From yet another case of life imitating humor, there is this article on how Caucho's Quercas PHP implementation in Java was used to run Drupal.

Another post by Dave Herron on Resin's PHP support, and using Drupal as a test for that.

What is scary is that this is indeed a case of life imitating humor. A running joke in the community is that Dries is secretly working on a Java port of Drupal, and that Subversion is being used as a repository for that project. A recent example can be found here.

However, this is not exactly "Drupal running in Java", since Java is used to reimplement PHP. Drupal is not yet written in native Java. But how soon would you think this would happen?

Thanks to Karoly's post on the topic for the first two links. He also mentions some jokes on IRC that ensued.