To me the biggest benefit of ActiveRecord is the database introspection to dynamically create the objects attributes based on the columns in the database.

This doesn't do that in Java, it parses rails migrations to generate the java classes for objects with these attributes.

Why is this a problem?

We have a similar setup up where I work, but rails migrations doesn't manage the database schema, that is managed on the java side. Rails still gets all changes to the DB in its objects, but using this setup wouldn't work. I was all excited by the title too, still it's a good solution to their particular problem.