The following code:

  public static final Foo {
    private static final Foo INSTANCE = new Foo();

    private Foo() {
        // hidden on purpose
    }

    public static Foo instance() {
        return INSTANCE
    }
 }

triggers a SING_SINGLETON_GETTER_NOT_SYNCHRONIZED (at https://github.com/opendaylight/yangtools/blob/master/common/yang-common/src/main/java/org/opendaylight/yangtools/yang/common/Empty.java#L40 in my case).

I believe this is a false positive: INSTANCE is guaranteed to be initialized before a call to Foo.instance() can be made. It is therefore superfluous to require synchronized() on instance() -- and in fact doing so creates a synchronization hotspot, leading to abysmal scalability.