README.md

Aspectj + annotation processing

This project is an example of how you can combine annotation processing with aspectj.

Problem

This code solves a specific problem which can be described as follows: we want to have an opportunity to add callbacks to arbitrary methods invocations (before annotated method or after). For example lets consider a simple class:

    public class Application {

        public void someMethod1() {
            System.out.println("someMethod1 is invoked");
        }
    }

What we want is to write code like this:

    Application app = new Application();
    app.OnSomeMethod1BeforeEvent.add((emmiter) -> {
        System.out.println("some before-callback for method \"someMethod1\"");
    });

Without Aspectj we would have to add new field per method to accumulate callbacks and moreover we need to modify the method itself.

Annotation processing

Firstly we mark some methods with apt.Event annotation from module annotation-processors. After that APT will generate 2 files:

1. {$className}Callback.java - this class contains only callbacks interfaces
2. {$className}EventsAspect.aj - this is aspect declaration for our class. It injects new fields to class for each annotated method for holding callbacks which will be called after/before method will be invoked respectively to annotation type:
   • apt.Event#Order.Before
   • apt.Event#Order.After

Usage

How to run my.Application:

gradlew app:run -i

To clean all:

gradlew clean