CSC/ECE 517 Summer 2008/wiki1 3 jb

This wiki will explore how reflection is implemented with both Java and Ruby, with the goal of showing which language's implementation makes it easier to write, easier to understand, and more efficient. Reflection refers to the ability of a program to peer inside itself and observe the components that it is comprised of. Reflection, as supported in Object Oriented languages such Java and Ruby, provides facilities to query about the methods and attributes of a specified class, and to execute methods that are discovered at runtime. These two features of reflection in Java and Ruby will explored in detail below.

To illustrate the concept of reflection, a simple example will be implemented in both Java and Ruby. The data model in the example will consist of a mountain bike class and disc brake class. A mountain bike in this example is composed of disc brakes. The mountain bike class will expose a public setter method method to set the disc brakes object. This setter method will be queried for by name at runtime, and executed.

Reflection in Java

The following is an implementation of the example in Java. A detailed description of the code follows.

import java.lang.reflect.*;

class DiscBrakes { }

class MountainBike
{
	private DiscBrakes m_discBrakes = null;

	public boolean hasDiscBrakes() { return m_discBrakes != null; }
	public void setDiscBrakes(DiscBrakes discBrakes) { m_discBrakes = discBrakes; }

	public static void main (String argv[])
	{
		Class c = MountainBike.class;

		MountainBike mb = new MountainBike();
		DiscBrakes db = new DiscBrakes();
		Method meth = null;

		System.out.println(mb.hasDiscBrakes());

		try 
		{
			meth = c.getMethod("setDiscBrakes", DiscBrakes.class);
		}
		catch (NoSuchMethodException e) { }

		try
		{
			meth.invoke(mb, db);
		}
		catch (IllegalAccessException e) { }
		catch (InvocationTargetException e) { }
			
		System.out.println(mb.hasDiscBrakes());
	}
}

The first thing to note in the Java implementation is the import of the java.lang.reflect package. This package provides the method class.

Reflection in Ruby

class DiscBrakes
end

class MountainBike
  @m_discBrakes = nil
  def hasDiscBrakes 
      return @m_discBrakes != nil
  end
  def setDiscBrakes(discBrakes)
    @m_discBrakes = discBrakes
  end
end

mb =  MountainBike.new

puts mb.hasDiscBrakes

db = DiscBrakes.new
setter = mb.method("setDiscBrakes")
setter.call(db)

puts mb.hasDiscBrakes

Java Reflection Links

Pros and cons of reflection, written for Java but pretty generic
Using Java reflection
Javadoc for the java.lang.Class class
Javadoc for the java.lang.reflect package

Ruby Reflection Links

Good overview of reflection in Ruby