JBcRs

JBcRs is a Library, written in rust, to support reading and writing of java class files.

This library is not close from being finished, but certain features have already been implemented:

• Basic parsing: A class file is parsed rather primitive:
  • You have access to the constant pool. Names, Descriptors and more are represented as u16, Indexing into the pool must be done manually. No validation of indices when parsing will be done.
  • Access Flags are decoded using the bitflags crate to provide a better experience.
  • Attributes are always parsed. This might be changed since it allows attackers to craft invalid Debug Attributes, which don't play a serious role in executing the code, Parsing the entire class file then might not work, since an error will be returned.
• More will be coming soon™.

Getting Started:

First, add this library as a dependency to your Cargo.toml

[dependencies]
jbcrs = "0.1.0"

Now, you should choose if you want to use basic or advanced, but since advanced is not yet implemented, basic must be used.

Basic

We want to parse a class from a byte array and print its version, access flags and name. Of course you could use std::fs::File or a zip library, but showing this is not the purpose of this tutorial.

use jbcrs::basic;

// You got the bytes from any possible source.
let bytes: &[u8] = [0xCA, 0xFE, 0xBA, 0xBE];

// After parsing the class file,
// you will get the constant pool
// and the class itself.
// You don't have to annotate the types here.
let (constant_pool, class): (basic::Pool, basic::Class) = basic::parse(bytes)
    .expect("could not parse class file");

// Print its major and minor version:
println!("version: {}.{}", class.major_version, class.minor_version);

// Access Flags can be printed human readable
println!("access: {}", class.access_flags);

// Printing the name requires us to use the constant pool.
println!("name: {}", constant_pool.get_class_name(class.name).expect("could not get class name"));

Resources

Java Virtual Machine Specification (Java SE 9)