我正在制作一个需要监视 Gmail 帐户是否有新邮件的程序，为了尽快收到它们，我正在使用 JavaMail 的闲置功能。这是我用来调用 folder.idle() 的线程中的代码片段:

//Run method that waits for idle input. If an exception occurs, end the thread's life.
public void run() {

    IMAPFolder folder = null;

            try {
                folder = getFolder();
                while(true)
                {
                  //If connection has been lost, attempt to restore it
                  if (!folder.isOpen())
                      folder = getFolder();
                  //Wait until something happens in inbox
                  folder.idle(true);
                  //Notify controller of event
                  cont.inboxEventOccured();
                }
            }
            catch (Exception ex) {
                ex.printStackTrace();
            }
             System.out.println("MailIdleWaiter thread ending.");
}

getFolder() 方法主要是打开与 IMAP 服务器的连接并打开收件箱。

这工作了一段时间，但在 10 分钟左右后它停止获取更新(没有抛出异常)。

我正在寻找保持连接有效的建议。我是否需要第二个线程，其唯一作用是每 10 分钟 hibernate 并更新 idle() 线程，还是有更简单/更好的方法？

提前致谢。

最佳答案

一个常见的错误是假设 IDLE 命令会无限期地发布更新。然而，RFC 2177 ，它定义了 IDLE 扩展状态:

The server MAY consider a client inactive if it has an IDLE command running, and if such a server has an inactivity timeout it MAY log the client off implicitly at the end of its timeout period. Because of that, clients using IDLE are advised to terminate the IDLE and re-issue it at least every 29 minutes to avoid being logged off. This still allows a client to receive immediate mailbox updates even though it need only "poll" at half hour intervals.

特别是 GMail，超时时间要短得多，如您所说，大约 10 分钟。

我们只需要每 9 分钟左右重新发出一次 IDLE 命令，它就可以工作了。 javax.mail API 无法为 IDLE 命令设置超时，因此您需要第二个线程来解决这个问题。

第一种方法是让第二个线程中断第一个线程，处理异常并忽略它。然而，这将不允许以干净的方式关闭线程，所以我不会推荐它。一种更简洁的方法是让第二个线程向服务器发出 NOOP 命令。这根本不执行任何操作，但足以让 IDLE 中止并重新发布。

我在这里提供了一些代码来做到这一点:

public void startListening(IMAPFolder imapFolder) {
    // We need to create a new thread to keep alive the connection
    Thread t = new Thread(
        new KeepAliveRunnable(imapFolder), "IdleConnectionKeepAlive"
    );

    t.start();
    
    while (!Thread.interrupted()) {
        LOGGER.debug("Starting IDLE");
        try {
            imapFolder.idle();
        } catch (MessagingException e) {
            LOGGER.warn("Messaging exception during IDLE", e);
            throw new RuntimeException(e);
        }
    }
    
    // Shutdown keep alive thread
    if (t.isAlive()) {
        t.interrupt();
    }
}

/**
 * Runnable used to keep alive the connection to the IMAP server
 * 
 * @author Juan Martín Sotuyo Dodero <jmsotuyo@monits.com>
 */
private static class KeepAliveRunnable implements Runnable {

    private static final long KEEP_ALIVE_FREQ = 300000; // 5 minutes

    private IMAPFolder folder;

    public KeepAliveRunnable(IMAPFolder folder) {
        this.folder = folder;
    }

    @Override
    public void run() {
        while (!Thread.interrupted()) {
            try {
                Thread.sleep(KEEP_ALIVE_FREQ);
                
                // Perform a NOOP just to keep alive the connection
                LOGGER.debug("Performing a NOOP to keep alvie the connection");
                folder.doCommand(new IMAPFolder.ProtocolCommand() {
                    public Object doCommand(IMAPProtocol p)
                            throws ProtocolException {
                        p.simpleCommand("NOOP", null);
                        return null;
                    }
                });
            } catch (InterruptedException e) {
                // Ignore, just aborting the thread...
            } catch (MessagingException e) {
                // Shouldn't really happen...
                LOGGER.warn("Unexpected exception while keeping alive the IDLE connection", e);
            }
        }
    }
}

关于JavaMail : Keeping IMAPFolder. 闲置()还活着，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/4155412/