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Lombok acts as an annotation processor that “adds” code to your classes at compile time. Annotation processing is a feature added to the Java compiler at version 5. The idea is that users can put annotation processors (written by oneself, or via third-party dependencies, like Lombok) into the build classpath.
Lombok is used to reduce boilerplate code for model/data objects, e.g., it can generate getters and setters for those object automatically by using Lombok annotations. The easiest way is to use the @Data annotation.
Lombok is an awesome Java library. It helps to reduce the amount of 'infrastructural code'. You don't need to write constructors, getters, setters, and even builders anymore. All you have to do is to put the proper annotations and the plugin will generate everything for you.
Lombok does indeed code against internal API, as Sean Patrick Floyd said. However, as lombok is ONLY involved in the compilation phase, its misleading to claim Lombok will only run on a sun VM. It'll only compile on ecj or sun's javac. However, the vast majority of VMs out there, if they ship a compiler at all, are one of those two. For example, the Apple VM ships with stock sun javac, and as such lombok works just fine on macs. Same goes for the soylatte VM, for example.
While for javac we really do have to stick with their updates, partly because of a lot of ongoing work on their compiler right now, we've had to make just 1 minor adjustment to our eclipse support over many many versions of eclipse. So, while we do code against internal API, they are relatively stable bits.
If what lombok does could be done without resorting to internal API, we'd have done something else, but it can't be done, so we resort to internal API usage.
NB: I'm one of the lead developers of lombok, so, I'm probably a little biased :P
It uses JSR 269 Pluggable Annotation Processing API available in Java 6.
Note that lombok.jar contains a file named /META-INF/services/javax.annotation.processing.Processor. When javac sees this file in a compilation classpath, it runs annotation processors defined there during compilation.
In addendum to axtavt's answer: Lombok uses a lot more than the JSR 269 api exposes. Lombok codes against a) internal javac apis and b) internal eclipse apis (in a separate processor). JSR 269 does not let you modify existing source code, but when you cast an Element to the underlying AST node, you can actually modify the AST (which is what project Lombok does).
So Lombok is a huge hack that will only compile using javac or eclipse's compiler. It's a great piece of software, but it's also hated by many for being such a non-standard hack.
Project Lombok: Creating Custom Transformations is some helpful.
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