public void stop() {
    synchronized (startStopLock) {
      if (!started) {
        throw new IllegalStateException("calling `stop` without first calling `start`.");
      }
      if (stopped) {
        return;
      }
      pool.shutdownNow(); // shutdownNow to interrupt the worker thread sitting on `take()`
      stopped = true;
    }
  }
This class encapsulates the details of the {@link RegionNormalizer} subsystem.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/normalizer/RegionNormalizerManager.java