@Override
  public int getNumDrainingRegionServers() {
    ServerManager serverManager = this.master.getServerManager();
    if (serverManager == null) {
      return 0;
    }
    return serverManager.getDrainingServersList().size();
  }
Impl for exposing HMaster Information through JMX
hbase-server/src/main/java/org/apache/hadoop/hbase/master/MetricsMasterWrapperImpl.java