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： 

<span style="font-size:16px;">    /**
     * 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     * this applet has been replaced by another page, and also just before the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() {
    }
</span>

 

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：

<span style="font-size:16px;">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);
    }
}
</span>

下面的例子用html显示Applet

<span style="font-size:16px;">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);
    }
}
</span>

 

<span style="font-size:16px;"><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></span>

运行结果：