CSC/ECE 517 Fall 2007/wiki1b 5 b4

Assignment

AspectR is a very useful Ruby module, but it is not easy to find documentation on it that is appropriate for students taking this class. Find, or construct, documentation that explains what it does without presuming previous knowledge of AspectJ, that describes many or all methods of the module and how they work. Also find or produce an easy-to-understand example that does not involve logging.

Topics: What is AspectR? What is AOP? Benefits to using.

Downloading and Installing AspectR

In order to take advantage of the features of AspectR, you must first download and install it.

1. Go to http://aspectr.sourceforge.net/ and download the tarball aspectr-0-3-5.tar.gz.
2. Unpack the tarball. If you are on Windows, this can be unzipped using Winzip.
3. From the newly created directory aspectr-0-3-5, execute "ruby install.rb".

These steps will place the AspectR module (aspectr.rb) in your Ruby library.

AspectR RubyDoc

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wrap

Arguments: target, pre, post, *args

Adds the advice specified by pre and post to all methods specified in args on the class or object target. To specify more than one method, pass a regular expression to args.

Example:

MyAspect.new.wrap(MyClass, :log_entry, :log_exit, /method/)

In this case, all calls to methods on MyClass with names beginning with "method" will be wrapped by calls to log_entry and log_exit.

unwrap

Arguments: target, pre, post, *args

Removes the advice specified by pre and post from all methods specified in args on the class or object target. To specify more than one method, pass a regular expression to args.

Example:

MyAspect.new.unwrap(MyClass, :log_entry, :log_exit, /method/)

In this case, the advice log_entry and log_exit will no longer be executed when calls to methods on MyClass with names beginning with "method" are made.

wrap_with_code

Arguments: target, preCode, postCode, *args

Wraps the code in preCode and postCode around the methods specified in args on the class or object target. This method results in faster execution than the wrap method and cannot be undone (i.e. there is no unwrap_with_code).

Example:

class SomeClass
  def method1(str)
    puts "executing SomeClass.method1 #{str}"
  end
end

Aspect.new.wrap_with_code(SomeClass, %q{puts "entering method"}, %q{puts "exiting method"}, /method/)

SomeClass.new.method1 "test"

Output:

entering method
executing SomeClass.method1 test
exiting method

add_advice

Arguments: target, joinpoint, method, advice

Adds the advice specified by advice to the method method on the class or object target. Use a joinpoint of Aspect::PRE if the advice should be executed prior to the base method; otherwise, use Aspect::POST to execute the advice after the base method returns.

Example:

MyAspect.new.add_advice(MyClass, Aspect::PRE, :method1, :log_entry)

In this case, the advice log_entry will be executed just before the code in MyClass.method1.

remove_advice

Arguments: target, joinpoint, method, advice

Removes the advice specified by advice from the method method on the class or object target. Use the same value for joinpoint that was specified when add_advice was called (Aspect::PRE or Aspect::POST).

Example:

MyAspect.new.remove_advice(MyClass, Aspect:PRE, :method1, :log_entry)

In this case, the advice log_entry will no longer be executed just before the code in MyClass.method1.

Example: Profiler Aspect

One common use of AOP is profiling. Profiling refers to the collection of data about the dynamic behavior of a system. It is done to determine a breakdown of where time and memory are being consumed at runtime and make optimizations based on the results. In the example below, which is based off a sample found in make a link -- Ruby Developer's Guide, we create an aspect that will determine the number of (milli)seconds spent in a single method. It is kept very simple to clearly illustrate the functionality of AspectR; a full-featured profiler would need to account for non-serialized and nested method invocations, aggregate the results, etc.

The Profiler class, which inherits Aspect, contains two methods: profiler_enter and profiler_exit. The before advice, profiler_enter, records the time immediately before the method is invoked. The after advice, profiler_exit, records the time immediately after the method exits and prints out the duration of the method invocation.

require 'aspectr'
include AspectR
class Profiler < Aspect
   
  def profiler_enter(method, object, exitstatus, *args) 
    @enter_time = Time.now
    puts "#{@enter_time.strftime('%Y-%m-%d %X')} #{self.class}##{method}: #{args.inspect}" 
   end
   
  def profiler_exit(method, object, exitstatus, *args) 
    @exit_time = Time.now
    print "#{@exit_time.strftime('%Y-%m-%d %X')} #{self.class}##{method}: exited " 
    if exitstatus.kind_of?(Array) 
      print "normally returning #{exitstatus[0].inspect} " 
    elsif exitstatus == true 
      print "with exception '#{$!}' " 
    else 
      print "normally " 
    end 
    puts "after #{@exit_time.to_f - @enter_time.to_f} seconds"
  end
end

A user of Profiler would need to wrap all methods that she is interested in profiling. The program below uses the Profiler aspect to calculate the amount of time spent in the SomeClass.method1 method call.

class SomeClass
  def method1(str)
    puts "entering SomeClass.method1 #{str}"
    sleep 5.3
    puts "exiting SomeClass.method1"
  end
end

Profiler.new.wrap(SomeClass, :profiler_enter, :profiler_exit, :method1)
SomeClass.new.method1 "test"

This program generates the following output:

2007-09-30 22:05:51 Profiler#method1: ["test"]
entering SomeClass.method1 test
exiting SomeClass.method1
2007-09-30 22:05:56 Profiler#method1: exited normally returning nil after 5.29699993133545 seconds

You can see from the Profiler output that SomeClass.method1 took approximately 5.3 seconds to execute, as expected.

Note: A profiler for Ruby programs, RbProf, is included in the AspectR distribution.

References

1. CSC517 Lecture 08: Reflection and metaprogramming
2. AspectR README