java常用類解析九：Applet（JApplet）詳解及示例

1、Applet類及各個方法說明

     Applet類提供一個基本框架，使得applet可以通過Web瀏覽器來運行，applet沒有main方法，它依靠瀏覽器調用Applet類中的方法。Applet不安全。下面是截取的一段Applet類的源代碼： 

<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.
     * 補充：JVM加載applet類，瀏覽器創建applet，瀏覽器調用init方法進行初始化，如果Applet的子
     * 類具有初始化操作應覆蓋此方法。通常，該方法實現的功能包括創建用戶界面組件、裝載圖像和音頻等資源
     * 以及從HTML網頁的<applet>標記中獲取參數
     */
    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.
     * 補充：init方法完成后，調用start方法，瀏覽過別的網頁之后回來也調用此方法
     */
    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     * this applet has been replaced by another page, and also just before the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.
     * 補充：離開頁面時調用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>

 

從以上描述和代碼中可以看出，瀏覽器通過init、start、stop、destroy方法控制Applet，通常這些方法都是空方法，一般要覆蓋這些方法實現操作。

2、JApplet類示例

     Applet類沒有考慮與Swing組件一起工作，所以從Applet類擴展出了一個JApplet類。JApplet的內容窗格使用BorderLayout布局管理器。下面是一個JApplet的實際例子：

<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方法中接收來自html頁面上的參數
        //String message = getParameter("MESSAGE");
        add(new JLabel("welcome to touch's blog", JLabel.CENTER));
    }

    // 用main方法運行JApplet
    public static void main(String[] args) {
        JFrame frame = new JFrame("Applet is in the frame");
        MyJApplet myJApplet = new MyJApplet();
        // main方法里創建一個框架來放置applet，applet單獨運行時，
        // 要完成操作必須手動調用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顯示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方法中接收來自html頁面上的參數
        String message = getParameter("MESSAGE");
        add(new JLabel(message, JLabel.CENTER));
    }

    // 用main方法運行JApplet
    public static void main(String[] args) {
        JFrame frame = new JFrame("Applet is in the frame");
        MyJApplet myJApplet = new MyJApplet();
        // main方法里創建一個框架來放置applet，applet單獨運行時，
        // 要完成操作必須手動調用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>

運行結果：