Noak

A library for reading and writing java class files.

Example

use noak::reader::Class;

let mut class = Class::new(&bytes)?;

println!("Major Version: {}", class.version().major);
println!(" Access Flags: {:?}", class.access_flags()?);
println!("   Class Name: {}", class.this_class_name()?.display()?);

Why should you use noak?

• Reading:
  • You want to parse class files without losing information.
  • You don't want to parse the entire class file.
  • Any valid class file is accepted by noak. Many invalid class files are accepted as well (this can be useful for reading heavily obfuscated code).
• Writing:
  • You want to write your own class files (limitations apply; see below).

Why wouldn't you use noak?

Many of these issues are in the process of being resolved, but that may take some time.

• You want more than just a low-level class file reader and writer.
• Noak makes heavy use of the type system and macros. This can cause problems with your IDE or make some aspects hard to understand.
• Documentation is virtually non-existent.
• The API is very unstable.
• The code is not heavily tested.
• Reading:
  • Some code may be repetitive (e.g. retrieving values from the constant pool).
• Writing:
  • Modifying existing class files can be very tedious.
  • Not every attribute can be written at the moment (related issue)
  • Jumps above 65535 bytes may fail to be written.
  • The builder API isn't flexible enough for your use case.
  • Custom errors are quite restricted.
  • Stack Map Frames are not automatically generated.

Alternatives

This is not an exhaustive list. The statements below may not accurately reflect reality.

Libraries written in Rust:

• cafebabe
  • Parses the entire file at once.
  • Accesses the constant pool during parsing. No explicit retrieval from the user side required.
• classreader-rs
  • Parses the entire file at once.
  • Only supports parsing.
  • Does not support any attributes from version 53.0 and above.
  • Does not parse strings containing unpaired surrogates.
• jbcrs
  • Parses the entire file at once.
  • Only supports parsing.
  • Does not support any attributes from version 53.0 and above.
  • Does not parse files containing UTF-8 constants with unpaired surrogates.
• frappe/classfile
  • Parses the entire file at once.
  • Only supports parsing.
  • Does not support any attributes from version 53.0 and above.
  • Does not parse files containing UTF-8 constants with surrogates.
• classfile-rs
  • Seems to be very incomplete.
  • Supports writing as well as parsing.
  • Parses the entire file at once.
  • Reads UTF-8 constants lossily.
• classfile-parser
  • Seems to be very incomplete.
  • Parses the entire file at once.
  • Only supports parsing.
• classfmt
  • Seems to be very incomplete.
  • Parses the entire file at once.
  • Only supports parsing.
• javabc
  • Seems to be very incomplete.
  • Parses the entire file at once.
  • Only supports parsing.

A small fraction of libraries written in languages that are not Rust:

• ASM (Java)
  • The most comprehensive and best supported class file manipulation and analysis framework.
  • Use this if noak does not suit your requirements and probably even if it does suit them.
• javassist (Java)
  • A high-level bytecode library.
• BCEL (Java)

License

This project is licensed under the MIT license.