import org.apache.hadoop.yarn.server.applicationhistoryservice.FileSystemApplicationHistoryStore; //导入依赖的package包/类
protected ApplicationHistoryStore createApplicationHistoryStore(
    Configuration conf) {
  // If the history writer is not enabled, a dummy store will be used to
  // write nothing
  if (historyServiceEnabled) {
    try {
      Class<? extends ApplicationHistoryStore> storeClass =
          conf.getClass(YarnConfiguration.APPLICATION_HISTORY_STORE,
            FileSystemApplicationHistoryStore.class,
            ApplicationHistoryStore.class);
      return storeClass.newInstance();
    } catch (Exception e) {
      String msg =
          "Could not instantiate ApplicationHistoryWriter: "
              + conf.get(YarnConfiguration.APPLICATION_HISTORY_STORE,
                FileSystemApplicationHistoryStore.class.getName());
      LOG.error(msg, e);
      throw new YarnRuntimeException(msg, e);
    }
  } else {
    return new NullApplicationHistoryStore();
  }
}