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Designing with Dynamic Extension
How Dynamic Extension Works in Java and How to Use it in Your Designs
by Bill Venners
First Published in JavaWorld, December 1998

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In this installment of Design Techniques, I take a look at a less commonly understood aspect of Java's architecture: dynamic extension. I discuss the two kinds of dynamic extension, forName() and class loaders, and offer guidelines on how to use these tools to make your programs more customizable.

At the Second Annual JavaOne Developer Conference in 1997, I met a programmer from Mexico named Gerardo Horvilleur. Gerardo had created a product that he was publicizing from a booth on the convention floor. His product interested me because in those days I was working on my Java virtual machine book, and his product was one of the few I'd seen that seized new design opportunities made possible by Java's architecture.

Gerardo's product, a Java library called Montage, enabled Java applets or applications to display a special kind of image. The image was special because it was defined not just by data, but also by code.

To display one of these images, the Montage library would first show a background image (stored as plain old image data), then it would dynamically load and execute some Java code (called painters) that massaged the background image. To massage the background image, a painter could process the image to produce a special effect or animate the image -- anything that could be done to an image with Java code a painter could do. The result was the Montage image.

This product stoked my inner nerd sufficiently enough that I wanted to know the details of how he did it. I arranged a meeting with Gerardo in the Hacker's Lounge, a social gathering spot at JavaOne.

There he described the inner workings of the class loader he used to load painters. Our conversation drifted to class loaders, in general, and to Java's overall architecture. Gerardo was very enthusiastic about Java and excited in particular about the new opportunities made possible by its architecture.

Eventually, Gerardo and I left the Hacker's Lounge and headed for a JavaOne session. As we made our way through the crowd, he continued his animated discussion of the benefits of Java.

One thing he said that stuck in my mind was: "When you look at C++'s linking model, it is really just C's linking model with mangled names tacked on. If you look at C's linking model, it is really just Fortran's linking model. But when you look at Java's linking model, it doesn't look like any of those. That's the significant difference about Java!", he exclaimed. "It's the linking model!"

In this article, I'll discuss how to take advantage of one of the more interesting design opportunities made possible by Java's linking model: dynamic extension. As a background to the design guidelines, I'll give an overview of Java's linking model, touching on topics such as name spaces, forName(), and class loaders.

For the full details on these topics, I'll refer you to Chapter 7 of my book Inside the Java Virtual Machine, which, partly in honor of my conversation with Gerardo, I named "The Linking Model." In this chapter, I explain in painstaking detail the process of dynamic linking in the JVM, and show how to write and use class loaders.

But you needn't rush to the bookstore to read this chapter; I have excerpted it in its entirety on my Web site. (See the Resources section for a link to this material.)

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