java常用类解析九：Applet（JApplet）详解及示例

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	 * this applet has been replaced by another page, and also just before the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;

	@Override
	public 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JApplet
	public 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;

	@Override
	public 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JApplet
	public 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>
  <applet
     code ="MyJApplet.class"
	 width=200
	 height=50
	 alt="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：