2009-11-21

Receiving email in Google App Engine + Spring

Here's my Spring Controller which, at the moment, receives an email and prints various elements of the message to the log. The instructions here are very good and I only adapted it slightly for Spring.

Firstly I added the inbound-services section to my appengine-web.xml file. Next I added a url mapping to my Spring config:

<bean id="publicUrlMapping" 
  class="org.springframework.web.servlet.handler.SimpleUrlHandlerMapping">
    <property name="mappings">
        <props>
            <prop key="/_ah/mail/*">mailController</prop>
        </props>
    </property>
</bean>

Here is the MailController class:

import java.io.InputStream;
import java.io.UnsupportedEncodingException;
import java.util.Properties;

import javax.activation.DataHandler;
import javax.activation.DataSource;
import javax.mail.Address;
import javax.mail.Part;
import javax.mail.Session;
import javax.mail.internet.MimeMessage;
import javax.mail.internet.MimeMultipart;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.apache.commons.io.IOUtils;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.springframework.web.servlet.ModelAndView;
import org.springframework.web.servlet.mvc.AbstractController;

public class MailController extends AbstractController {
    protected final Log logger = LogFactory.getLog(getClass());

    @Override
    protected ModelAndView handleRequestInternal(HttpServletRequest request, HttpServletResponse response) throws Exception {
        Properties props = new Properties(); 
        Session session = Session.getDefaultInstance(props, null); 
        MimeMessage message = new MimeMessage(session, request.getInputStream());

        Address[] from = message.getFrom();
        String fromAddress = "";
        if (from.length > 0) {
            fromAddress = from[0].toString();
        }

        String contentType = message.getContentType();
        InputStream inContent = null;
        logger.info("Message ContentType: ["+contentType+"]");

        if (contentType.indexOf("multipart") > -1) {
            //Need to get the first part only
            DataHandler dataHandler = message.getDataHandler();
            DataSource dataSource = dataHandler.getDataSource();
            MimeMultipart mimeMultipart = new MimeMultipart(dataSource);
            Part part = mimeMultipart.getBodyPart(0);            
            contentType = part.getContentType();
            logger.info("Part ContentType: ["+contentType+"]");
            inContent = (InputStream)part.getContent();
        } else {
            //Assume text/plain
            inContent = (InputStream)message.getContent();
        }
        
        String encoding = "";
        if (contentType.indexOf("charset=") > -1) {
            encoding = contentType.split("charset=")[1];
        }
        logger.info("Encoding: ["+encoding+"]");
        
        String content;
        try {
            content = IOUtils.toString(inContent, encoding.toUpperCase());
        } catch (UnsupportedEncodingException e) {
            content = IOUtils.toString(inContent);
        }
        
        logger.info("Received email from=["+fromAddress+"] subject=["+message.getSubject()+"]");
        logger.info("Content: "+content);
        
        return null;
    }
}

Note that on the Google page it says
The getContent() method returns an object that implements the Multipart interface. You can then call getCount() to determine the number of parts and getBodyPart(int index) to return a particular body part.
This doesn't appear to be quite true. If you call getContent() on MimeMessage it returns a ByteArrayInputStream of the raw bytes for the whole message. According to the documentation here an input stream should only be returned if the content type is unknown. I think this is a bug.

You can get around this by parsing the content type yourself as I have done in my example. If message.getContentType() returns a string containing "multipart" then I parse it as a multipart message, otherwise I assume it is "text/plain".

In order to extract a single part of the Multipart content you have to pass through a MimeMultipart object. It's here that you can call getCount() and extract the Parts that you want. In my example I just get the first part.

Calling getContent() on the Part still returns a stream of bytes so you have to convert it with the correct encoding. You can extract the encoding from the ContentType of the Part. I added a try..catch block around the conversion to a string in case the encoding was not recognized - in which case it falls back to the default.

It is vital that you determine whether you have multipart content or not. If you try to parse a "text/plain" message as a multipart then you may well encounter an error like this:
Nested in org.springframework.web.util.NestedServletException: Handler processing failed; nested exception is java.lang.OutOfMemoryError: Java heap space:
java.lang.OutOfMemoryError: Java heap space
 at java.util.Arrays.copyOf(Unknown Source)
 at java.io.ByteArrayOutputStream.write(Unknown Source)
 at javax.mail.internet.MimeMultipart.readTillFirstBoundary(MimeMultipart.java:244)
 at javax.mail.internet.MimeMultipart.parse(MimeMultipart.java:181)
 at javax.mail.internet.MimeMultipart.getBodyPart(MimeMultipart.java:114)
The readTillFirstBoundary method fails because in a "text/plain" message there are no boundaries!

Note that the development server always sends a multipart message with two parts: text/plain and text/html. GMail also sends emails in this format but lots of other servers just send text/plain.
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