void handle(final String path) {
    if (path.equals(watcher.getZNodePaths().masterAddressZNode) && !master.isStopped()) {
      handleMasterNodeChange();
    }
  }
@param watcher ZK watcher
@param sn      ServerName
@param master  In an instance of a Master.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/ActiveMasterManager.java