如何确保只启动一个线程

时间:2013-11-05 18:01:08

标签: java multithreading countdownlatch

情况是,我必须确保当我尝试getConnection时只创建一个RecoveryThread,如果它在PrimaryData Source上的getConnection失败,那么我的代码是:

public Connection getConnection() throws SQLException {
        if (isFailedOver()) {
            try {
                return failoverDataSource.getConnection();
            } catch (SQLException e) {
                throwBigError();
            }
        }
        Connection connection = null;
        try {
            connection = dataSource.getConnection();
            return connection;
        }
        catch (SQLException unexpected) {
            return requestFailover();
        }
    }

    private Connection requestFailover() throws SQLException {
        this.dbFailoverMutex.requestFailover();
        DBFailoverRecoveryService recoveryService = new DBFailoverRecoveryService(this.dbFailoverMutex,this.dataSource);
        Thread recoveryServiceThread = new Thread(recoveryService, "DBFailover Recovery Service");
        recoveryServiceThread.start();
        try {
            return failoverDataSource.getConnection();
        } catch (SQLException e) {
            throwBigError();
        }
        return null;
    }

如果有两个不同的线程尝试getConnection,这可能最终调用requestFailover()方法两次,当它被调用两次时,这将最终创建两个recoveryService线程,我该怎么做以确保永远不会发生?

提前感谢您的帮助。

2 个答案:

答案 0 :(得分:5)

  

我该怎么做以确保永远不会发生?

要考虑的一件事是切换到使用Executors.newSingleThreadExecutor(),它只会分叉一个线程来运行。然后,您可以根据需要提交任意数量的任务,而无需担心它们重叠。

private final ExecutorService threadPool =
      Executors.newSingleThreadExecutor(/* pass in ThreadFactory to set name */);
...
DBFailoverRecoveryService recoveryService =
       new DBFailoverRecoveryService(this.dbFailoverMutex, this.dataSource);
threadPool.submit(recoveryService);

ExecutorService一样,您需要在将最后一项任务提交到池后调用threadPool.shutdown(),否则会挂起您的应用程序。您可以添加Datasource.destroy();方法来执行此操作。

答案 1 :(得分:0)

您可以将“synchronized”修饰符添加到getConnection方法,然后添加一个布尔标志来指示是否已经请求了故障转移,例如

public synchronized getConnection(){ throws SQLException
  if(alreadyRequestedFailover){
    return;
  }
  ....
  catch (SQLException unexpected) {
     alreadyRequestedFailover = true;
     return requestFailover();
  }
}

这将阻止两个线程同时进入getConnection()方法,并确保如果线程请求故障转移,它将在允许另一个线程进入getConnection()之前更新alreadyRequestedFailover标志。