Java

One plug-in that was actually integrated into the browsers almost as soon as it was released, was Java from Sun Microsystems. Java was a programming language that could be used by anyone to code add on features that would be executed within the context of the browser. The browsers added Java Virtual Machines to interpret the Java code downloaded from web servers.

In fact, the browser became a sort've worldwide desktop upon which application developers could distribute their own full featured applications with high performance scaleable widgets such as trees, grids, tables, image buttons, etc.... Consider the following screen shot of an embedded applet that looks to be a fancy looking HTML page but which is much, much more.

Java was originally developed at Sun Microsystems in 1991 to provide a platform-independent programming language and operating system for consumer electronics (TV sets, toasters and VCRs).

In syntax and execution, Java is a lot like a simplified version of C++. ("simplified" should be read in the previous sentence as "an improved"). It is a highly robust, distributed, high performance, object-oriented, multi-threaded language with all of the usual features. As such, it builds upon years of C++ development, taking the good and dispensing with the bad.

As it so happened however, Java did not make it into the consumer electronics market. Instead it wound up in our web browsers.

Java seemed to be a perfect fit for the web. The language itself was extremely small (as it was built to go inside toasters and alarm clocks with tiny amounts of memory). Thus it could quickly be transferred over the web.

Further, Java was platform independent. That is, any computer with a Java virtual machine can run a Java program. Programs can be written anywhere and be run anywhere. This is crucial because if a language can not run on any machine, it cannot be used on the web that must service every machine, language, and environment imaginable.

Platform independence works because Java is an interpreted rather than a compiled language. Unlike C or C++ code, when Java is compiled, it is not compiled into platform specific machine code, but into platform independent byte code. This byte code is distributed over the web and interpreted by a virtual machine (typically built right into a web browser these days) on whichever platform it is being run.

Thus, as a programmer, you need only concern yourself with the generic Java programming language and compile your applications into bytecode on whatever system you are using. You can then be assured that your bytecode will be executed correctly whether your clients are using Macs, PCs, Unix boxes or anything else.

What's more, Java was a full-featured programming language complete with a more complete collection of user interface widgets in its AWT library, networking capabilities, database connectivity, internationalization features, and much, much more.

Though Java was sadly reduced to tickers and bouncing balls for the first few years of its life, by now, many high quality, production applications have been completed and are running happily with Java served through web pages.

NOTE: To a certain degree, I also like to think of client-side Active-X components as plug-ins. Active-X technology allows web browsers on Microsoft systems to leverage the vast resource of Windows. Like Java applets, Active-X components can be instantiated within the context of the web browser itself making it appear that the web browser is providing the functionality when in fact the Active-X component is simply piggy-backing off the browser. Since any COM object in Windows can be an Active-X control, almost every Windows resource can thus be embedded in the browser window. Controls can be as simple as a custom stock ticker or as complex as an application like Microsoft Excel. Consider the following screen shot of an Excel spreadsheet embedded directly into Windows Internet Explorer. Of course, beware that Active-X is a Microsoft-specific technology so it may not ever work on Netscape or on Macintosh.

Extending the Browser with Plug-ins
Introduction to the Web Application Development Environment (Tools)
Exposing the Browser API with Client-side Scripting