JAX-WS RI provides all necessary tools for doing such tasks. Though to get the WSDL generated, you need to write interfaces and its implementation, which can work just as a wrapper around your existing code. See here http://blog.vinodsingh.com/2008/09/building-jax-ws-web-service.html how to develop JAX-WS web services.
Do you know XFire? As long as your Java app uses well defined Java Interfaces, you may not need to write a single line of code to covert it to web services.
Actually, XFire got sucked up into the Apache CXF project and no longer has a separate identity. As I understand it, CXF still includes simple tools for turning POJO (Plain Old Java Objects) into web services with a minimum of fiddling.