Real world example of Memory Consistency Errors in multi-threading?

xingbin :

In the tutorial of java multi-threading, it gives an exmaple of Memory Consistency Errors. But I can not reproduce it. Is there any other method to simulate Memory Consistency Errors?

The example provided in the tutorial:

Suppose a simple int field is defined and initialized:

int counter = 0;

The counter field is shared between two threads, A and B. Suppose thread A increments counter:

counter++;

Then, shortly afterwards, thread B prints out counter:

System.out.println(counter);

If the two statements had been executed in the same thread, it would be safe to assume that the value printed out would be "1". But if the two statements are executed in separate threads, the value printed out might well be "0", because there's no guarantee that thread A's change to counter will be visible to thread B — unless the programmer has established a happens-before relationship between these two statements.

John Vint :

I answered a question a while ago about a bug in Java 5. Why doesn't volatile in java 5+ ensure visibility from another thread?

Given this piece of code:

public class Test {
    volatile static private int a;
    static private int b;

    public static void main(String [] args) throws Exception {
        for (int i = 0; i < 100; i++) {
            new Thread() {

                @Override
                public void run() {
                    int tt = b; // makes the jvm cache the value of b

                    while (a==0) {

                    }

                    if (b == 0) {
                        System.out.println("error");
                    }
                }

            }.start();
        }

        b = 1;
        a = 1;
    }
}

The volatile store of a happens after the normal store of b. So when the thread runs and sees a != 0, because of the rules defined in the JMM, we must see b == 1.

The bug in the JRE allowed the thread to make it to the error line and was subsequently resolved. This definitely would fail if you don't have a defined as volatile.