Annotations, introduced in Java 5 two decades ago, have revolutionised how we develop Java applications. Traditional frameworks like Spring and Hibernate rely on runtime annotation processing using reflection to perform tasks such as dependency injection or implement persistence operations.

Alternatively, the Java Annotation Processing API, introduced in Java 6 in 2006, allows developers to hook into the compilation phase to process sources and react to annotations present on them. This was leveraged by libraries like Lombok and Dagger, and frameworks such as Micronaut, although their way of processing annotations varies. Other frameworks like Quarkus perform the annotation processing during build time, instead of the compilation time.

In this session, Micronaut Framework committer Álvaro Sánchez-Mariscal will explain with examples the different techniques that can be used for both runtime and compile-time processing of annotations, revealing the magic behind popular open-source projects.

https://speakerdeck.com/alvarosanchez/java-annotations-magic
https://www.youtube.com/watch?v=mAXNKkejl38