[cess11]

There is metaprogramming support in Java, but it's not as inviting and friendly as hygienic macros or Ruby patching. The obvious example is reflection, with which you can do a lot of bizarre things, some of which are useful sometimes. Another is annotations, which is heavily used by libraries in a way similar to how macros are used in certain Lisp like languages.

https://www.baeldung.com/java-reflection

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

Then you've got the byte code itself, and there be dragons: <https://aphyr.com/posts/341-hexing-the-technical-interview>

While you rarely see byte code shenanigans in Java code bases, it's how some other languages on the JVM achieve things like runtime metaprogramming.