JavaMail IMAP over SSL quite slow - Bulk fetching multiple messages

after a lot of work, and assistance from the people at JavaMail, the source of this "slowness" is from the FETCH behavior in the API. Indeed, as pjaol said, we return to the server each time we need info (a header, or message content) for a message.

If FetchProfile allows us to bulk fetch header information, or flags, for many messages, getting contents of multiple messages is NOT directly possible.

Luckily, we can write our own IMAP command to avoid this "limitation" (it was done this way to avoid out of memory errors: fetching every mail in memory in one command can be quite heavy).

Here is my code:

import com.sun.mail.iap.Argument;
import com.sun.mail.iap.ProtocolException;
import com.sun.mail.iap.Response;
import com.sun.mail.imap.IMAPFolder;
import com.sun.mail.imap.protocol.BODY;
import com.sun.mail.imap.protocol.FetchResponse;
import com.sun.mail.imap.protocol.IMAPProtocol;
import com.sun.mail.imap.protocol.UID;

public class CustomProtocolCommand implements IMAPFolder.ProtocolCommand {
    /** Index on server of first mail to fetch **/
    int start;

    /** Index on server of last mail to fetch **/
    int end;

    public CustomProtocolCommand(int start, int end) {
        this.start = start;
        this.end = end;
    }

    @Override
    public Object doCommand(IMAPProtocol protocol) throws ProtocolException {
        Argument args = new Argument();
        args.writeString(Integer.toString(start) + ":" + Integer.toString(end));
        args.writeString("BODY[]");
        Response[] r = protocol.command("FETCH", args);
        Response response = r[r.length - 1];
        if (response.isOK()) {
            Properties props = new Properties();
            props.setProperty("mail.store.protocol", "imap");
            props.setProperty("mail.mime.base64.ignoreerrors", "true");
            props.setProperty("mail.imap.partialfetch", "false");
            props.setProperty("mail.imaps.partialfetch", "false");
            Session session = Session.getInstance(props, null);

            FetchResponse fetch;
            BODY body;
            MimeMessage mm;
            ByteArrayInputStream is = null;

            // last response is only result summary: not contents
            for (int i = 0; i < r.length - 1; i++) {
                if (r[i] instanceof IMAPResponse) {
                    fetch = (FetchResponse) r[i];
                    body = (BODY) fetch.getItem(0);
                    is = body.getByteArrayInputStream();
                    try {
                        mm = new MimeMessage(session, is);
                        Contents.getContents(mm, i);
                    } catch (MessagingException e) {
                        e.printStackTrace();
                    }
                }
            }
        }
        // dispatch remaining untagged responses
        protocol.notifyResponseHandlers(r);
        protocol.handleResult(response);

        return "" + (r.length - 1);
    }
}

the getContents(MimeMessage mm, int i) function is a classic function that recursively prints the contents of the message to a file (many examples available on the net).

To avoid out of memory errors, I simply set a maxDocs and maxSize limit (this has been done arbitrarily and can probably be improved!) used as follows:

public int efficientGetContents(IMAPFolder inbox, Message[] messages)
        throws MessagingException {
    FetchProfile fp = new FetchProfile();
    fp.add(FetchProfile.Item.FLAGS);
    fp.add(FetchProfile.Item.ENVELOPE);
    inbox.fetch(messages, fp);
    int index = 0;
    int nbMessages = messages.length;
    final int maxDoc = 5000;
    final long maxSize = 100000000; // 100Mo

    // Message numbers limit to fetch
    int start;
    int end;

    while (index < nbMessages) {
        start = messages[index].getMessageNumber();
        int docs = 0;
        int totalSize = 0;
        boolean noskip = true; // There are no jumps in the message numbers
                                           // list
        boolean notend = true;
        // Until we reach one of the limits
        while (docs < maxDoc && totalSize < maxSize && noskip && notend) {
            docs++;
            totalSize += messages[index].getSize();
            index++;
            if (notend = (index < nbMessages)) {
                noskip = (messages[index - 1].getMessageNumber() + 1 == messages[index]
                        .getMessageNumber());
            }
        }

        end = messages[index - 1].getMessageNumber();
        inbox.doCommand(new CustomProtocolCommand(start, end));

        System.out.println("Fetching contents for " + start + ":" + end);
        System.out.println("Size fetched = " + (totalSize / 1000000)
                + " Mo");

    }

    return nbMessages;
}

Do not that here I am using message numbers, which is unstable (these change if messages are erased from the server). A better method would be to use UIDs! Then you would change the command from FETCH to UID FETCH.

Hope this helps out!

You need to add a FetchProfile to the inbox before you iterate through the messages. Message is a lazy loading object, it will return to the server for each message and for each field that doesn't get provided with the default profile. e.g.

for (Message message: messages) {
  message.getSubject(); //-> goes to the imap server to fetch the subject line
}

If you want to display like an inbox listing of say just From, Subject, Sent, Attachement etc.. you would use something like the following

    inbox.open(Folder.READ_ONLY);
    Message[] messages = inbox.getMessages(start + 1, total);

    FetchProfile fp = new FetchProfile();
    fp.add(FetchProfile.Item.ENVELOPE);
    fp.add(FetchProfileItem.FLAGS);
    fp.add(FetchProfileItem.CONTENT_INFO);

    fp.add("X-mailer");
    inbox.fetch(messages, fp); // Load the profile of the messages in 1 fetch.
    for (Message message: messages) {
       message.getSubject(); //Subject is already local, no additional fetch required
    }

Hope that helps.