JavaMail : Keeping IMAPFolder. 闲置()还活着

Question

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。

Answer 1

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 commandrunning, and if such a server has an inactivity timeout it MAY logthe client off implicitly at the end of its timeout period. Becauseof that, clients using IDLE are advised to terminate the IDLE andre-issue it at least every 29 minutes to avoid being logged off.This still allows a client to receive immediate mailbox updates eventhough 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);
            }
        }
    }
}