I am looking for an open source Java CMS (Web CMS, actually) to run on Google App Engine. I have looked at related older questions on this topic (What CMS runs on Google AppEngine?, CMS over Google App Engine, with SEO etc.) but the problem is that they all largely list Python-based CMSes. Plus these questions are pretty old, and since GAE is a fast-moving target, I thought it might be worthwhile to ask again.

I want a CMS for creating some websites (for myself and for others), but would rather not start writing one from scratch. A "good" (very subjective, I know) open source WCMS allows me to start using a product, while still being able to add to/extend the product/project.

On the one hand I am looking for a somewhat mature product/project, on the other hand it's easier to start contributing to the development cycle of a young product/project (conflicting, I know :-).

Here are some features that would be preferable:

• [X]HTML/XML/CSS based templating
• Ability to create multiple blogs
• Galleries
• Ability to create a "Downloads" section (is this pretty much standard?)
  • Separate management for digital assets (images, PDFs, binary files etc.)
• Roles like "Administrator", "Editor", "Contributor" etc. (or their equivalents)
• Ability to move/reorganize pages
• Export to PDF
• Reformat content for printing

Is the CMS you are about to suggest especially well-suited to publishing an online book? My idea is that while the book may be offered as a downloadable eBook, the latest, most current version will be the one available on the website.