Volatile Keyworld

[Salil Vverma]

As per my understanding, we use volatile keyword to make sure that all the threads access the value of that specific instance variable from main memory rather than local cache. In this case, value updated by one thread would be visible to all other threads as well. But I could not see this happening in the below example.

   
class ExampleThread extends Thread {
	private volatile int testValue;

	public ExampleThread(String str) {
		super(str);
	}

	@Override
	public void run() {
		for (int i = 0; i < 3; i++) {
			try {
				System.out.println(getName() + " : " + i);
				if (getName().equals("Thread 1 ")) {
					testValue = 10;
				}

				if (getName().equals("Thread 2 ")) {
					System.out.println("Test Value : " + testValue);
				}
				Thread.sleep(1000);
			} catch (InterruptedException exception) {
				exception.printStackTrace();
			}
		}
	}
}

public class VolatileExample {
	public static void main(String args[]) {
		new ExampleThread("Thread 1 ").start();
		new ExampleThread("Thread 2 ").start();
	}
}

Each time, it prints Test Value : 0 while, as per my understanding, it should printTest Value : 10. Can someone please let me know where I have gap in my understanding.

[Ankit Garg]

There are two different instances of ExampleThread class. And the field testValue is an instance variable. So both the threads will have a different copy of the testValue field. Make testValue static and then see...

[Salil Vverma]

Hey Ankit,

If I declare testValue as static member, it returns the Test Value : 10 irrespective of testValue being volatile or non volatile. Could you please explain the reason behind it ?

I am mainly trying to make an example where I could see the impact of volatile keyword.

[Ankit Garg]

Look at a tutorial like this to understand the actual use of volatile and this tutorial explains when you might use volatile keyword...

[Raghavan Muthu]

Thats a good link Ankit. Thanks!

[Siva Masilamani]

I thnk volatile has no effect on static variable.

As far as i know volatile and transient modifiers applies to instance variable not to static variable.