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* this applet has been replaced by another page, and also just before the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