Android JavaMail检测服务器是否脱机

Question

我目前正在开发带有后台服务和JavaMail Idle功能一起使用的电子邮件应用程序。 后台服务通过每29分钟发出一次检查来保持空闲功能正常工作（因为正在使用的服务器（Exchange服务器））有时在连接30分钟后会断开连接。

尽管这可以正常运行，但如果Exchange服务器处于脱机状态，则应用程序将继续尝试无限期地重新连接到IMAP文件夹。 我注意到3AM和6AM（典型的Exchange更新时间）之间的数据使用量激增。

为了避免增加数据使用量，我正在寻求实现一些功能，使应用程序应尝试三遍重新连接到IMAP文件夹，然后向用户显示警告，表明服务器处于脱机状态，并且将在30分钟内重试一次新的连接。

为了实现此目的，如果应用程序无法连接到IMAP文件夹，我将如何检测Exchange服务器是否实际上处于脱机/更新状态，并且会引发任何异常？ 好像会引发异常，那么我可以保存一个本地int变量，并在每次引发异常时将其递增1，然后在第三次向用户显示警报。

我当前的代码实现如下所示：

public void checkInboxEmail(final String host, final String user, final String password) {

    Log.d(TAG, "checkEmail");

    this.host = host;
    this.user = user;
    this.password = password;

    new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                Log.d(TAG, "checkEmail - run()");

                long databaseRecords;

                //create properties field
                Properties properties = new Properties();
                properties.put("mail.store.protocol", "imaps");
                properties.put("mail.imaps.ssl.trust", "*");
                properties.put("mail.debug", "true");

                emailSession = Session.getInstance(properties, new Authenticator() {
                    protected PasswordAuthentication getPasswordAuthentication() {
                        return new PasswordAuthentication(user, password);
                    }
                });


                IMAPStore imapStore = (IMAPStore) emailSession.getStore("imaps");
                // imapStore.connect();

                imapStore.connect(host, user, password);

                if (imapStore.isConnected()) {
                    Log.d("MailPush", "Successfully connected to IMAP");
                } else {
                    Log.d("MailPush", "Not connected to IMAP");
                }

                final IMAPFolder folder = (IMAPFolder) imapStore.getFolder("Inbox");
                folder.open(IMAPFolder.READ_WRITE);

                databaseRecords = dbManager.getReceivedEmailRecordsCount();

                if (databaseRecords < folder.getMessageCount()) {
                    Log.d(TAG, "Receiving Mail...");
                    receiveMail(folder.getMessages());
                } else {
                    Log.d(TAG, "Records match.");
                }

                Folder[] fdr = imapStore.getDefaultFolder().list();
                for (Folder fd : fdr)
                    System.out.println(">> " + fd.getName());

                folder.addMessageCountListener(new MessageCountListener() {

                    public void messagesAdded(MessageCountEvent e) {

                        System.out.println("Message Added Event Fired");
                        Log.d(TAG, "MESSAGE TYPE: " + e.getType());
                        //ADDED = 1 & REMOVED = 2

                        try {
                            Message[] messages = e.getMessages();

                            System.out.println("messages.length---" + messages.length);

                            for (Message message : messages) {
                                if (!message.getFlags().contains(Flags.Flag.SEEN)) {

                                    //Message is new (hasn't been seen) > Message Details
                                    System.out.println("---------------------------------");
                                    System.out.println("Email Number " + (message.getMessageNumber()));
                                    System.out.println("Subject: " + message.getSubject());
                                    System.out.println("From: " + message.getFrom()[0]);
                                    System.out.println("Text: " + message.getContent().toString());

                                    String from = message.getFrom()[0].toString();

                                    String cc = InternetAddress.toString(message.getRecipients(Message.RecipientType.CC));
                                    Log.d(TAG, "CC 1: " + cc);

                                    Address[] recipients = message.getRecipients(Message.RecipientType.CC);
                                    cc = InternetAddress.toString(recipients);
                                    Log.d(TAG, "CC 2: " + cc);

