Plugging in to Java compilers

Question

I have a post-compilation step that manipulates the Java bytecode of generated classes. I'd like to make life as painless as possible for library consumers, so I'm looking at ways I can make this process automatic and (if possible) compiler agnostic.

The Annotation Processing API provides many of the desired features (automatic service discovery; supported by Eclipse). Unfortunately, this is aimed at code generators and doesn't support manipulation of existing artefacts:

The initial inputs to the tool are considered to be created by the zeroth round; therefore, attempting to create a source or class file corresponding to one of those inputs will result in a FilerException.

The Decorator pattern recommended by the API is not an option.

I can see how to perform the step with a runtime agent/instrumentation, but this is a worse option than a manual build step as it would require anyone even peripherally touched by the API to configure their JVMs in a non-obvious manner.

Is there a way to plug into or wrap the compiler tool as invoked by javac? Has anyone successfully subverted the annotation processors to manipulate bytecode, no matter what the doc says?

Answer 1

The Groovy compiler is the only bytecode compiler which allows to hook into the compilation process (example: Generate bytecode to support the Singleton pattern)

The Annotation Processing API is not meant to change the code. As you have already found out, all you can do is install a classloader, examine the bytecode at runtime and manipulate it. It's braindead but it works. This follows the general "we're afraid that a developer could try something stupid" theme which you will find throughout Java. There is no way to extend javac. The relevant classes are either private, final or will change with the next version of Java.

Another option is to write annotated Java, for example you write a class "ExampleTpl.java". Then, you use a precompiler which expands the annotations in that file to get "Example.java". In the rest of the code, you use Example and ignore ExampleTpl.

For Eclipse, there is a bug report to automate this step. I'm not aware of any other work in this area.

Answer 2

It can be done.

Take a look at my blog post Roman Numerals, in our Java where an annotation processor is used to rewrite code. Limitation being that it works with Sun's javac only.