@Override
  public synchronized MetricsUserAggregateSourceImpl getUserAggregate() {
    synchronized (FactoryStorage.INSTANCE.aggLock) {
      if (FactoryStorage.INSTANCE.userAggImpl == null) {
        FactoryStorage.INSTANCE.userAggImpl = new MetricsUserAggregateSourceImpl();
      }
      return FactoryStorage.INSTANCE.userAggImpl;
    }
  }
Factory to create MetricsRegionServerSource when given a MetricsRegionServerWrapper
hbase-hadoop-compat/src/main/java/org/apache/hadoop/hbase/regionserver/MetricsRegionServerSourceFactoryImpl.java