private void shutDownSyncRunners() {
    SyncRunner[] syncRunnersToUse = this.syncRunners;
    if (syncRunnersToUse != null) {
      for (SyncRunner syncRunner : syncRunnersToUse) {
        syncRunner.shutDown();
      }
    }
    this.syncRunners = null;
  }
This method allows subclasses to inject different writers without having to extend other
methods like rollWriter().
@return Writer instance
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/FSHLog.java