CSC/ECE 517 Summer 2008/wiki1 5 bk

Hook Concept

The hooking mechanism in Ruby allows functional events inside a running program to be identified while providing an action that will occur upon detection of the specified event. In this way, hooks serve as a detector of actions within the code, when event x is encountered, y will be performed. Examples where such a technique would prove useful include:

• Recording when or how many times a method is called.
• Recording when an object is created.
• Debugging problematic code.

Example of Hooking System Calls (citation)

The following is an example of a hook being inserted to perform a certain action when the program makes a system call. In order for this to occur, an alias must be created so that the original functionality can still be accessed. In this case, original_system is aliased to the unaltered system. Once this is done, we can freely modify the definition of system to perform just about any operation. This is similar to the way in which a nefarious hook would work by still performing the intended operation, but also appending something extra...

alias_method :original_system, :system

The following code will then proceed to print the command executed ("ls -al" in this case), the timestamp, and then finally the output of the command to standard output.

module Kernel
  alias_method :original_system, :system
  def system(*args)
    puts "Your program executed the \"#{args.join(', ')}\" command." 
    puts "It was executed at: " + `date`
    puts "The command output the following: "
    original_system(*args)
  end
end
system("ls -al")

Your program executed the "ls -al" command. It was executed at: Mon Jun 2 21:19:27 EDT 2008 The command output the following: total 20 drwxr-xr-x 2 brking brking 4096 2008-06-02 21:19 . drwxr-xr-x 18 brking brking 4096 2008-06-02 21:19 .. -rw-r--r-- 1 brking brking 134 2008-06-01 22:01 fib.rb -rw-r--r-- 1 brking brking 235 2008-06-02 20:29 hooking.rb -rw-r--r-- 1 brking brking 287 2008-06-02 21:19 moreHooking.rb

Similarly, a method could be hooked to perform additional tasks upon calling the method. The method would still carry out its original functionality, but the new instructions would precede or follow. It is important to point out that while Ruby supports hooking as part of the language, Java does not. However, similar functionality to hooking is provided through Aspect Oriented tools in both languages. A discussion of the specific implementations as well as their advantages and disadvantages with respect to native hooking follows.