@RestrictedApi(explanation = "Should only be called in tests", link = "",
      allowedOnPath = ".*/src/test/.*")
  public boolean isShutdownHookInstalled() {
    return isShutdownHookInstalled;
  }
In order to register ShutdownHook, this method is called when HMaster and HRegionServer are
started. For details, please refer to HBASE-26951
hbase-server/src/main/java/org/apache/hadoop/hbase/HBaseServerBase.java