import org.apache.hadoop.yarn.server.applicationhistoryservice.NullApplicationHistoryStore; //导入依赖的package包/类
@Override
protected synchronized void serviceInit(Configuration conf) throws Exception {
  historyServiceEnabled =
      conf.getBoolean(YarnConfiguration.APPLICATION_HISTORY_ENABLED,
        YarnConfiguration.DEFAULT_APPLICATION_HISTORY_ENABLED);
  if (conf.get(YarnConfiguration.APPLICATION_HISTORY_STORE) == null ||
      conf.get(YarnConfiguration.APPLICATION_HISTORY_STORE).length() == 0 ||
      conf.get(YarnConfiguration.APPLICATION_HISTORY_STORE).equals(
          NullApplicationHistoryStore.class.getName())) {
    historyServiceEnabled = false;
  }

  // Only create the services when the history service is enabled and not
  // using the null store, preventing wasting the system resources.
  if (historyServiceEnabled) {
    writer = createApplicationHistoryStore(conf);
    addIfService(writer);

    dispatcher = createDispatcher(conf);
    dispatcher.register(WritingHistoryEventType.class,
      new ForwardingEventHandler());
    addIfService(dispatcher);
  }
  super.serviceInit(conf);
}
 

import org.apache.hadoop.yarn.server.applicationhistoryservice.NullApplicationHistoryStore; //导入依赖的package包/类
protected ApplicationHistoryStore createApplicationHistoryStore(
    Configuration conf) {
  try {
    Class<? extends ApplicationHistoryStore> storeClass =
        conf.getClass(YarnConfiguration.APPLICATION_HISTORY_STORE,
            NullApplicationHistoryStore.class,
            ApplicationHistoryStore.class);
    return storeClass.newInstance();
  } catch (Exception e) {
    String msg =
        "Could not instantiate ApplicationHistoryWriter: "
            + conf.get(YarnConfiguration.APPLICATION_HISTORY_STORE,
                NullApplicationHistoryStore.class.getName());
    LOG.error(msg, e);
    throw new YarnRuntimeException(msg, e);
  }
}
 

import org.apache.hadoop.yarn.server.applicationhistoryservice.NullApplicationHistoryStore; //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();
  }
}