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