java常用类解析九：Applet（JApplet）详解及示例

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1、Applet类及各个方法说明

Applet类提供一个基本框架，使得applet可以通过Web浏览器来运行，applet没有main方法，它依靠浏览器调用Applet类中的方法。Applet不安全。下面是截取的一段Applet类的源代码：

/*** Called by the browser or applet viewer to inform this applet that it has* been loaded into the system. It is always called before the first time* that the <code>start</code> method is called.* <p>* A subclass of <code>Applet</code> should override this method if it has* initialization to perform. For example, an applet with threads would use* the <code>init</code> method to create the threads and the* <code>destroy</code> method to kill them.* <p>* The implementation of this method provided by the <code>Applet</code>* class does nothing.* 补充：JVM加载applet类，浏览器创建applet，浏览器调用init方法进行初始化，如果Applet的子* 类具有初始化操作应覆盖此方法。通常，该方法实现的功能包括创建用户界面组件、装载图像和音频等资源* 以及从HTML网页的<applet>标记中获取参数*/public void init() {}/*** Called by the browser or applet viewer to inform this applet that it* should start its execution. It is called after the <code>init</code>* method and each time the applet is revisited in a Web page.* <p>* A subclass of <code>Applet</code> should override this method if it has* any operation that it wants to perform each time the Web page containing* it is visited. For example, an applet with animation might want to use* the <code>start</code> method to resume animation, and the* <code>stop</code> method to suspend the animation.* <p>* Note: some methods, such as <code>getLocationOnScreen</code>, can only* provide meaningful results if the applet is showing. Because* <code>isShowing</code> returns <code>false</code> when the applet's* <code>start</code> is first called, methods requiring* <code>isShowing</code> to return <code>true</code> should be called from* a <code>ComponentListener</code>.* <p>* The implementation of this method provided by the <code>Applet</code>* class does nothing.* 补充：init方法完成后，调用start方法，浏览过别的网页之后回来也调用此方法*/public void start() {}/*** Called by the browser or applet viewer to inform this applet that it* should stop its execution. It is called when the Web page that contains* this applet has been replaced by another page, and also just before the* applet is to be destroyed.* <p>* A subclass of <code>Applet</code> should override this method if it has* any operation that it wants to perform each time the Web page containing* it is no longer visible. For example, an applet with animation might want* to use the <code>start</code> method to resume animation, and the* <code>stop</code> method to suspend the animation.* <p>* The implementation of this method provided by the <code>Applet</code>* class does nothing.* 补充：离开页面时调用stop方法*/public void stop() {}/*** Called by the browser or applet viewer to inform this applet that it is* being reclaimed and that it should destroy any resources that it has* allocated. The <code>stop</code> method will always be called before* <code>destroy</code>.* <p>* A subclass of <code>Applet</code> should override this method if it has* any operation that it wants to perform before it is destroyed. For* example, an applet with threads would use the <code>init</code> method to* create the threads and the <code>destroy</code> method to kill them.* <p>* The implementation of this method provided by the <code>Applet</code>* class does nothing.*/public void destroy() {}

从以上描述和代码中可以看出，浏览器通过init、start、stop、destroy方法控制Applet，通常这些方法都是空方法，一般要覆盖这些方法实现操作。

2、JApplet类示例

Applet类没有考虑与Swing组件一起工作，所以从Applet类扩展出了一个JApplet类。JApplet的内容窗格使用BorderLayout布局管理器。下面是一个JApplet的实际例子：

package demo.others;import java.awt.BorderLayout;import javax.swing.JApplet;
import javax.swing.JFrame;
import javax.swing.JLabel;public class MyJApplet extends JApplet {private static final long serialVersionUID = 1L;@Overridepublic void init() {// 在init方法中接收来自html页面上的参数//String message = getParameter("MESSAGE");add(new JLabel("welcome to touch's blog", JLabel.CENTER));}// 用main方法运行JAppletpublic static void main(String[] args) {JFrame frame = new JFrame("Applet is in the frame");MyJApplet myJApplet = new MyJApplet();// main方法里创建一个框架来放置applet，applet单独运行时，// 要完成操作必须手动调用init和start方法frame.add(myJApplet, BorderLayout.CENTER);myJApplet.init();frame.setLocationRelativeTo(null);frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);frame.setSize(300, 300);frame.setVisible(true);}
}

下面的例子用html显示Applet

import java.awt.BorderLayout;import javax.swing.JApplet;
import javax.swing.JFrame;
import javax.swing.JLabel;public class MyJApplet extends JApplet {private static final long serialVersionUID = 1L;@Overridepublic void init() {// 在init方法中接收来自html页面上的参数String message = getParameter("MESSAGE");add(new JLabel(message, JLabel.CENTER));}// 用main方法运行JAppletpublic static void main(String[] args) {JFrame frame = new JFrame("Applet is in the frame");MyJApplet myJApplet = new MyJApplet();// main方法里创建一个框架来放置applet，applet单独运行时，// 要完成操作必须手动调用init和start方法frame.add(myJApplet, BorderLayout.CENTER);myJApplet.init();frame.setLocationRelativeTo(null);frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);frame.setSize(300, 300);frame.setVisible(true);}
}

<html><head><title>passing string to java Applets</title></head><body><p>this applet gets a message from the HTML</p><appletcode ="MyJApplet.class"width=200height=50alt="you must have a java 2-enable browser to view the applet"><param name=MESSAGE value="Welcome to touch's blog"></applet>
</html>

运行结果：