Question

This code is used to demonstrate double-checked-locking anti-pattern:

@NotThreadSafe
public class DoubleCheckedLocking {
    private static Resource resource;

    public static Resource getInstance() {
        if (resource == null) {
            synchronized (DoubleCheckedLocking.class) {
                if (resource == null)
                    resource = new Resource();
            }
        }
        return resource;
    }
}

Can I just avoid this problem by modify it to:

@NotThreadSafe
public class DoubleCheckedLocking {
    private static Resource resource;

    public static Resource getInstance() {
        if (resource == null) {
            synchronized (DoubleCheckedLocking.class) {
                if (resource == null){
                    Resource r=new Resource();
                    resource = r;
                }
            }
        }
        return resource;
    }
}

As far as I know,

Resource r=new Resource();
resource = r;

That compiler should provide happen-before relationship for that.

Answer 1

As far as I know, the only known implementation of the double-checked locking pattern that works (for JDK5 and above) makes use of the ‘volatile’ keyword. See Fixing Double-Checked Locking using Volatile