Is there any point in using a volatile long?

Question

I occasionally use a volatile instance variable in cases where I have two threads reading from / writing to it and don\'t want the overhead (or potential deadlo

Answer 1

This can be demonstrated by example

• constantly toggle two fields, one marked volatile and one not between all bits set and all bits clear
• read the field values on another thread
• see that the foo field (not protected with volatile) can be read in an inconsistent state, this never happens to the bar field protected with volatile

Code

public class VolatileTest {
    private long foo;
    private volatile long bar;
    private static final long A = 0xffffffffffffffffl;
    private static final long B = 0;
    private int clock;
    public VolatileTest() {
        new Thread(new Runnable() {
            @Override
            public void run() {
                while (true) {
                    foo = clock % 2 == 0 ? A : B;
                    bar = clock % 2 == 0 ? A : B;
                    clock++;
                }
            }

        }).start();
        while (true) {
            long fooRead = foo;
            if (fooRead != A && fooRead != B) {
                System.err.println("foo incomplete write " + Long.toHexString(fooRead));
            }
            long barRead = bar;
            if (barRead != A && barRead != B) {
                System.err.println("bar incomplete write " + Long.toHexString(barRead));
            }
        }
    }

    public static void main(String[] args) {
        new VolatileTest();
    }
}

Output

foo incomplete write ffffffff00000000
foo incomplete write ffffffff00000000
foo incomplete write ffffffff
foo incomplete write ffffffff00000000

Note this only happens for me when running on a 32 bit VM, on 64 bit VM I couldn't get a single error in several minutes.