CSC/ECE 517 Summer 2008/wiki1 5 a5

Introduction

Ruby has hooks that allows trapping a certain event (e.g., object creation) and running a particular code sequence whenever the event occurs. There's no comparable facility in Java. But both Ruby and Java have support for aspect-oriented programming (AspectR and AspectJ, respectively). What's the difference between simply providing hooks, and supporting full AOP, and why is it more convenient to program this way in Ruby than Java? Give a few code sequences to justify your conclusions.

Hooks

There is a strong desired from developers to have control on object life cycle. They would like to know when an object is created or destroyed, or some specific method is executed. They would like to monitor what application is doing inside and look for possibility to change application behavior or add new functionality without making big changes in object model. Some time programming languages provide hooks to monitor such events. Hooks are set of code that automatically execut when a particular event triggers.

Hooks Implementation

Most of dynamic languages provide some ways to execute custom code at different steps of object life cycle. Most of them built this functionality as part of language design. They use interceptor design pattern as implementation guideline.

Hooks in Ruby

Ruby is a dynamic and pure object oriented language. It has a very strong support for metaprogramming. It provides system hooks for monitoring events like object creation. The technique use by Ruby to provide this functionality is a simple example of interceptor design pattern. By intercepting calls to system classes developer could modify the system behavior without changing application code.

Example Code:

Following is an example code from Programming Ruby - The Pragmatic Programmer's Guide (First edition is freely available on web) for hooking objects creation event. This code modifies two Ruby system classes (Class and Object). It renames and redefines the Class new method and modifies the Object class to store timestamp.

  class Class 
           alias_method :old_new, new
           def new (*args)
                       result = old_new(*args)
                       result.timestamp = Time.now
                       return result
           end
  end

  class Object
           def timestamp 
                       return @timestamp
           end 
           def timestamp = (aTime)
                       @timestamp = aTime
           end
  end

Now, lets run some tests:

  class Test
  end

  obj1 = Test.new
  sleep 2
  obj2 = Test.new

  obj1.timestamp 
  obj2.timestamp

Hooks in Java

Java is statically type language. It doesn’t provide any language level ability to monitor events like object creation etc. However, the latest JVM version through JVMTI allow some hooks for monitoring performance but there is no way for the developer to change the behavior of an application through the available hooks.

Benefits of Hooks

Hooks is an elegant way of changing application behavior in dynamic languages. It is simple and compatible with dynamic languages philosophy. Since most time it is part of language, developer does not have to learn new API to take advantage of this.

AOP vs Hooks

From earlier days of object-oriented programming languages, programmers are using different techniques to execute code at different phases of object life cycle. They have successfully used programming concepts like subjects, interception and delegation to deal with such requirements. All modern object-oriented dynamic languages now support hooking mechanism for such situations.

Aspect Oriented Programming on other hand is a new concept. It provides more comprehensive solution for dealing with complex requirements related to cross-cutting concerns and modularity. It has its own domain-specific language to deal with such problems. There is no comparison between AOP’s extensive semantics with hooks implementation. Hooks could be use as AOP implementation mechanism but they will never be able to compete with AOP functionality.

It is hard for us to come up with some good example that shows the limitation of hooks compare to AOP. Since both of us has no experience in Ruby and AOP. But while searching on web we come across a specification for implementing Cut-based AOP in Ruby. This specification discusses how we could add additional functionality to existing class without subclassing it. It introduces a new class called Cut. It is used to encapsulate advice for a single class. Following is an example for adding new functionality to existing class C without subclassing it using new Cut class,

  class C
  	def f(*args); 1; end	
  	def g(*args): 2; end
  end

In order to add new functionality we would subclass C. i.e.

  class A<C
  	def f
  		print ‘(’, super, ‘)’
  	end
  end

Now with AOP extension for Ruby,

  cut A < C
  	def f
  		print ‘(’, super, ‘)’ 
  	end
  end

Now instead of subclassing C “we have cut-across behavior of C with A”.

Now we will discuss how Ruby hooks are still better. If we compare above two examples (Ruby Hook and Java AOP) we could see using AOP is over kill for such simple problem. Ruby hooks are more elegant solution in such situation. AOP are more suitable for statically type languages like Java. Ruby as dynamic language needs simpler syntax for dealing with simple cross-cutting concern problems. It supports very strong metaprogramming functionalities using interceptors, hotswapping, mixins and hooks that make lot easier for injecting custom code during runtime. Different Ruby experts also share the same opinion,

“For Ruby developers, AOP is not quite as urgent, because you’ve already got robust tools to deal with these kinds of concerns.” Bruce Tate – Beyond Java.

“A standardized AOP framework has never really taken off in Ruby because the language itself already supports most of the desirable functionality of AOP.” David Heinemeier Hansson – Rail Architect

Links

• Ruby
• AspectR
• AspectJ
• Dynamic Languages
• interceptor design pattern
• Programming Ruby - The Pragmatic Programmer's Guide (First edition)
• JVMTI
• Does Ruby need AOP ?