If you already know Java, why not use one of the rich Web Application frameworks on Java, such as Spring or Struts, or Wicket, or Google Web Toolkit. Or even just servlets and jsp?
Edit: I suppose part of the answer depends on your goal. Are you looking for a new career? Or, are you simply looking to broaden your horizons, learn a new facet of software development?
A lot of the concepts carry over from language to language - things like MVC are pretty universal. Web application architecture design is also pretty universal - caching, scaling, sharding, etc. are all more or less the same, no matter your underlying language.
For pure learning and fun, interpreted languages are hard to beat - no recompiling, instant changes, etc.