public static QueryObserver setInstance(QueryObserver observer) {
    Support.assertArg(observer != null, "setInstance expects a non-null argument!");
    QueryObserver oldObserver = _instance;
    _instance = observer;
    return oldObserver;
  }
Set the given observer to be notified of query events. Returns the current observer.
geode-core/src/main/java/org/apache/geode/cache/query/internal/QueryObserverHolder.java