getter上的volatile和setter上的同步与两者上的同步相同吗？

Question

In a concurrent program, is this safe:

private volatile int i;

public int getI() {
    return i;
}

public synchronized incrementI() {
    i++;
}

?

From what I know about synchronize , the changes are guaranteed to be available on i only for threads that acquires the lock monitor on the same object. So I think, the code above is not safe. Please just confirm this if true, otherwise, please explain. Thanks.

Answer 1

This code is thread safe. Changes inside a synchronized method are passed on to all threads. And as you have made it volatile, all threads will not cache it, so will get latest copy.

Also this thread will be helpful Java volatile modifier and synchronized blocks

Answer 2

Almost the exact example is given in "JMM Pragmatics" here .

volatile provides the happens-before edges to make the changes before the setter visible to all things that happen after the getter, assuming the getter actually observed the set value. synchronized in setter additionally gives the mutual exclusion, that volatile does not guarantee alone.

Answer 3

Since Integer i is the private instance variable of the class the synchronise instance method will work. You need to make sure all methods you add to class which modifies the i must synchronise using same object monitor.

Please note i++ is not automic expression so only volatile will not work.

You can use AtomicInteger for single value like this.