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This is pretty exciting... I've used them all libraries at this point in my career: CGLib, ASM, BCEL, ByteBuddy, Javassist, etc... each has its pluses and minuses. I've designed everything from profiling agents, to systems that pack decimals into EBCDIC and invoke COBOL programs on big IBM iron, to lightweight JIT compilers, all using these libraries.
> In 2002, the visitor approach used by ASM seemed clever
I couldn't agree more. The visitor pattern was very hard to explain/justify back then, and still difficult to explain to newbie programmers just entering the profession.
Looking at the examples, I think this is going to be an official replacement for ASM, meaning it's going to be pretty low level. The use of streams pretty straightforward.
If anyone from the JEP is reading this: I have two pieces of feedback!
First, take some inspiration from the way CDI Portable Extensions work. This is probably the most delightful extension API I've ever used. The @Observe callbacks are super simple to explain to people and it's really easy to write extensions for the framework.
Next, I wouldn't ignore the need for a higher-level API akin to ByteBuddy or Javassist. Sometimes I just want to write an interpreter or intercept a method call and thats it.
For example in my Junit/Mockito extension https://github.com/exabrial/mockito-object-injection I need to intercept a call to the class under test in order to lazily inject dependencies at the last possible moment. While I certainly could do this with ASM, Javassist makes this fairly simple with it's MethodHandler api.
Side note, it's a damn shame we don't have a mobile operating system that is JVM native :/ All this cool APIs simply never reach a huge number of devices.
Lombok is not a compiler extension. Compiler extensions, aka annotation processors, are offered only specific capabilities that ensure that they preserve the Java language specification. Particularly, code that compiles successfully with an extension also compiles without it (perhaps requiring other classes to be available) and it compiles down to the same bytecode. Annotation processors are used to implement pluggable type systems (e.g. https://checkerframework.org) or to generate other classes (e.g. https://immutables.github.io/).
Unlike compiler extensions, Lombok compiles source files that do not conform to the Java language specification. Lombok is an alternative Java Platform language, like Clojure or Kotlin or Scala, except that it's a superset of the Java language. However, rather than forking `javac` source code and modifying it to compile Lombok source files, the Lombok compiler modifies `javac`'s operation by hacking into its internals and modifying them as it runs to compile Lombok sources rather than Java sources.
Having alternative Java Platform languages is perfectly fine. The problem with Lombok is that it doesn't present itself as such but as a library or a compiler extension even though it violates the Java language specification in ways that compiler extensions are forbidden from doing.