Right way to poll GMail inbox for incoming mail from stand-alone application

Question

I am trying to poll GMail inbox for incoming mail. Here is what I have:

import java.util.Properties;

import javax.mail.Authenticator;
import javax.mail.Folder;
import javax.mail.Message;
import javax.mail.MessagingException;
import javax.mail.PasswordAuthentication;
import javax.mail.Session;
import javax.mail.event.MessageCountEvent;
import javax.mail.event.MessageCountListener;

import com.sun.mail.imap.IMAPFolder;
import com.sun.mail.imap.IMAPStore;

public class GmailIncomingTest {

    public static void main(String[] args) {
        try {
            String username = "[email protected]";
            String password = "mypassword";

            Properties properties = new Properties();
            properties.put("mail.smtp.auth", "true");
            properties.put("mail.smtp.host", "smtp.gmail.com");
            properties.put("mail.smtp.port", "587");

            Session session = Session.getDefaultInstance(properties, new Authenticator() {

                protected PasswordAuthentication getPasswordAuthentication() {
                    return new PasswordAuthentication(username, password);
                }
            });

            IMAPStore store = (IMAPStore) session.getStore("imaps");
            store.connect("smtp.gmail.com", username, password);

            IMAPFolder inbox = (IMAPFolder) store.getFolder("inbox");
            inbox.open(Folder.READ_ONLY);

            inbox.addMessageCountListener(new MessageCountListener() {

                @Override
                public void messagesRemoved(MessageCountEvent event) {

                }

                @Override
                public void messagesAdded(MessageCountEvent event) {
                    Message[] messages = event.getMessages();

                    for (Message message : messages) {
                        try {
                            System.out.println("Mail Subject:- " + message.getSubject());
                        } catch (MessagingException e) {
                            e.printStackTrace();
                        }
                    }
                }
            });

            new Thread(new Runnable() {
                private static final long KEEP_ALIVE_FREQ = 10000;

                @Override
                public void run() {
                    while (!Thread.interrupted()) {
                        try {
                            inbox.idle();
                            Thread.sleep(KEEP_ALIVE_FREQ);                                  
                        } catch (InterruptedException e) {
                        } catch (MessagingException e) {
                        }
                    }
                }
            }).start();                 
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

I would like to know if this is a right way to do this or if there is any other better way.

Solution 1

As long your server supports IDLE (gmail will) and you want only notified about new mails (since programm is running) than the below programm should fit your needs. If server does not support IDLE and/or older message are relevant (or other events like folder rename etc) then you need to modify the code. Have a look here for some hints: https://github.com/salyh/elasticsearch-imap/blob/master/src/main/java/de/saly/elasticsearch/importer/imap/mailsource/ParallelPollingIMAPMailSource.java

import java.util.Properties;

import javax.mail.Folder;
import javax.mail.Message;
import javax.mail.MessagingException;
import javax.mail.Session;
import javax.mail.Store;
import javax.mail.event.MessageCountAdapter;
import javax.mail.event.MessageCountEvent;

import com.sun.mail.imap.IMAPFolder;
import com.sun.mail.imap.IMAPStore;

public class GmailIncomingTest {

    private static final String username = "[email protected]";
    private static final String password = "passwd";

    public static void main(String[] args) {

        Properties properties = new Properties();
        // properties.put("mail.debug", "true");
        properties.put("mail.store.protocol", "imaps");
        properties.put("mail.imaps.host", "imap.gmail.com");
        properties.put("mail.imaps.port", "993");
        properties.put("mail.imaps.timeout", "10000");

        Session session = Session.getInstance(properties); // not
                                                           // getDefaultInstance
        IMAPStore store = null;
        Folder inbox = null;

        try {
            store = (IMAPStore) session.getStore("imaps");
            store.connect(username, password);

            if (!store.hasCapability("IDLE")) {
                throw new RuntimeException("IDLE not supported");
            }

            inbox = (IMAPFolder) store.getFolder("INBOX");
            inbox.addMessageCountListener(new MessageCountAdapter() {

