Learning Java, 4th Edition

Chapter 23. Applets

One of the original promises of Java was that applications could be delivered over the network to your computer as needed. Instead of the old days of buying a shrink-wrapped box containing a word processor, installing it, and upgrading it every few years, it would now be possible to obtain and use software directly from the Internet, safely and on any platform. Today, we take networked distribution of software for granted. Mobile devices have driven the advent of app stores for installed software, and many (if not the majority) of the most highly used applications are now purely browser-based. Unfortunately for Java fans, these advances took different paths, arguably less elegant, than the Java platform offered and took longer to arrive than they likely could have.

This chapter is about the applet API, which was Java’s earliest mechanism for delivering applications to the web browser. Applets are not widely used today, but they are part of the vernacular and are still doing well in some niches. While you may not wish to write applets going forward, it is useful to understand their basic functionality. As we review applets, we will also touch on general areas of interest along the way, such as the Java security model.

An applet is a Java program that runs within the context of a web page. Like an image or hyperlink, it “owns” some rectangular area of the user’s screen. When the web browser loads a page that contains a Java applet, it knows how to load the classes ...