Friday, April 17, 2009

Running Spring on Google App Engine

In case you've been living under a rock the last couple of weeks, Google recently announced the addition of Java support to its App Engine. I have written a small sample application that leverages Spring and each of the Google infrastructure services that are exposed either as proprietary APIs or serves as backend to standard APIs.

The application itself, Feeling Lucky Pictures, is very simple: you login using your Google account, and you import images from URLs into your personal gallery. You can also send an email with your pictures attatched.

Integration with the Google infrastructure is as follows:
  • Authentication is of course done with the Google Accounts API.

  • Image import uses a regular input stream, but on the GAE runtime that class is backed by the URL Fetch API.

  • Storage is implemented using JDO, which is backed by a BigTable data store.

  • Reading objects from the data store uses the JSR 107 javax.cache API, which is backed by Memcache.

  • Imported images are enhanced with the I'm feeling lucky filter, part of the Images API.

  • Email is sent with the standard JavaMail API, which also is backed by a custom mail service on GAE.
As I said, the application is built on Spring as I wanted to see what problems, if any, I would run into if I were to run a regular Spring application on GAE. Most things worked out fine, but there were a handful of issues that I discovered and resolved, which is the real value of this application.
  • I decided to make as heavy use of the annotation configuration option as possible, including automatic classpath scanning. However, Spring will then attempt to scan for JPA annotations if certain conditions are met, which in turn will cause the javax.naming.NamingException class to load.

    This class is not on the GAE class whitelist, and working around the missing class by providing a JNDI API jar or a dummy class won't work either. My solution was to roll a JDO-only version of the spring-orm jar. This problem is also discussed in this thread, with alternative solutions.

  • When configuring the LocalPersistenceManagerFactoryBean, don't specify a configuration file property (i.e. "classpath:META-INF/jdoconfig.xml"). Instead, set the persistenceManagerFactoryName property to "transactions-optional", which is the name used in the default configuration file provided by the Eclipse GAE plugin.

  • I wanted to avoid using the various Google API static factories programmatically in my MVC controllers, instead injecting service interfaces like ImageService to improve testability. This worked fine for the most part, simply using the factory-method attribute in the bean definition, with the exception of the Cache interface.

    Cache extends java.util.Map, and for some reason the @Autowire mechanism requires generic key and value types on constructor parameters of Map type. Unfortunately the Cache interface is not possible to parameterize, so my workaround was to embed it in a very simple CacheHolder one-property class, which is produced by a factory bean and injected.

  • I'm caching picture ids per email address, like this: "":[1,5,7]. In a regular in-memory HashMap cache you can add elements to a collection map value, but on GAE you need to overwrite the old entry with a new collection that contains the old elements plus the new one. See JdoRepository for details.

  • Spring provides a set of abstractions on top of JavaMail, which I wanted to keep. In particular, the JavaMailSender interface and MimeMessageHelper class, for sending emails with attachments. This turned out to be a bit cumbersome, since the GAE mail backend is wired into the implementation of the JavaMail API in an unusual way. There's no SMTP transport, for example.

    What I had to do was to override Spring's implementation of JavaMailSender and align creation and usage of javax.mail.Session and javax.mail.Transport exactly with the howto instructions for the GAE mail service. Basically you can't let the Transport connect and then send the message on the established connection, you need to do it all in one pass using the static Transport.send() method.

    Oh, and don't leave the body of an email empty, or you'll get a really poor error message.

Other than that it's business as usual. The complete source code is available on Google Code, and it's MIT licensed so you can do whatever you want with it. If it helped you out, or if you've found a better or simpler solution to any of the problems above, please drop a comment on this article.