Declarative Dependency Injection via Comments

[James Talmage <ja...@jrtechnical.com>]

Intellij Idea will let you define the class type of context elements in a
multi-line comments like this:

       #* @vtlvariable name="service"
type="com.google.appengine.api.users.UserService" *#

this provides very handy code completion, but I thought it would be cool to
let that same line of code double as a way to grab objects from my
dependency injection framework.

I have modified the velocity 1.6.4 code base to find "Annotations" and
"Attributes" in multi-line comments, and provide hooks for processing them.
 The following line:

       #* @inject @vtlvariable name="service"
type="com.google.appengine.api.users.UserService" *#

Would produce a callback containing two annotations (@inject &
@vtlvariable), and two attributes (name & type).  It's then pretty straight
forward to create code that will do something with those values (in my case
- grab the specified type from a google guice context, and put it in the
context using the specified name).

I could see other uses for it:  Instantiating tools objects, ensuring the
context contains a specific type (@typesafe), require that a context has a
certain value (@required).

It was a fun weekend project that I'd be happy to donate back to the
community, if there is interest.