public void registerObserver(ConfigurationObserver observer) {
    synchronized (configurationObservers) {
      configurationObservers.add(observer);
      if (observer instanceof PropagatingConfigurationObserver) {
        ((PropagatingConfigurationObserver) observer).registerChildren(this);
      }
    }
  }
Register an observer class
@param observer observer to be registered.
hbase-common/src/main/java/org/apache/hadoop/hbase/conf/ConfigurationManager.java