Adding arbitrary web app debug code at runtime

"Debugging" can mean a lot of different things. The classic use case has a developer sitting at his or her machine, using a low-level debugger like jdb or IDE-based equivalents to inspect the state of variables, threads, objects, etc. And that's great. But there's also a sense of debugging involved in seeing a production system misbehaving and trying to find some way of determining what it's doing and why. Particularly with web apps, these situations aren't amenable to hooking up a debugger, because of the remote deployment, the fact that debugging may not be enabled, etc. And that's assuming the problem is amenable to that style of debugging.

Admit it: at least once, you've probably wanted to throw an arbitrary System.out.println() into a remote, running application, if only to start to figure out what the heck's going on. But of course, that means recompiling, restarting, and lots of other impracticalities. What you could really use is a maintenance servlet that would let you get in and do stuff at runtime.

This is Lorenzo Puccetti's line of thinking. In today's Featured Article, Dynamic Interaction with Your Web Application, he writes:

The servlet we want to write needs to be able to hook into any resource provided by the web server and allow the user to inspect any part of it. To be able to gather the required information, it might require flow-control constructs and loops. This leads to one solution: to make the servlet able to execute a script sent by the client (i.e., the browser). The script will have not only the ability to access any server resource, but by manipulating host objects representing a HTTP request and response, it will be capable of communicating back to the client.

He goes on to deploy the Rhino JavaScript interpreter behind this servlet, and shows how JavaScript code can access the Java runtime, allowing you to dig into your web application's behavior interactively, sending the results back to your browser.

This is a wide-open technique, which has some obvious benefits and hazards -- check it out, think it over, then let us know what you think in the article comments.