(thanks to E Hauser)

Here's how I debug Resin with Eclipse. I launch the Resin start script with the arguments:

-Xdebug -Xnoagent -Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=12345

Then in Eclipse, go to Run -> Debug -> Remote Application. Click new and make sure the project is set to the application you want to debug. Connection type should be "Standard (Socket Attach)", and your connection host should be "localhost" with the port "12345".

You can launch Resin internally from Eclipse by launching the main Resin class with the correct parameters, but socket attach has always worked fine for me.

Arnaud Peyre has developed a comprehensive Resin plugin for Eclipse.

Resin For Eclipse plugin provides tools for developing web projects:

Resin management : a graphical user interface for managing Resin servers within Eclipse. The interface allows you to create servers, add web projects, edit classpath and system properties, manage https, launch, debug and stop the server. It should work with web projects created by any other plugin (such as WSAD ones).
Web projects : the plugin allows you to create generic web projects (you don't need to install Resin) or projects optimized for Resin. Web projects include autodetection of jar files added to the "WEB-INF/lib" directory.
War exporation : a wizard to export web projects into war files.

Resin for Eclipse Home page