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
Object-Oriented Programming (OOP) was a big shift from procedural programming. Now instead of concentrating on procedures and data separately, developers start working with objects and their interactions to design applications. This allowed them to deal with large and more complicated system and develop them in less time ever before. Later Aspect-oriented Programming made developers life easier by allowing them to dynamically modify the static OO model to create a system that can grow to meet new requirements.
Aspect-Oriented Programming helped the developers in separation of concerns, specifically [1] cross-cutting concern, as an advance in modularization. – Wikipedia
AOP is look very simple in definition but it is quite complex topic. Before going further, it is better to familiarize ourselves with some related terminology.
AOP Implementation
AOP in Ruby
AOP in Java
Benefits of AOP
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