Introduction to Java Beans

Java Bean

A Java Bean is a software component that has been designed to be reusable in a variety of different environments. There is no restriction on the capability of a Bean. It may perform a simple function, such as checking the spelling of a document, or a complex function, such as forecasting the performance of a stock portfolio. A Bean may be visible to an end user. One example of this is a button on a graphical user interface.

A Bean may also be invisible to a user. Software to decode a stream of multimedia information in real time is an example of this type of building block. Finally, a Bean may be designed to work autonomously on a user’s workstation or to work in cooperation with a set of other distributed components. Software to generate a pie chart from a set of data points is an example of a Bean that can execute locally.

However, a Bean that provides real-time price information from a stock or commodities exchange would need to work in cooperation with other distributed software to obtain its data.

Advantages of Java Beans

A software component architecture provides standard mechanisms to deal with software building blocks. The following list enumerates some of the specific benefits that Java technology provides for a component developer:

■ A Bean obtains all the benefits of Java’s “write-once, run-anywhere” paradigm. 

■ The properties, events, and methods of a Bean that are exposed to an application builder tool can be controlled.

■ A Bean may be designed to operate correctly in different locales, which makes it useful in global markets.

■ Auxiliary software can be provided to help a person configure a Bean. This software is only needed when the design-time parameters for that component are being set. It does not need to be included in the run-time environment.

■ The configuration settings of a Bean can be saved in persistent storage and restored at a later time.

■ A Bean may register to receive events from other objects and can generate

events that are sent to other objects.

Bean Developer Kit (BDK)

The Bean Developer Kit (BDK), available from the JavaSoft site, is a simple example of a tool that enables you to create, configure, and connect a set of Beans. There is also a set of sample Beans with their source code.

Installing the BDK

The Java 2 SDK must be installed on your machine for the BDK to work. Confirm that the SDK tools are accessible from your environment.

The BDK can then be downloaded from the JavaSoft site (http://java.sun.com). It is packaged as one file that is a self-extracting archive. Follow the instructions to install it on your machine. The discussion that follows assumes that the BDK is installed in a directory called bdk. If this is not the case with your system, substitute the proper directory.

Starting the BDK

To start the BDK, follow these steps:

1. Change to the directory c:\bdk\beanbox.

2. Execute the batch file called run.bat. This causes the BDK to display the three windows. ToolBox lists all of the different Beans that have been included with the BDK. BeanBox provides an area to lay out and connect the Beans selected from the ToolBox. Properties provides the ability to configure

a selected Bean. You may also see a window called Method Tracer, but we won’t be using it.

Using the BDK

This example is to create an application by using some of the Beans provided with the BDK. First, the Molecule Bean displays a three-dimensional view of a molecule. It may be configured to present one of the following molecules: hyaluronic acid, benzene, buckminsterfullerine, cyclohexane, ethane, or water. This component also has methods that allow the molecule to be rotated in space along its X or Y axis.

Create and Configure an Instance of the Molecule Bean

Follow these steps to create and configure an instance of the Molecule Bean:

1. Position the cursor on the ToolBox entry labeled Molecule and click the leftmouse button. You should see the cursor change to a cross.

2. Move the cursor to the BeanBox display area and click the left mouse button in approximately the area where you wish the Bean to be displayed. You should see a rectangular region appear that contains a 3-D display of a molecule. This area is surrounded by a hatched border, indicating that it is currently selected.

3. You can reposition the Molecule Bean by positioning the cursor over one of the hatched borders and dragging the Bean.

4. You can change the molecule that is displayed by changing the selection in the Properties window. Notice that the Bean display changes immediately when you change the selected molecule.

Second, the OurButton Bean provides a push-button functionality. We will have one button labeled “Rotate X” to rotate the molecule along its X axis and another button labeled “Rotate Y” to rotate the molecule along its Y axis.

Create and Configure an Instance of the OurButton Bean

Follow these steps to create and configure an instance of the OurButton Bean and connect it to the Molecule Bean:

1. Position the cursor on the ToolBox entry labeled OurButton and click the left mouse button. You should see the cursor change to a cross.

2. Move the cursor to the BeanBox display area and click the left mouse button in approximately the area where you wish the Bean to be displayed. You should see a rectangular region appear that contains a button. This area is surrounded by a hatched border indicating that it is currently selected.

3. You may reposition the OurButton Bean by positioning the cursor over one of the hatched borders and dragging the Bean.

4. Go to the Properties window and change the label of the Bean to “Rotate X”. The button appearance changes immediately when this property is changed.

5. Go to the menu bar of the BeanBox and select Edit | Events | action | actionPerformed. You should now see a line extending from the button to the cursor. Notice that one end of the line moves as the cursor moves. However, the other end of the line remains fixed at the button.

6. Move the cursor so that it is inside the Molecule Bean display area, and click the left mouse button. You should see the Event Target Dialog dialog box.

7. The dialog box allows you to choose a method that should be invoked when this button is clicked. Select the entry labeled “rotateOnX” and click the OK button. You should see a message box appear very briefly, stating that the tool is “Generating and compiling adaptor class.”

Test the application. Each time you press the button, the molecule should move a few degrees around one of its axes.