JavaMail： Keep IMAPFolder.idle（） alive

一个常见的错误是假设 IDLE 命令将无限期地保持发布更新。但是，定义 IDLE 扩展的 RFC 2177 声明：

如果服务器正在运行 IDLE 命令，则服务器可能会认为客户端处于非活动状态，并且如果此类服务器具有不活动超时，它可能会在其超时期限结束时隐式注销客户端。因此，建议使用 IDLE 的客户端终止 IDLE，并至少每 29 分钟重新发出一次，以避免注销。这仍然允许客户端接收即时邮箱更新，即使它只需要以半小时为间隔进行“轮询”。

特别是GMail，正如你所说，超时要低得多，大约10分钟。

我们只需要每隔 9 分钟左右重新发出 IDLE 命令，它就可以正常工作。API 无法为 IDLE 命令设置超时，因此您需要第二个线程来移动此线程。javax.mail

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。但是，这将不允许使用干净的方法来关闭线程，因此我不建议这样做。一个更干净的方法是让第二个线程向服务器发出 NOOP 命令。这根本不会做任何事情，但足以使 IDLE 中止并重新发布。

我在这里提供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);
            }
        }
    }
}

实际上，Java 邮件示例包括一个 IMAP IDLE 示例，如下所示。除此之外，IdleManager类可能很有趣。

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 */

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();
    }
    }
}