Accessing class bytes?

Question

My goal is to access the byte[] representing the bytecode of a class without specifically knowing the location of the class files at runtime.

I have looked into two solutions and was able to get mild success out of one of them, but I was wondering if there might be other ways to accomplish it (or how I went wrong in the second solution that I couldn't get to work).

My first (mildly) successful solution was to use the java.lang.instrumentation ClassFileTransformer class to access the byte[] of the classes. Though this workek, I assumed that there must be a cleaner way to accomplish this.

My second solution was to use the -Xbootclasspath JVM argument to replace java.lang.ClassLoader with my own allowing it to have access to the byte[] of the classes loaded. I added a simple System.out.println debug message to confirm that the overriding of the ClassLoader was working, but it wasn't. I got this idea from this paper on the same subject. My class was made similarly to how the Integer class was remade in the linked paper. I also used a similar directory setup for the JVM argument looking something like this:

java -Xbootclasspath/p:.\out\production\boot\java\lang TestLoader

My thought is that the ClassLoader class specifically cannot be overridden using the method in the paper I linked.

I would be interested seeing why my attempt at overriding the ClassLoader did not work and also in hearing what else I could do to access the byte[] of classes.

Answer 1

Could you just read the class bytes using getResourceAsStream()?

InputStream is = String.class.getResourceAsStream("String.class");

Edit adding alternative:

(copied from comment)

Given all the possibilities that need to be covered, ClassFileTransformer and instrumentation API might be the way to go. I don't know what the requirements are for 'clean', but if the issue is having to specify command line arguments to the JVM you could try using the Attach API - you can attach to an already running Java process, push in your ClassFileTransformer, and look at all the classes already loaded in the JVM plus any that are loaded thereafter.