public void start() {
    synchronized (startStopLock) {
      if (started) {
        return;
      }
      if (worker != null) {
        // worker will be null when master is in maintenance mode.
        pool.submit(worker);
      }
      started = true;
    }
  }
This class encapsulates the details of the {@link RegionNormalizer} subsystem.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/normalizer/RegionNormalizerManager.java