One of the reasons why Java-based, open-source CMS projects have not really achieved lift-off, we think, is their lack of adherence to standard Java patterns and practices. In its newest version, the most well-known such package, OpenCMS, heads in the right direction by standardizing around JSP and Servlets. Before the latest release (V. 5.0), you had to use a funky (and proprietary) system of XML-based templates for the presentation tier. Several other improvements help move the project more into the Java mainstream, which could speed adoption substantially. We'll have more to say on the topic of open-source CMS within the next few days...
Check out OpenCMS