So the summary is: do not store nulls into Lists, otherwise you will get into a problem in future.

A join model is a data model that models the relationship between two other models. For example, you might have Person entities and Group entities, and you want Persons to be in Groups and Groups to "have" Persons. The relationship between Persons and Groups is one of Memberships. A Person is "in" a Group "through" a Membership. They may belong to multiple Groups, i.e. have multiple Memberships.

class Membership(db.Model): person = db.ReferenceProperty(Person) group = db.ReferenceProperty(Group)

If certain properties of your joined entities (Person and Group in this example) don't change much, but get queried often via the join model, you may find it worth caching those properties on the join model itself.

You can study the JavaMail API in more detail to understand how to do things like sending attachments in your email too. You can do that using the Google App Engine Email Service.

Google app engine uses the concept of static files. This is a performance optimization. Using the file appengine-web.xml you can configure the way google handles static files. You can include and exclude certain files using their extension or name. More information can be found here at google. This all works nice in the online version, however there seems to be a problem with the development server. Some solutions try to configure the local version as well, still that did not work for me. I decided to look for a servlet that serves static files.

That is it, now you can test your stuff locally and all your scripts, images, styles are loaded by your application. Of course you have to remove this servlet before uploading your application. Hope it helps people with their local debugging of jquery scripts or other javascript things.

To get the development environment working the plugin also requires access to the unzipped SDK as packaged by Google. The plugin tries to help you set this up (“gae:unpack”) but that failed for me. I was able to get stuff working by manually unzipping the SDK artifact downloaded directly from Google to the following directory: ~/.m2/repository/com/google/appengine/appengine-java-sdk/1.3.0/appengine-java-sdk-1.3.0

You’ll also notice in our appengine-web.xml that we substitute in our application name. By default this comes from the properties section of the pom.xml file (line 11).

Cron jobs / task queues Instead of using a framework like Quartz to schedule jobs, Google App Engine takes care of executing jobs for you. You simply enter a cron-like expression and a URL to call and your job is configured. You also have a task queue at your disposal. Your application code can add tasks to a task queue which will be executed later in the future, asynchronously. An example use case is that you don’t want clients to wait for an email to be sent before he sees the next page. Instead you can put the email task on the task queue and the email will be sent asynchronously.

When an email is received Google App Engine does a post on an URL in your application you configured. The HTTP body of the POST request contains the exact mime message as it was received by Google. To parse this mime message you can use the MimeMessage class provided by the JDK.