Good article about how to create and process Java annotations from scratch
https://dzone.com/articles/creating-custom-annotations-in-java
https://dzone.com/articles/creating-custom-annotations-in-java
DZone
Creating Annotations in Java
Learn not only how to create annotations in Java, but also how the JVM processes them. Additionally, get some advice for implementing them in your code.
To be honest knowledge about Java bytecode is not a necessity for the most Java developers. But if you want to know something about Java bytecode - here you can read about very basics.
https://dzone.com/articles/introduction-to-java-bytecode
https://dzone.com/articles/introduction-to-java-bytecode
DZone
Introduction to Java Bytecode
Follow along this deep dive into JVM internals and Java bytecode to see how you can disassemble your files for in-depth inspections.