可以使用volatile字段修饰符替换同步读取吗？

Question

While reading the accepted answer How to synchronize or lock upon variables in Java? , it occurred to me if the behavior would change if one was to use volatile in the below example instead of synchronized block. I would like to make sure message always return the correct value.

Let me use this same sample: Note now I've removed the synchronized block in the get method and marked the variable message as volatile.

class Sample {
    private volatile String message = null;
    private final Object lock = new Object();

    public void newMessage(String x) {
        synchronized (lock) {
            message = x;
        }
    }

    public String getMessage() {
            return message;
        }
    }
}

Would it warrant the same behavior if one was to change the code as shown above ?. Are there any differences between two approaches ?

Thank you in advance

Answer 1

The volatile keyword in this case is enough because it establishes a happens-before relation for the reads and writes, guaranteeing that the write in one thread will be observed by a subsequent read in another thread.

The synchronized block is not incorrect but it is redundant. Also, if you do use synchronized , you can mark your message variable as non-volatile. But be careful and make sure the read is also in a synchronized block, otherwise you have a data race. Non-volatile writes made by one thread in a synchronized block are guaranteed to be visible for threads subsequently entering a synchronized block on the same monitor.

As a final note, the synchronized block has different progress conditions than volatile reads and writes. The former is blocking (mutual exclusion) while the latter is wait-free.