[java] NonThreadSafeSingleton false positive with double-checked locking

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Affects PMD Version: 6.x

Rule: NonThreadSafeSingleton

Description:

The rule reports a violation even for double checked locking, that is implemented correctly.

Code Sample demonstrating the issue:

public class Foo {
    private static volatile Object o;
    public static Object getO() {
        if (o == null) {   // violation
            synchronized(Foo.class) {
                if (o == null) {
                    o = new Object();
                }
            }
        }
        return o;
    }
}

Expected outcome: No violation

Note: This has already been fixed for PMD 7.0.0-rc1 via #3474. This issue is just there for documentation.