volatile test did not meet its happen-before feature in JLS - volatile

Question

Run main
public class ThreadTest {
volatile int p = 0, q = 0;
public void test() throws InterruptedException {
Thread writeThread = new Thread(){
public void run(){
while (!isInterrupted()) {
p++;
q++;
}
}
};
Thread readThread = new Thread(){
public void run(){
while (!isInterrupted()) {
//p should not be less than q
if(p<q){
System.out.println("happen before violation p = " + p + ";q = " + q);
}
}
}
};
writeThread.start();
readThread.start();
Thread.currentThread().sleep(2);
writeThread.interrupt();
readThread.interrupt();
}
public static void main(String[] args) throws InterruptedException {
new ThreadTest().test();
}
}
output
happen before violation p = 736;q = 827
happen before violation p = 4635;q = 4657
happen before violation p = 6421;q = 6440
happen before violation p = 8719;q = 8803

Answer 1

You should read p AFTER q for being sure that their values are in "not less than" order:
int q_value = q; // Read 'q' once
int p_value = p; // Read 'p' AFTER q
if(p_value < q_value) // ... happens-before violation
In that case volatile modifier is required only for q, and it can be omitted for p without constraints' violation.
In you original code nothing prevents the compiler from reading p BEFORE q. So readThread can see p value from 736-th iteration of the writeThread, but see q value from 827-th iteration.
Or readThread can see p value from the beginning of the 100-th iteration(that is, p=100), but see q value from the end of that iteration(that is, q=101).