Aquarium 0.4.2: Aspect-Oriented Programming for Ruby

Overview

Aquarium is a framework that implements Aspect-Oriented Programming (AOP) for Ruby. The premise of AOP is that some concerns in an application will cut across the natural object boundaries of the problem domain. Rather than scatter duplicated code in each object to handle the cross-cutting concern, AOP modularizes the specification of which execution points are affected (called join points) and the actions that should be invoked at those points.

For example, persistence of “model” objects is a cross-cutting concern, in the sense that the desired persistence approach (database, flat files, replication, etc.) is independent of the domain logic represented by the model. So, why should the model code have any persistence logic? Instead, capture the details of mapping the domain to the persistence approach in separate “components” and programmatically or declaratively modify the model objects to synchronize state changes with the persistent memory of the state.

New in V0.4.0: Preliminary support for advising Java classes in JRuby! See the discussion here.

See also the RubyForge project page.

How to Use Aquarium

Aquarium provides a Domain Specific Language (DSL) with which you can express “aspectual” system behaviour in a modular way, i.e., using a succinct language and without repeating yourself all over your code base!

Imagine you want to trace all invocations of the public, instance methods in all classes whose names end with “Service”. Here’s how you can implement that behavior in Aquarium:

class ServiceTracer
    include Aquarium::Aspects::DSL::AspectDSL
    before :calls_to => :all_methods, :in_types => /Service$/ do |join_point, object, *args|
      log "Entering: #{join_point.target_type.name}##{join_point.method_name}: object = #{object}, args = #{args}" 
    end
    after :calls_to => :all_methods, :in_types => /Service$/ do |join_point, object, *args|
      log "Leaving: #{join_point.target_type.name}##{join_point.method_name}: object = #{object}, args = #{args}" 
    end
end

The #before advice adds behavior that is invoked before each method is invoked, in this case, it logs a message with the name of the executing class and method, followed by the list of arguments passed to the method.

The #after advice adds similar behavior the is invoked after each method is invoked.

A more succinct implementation of this behavior uses #around advice:

class ServiceTracer
    include Aquarium::Aspects::DSL::AspectsDSL
    around :calls_to => :all_methods, :in_types => /Service$/ do |join_point, object, *args|
      log "Entering: #{join_point.target_type.name}##{join_point.method_name}: object = #{object}, args = #{args}" 
      result = join_point.proceed
      log "Leaving: #{join_point.target_type.name}##{join_point.method_name}: object = #{object}, args = #{args}" 
      result  # block needs to return the result of the "proceed"!
    end
end

The special method #proceed invokes the advised method, passing it the args list (by default). For #around advice, you must call #proceed unless you specifically don’t want the original method called!

See the Examples and the API section for more details.

Start Here

$ gem install -y aquarium

See the download page for different options or go directly to Rubyforge download page.