I currently have a web app that is built using Play framework (1.2.4) with Java. I'm looking to rebuild it using node.js. I was wondering if the same Java classes can be used, or if all of the code will have to be in javascript?
Thanks
Short answer: no, you will need to rewrite that code in javascript. Since you say you are "looking to rebuild it using node.js", there you go. Full steam ahead.
Longer answer: well, you could run 2 apps, 1 java in the JVM and 1 javascript in node and have them talk via some interprocess communication mechanism such as a REST/HTTP API. This is a fairly common pattern and node.js lends itself well to serving as a web app server that connects to one or more back end APIs for supporting services. That would allow you to maintain your existing java code base. This pattern is sometimes called a "Service Oriented Architecture".