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 = "my.user@gmail.com";
            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.

Answer 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 = "xxx@gmail.com";
    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());
        }

    }
}

Answer 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.