同步作为易失性声明的替代方法

Question

In this tutorial ( link ) on Java's volatile declaration it is said that the volatile declaration can be an alternative to synchronization.

The author's example of a use of the volatile declaration:

volatile boolean shutdownRequested;

...

public void shutdown() { shutdownRequested = true; }

public void doWork() { 
    while (!shutdownRequested) { 
        // do stuff
    }
}

My naive implementation of synchronization would be:

volatile boolean shutdownRequested;

...

synchronized public void shutdown() { shutdownRequested = true; }

synchronized public void doWork() { 
    while (!shutdownRequested) { 
        // do stuff
    }
}

It seems that if thread A invokes shutdown while thread B has already invoked and is running doWork it would seem that thread B will continually lock execution, because it is a loop. This seems to give no opportunity for thread A to access the boolean. What did the author have in mind as the synchronized alternative to a volatile declaration?

Answer 1

The // do stuff part would be outside the synchronized block. For example, instead of doWork being synchronized and having while (!shutdownRequested) , you might have while (!getShutDown()) where getShutDown is the synchronized method that returns shutdownRequested .

Answer 2

Here's one way it could work:

public void doWork() { 
    while (true) {
        synchronized (this) {
            if (shutdownRequested) {
                break;
            }
        }
        // do stuff
    }
}