Applet 的生命周期

An applet can react to major events in the following ways:

• It can initialize itself.
• It can start running.
• It can stop running.
• It can perform a final cleanup, in preparation for being unloaded.

This section introduces a new applet, Simple, that uses all of these methods. Unlike Java applications, applets do not need to implement a main method.

Here is the Simple applet.

The following is the source code for the Simple applet. This applet displays a descriptive string whenever it encounters a major milestone in its life, such as when the user first visits the page the applet is on.

import java.applet.Applet;
import java.awt.Graphics;

//No need to extend JApplet, since we don't add any components;
//we just paint.
public class Simple extends Applet {

    StringBuffer buffer;

    public void init() {
        buffer = new StringBuffer();
        addItem("initializing... ");
    }

    public void start() {
        addItem("starting... ");
    }

    public void stop() {
        addItem("stopping... ");
    }

    public void destroy() {
        addItem("preparing for unloading...");
    }

    private void addItem(String newWord) {
        System.out.println(newWord);
        buffer.append(newWord);
        repaint();
    }

    public void paint(Graphics g) {
	//Draw a Rectangle around the applet's display area.
        g.drawRect(0, 0, 
		   getWidth() - 1,
		   getHeight() - 1);

	//Draw the current string inside the rectangle.
        g.drawString(buffer.toString(), 5, 15);
    }
}

Loading the Applet

As a result of the applet being loaded, you should see the text "initializing... starting...". When an applet is loaded, here's what happens:

• An instance of the applet's controlling class (an Applet subclass) is created.
• The applet initializes itself.
• The applet starts running.

Leaving and Returning to the Applet's Page

When the user leaves the page, for example, to go to another page, the browser stops and destroys the applet. The state of the applet is not preserved. When the user returns to the page, the browser intializes and starts a new instance of the applet.

Reloading the Applet

When you refresh or reload a browser page, the current instance of the applet is stopped and destroyed and a new instance is created.

Quitting the Browser

When the user quits the browser, the applet has the opportunity to stop itself and perform a final cleanup before the browser exits.

Download source codefor the Simple Applet example to experiment further.