Take command of your software

The Command pattern benefits both the client and the server

By David Geary, JavaWorld.com, 06/28/02

If you've been developing software long enough, you might recall a time before object-oriented (OO) development went mainstream when software development proved much more difficult than it is today. In those days, developers wrote software using archaic function libraries to develop applications entirely from scratch, using low-level functions from those libraries to develop software systems. For the fruits of their labors, developers produced software such as Microsoft Windows 3.1, which ever since has been known as the poster boy for clunky software that crashed soon after your last reboot.

Today, of course, things have improved considerably. Instead of Windows 3.1, we now have Windows XP, a vast improvement over its predecessors, and other software marvels, such as the amazing Mac OS X. Many factors contributed to the increase of software productivity and reliability over the years, but the overriding factor has been the widespread adoption of OO design and development, which turned the paradigm of developing software from scratch with function libraries completely inside-out. With OO design, developers employ application frameworks that provide the majority of an application's code and implement applications by plugging reusable components into those frameworks.

In the early days, when application frameworks first appeared, developers faced a major hurdle. Application frameworks must issue requests to application-specific objects without knowing anything about those objects or the operations they perform. For example, the Swing application framework issues requests to objects when menu items are activated or buttons are pressed. Swing can't implement those requests because they are application-specific. So how does an application framework like Swing issue requests to objects without knowing anything about the requested operation or the objects themselves? The answer: the Command design pattern.

In this article, I discuss the Command pattern, sometimes known as the Action pattern, from two different perspectives. First, I show how the Command pattern works in client-side Java with Swing's implementation of the Command pattern for menu items. Second, I discuss how the Command pattern works in server-side Java to parameterize HTTP requests with the Apache Struts JSP (JavaServer Pages) framework. Along the way, you will learn a bit about Swing, Struts, and the newest kid on the server-side Java block, the JSP Standard Tag Library (JSTL).

Note: You can download this article's example source code from Resources.

The Command pattern

In Design Patterns, the authors define the Command pattern as:

Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.

By implementing commands as objects, the Command pattern lets application frameworks invoke methods of application-specific commands. Figure 1 shows a Command pattern class diagram.

Figure 1. Command pattern class diagram

In Figure 1, the Invoker class represents an object in an application framework, such as a button or menu item. The Invoker maintains an association with a command through a reference to an object that implements a Command interface, which defines an execute() method. Concrete command classes implement that interface, typically by using an object known as a receiver—application-specific objects that carry out the request. Figure 2 shows a Command pattern sequence diagram.

Figure 2. Command pattern sequence diagram

At runtime, the Invoker calls the Command's execute() method, and the Command invokes a Receiver's method—modeled here as an action() method—that carries out the request.

Swing actions

Almost all OO application frameworks implement the Command pattern, and Swing is no different. This section illustrates how Swing uses the Command pattern to perform application-specific functionality tied to Swing buttons serving as the building blocks for other components, such as menu items or toolbar buttons.

Swing implements the Command pattern with action objects. Whenever you select a Swing menu item or activate a Swing button, those action objects issue a request to an application-specific action. Figure 3 shows a class diagram for Swing actions and how they relate to buttons and menus.

Buttons, defined by the AbstractButton class, serve as the Swing workhorses. For simplicity's sake, Figure 3's class diagram does not show all of the classes that ultimately extend AbstractButton because there are many AbstractButton ancestor classes. (Note: Figure 3 also omits most of the AbstractButton methods.) Swing buttons, radio buttons, toggle buttons, check boxes, menus, menu items, check-box menu items, and radio-button menu items are all AbstractButton class extensions. That class maintains a reference to an action defined by the Action interface. You can set or get that reference with the AbstractAction setAction() and getAction() methods, respectively.

Whenever one of the aforementioned Swing components activates, it calls its action's actionPerformed() method, which implements some application-specific functionality. As a convenience, Swing's AbstractAction class eases the implemention of application-specific actions. Let's see how.

Figure 4 shows a Swing application with a single menu featuring two menu items: "show dialog" and "exit." When you activate the "show dialog" menu item, as depicted in Figure 4's top picture, an application-specific action displays the dialog box shown in Figure 4's bottom picture. When you activate the "exit" menu item, another application-specific action terminates the application.

Figure 4. Swing actions in action

The application shown in Figure 4 is listed in Example 1:

Example 1. Two simple Swing actions

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class Test extends JFrame {
   public static void main(String args[]) { 
      Test frame = new Test();
      frame.setTitle("Swing Actions");
      frame.setSize(500, 400);
      frame.setLocation(400, 200);
      frame.show();
   }
   public Test() {
      JMenuBar mb = new JMenuBar();
      JMenu fileMenu = new JMenu("File");
     fileMenu.add(new ShowDialogAction());
      fileMenu.add(new ExitAction());

      mb.add(fileMenu);
      setJMenuBar(mb);
   }
}
class ShowDialogAction extends AbstractAction {
   public ShowDialogAction() {
      super("show dialog");
   }
   public void actionPerformed(ActionEvent e) {
      JOptionPane.showMessageDialog((Component)e.getSource(),
                               "An action generated this dialog");
   }
}
class ExitAction extends AbstractAction {
   public ExitAction() {
      super("exit");
   }
   public void actionPerformed(ActionEvent e) {
      System.exit(0);
   }
}

The preceding application implements two actions—ShowDialogAction and ExitAction—both of which extend the AbstractAction class. Those classes implement their actionPerformed() methods to implement application-specific functionality; namely, showing a dialog and exiting the application, respectively. The application creates a file menu that's a JMenu class instance and passes instances of ShowDialogAction and ExitAction to that menu's add() method. The add() method creates a menu item—a JMenuItem instance—and associates the action it is passed with the newly created menu item by invoking the menu item's add() method, inherited from AbstractAction, as Figure 3 shows. From then on, the menu items and their actions are wired together: when the menu item is selected, it invokes its action's actionPerformed() method. That method is passed an action event, which provides information about the event, including the component that generated the event. The ShowDialogAction.actionPerformed() method uses that component to create a message dialog. (The ExitAction.actionPerformed() method has no use for the action event it is passed because it simply exits the application.)

The Swing Command pattern implementation closely adheres to the classic pattern's description in Design Patterns. In Swing, an invoker (in this case a menu item) maintains a reference to a command (in this case an action). That reference points to a command's abstract definition (the AbstractAction class), so that the command can represent any application-specific command (in this case a class that extends AbstractAction). When a trigger event occurs (in this case the menu item is selected), the invoker sends a request to the command and the command implements its application-specific functionality.

Now that we've discussed a classic Command pattern implementation for client-side Java, let's take a look at another Command pattern implementation for server-side Java that deviates a bit from the classic Command pattern definition.

Struts actions

Struts, a popular open source JSP application framework from the Apache Software Foundation, implements numerous design patterns, including the Model-View-Controller (MVC) pattern and, of course, the Command pattern. By applying design patterns to server-side Java components, such as servlets and JSPs, Struts makes it simple to implement modular, easily maintainable, and extensible Web applications.

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