cafebabe

cafebabe is a parser for .class files, which are generated by the javac compiler and other compilers targeting the JVM. It supports the class file format from Java 21 (September 2023).

There are a bunch of different class file parsers available on crates.io. Main differentiators for cafebabe are:

• Support for Java 21.
• A better user API. Most other parsers expose the raw u16 values of the constant pool, which means you as the user end up being responsible for doing the constant pool lookup. This also means you have to do type validation (e.g. make sure the constant pool entry you're reading is actually a classinfo type, or whatever). cafebabe does all this internally and returns the "primitive" type you get after doing the constant pool lookups. Consider the example of reading a classinfo member. With most other parsers, you get a u16 from the parser, with which you have to: (a) do a constant pool lookup, (b) verify it's a classinfo type, (c) extract the u16 from the classinfo, (d) do another constant pool lookup, (e) verify it's a utf8 type, (f) read the string. With cafebabe you just get the string right off the bat, since all the type checking and lookups were done during the parsing phase.
• Zero-copy of string data. All strings are returned as Cow<'a, str> which are tied to the lifetime of your input class bytes. This means there's zero copying for strings anywhere in cafebabe itself, except in the rare case where a string is in the subset of modified "java utf-8" that isn't regular utf-8.
• Minimal dependencies. The crate only has a couple of dependencies, and neither of those pull in any transitive dependencies.

Current status

The main parsing code is fully implemented. All structures (including attributes) described in Chapter 4 of the JVM spec are supported. The entire modules file of the OpenJDK 16/17/21 distributions can be parsed without errors. cafebabe will do some kinds of validation/checking at parse time, but not everything described in Chapter 4 of the JVM spec. It does all of section 4.8 validation ("Format checking") except possibly some of the attribute validations (the Signature attribute, for instance). A reasonable goal for this project is to fully implement section 4.8, so patches would be accepted for additional format checking.

Q&A

Why is the project called cafebabe?

Because the first 4 bytes in any valid class file are a magic identifier with the value 0xCAFEBABE. ☕