Are Java annotations used for adding functionality to Java code besides just adding documentation about what's going on in the code? What's the most advanced/complex functionality you could add to your code through an annotation?
4 Answers
Annotation are basically not more than a tag (with optional additional data) on a class/method/field. Other code (libraries or tools) can discover these tags and execute functionality dependant on the annotations found. I don't see a real limit on the complexity of the functionality possibly added by annotations. This can for example emulate AOP (adding functionality before or after a method with an annotation).
Comments
Annotations as such only add information (metadata) to a class.
One can easily build a system that uses that metadata to provide additional functionality, however.
For example you can use apt to generate classes based on the information provided by the annotation.
Comments
An annotation needs a tool to react to it. If such a tool does not exist the annotation is merely a notation. The "tool" can be an APT based agent or some piece of code that uses reflection (for instance, JUnit's @Test).
Several annotations are recognized by the Java compiler and thus have pre-defined semantics: @Override, @Deprecated, @Target.
Comments
I would understand Annotations as a way to document your code in a machine readable way. For example in Hibernate you can specify the whole persistence information for your objects as annotations. This is directly readable for you and not in a distant xml file. But is also readable for the tool to generate configurations, database schemes etc.
