Learning Java, 4th Edition

The JApplet Class

A JApplet is a Swing JPanel with a mission. It is a GUI container that has some extra structure to allow it to be used in the “alien” environment of a web browser. Applets also have a lifecycle that lets them act more like an application than a static component. Although applets tend to be relatively simple, there’s no inherent restriction on their complexity other than the issues of downloading and caching their content. Historically, applets have tended to be small “widgets.”

The javax.swing.JApplet class defines the core functionality of an applet. (java.awt.Applet is the older, AWT-based form.)

Structurally, an applet is a wrapper for your Java code. In contrast to a standalone graphical Java application, which starts up from a main() method and creates a GUI, an applet itself is a component that expects to be dropped into someone else’s GUI. Thus, an applet can’t run by itself; it runs in the context of a web browser or a special applet-viewer program (which we’ll talk about later). Instead of having your application create a JFrame to hold your GUI, you stuff your application inside a JApplet (which itself extends Container) and let the browser add your applet to the page.

Applets are placed on web pages with the <applet> HTML tag, which we’ll cover later in this chapter. At its simplest, you just specify the name of the applet class and a size in pixels for the applet:

    <applet code="AnalogClock" width="100" height="100"></applet>

Pragmatically, an applet is ...