Using alias_method to implement AOP in Ruby

This week I just realize the power of ruby alias_method, you use alias_method to intercept the existing method, and modify its behaviour,  this is exactly what AOP does the jobs.

The following code example tell you how to use the alias_method, to add a logging logic to the exit() method.

	module Mod
	alias_method :orig_exit, :exit
	def exit(code=0)
	puts "Exiting with code #{code}"
	orig_exit(code)
	end
	end
	include Mod
	exit(99)

view raw alias_method hosted with ❤ by GitHub

This reminds me the javaasist framework, I used before, which is a powerful tool to do bytecode manipulation, it can intercept the method by renaming the existing method to another method,  and modifying the existing method, which is the exactly similar strategy.
the code example:

	private static void addTiming(CtClass clas, String mname)
	throws NotFoundException, CannotCompileException {
	// get the method information (throws exception if method with
	// given name is not declared directly by this class, returns
	// arbitrary choice if more than one with the given name)
	CtMethod mold = clas.getDeclaredMethod(mname);
	// rename old method to synthetic name, then duplicate the
	// method with original name for use as interceptor
	String nname = mname+"$impl";
	mold.setName(nname);
	CtMethod mnew = CtNewMethod.copy(mold, mname, clas, null);
	// start the body text generation by saving the start time
	// to a local variable, then call the timed method; the
	// actual code generated needs to depend on whether the
	// timed method returns a value
	String type = mold.getReturnType().getName();
	StringBuffer body = new StringBuffer();
	body.append("{\nlong start = System.currentTimeMillis();\n");
	if (!"void".equals(type)) {
	body.append(type + " result = ");
	}
	body.append(nname + "($$);\n");
	// finish body text generation with call to print the timing
	// information, and return saved value (if not void)
	body.append("System.out.println(\"Call to method " + mname +
	" took \" +\n (System.currentTimeMillis()-start) + " +
	"\" ms.\");\n");
	if (!"void".equals(type)) {
	body.append("return result;\n");
	}
	body.append("}");
	// replace the body of the interceptor method with generated
	// code block and add it to class
	mnew.setBody(body.toString());
	clas.addMethod(mnew);
	// print the generated code block just to show what was done
	System.out.println("Interceptor method body:");
	System.out.println(body.toString());
	}

view raw javaassist_sample hosted with ❤ by GitHub

Now when you compare two solution, you will tell how easy to do it in Ruby!

Resource
1. alias_method API document
2. IBM Javaassist tutorial 
3. Spring AOP document