16.1 Introduction to Applets

Applets differ from regular applications in a number of ways. One of the most important is that there are a number of security restrictions on what applets are allowed to do. An applet often consists of untrusted code, so it cannot be allowed access to the local filesystem, for example.

All applets subclass java.applet.Applet, which inherits from java.awt.Panel and java.awt.Component. (In Java 1.2 and later, you can also subclass an applet from the JApplet Swing component.) So creating an applet is more like subclassing a GUI component than it is like writing an application. In particular, an applet does not have a main( ) method or other single entry point from which the program starts running. Instead, to write an applet, you subclass Applet and override a number of standard methods. At appropriate times, under well-defined circumstances, the web browser or applet viewer invokes the methods you have defined. The applet is not in control of the thread of execution; it simply responds when the browser or viewer tells it to. For this reason, the methods you write must take the necessary action and return promptly; they are not allowed to enter time-consuming (or infinite) loops. To perform a time-consuming or repetitive task, such as animation, an applet must create its own thread, over which it does have complete control.

The task of writing an applet, then, comes down to defining the appropriate methods. A number of these methods are defined by the Applet class:

init( )

Called when the applet is first loaded into the browser or viewer. It typically performs applet initialization, in preference to a constructor method. (An applet is allowed to have a constructor, but it must not require any arguments.)

destroy( )

Called when the applet is about to be unloaded from the browser or viewer. It should free any resources, other than memory, that the applet has allocated. Applets often do not need to define this.

start( )

Called when the applet becomes visible and should start doing whatever it does. Often used to start animation threads, for example.

stop( )

This method tells the applet to stop doing whatever it does. A web browser might invoke this method when an applet is scrolled off the visible region of the web page, for example.

getAppletInfo( )

Called to get information about the applet. Should return a string suitable for display in a dialog box. Most web browsers never call this method, so most applets don't implement it.

getParameterInfo( )

Called to obtain information about the parameters the applet responds to. Should return strings describing those parameters. Like getAppletInfo( ), this method is usually not implemented.

In addition to these Applet methods, there are a number of other methods, inherited from superclasses of Applet, that the browser invokes at appropriate times, and which an applet should override. The most obvious of these methods is paint( ) (or paintComponent( ) for Swing-based applets that extend JApplet), which the browser or viewer invokes to ask the applet to draw itself on the screen. A related method is print( ), which an applet should override if it wants to display itself on paper differently than it does on the screen.

The Applet class also defines some methods that are commonly used (but not overridden) by applets:

getImage( )

Loads an image file from the network and returns a java.awt.Image object.

getAudioClip( )

Loads a sound clip from the network and returns a java.applet.AudioClip object.

getParameter( )

Looks up and returns the value of a named parameter, specified in the HTML file that refers to the applet with the <param> tag.

getCodeBase( )

Returns the base URL from which the applet class file was loaded.

getDocumentBase( )

Returns the base URL of the HTML file that refers to the applet.

showStatus( )

Displays a message in the status line of the browser or applet viewer.

getAppletContext( )

Returns the java.applet.AppletContext object for the applet. AppletContext defines the useful showDocument( ) method that asks the browser to load and display a new web page.