                                    //Check Encryption Details > Add SEEN Flag > Add to database
                                    checkEncryption((MimeMessage) message, from, cc);
                                }
                            }
                        } catch (Exception ex) {
                            ex.printStackTrace();
                        }
                    }

                    public void messagesRemoved(MessageCountEvent e) {
                        System.out.println("Message Removed Event fired");
                    }
                });

                folder.addMessageChangedListener(new MessageChangedListener() {

                    public void messageChanged(MessageChangedEvent e) {
                        System.out.println("Message Changed Event fired");
                    }
                });

                startListening(folder);

                //close the store and folder objects
                //   emailFolder.close(false);
                //   store.close();

            } catch (NoSuchProviderException e) {
                e.printStackTrace();
            } catch (MessagingException e) {
                e.printStackTrace();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }).start();
}

private void startListening(IMAPFolder imapFolder) {
    Log.d(TAG, "startListening");

    // 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()) {
        Log.d(TAG, "Starting IDLE");
        try {
            Log.d(TAG, "Setting IDLE");
            imapFolder.idle();
        } catch (FolderClosedException fex) {
            //Server closes connection.
            Log.d(TAG, "FolderClosedException. Server potentially dropped connection. Retrying connection...");
            fex.printStackTrace();

            if (!isServiceRunning(MyService.class)) {
                Log.d(TAG, "Service isn't running. Starting service...");

                //Start service
                Intent intent = new Intent(context, MyService.class);
                intent.putExtra("host", host);
                intent.putExtra("email", user);
                intent.putExtra("password", password);

                context.startService(intent);
            } else {
                Log.d(TAG, "Service is already running. Checking email...");
                checkInboxEmail(host, user, password);
            }
        } catch (MessagingException e) {
            //Idle isn't supported by server.
            Log.d(TAG, "Messaging exception during IDLE: ");
            e.printStackTrace();
        }
    }

    // Shutdown keep alive thread
    if (t.isAlive()) {
        Log.d(TAG, "Interrupting thread");
        t.interrupt();
    }
}

private static class KeepAliveRunnable implements Runnable {

    private final String TAG = getClass().getName();

    private static final long KEEP_ALIVE_FREQ = 60000 * 29; // 29 minutes (Exchange connection drops after 20-30 minutes)

    private IMAPFolder folder;

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

    @Override
    public void run() {
        while (!Thread.interrupted()) {
            try {
                Thread.sleep(KEEP_ALIVE_FREQ);

                // Perform a messageCount check just to keep alive the connection
                Log.d(TAG, "Performing a messageCount check to keep the connection alive");
                folder.getMessageCount();
            } catch (InterruptedException e) {
                // Ignore, just aborting the thread...
                Log.d(TAG, "Interrupted...");
                e.printStackTrace();
            } catch (MessagingException e) {
                // Shouldn't really happen...
                Log.d(TAG, "Unexpected exception while keeping alive the IDLE connection");
                e.printStackTrace();
            }
        }
    }
}

private void receiveMail(Message[] messages) {
    try {

        System.out.println("messages.length---" + messages.length);

        for (Message message : messages) {
            if (!message.getFlags().contains(Flags.Flag.SEEN)) {

                //Message is new (hasn't been seen) > Message Details
                System.out.println("---------------------------------");
                System.out.println("Email Number " + (message.getMessageNumber()));
                System.out.println("Subject: " + message.getSubject());
                System.out.println("From: " + message.getFrom()[0]);
                System.out.println("Text: " + message.getContent().toString());

                String from = message.getFrom()[0].toString();
                String cc = InternetAddress.toString(message.getRecipients(Message.RecipientType.CC));

                //Check Encryption Details > Add SEEN Flag > Add to database
                checkEncryption((MimeMessage) message, from, cc);
            }
        }
    } catch (Exception ex) {
        ex.printStackTrace();
    }
}

Answer 1

如果服务器已启动但不接受连接，则连接将立即失败（例外）。 如果服务器已关闭，并且您设置了连接超时，则超时后连接将失败（例外）。

Answer 2

为了阐明@BillShannon的答案，如果服务器主机已启动，但Exchange不接受连接，则连接将立即失败，并显示ConnectException: connection refused 。 如果服务器主机已关闭，则连接将在超时后失败，并显示ConnectException: connect timeout （或可能是SocketTimeoutException ）， 无论您是否设置了连接超时，因为平台始终都有一个连接超时。