软件系统分析与设计

一般地，面向对象分析与设计中存在三种事件处理的机制，出了普通的方法调用外，常常用到回调函数，而J2EE中还提供了一种基于监听方式的时间处理机制，请查阅资料，对Action以及ActionListener的机制进行分析，完成一个分析示例。

ActionListener

网上对ActionListener的描述:

Use actionListener if you want have a hook before the real business action get executed, e.g. to log it, and/or to set an additional property (by f:setPropertyActionListener), and/or to have access to the component which invoked the action (which is available by ActionEvent argument). So, purely for preparing purposes before the real business action gets invoked.

来自docs.oracle.com上的例子:

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package events;

/*
 * Beeper.java requires no other files.
 */

import java.awt.*;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JButton;
import javax.swing.JComponent;

import java.awt.Toolkit;
import java.awt.BorderLayout;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

public class Beeper extends JPanel 
                    implements ActionListener {
    JButton button;

    public Beeper() {
        super(new BorderLayout());
        button = new JButton("Click Me");
        button.setPreferredSize(new Dimension(200, 80));
        add(button, BorderLayout.CENTER);
        button.addActionListener(this);
    }

    public void actionPerformed(ActionEvent e) {
        Toolkit.getDefaultToolkit().beep();
    }

    /**
     * Create the GUI and show it.  For thread safety,
     * this method should be invoked from the
     * event-dispatching thread.
     */
    private static void createAndShowGUI() {
        //Create and set up the window.
        JFrame frame = new JFrame("Beeper");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        //Create and set up the content pane.
        JComponent newContentPane = new Beeper();
        newContentPane.setOpaque(true); //content panes must be opaque
        frame.setContentPane(newContentPane);

        //Display the window.
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        //Schedule a job for the event-dispatching thread:
        //creating and showing this application's GUI.
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }
}

上例实现了一个点击后发出响声的小程序
ActionListener的使用需要3个小步骤：
1、Declare an event handler class and specify that the class either implements an ActionListener interface or extends a class that implements an ActionListener interface. 在本例中相应代码:
public class Beeper extends JPanel
implements ActionListener {

2、Register an instance of the event handler class as a listener on one or more components. 本例中的相应代码:
button.addActionListener(this);
3、Include code that implements the methods in listener interface. 本例中:
public void actionPerformed(ActionEvent e) {
Toolkit.getDefaultToolkit().beep();
}

Action

网上对于Action的描述：

Use action if you want to execute a business action and if necessary handle navigation. The action method can (thus, not must) return a String which will be used as navigation case outcome (the target view). A return value of null or void will let it return to the same page and keep the current view scope alive. A return value of an empty string or the same view ID will also return to the same page, but recreate the view scope and thus destroy any currently active view scoped beans and, if applicable, recreate them.
一个创建Action对象的小例子:

leftAction = new LeftAction("Go left", anIcon,
             "This is the left button.",
             new Integer(KeyEvent.VK_L));
...
class LeftAction extends AbstractAction {
    public LeftAction(String text, ImageIcon icon,
                      String desc, Integer mnemonic) {
        super(text, icon);
        putValue(SHORT_DESCRIPTION, desc);
        putValue(MNEMONIC_KEY, mnemonic);
    }
    public void actionPerformed(ActionEvent e) {
        displayResult("Action for first button/menu item", e);
    }
}

即创建了一个AbstractAction的子类，并重写ActionPerformed方法。
接下来就可以创建相同功能的button和menuitem了

Action leftAction = new LeftAction(); //LeftAction code is shown later
...
button = new JButton(leftAction)
...
menuItem = new JMenuItem(leftAction);