Fun with Classloading in Java

With my CF Groovy project, I've been doing a lot of Java coding of late. It's all been written in CFML, but it's mostly Java code in CFML syntax. As an aside, doing that sucks. CFML is horrible for writing anything more than the simplest Java code. But on to the topic at hand….

To this point in my career, basically all of the Java code I've written has been application code. Most of the CFML, JavaScript, and ActionScript I write is also application code, but I've contributed significantly to Fusebox and built a number of micro-frameworks in all three languages. However, none of that touches on the stuff I've done in the past couple weeks. The primary source of my issues getting Groovy and Hibernate to play together was classloading.

Unlike Grails (GORM, really), any of the various examples on the web, and I'm guessing Joe Rinehart's Hiberailooving project, three of the inviolate framework objectives were zero dependency on an IDE beyond text editing, no manual compilation, and no container restarts. Obviously, Hibernate provides a set of IDE tooling that can be brought to bear, but I'm talking about just the CF integration portion. I want to be productive with emacs and a web browser.

This presented a number of issues with classloading, because Groovy can only be loaded by the GroovyClassLoader (and subclasses). Basically this forced my hand to using compiled Groovy code, and ensuring it and Hibernate are loaded by the same ClassLoader. No GroovyClassLoader at all.

Now all that's well and good, but you have to be a bit careful, because two of the goals where no manual compilation and no container restarts. Doing compilation from CF is fairly simple (instantiate FileSystemCompiler and call 'main' with the right args), though again, getting all the right stuff on the classpath in the right order proved a nightmare. This is further complicated by the while JEE classloader situation, which isn't straightforward at all.

The details of this intricate dance completely escaped me for a week and a half. Then one night, while lying in bed, it all clicked together (thank you, Heather, for going to England). The solutions seem fairly obvious now, but they took a lot of fussing and fighting to deduce. I'd never even instantiated a ClassLoader before, aside from a couple one-off URLClassLoaders in CFML to lazy-load some JARs. Now I've got stacks of ClassLoaders, in some places two or three deep.

After my initial post-epiphany implementation was complete, I realized I can do some streamlining, but that's for after the release. I think I can even remove the need to drop the Groovy JAR in your /WEB-INF/lib folder, without adding any code complexity or runtime overhead, which would a nice bonus for ease of deployment. I.e., unzip the engine, write an entity, save it to the DB. No JARs, no restart the container, nothing.

It's been a very interesting (and pleasurable) experience, since it's so different from my day job of banging out boring little presentation apps all day. I've learned a lot, and even just the past few days of thinking about how other Java apps/frameworks work, I've realized how applicable that knowledge really is.

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