By alexismp on Jan 20, 2010
While GlassFish v3 offers a lot of new features (Java EE 6 and OSGi-based modularity being the most obvious ones), we take compatibility very seriously and want your existing applications currently running in GlassFish v2.1.x or GlassFish v3-Prelude to run unmodified in our latest release.
To that effect GlassFish can detect earlier versions of 'domain.xml' (the all-in-one config file for GlassFish) and upgrade it for you (when you start GlassFish v3 as one option). Please read Bobby's recent post for more on this.
The official documentation can be found in the upgrade guide. That document also covers the 'asupgrade' graphical tool (which ships with the appserver but which wasn't there in Prelude). If you want more technical details on the changes and why they were required, please read this piece. Remember that it was not supported to edit domain.xml by hand back then. It's still not supported today, but who would want to do that when you have 'asadmin', the web console, and now the RESTful API?
As a reminder, when we released GlassFish v3 on December 10th 2009 (together with Java EE 6), we had a blog storm with many many interesting posts about everything-GlassFish v3 which are summarized in previous entries: Part I and Part II.
Expanded from @glassfish.