Wednesday, July 14, 2010

Tip Debugging External Java Dependencies

Ever spent time debugging 3rd party java libraries? Decompiling is usually the first step. Attempting to walk through the code can be tedious but it's usually the first line of defense. But what if you want to deploy a slightly modified version? In the past, I've checked out the project and built it with my modifications. Since most open source projects don't support "virgin builds", this has a success rate of about 10%. Fortunately, there is a better way. I'm just disappointed I didn't think of it.

In our project we deploy a wiki that is based on JSPWiki using maven overlays. In the version we are using, there isn't any support for being able to configure the wiki files directory outside of a properties file in the WAR. In order to point JSPWiki to a different directory, you would basically have to unzip the WAR, update the file, and then zip the WAR back together (#fail). So, someone on our team discovered we could basically override this behaviour by providing our own implementation of the same class.

To be more specific, the class under question is com.ecyrd.jspwiki.PropertyReader. It's included in the JSPWiki.jar file under /WEB-INF/lib. It's default behaviour is not suitable for our needs, so we get an original copy of, and place it under our maven projects /src/main/java directory under the same package of com.ecyrd.jspwiki. Once the projects builds, we now have our version of PropertyReader.class under /WEB-INF/classes, which is important because the ClassLoader will first look under /WEB-INF/classes first before looking in /WEB-INF/lib. This means our class is used instead of the one provided by JSPWiki in /WEB-INF/lib/JSPWiki.jar.

Now I know what your thinking: that's a horrible idea James. And for the most part I agree, but it's not my fault this ability doesn't already exist in JSPWiki. So if you want to keep your conscience clean, go ahead and continue unpacking and repacking that WAR. I'll be happy getting important things done. Obviously, practicing this is the exception and not the rule. And one should provide the patch as an improvement back to the 3rd party for all to enjoy. And before you start asking yourself why you can't just extend the real PropertyReader and override the necessary methods, which I agree would be more ideal, it's not possible because you'd basically be extending yourself since the modified class is the first class in the classpath.

This technique has actually helped me twice debug environment specific issues. It'd saved me a huge amount of time not having to build an external library. In fact, if you check out the exact version, you could even perform remote debugging with breakpoints.

So next time you need to debug an external 3rd party library, consider using this technique before attempting to build it.

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