CopyOnWriteArraySet<HandleListener> copyListeners() {
        return new CopyOnWriteArraySet<>(handleListeners);
    }
Returns the collection of {@link HandleListener} objects. This set is immutable.

@return A set of {@link HandleListener} objects. The set is never null, can be empty and is immutable.
core/src/main/java/org/jdbi/v3/core/Handles.java