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Developing Java Beans by Robert Englander

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The JavaBeans Architecture

JavaBeans is an architecture for both using and building components in Java. This architecture supports the features of software reuse, component models, and object orientation. One of the most important features of JavaBeans is that it does not alter the existing Java language. If you know how to write software in Java, you know how to use and create Beans. The strengths of Java are built upon and extended to create the JavaBeans component architecture.

Although Beans are intended to work in a visual application development tool, they don’t necessarily have a visual representation at run-time (although many will). What this does mean is that Beans must allow their property values to be changed through some type of visual interface, and their methods and events should be exposed so that the development tool can write code capable of manipulating the component when the application is executed.

Creating a Bean doesn’t require any advanced concepts. So before I go any further, here is some code that implements a simple Bean:

public class MyBean implements java.io.Serializable
{
   protected  int theValue;

   public MyBean()
   {
   }

   public void setMyValue(int newValue)
   {
      theValue = newValue;
   }

   public int getMyValue()
   {
      return theValue;
   }
}

This is a real Bean named MyBean that has state (the variable theValue) that will automatically be saved and restored by the JavaBeans persistence mechanism, and it has a property named MyValue that is usable by a visual programming environment. This Bean doesn’t have any visual representation, but that isn’t a requirement for a JavaBean component.

JavaSoft is using the slogan “Write once, use everywhere.” Of course “everywhere” means everywhere the Java run-time environment is available. But this is very important. What it means is that the entire run-time environment required by JavaBeans is part of the Java platform. No special libraries or classes have to be distributed with your components. The JavaBeans class libraries provide a rich set of default behaviors for simple components (such as the one shown earlier). This means that you don’t have to spend your time building a lot of support for the Beans environment into your code.

The design goals of JavaBeans are discussed in Sun’s white paper, “Java Beans: A Component Architecture for Java.” This paper can be found on the JavaSoft web site at http://splash.javasoft.com/beans/WhitePaper.html. It might be interesting to review these goals before we move on to the technology itself, to provide a little insight into why certain aspects of JavaBeans are the way they are.

Compact and Easy

JavaBeans components are simple to create and easy to use. This is an important goal of the JavaBeans architecture. It doesn’t take very much to write a simple Bean, and such a Bean is lightweight—it doesn’t have to carry around a lot of inherited baggage just to support the Beans environment. If a Bean does not require the advanced features of the architecture, it doesn’t get them, nor does it get the code that goes with them. This is an important concept. The JavaBeans architecture scales upward in complexity, not downward like other component models. This means it really is easy to create a simple Bean. (The previous example shows just how simple a Bean can be.)

Portable

Since JavaBeans components are built purely in Java, they are fully portable to any platform that supports the Java run-time environment. All platform specifics, as well as support for JavaBeans, are implemented by the Java virtual machine. You can be sure that when you develop a component using JavaBeans it will be usable on all of the platforms that support Java (version 1.1 and beyond). These range from workstation applications and web browsers to servers, and even to devices such as PDAs and set-top boxes.

Leverages the Strengths of the Java Platform

JavaBeans uses the existing Java class discovery mechanism. This means that there isn’t some new complicated mechanism for registering components with the run-time system.

As shown in the earlier code example, Beans are lightweight components that are easy to understand. Building a Bean doesn’t require the use of complex extensions to the environment. Many of the Java supporting classes are Beans, such as the windowing components found in java.awt.

The Java class libraries provide a rich set of default behaviors for components. Use of Java Object Serialization is one example—a component can support the persistence model by implementing the java.io.Serializable interface. By conforming to a simple set of design patterns (discussed later in this chapter), you can expose properties without doing anything more than coding them in a particular style.

Flexible Build-Time Component Editors

Developers are free to create their own custom property sheets and editors for use with their components if the defaults aren’t appropriate for a particular component. It’s possible to create elaborate property editors for changing the value of specific properties, as well as create sophisticated property sheets to house those editors.

Imagine that you have created a Sound class that is capable of playing various sound format files. You could create a custom property editor for this class that listed all of the known system sounds in a list. If you have created a specialized color type called PrimaryColor, you could create a color picker class to be used as the property editor for PrimaryColor that presented only primary colors as choices.

The JavaBeans architecture also allows you to associate a custom editor with your component. If the task of setting the property values and behaviors of your component is complicated, it may be useful to create a component wizard that guides the user through the steps. The size and complexity of your component editor is entirely up to you.

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