Are primitive datatypes thread-safe in Java

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 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.

Answer 2

Primitive types are not thread safe. Check this tutorial.