proguard-core VS Byte Buddy

Anyways, have you looked at Proguard core?

For generating Java bytecode/class files you could take a look at ProGuardCORE . It's a bytecode manipulation and analysis library that grew out of ProGuard.

The MethodBuilder "low-level" example looks like the ProGuardCORE API for building classes & code sequences, which can be written as:

Something else that could make your code generation for your JVM language easier: ProGuardCORE (https://github.com/Guardsquare/proguard-core). It can be used to read, generate and analyse Java bytecode.

Some examples for code generation: ProGuard where the project originated (https://github.com/Guardsquare/proguard), Brainf*ck compiler (https://github.com/mrjameshamilton/bf), Lox compiler (https://github.com/mrjameshamilton/klox)

Disclaimer: I work at Guardsquare on ProGuardCORE so may be biased ;)

I implemented Lox in Kotlin based on the book's Java interpreter then added a JVM backend using ProGuardCORE: https://github.com/mrjameshamilton/klox

You can find out more about ProGuardCORE on GitHub and the manual contains some further examples.

Not a book or guide but a useful library for working with/generating JVM class files is ProGuardCORE. It's the underlying library used by ProGuard and I also recently used it to add a JVM backend to my implementation of Lox from the Crafting Interpreters book.

After completing the first part of the book (using Kotlin instead of Java), I added a JVM backend using ProGuardCORE (https://github.com/Guardsquare/proguard-core).

The biggest difference is our initial goal of creating a tool for application developers rather than a security team. This means we aim to exclude information that the application developer definitely knows and doesn't want to see again, e.g. the list of permissions an application is requesting. Furthermore we try to focus on nice and intuitive UX that is familiar to this audience, enabling an application developer to be very efficient in reading & interpreting the results (e.g. easy comparison of two builds). Also note that this is built on the same core technology as ProGuard. This foundation of compiler components (e.g. our partial evaluator) and the knowledge and experience at Guardsquare that comes with it will unlock many more in-depth code checks.

As seen above, the API exposes the user to low-level bytecode manipulation via byte arrays. It would be unwieldy to do it directly. Hence, real-life projects rely on bytecode manipulation libraries. ASM has been the traditional library for this, but it seems that Byte Buddy has superseded it. Note that Byte Buddy uses ASM but provides a higher-level abstraction.

Just a drive-by comment: ByteBuddy is worth a look https://bytebuddy.net/. It is built on top of ASM.

> can you show an example of how you'd parse, say, a .java.in

The canonical way to do such a thing is through the java annotation processing api [1] and using a tool like java poet [2]. Before you did that, you'd probably decide if you wanted to instead use bytecode generation with a library like bytebuddy [3]

But, assuming for some reason, you wanted to torture yourself and actually consume a java.in file and apply a regex, then you'd probably pull out the "maven-replacer-plugin" [4] and configure that for the task at hand. (or use your favorite templating language plugin. There's a million of them).

Though, to be fair, this really isn't something that comes up in regular java programming due to the nature of the ecosystem. Anything you'd want to codegen likely already has a library and anything you didn't would receive (legitimate) push back.

[1] https://www.baeldung.com/java-annotation-processing-builder

[2] https://github.com/square/javapoet

[3] https://bytebuddy.net/

[4] https://github.com/beiliubei/maven-replacer-plugin

Have a look at byte buddy.

If you've ever used libraries like https://github.com/JakeWharton/hugo or https://hibernate.org/ (if you've ever done some backend development) and wondered how do they seem to add some code/logic into your app just by adding some annotation to some method, or if you ever wondered how mocking frameworks like Mockito can change a class behavior for example, then most likely you're interested in a programming technique that allows to modify existing code, usually known as Aspect oriented programming (also known in Java as Bytecode instrumentation) which, even though it might sound intimidating at first, some really cool tools such as Byte Buddy or AspectJ make it quite easy to accomplish.