public static DistributionMessageObserver setInstance(DistributionMessageObserver instance) {
    DistributionMessageObserver oldInstance = DistributionMessageObserver.instance;
    DistributionMessageObserver.instance = instance;
    return oldInstance;
  }
Set the instance of the observer. Setting to null will clear the observer.

@return the old observer, or null if there was no old observer.
geode-core/src/main/java/org/apache/geode/distributed/internal/DistributionMessageObserver.java