Java Tip 57: Applet parameterization via class reflection

Making applets configurable from HTML docs can be a chore -- unless you know how to do it the easy way

By Yvon Sauvageau, JavaWorld.com, 08/01/98

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Therefore we need a list of all public fields that are declared in the applet. The class reflection mechanism provides this kind of information. This mechanism is located in the java.lang.reflect package, that's why we import it.

We obtain a reference to the applet's metaclass by calling applet.getClass(). Then we obtain the array of metafields by calling metaclass.getFields(). Then, for each metafield in the array, we check that it does have a corresponding PARAM tag in the HTML document. If it does, we check whether it represents a final field by calling:

   Modifier.isFinal(fields[i].getModifiers()) 

Method getModifiers() returns a mask of "modifiers" (like static, final, volatile, and so on) that are part of the field's declaration.

Once we are sure that the metafield represents a public non-final field, we convert the PARAM string to the appropriate field type.

Setting private fields

If you wish to automatically initialize your private fields, you must write a similar initializeApplet method inside the applet itself, because the Field.setXXX methods called in class Util can't have access to the applet's private fields. You must also replace getFields (returns public fields only) by getDeclaredFields (returns all fields).

Conclusion

This tip has shown you how the class reflection mechanism can be used to ease the programmer's task of developing configurable applets. Using a method that does all the parameter fetching for you, it may be awhile before you have to type getParameter again.

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