Compile to java bytecode (without using Java)

Question

My compilers class is creating a language that we intend to compile to Java Bytecode. We have made plenty of progress and are nearing the time where it's time for code generation.

We are having problems locating information on how to create .class files from our compiler. Do you have any resources that can give us some assistance? We already have plenty of documentation on the instruction set, but need information on how to directly fill out the class file/the writing of hex.

We do not need information or suggestions on decompiling the .class files.

Even a simple example of writing out a .class file from scratch would be excellent.

The JVM spec is not what we're after. What we really need is an example or a walkthrough.

Answer 1

There are a number of projects out there that provide a high level interface to creating Java class files without you having to write the class files yourself. Take a look at the following:

• ASM - http://asm.objectweb.org/
• BCEL - http://jakarta.apache.org/bcel/
• Trove - http://teatrove.sourceforge.net/trove.html

All provide an API to create class files. You could always look at the code they've written to do this and write some similar code for your compiler although I would imagine that it's a fair amount of work.

With BCEL take a look at ClassGen, that should enable you to write out class files in the format you want, a simple example follows:

ClassGen cg = new ClassGen("HelloWorld", "java.lang.Object",
                             "<generated>", ACC_PUBLIC | ACC_SUPER,
                             null);