Skip to main content

What is metaprogramming?

The word metaprogramming keeps coming up everywhere in stuff I read nowadays. Even though I sort of knew what it meant I wasn't really sure. So I decided to ask the question 'what exactly is metaprogramming' on SO.

I got some really nice answers and links to articles. I have written this blog post as an effort to summarize what I have learned and to be able to share it with others who might be interested.

According to Wikipedia

Metaprogramming is the writing of computer programs that write or manipulate other programs (or themselves) as their data, or that do part of the work at compile time that would otherwise be done at runtime. In many cases, this allows programmers to get more done in the same amount of time as they would take to write all the code manually, or it gives programs greater flexibility to efficiently handle new situations without recompilation."

The compiler is the simplest example of a metaprogram. It takes as input, code in some language, and creates code for another language. An example of metaprogramming in Java, is the use of generics. Here is a blog post I wrote (although for a different reason) some time back to show a simple example of how the compiler transforms Java code which has generics.

In languages like C and C++, metaprogramming can be done by the preprocessor.

In the Groovy programming language, there are two types of metaprogramming techniques: compile time metaprogramming, and runtime metaprogramming.

In compile time metaprogramming, Groovy allows us to hook into the compilation process at various stages, and modify the Abstract Syntax Tree which the compiler works on. (If you are interested, here is an Eclipse plugin to visualize the AST of a Java program)

An example of compile time metaprogramming is the use of the @Singleton annotation in Groovy. Whenever, we implement a Singleton, we always create a private constructor and a static method which gives us an instance of the Singleton. If we have several Singleton classes in our software, we have to implement this pattern for all of them. Using the @Singleton annotation gives us pattern reuse through AST transformations.

Let's say I want to create a Singleton class called MySingleton. In Groovy I can create it as follows without having to write a single line of plumbing code.

@Singleton class MySingleton {
//implementation of the class

During the compilation phase, some custom code will be invoked which will add the private constructor and a static instance() method to the class. Isn't this great? I think AST transformations can take reuse to a totally new level

Runtime metaprogramming can be done by using the Groovy MetaClass or more specifically the MetaObjectProtocol. Using the MetaClass, we can:
  • Add methods to objects at runtime
  • Determine if an object responds to a message or contains a property
  • Respond to calls made on non-existent methods
  • Respond to queries made on non-existent properties
Various builders in Groovy such as the MarkupBuilder and the SwingBuilder use the MetaObjectProtocol to achieve their magic. Here is some very simple code which uses SwingBuilder to create a simple GUI.

def swing = new SwingBuilder()
def gui = swing.frame(title:'GroovyTwitterClient',
defaultCloseOperation:WindowConstants.EXIT_ON_CLOSE) {
scrollPane {
tabbedPane = widget(new JTabbedPane())

What you see in the code above is a very simple DSL to create Swing GUI's. The code creates a JFrame, set's it's title, size, and defaultCloseOperation. Then it creates a JScrollPane and puts it in the JFrame. Finally it creates a JTabbedPane and puts it in the JScrollPane. If you look at the code carefully, you will realize that we call the method 'frame' on an instance of SwingBuilder and give it certain parameters and a closure. In the closure we call a method 'scrollPane' and give it another closure, and so on. Here each method call actually results in the creation of a Swing component. The parameters of the call are the properties which will be set on that component and the closure represents the component(s) to be added to that component. However, the most interesting thins is yet to come. The SwingBuilder class does not even have methods called 'frame', 'scollPane', and 'tabbedPane'. So then how does this work? The SwingBuilder class overrides invokeMethod(...) which is invoked for every single method call. So, when the 'frame' method is invoked on SwingBuilder, it goes through the MetaObjectProtocol to invokeMethod(...) . Here the call is intercepted and a swing component is created based on the name of the method.

This was just an overview. There is much more to metaprogramming and I will write more about it in future posts.


Popular posts from this blog

Running your own one person company

Recently there was a post on PuneTech on mom's re-entering the IT work force after a break. Two of the biggest concerns mentioned were : Coping with vast advances (changes) in the IT landscape Balancing work and family responsibilities Since I have been running a one person company for a good amount of time, I suggested that as an option. In this post I will discuss various aspects of running a one person company. Advantages: You have full control of your time. You can choose to spend as much or as little time as you would like. There is also a good chance that you will be able to decide when you want to spend that time. You get to work on something that you enjoy doing. Tremendous work satisfaction. You have the option of working from home. Disadvantages: It can take a little while for the work to get set, so you may not be able to see revenues for some time. It takes a huge amount of discipline to work without a boss, and without deadlines. You will not get the benefits (insuran

Some thoughts on redesigning education

Some time back I read a blog post on redesigning education. It asked some very good questions. Stuff which I had been thinking of myself. I left my thoughts on the blog, but I would also like to start a conversation around these ideas with those who read this blog as well. I would like to know what other people think of the issue of redesigning (college) education. I have often thought about how college education can be improved. To answer this question, we first have to ask a very basic question. What is the purpose of education? To me, we need education for 3 things: To learn more about the world around us To lead positive constructive lives To earn a good living / fulfill our ambitions I think education has to a large extent evolved to fulfill #3 (with a bias towards earning a comfortable living). The semester system, along with multiple choice tests, and grading, has made our education system into an assembly line. Students are pushed into the assembly line, given classes, admini

Testing Groovy domain classes

If you are trying to test Grails domain class constraints by putting your unit test cases in the 'test/unit' directory, then your tests will fail because the domain objects will not have the 'valdate' method. This can be resolved in two ways: Place the test cases inside test/integration (which will slow things down) Use the method 'mockForConstraintsTests(Trail)' to create mock method in your domain class and continue writing your test cases in 'test/unit' What follows is some example code around this finding. I am working on a Groovy on Grails project for a website to help programmers keep up and refresh their skills. I started with some domain classes and then moved on to write some unit tests. When we create a Grails project using grails create-app , it creates several directories, one of which is a directory called 'test' for holding unit tests. This directory contains two directories, 'unit', and 'integration' for uni