Google App Engine adds Java support (Review)

Last night Google announced Java support on Google App Engine.

After a bit of toying around, here are my findings.

The Eclipse plugin is pretty slick. Deploying and build is simple.

The dev server that you spin up locally looks to be jetty under the hood.

Objects intended for storage are JDO annotated and after compiling, you run the .class files through the DataNucleus Enhancer which adds additional metadata so Google can map it to BigTable. The Eclipse plugin automatically performs this step for you after compiling. The examples provide a bunch of ant macros to help facilitate building/deploying.

One issue that I had was that the project was building with Java 1.6 and I would get an error after compiling:

Caused by: java.lang.UnsupportedClassVersionError: Bad version number in .class file

Even though they say they support Java 1.5 and 1.6, I guess this doesn’t work on the Java 1.6 for the Mac. Switching the build to 1.5 allows the DataNucleus Enhancer to run successfully.

Even though they are using JPA, some features have not yet been implemented or supported ( see

Overall, I like what I see so far and think this would be great for quick prototypes of web apps/services.

Going through the tutorial, my awesome Guestbook application has been created and deployed.

Google App Engine adds Java support (Review)

Custom field names in Rails error messages

The defaults in Rails with ActiveRecord is beautiful when you are just getting started and are created everything for the first time. But once you get into it and your database schema becomes a little more solidified, the things that would have been easy to do by relying on the conventions of Rails require a little bit more work.

In my case, I had a form where there was a database column named “num_guests”, representing the number of guests. When the field fails to pass validation, the error messages is something like

Num guests is not a number

Not quite the text that we want. It would be better if it said

Number of guests is not a number

After doing a little bit of digging, I found the human_attribute_name method. You can override this method in your model class to provide alternative names for fields. To change our error message, I did the following

class Reservation < ActiveRecord::Base
  validates_presence_of :num_guests
      :num_guests    => "Number of guests"
  def self.human_attribute_name(attr)
      HUMAN_ATTRIBUTES[attr.to_sym] || super

Since Rails 2.2, this method is used to support internationalization (i18n). Looking at it, it reminds me of Java’s Resource Bundles and Spring MVC’s error messages. Messages are defined based off a key and there’s a chain of look ups that get applied to resolve an error’s message.

Although, I don’t see myself doing any i18n work in the near-term, it is cool that we have that option now in Rails.

Custom field names in Rails error messages