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Turn Java classes into JavaBeans

Reuse your existing Java classes as JavaBeans components

By Mark Johnson, JavaWorld.com, 06/01/98

Like many Java programmers, you've invested a lot of time and effort into your existing Java classes. You feel rightly proud of what you've accomplished. True, some of your classes may have been written for JDK 1.0.2, or even earlier, but they still run just fine, thank you very much. Wouldn't it be great if you could dust off those old applets and classes and repackage them as shiny new JavaBeans? Well, the good news is, you can. While some of the JavaBeans Specification depends on JDK 1.1 functionality, class files written to JDK 1.0.2 can still be modified to run as beans.

Beanify your applet

We're going to learn to "beanify" an applet by example. The applet we're going to beanify, ColorFadeBar, produces a title bar (perhaps for a Web page), with a color gradient, like this:

Figure 1: ColorFadeBar applet in action

You can see the full source code for this applet here. You may want to open another browser window (or print the applet) so you can follow along with this discussion.

Task 1: Identify your properties

The first task in beanifying an existing class (whether that class is an applet or not) is to identify what attributes of the class can be considered properties. (See my previous article on customization in the Resources below.) Properties allow the user of a class to customize how the class appears or behaves. For example, Figure 2 shows some attributes of the ColorFadeBar applet that are programmable (via the <PARAM> tags within an <APPLET> tag in HTML).

Figure 2: Programmable attributes of a ColorFadeBar

The font, font size, color, and alignment of the text message are all parameterized, as are the beginning and ending colors for the cool background fade effect, and the width and height of the applet. There are also two parameters (X0 and DY, not shown here; see Table 1 below) that allow you to nudge the text vertically or horizontally for various purposes.

These attributes of the applet aren't properties yet. A JavaBeans property consists of methods that allow the retrieval and/or modification of some internal state of a class. The applet currently doesn't have methods that provide that functionality.

Although applet parameters are often directly translatable to properties (since their purpose usually is to affect the appearance or behavior of an object), they are fundamentally different mechanisms. An applet accesses a parameter by calling java.applet.Applet.getParameter(), and then converting whatever string is returned to the appropriate type. A property, on the other hand, consists of a method or methods that the class exposes (that is, makes publicly available) in order to allow external agents to access encapsulated information.

If you used parameters while writing your applet, you were probably already thinking in terms of properties. In fact, the parameters to your applet are your first candidates for the properties of your new bean. Our ColorFadeBar applet has the following parameters:

Parameter	Meaning

WIDTH	The width of the applet
HEIGHT	The height of the applet
STARTCOLOR	The beginning color for the background fade
ENDCOLOR	The ending color for the background fade
TEXTCOLOR	The color in which to render the message
TEXT	The message text
FADEDIR	Background fade direction (left, right, up, down)
TEXTDIR	Text alignment (left, center, right)
FONT	The name of the font for TEXT
FONTSIZE	The size of the font for TEXT
X0	Text left or right margin offset from applet edge (depending on TEXTDIR; default 5)
DY	Text baseline offset (default 0)

Table 1. Parameters to existing applet

These parameters can be used to customize how the applet works. We first need to identify what the properties of the new bean will be, and then add accessor functions for them. Table 2 shows the properties I've chosen for the new bean, a description of each property, and the parameter or parameters to which the property corresponds.

 

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