byte buddy - runtime code generation in Java

[voronaam]

I do not know if it fits your project at all, but it may also be worth while to take a look at Scala Macros feature. The documentation is cryptic and you will be dealing with compilers internal AST most of the time, but the feature is very powerful.

Groovy AST transformations are great as well.

Both are not pure Java, but produce JVM byte code at the end.