed to be updated atomically (you could see half of a write from a different thread)
• The Java Memory Model doesn’t guarantee that you’ll see the latest updates from one thread in another thread, without extra memory barriers of some kind
• The act of incrementing a variable isn’t atomic anyway

Use AtomicInteger etc for thread-safe operations.