Question

我正在制作一个需要监控Gmail帐户以查找新邮件的程序，为了尽快获取它们，我正在使用JavaMail的空闲功能。这是我用来调用folder.idle（）的线程的代码片段：

//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（）方法基本上打开与IMAP服务器的连接并打开收件箱。

这种方法有一段时间了，但是大约10分钟后它就会停止更新（不会抛出任何异常）。

我正在寻找保持连接活着的建议。我是否需要第二个线程，其唯一的作用是每10分钟睡眠并更新idle（）线程，还是有更容易/更好的方法？

提前致谢。

Answer 1

一个常见的错误是假设IDLE命令将无限期地发布更新。但是，定义IDLE扩展名的RFC 2177表示：

如果客户端具有IDLE命令，则可以认为客户端处于非活动状态运行，如果这样的服务器有一个不活动超时，它可能会记录客户端在超时期限结束时隐式关闭。因为其中，建议使用IDLE的客户端终止IDLE和至少每29分钟重新发一次，以免被注销。这仍然允许客户端甚至接收即时邮箱更新虽然它只需要半小时的“轮询”。

特别是GMail，如你所说，大约10分钟就会超时。

我们只需每9分钟左右重新发出一次IDLE命令即可运行。 javax.mail API无法为IDLE命令设置超时，因此您需要第二个线程来移动它。

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，这将允许没有干净的方式来关闭线程，所以我不会推荐它。更简洁的方法是让第二个线程向服务器发出NOOP命令。这根本不做什么，但足以让IDLE中止并重新发布。

我在这里提供了一些代码来执行此操作：

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);
            }
        }
    }
}

Answer 2

@ user888307的建议是一个肮脏的黑客，最终惨遭失败。实际上只有一种正确的方法。

在当前选定的文件夹上调用idle（false）方法。理想情况下收件箱，因为它将收到所有邮件。

调用idle（false）将基本上挂起线程的运行时，因此最好将idle（false）放在新线程上。然后，一旦您使用messageCountChange收到新的电子邮件/通知，您必须重新运行此线程。

这是实现这一目标的唯一真正方法。当我编写一个名为JavaPushMail的程序时，我已经为你的显式问题写了一个包装器。您可以在我的网站上找到更多信息（http://www.mofirouz.com/wordpress），或者您可以在GitHub https://github.com/mofirouz/JavaPushMail上抓取应用程序（目前正在开发中）

Answer 3

实际上Java Mail samples包含IMAP IDLE示例，如下所示。除此之外，IdleManager class可能很有趣。

/*
 * Copyright (c) 1996-2010 Oracle and/or its affiliates. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 *   - Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 *
 *   - Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *
 *   - Neither the name of Oracle nor the names of its
 *     contributors may be used to endorse or promote products derived
 *     from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
 * IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

import java.util.*;
import java.io.*;
import javax.mail.*;
import javax.mail.event.*;
import javax.activation.*;

import com.sun.mail.imap.*;

/* Monitors given mailbox for new mail */

public class monitor {

    public static void main(String argv[]) {
    if (argv.length != 5) {
        System.out.println(
        "Usage: monitor <host> <user> <password> <mbox> <freq>");
        System.exit(1);
    }
    System.out.println("\nTesting monitor\n");

    try {
        Properties props = System.getProperties();

        // Get a Session object
        Session session = Session.getInstance(props, null);
        // session.setDebug(true);

        // Get a Store object
        Store store = session.getStore("imap");

        // Connect
        store.connect(argv[0], argv[1], argv[2]);

        // Open a Folder
        Folder folder = store.getFolder(argv[3]);
        if (folder == null || !folder.exists()) {
        System.out.println("Invalid folder");
        System.exit(1);
        }

        folder.open(Folder.READ_WRITE);

        // Add messageCountListener to listen for new messages
        folder.addMessageCountListener(new MessageCountAdapter() {
        public void messagesAdded(MessageCountEvent ev) {
            Message[] msgs = ev.getMessages();
            System.out.println("Got " + msgs.length + " new messages");

            // Just dump out the new messages
            for (int i = 0; i < msgs.length; i++) {
            try {
                System.out.println("-----");
                System.out.println("Message " +
                msgs[i].getMessageNumber() + ":");
                msgs[i].writeTo(System.out);
            } catch (IOException ioex) { 
                ioex.printStackTrace(); 
            } catch (MessagingException mex) {
                mex.printStackTrace();
            }
            }
        }
        });

        // Check mail once in "freq" MILLIseconds
        int freq = Integer.parseInt(argv[4]);
        boolean supportsIdle = false;
        try {
        if (folder instanceof IMAPFolder) {
            IMAPFolder f = (IMAPFolder)folder;
            f.idle();
            supportsIdle = true;
        }
        } catch (FolderClosedException fex) {
        throw fex;
        } catch (MessagingException mex) {
        supportsIdle = false;
        }
        for (;;) {
        if (supportsIdle && folder instanceof IMAPFolder) {
            IMAPFolder f = (IMAPFolder)folder;
            f.idle();
            System.out.println("IDLE done");
        } else {
            Thread.sleep(freq); // sleep for freq milliseconds

            // This is to force the IMAP server to send us
            // EXISTS notifications. 
            folder.getMessageCount();
        }
        }

    } catch (Exception ex) {
        ex.printStackTrace();
    }
    }
}

Answer 4

您可以将文件夹注册到 connectionListener。

var folder = store.getFolder("<FOLDER_NAME>");
folder.addConnectionListener(new javax.mail.event ConnectionAdapter() {
        public void closed(ConnectionEvent e) {
            try {
                log.info("Folder connection closed. Reconnect to server");
                
                // reopen connection 
                connectToServer();
            } catch (Exception exception) {
                log.error("Could not connect to server={0}", exception);
            }
        }
    });

Answer 5

每隔5分钟检查一次消息计数对我有用：

new Thread()
{
    @Override
    public void run()
    {
        startTimer();
    }
    private void startTimer()
    {
        int seconds = 0;
        while (true)
        {
            try
            {
                Thread.sleep(300000);
                int c = folder.getMessageCount();    
            }
            catch (InterruptedException ex)
            {
            }
            catch (MessagingException me)
            {
            }
        }
    }
}.start();

JavaMail：保持IMAPFolder.idle（）活着

5 个答案: