Adding Annotations to JRuby Using Ruby

I love how meta-programmable Ruby is.

JRuby doesn't support annotations because Ruby doesn't support annotations. So what! We can extend Ruby to add something like annotations:

class JPABean
def self.inherited(clazz)
@@annotations = {}
def self.anno(annotation)
@@last_annotation = annotation

def self.method_added(sym)
@@annotations[sym] = @@last_annotation

def self.get_annotation(sym)
return @@annotations[sym]
Given this, we can do things like the following:

class MyBean :sql => "SELECT * FROM stuff WHERE name = 'foo'"
def foo; end

anno :field => :hello_id
attr :hello

anno :field => :bar_in_table
attr_accessor :bar
And here's some output from the resulting class:

p MyBean.get_annotation(:foo)
p MyBean.get_annotation(:hello)
p MyBean.get_annotation(:bar=)


{:sql=>"SELECT * FROM stuff WHERE name = 'foo'"}
So as you can see we really do have all the necessary requirements to annotate classes. Now what if we just had MyBean be a java.lang.Object extension and stuffed the annotations into the resulting generated class? We can already create real Java classes in this way, but with the above syntax and a little magic in the JRuby Java integration later, we've got annotation support in on Java classes too. This should enable things like Hibernate, JPA, and JUnit 4 to work with JRuby's Ruby-based classes. Or at least, I believe it to be possible. It just requires a little work to add annotation information to the resulting generated classes.

I've planted the seed here and on the JRuby dev list...let's see if it germinates.
Written on May 28, 2007