                @Override
                public void messagesAdded(MessageCountEvent event) {
                    Message[] messages = event.getMessages();

                    for (Message message : messages) {
                        try {
                            System.out.println("Mail Subject:- " + message.getSubject());
                        } catch (MessagingException e) {
                            e.printStackTrace();
                        }
                    }
                }
            });

            IdleThread idleThread = new IdleThread(inbox);
            idleThread.setDaemon(false);
            idleThread.start();

            idleThread.join();
            // idleThread.kill(); //to terminate from another thread

        } catch (Exception e) {
            e.printStackTrace();
        } finally {

            close(inbox);
            close(store);
        }
    }

    private static class IdleThread extends Thread {
        private final Folder folder;
        private volatile boolean running = true;

        public IdleThread(Folder folder) {
            super();
            this.folder = folder;
        }

        public synchronized void kill() {

            if (!running)
                return;
            this.running = false;
        }

        @Override
        public void run() {
            while (running) {

                try {
                    ensureOpen(folder);
                    System.out.println("enter idle");
                    ((IMAPFolder) folder).idle();
                } catch (Exception e) {
                    // something went wrong
                    // wait and try again
                    e.printStackTrace();
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e1) {
                        // ignore
                    }
                }

            }
        }
    }

    public static void close(final Folder folder) {
        try {
            if (folder != null && folder.isOpen()) {
                folder.close(false);
            }
        } catch (final Exception e) {
            // ignore
        }

    }

    public static void close(final Store store) {
        try {
            if (store != null && store.isConnected()) {
                store.close();
            }
        } catch (final Exception e) {
            // ignore
        }

    }

    public static void ensureOpen(final Folder folder) throws MessagingException {

        if (folder != null) {
            Store store = folder.getStore();
            if (store != null && !store.isConnected()) {
                store.connect(username, password);
            }
        } else {
            throw new MessagingException("Unable to open a null folder");
        }

        if (folder.exists() && !folder.isOpen() && (folder.getType() & Folder.HOLDS_MESSAGES) != 0) {
            System.out.println("open folder " + folder.getFullName());
            folder.open(Folder.READ_ONLY);
            if (!folder.isOpen())
                throw new MessagingException("Unable to open folder " + folder.getFullName());
        }

    }
}

Solution 2

You might want to fix these common mistakes, and you need to connect to the imap server, not the smtp server (see the GMail example in the JavaMail FAQ). Otherwise, you've got the right general idea. You might want to look at the monitor.java sample program.

You probably also want to handle more error cases when the connection drops while you're using it.

Comments

[Tapas Bose]

I am using your code as you have provided. I have placed two logs in the method run(). One inside the while and another after the while. Now I can see the log inside the while only once. Also when I am calling your kill() method from outside I cannot see the log that I placed after the while? Do you know what is happening? Thanks for your sharing your work.

[salyh]

you will see more of your log statements when the idle() method returns (depends on server and mailbox activity). If you want to force this look here stackoverflow.com/questions/1326014/… .If you call kill() and then (for example) folder.close() youre done.

[Zigii Wong]

@salyh Hi, Where can I find some info about C++ or Objective-C about this resource?

[freitas]

@salyh which version of javax.mail did you use? Trying the 1.4 and IMAPFolder.idle() method is missing

[Admin]

@freitas 1.6.1 should have idle method . mvnrepository.com/artifact/javax.mail/javax.mail-api/1.6.1

[Al Grant]

@salyh is the messages array in messagesAdded only triggered on new messages event? (as opposed to messages deleted etc)

[Rohit Sehgal]

I am using your code in production system, but for some reason I am getting FolderClosedException at times. So lets say code process 100 emails out of which ~40 are causing email closed exception and when that is happening, the mail are getting lost.