But wait! There’s more you can do with the DD

Besides mapping URLs to actual servlets, you can use the DD to customize other aspects of your web application including security roles, error pages, tag libraries, initial configuration information, and if it’s a full J2EE server, you can even declare that you’ll be accessing specific enterprise javabeans.

Don’t worry about the details yet. The crucial point for now is that the DD gives you a way to declaratively modify your application without changing source code!

Think about this... it means that even those who aren’t Java programmers can customize your Java web application without having to drag you back from your tropical vacation.

The deployment descriptor (DD), provides a “declarative” mechanism for customizing your web applications without touching source code!

DD Benefits

  • Minimizes touching source code that has already been tested.

  • Lets you fine-tune your app’s capabilities, even if you don’t have the source code.

  • Lets you adapt your application to different resources (like databases), without having to recompile and test any code.

  • Makes it easier for you to maintain dynamic security info like access control lists and security roles.

  • Lets non-programmers modify and deploy your web applications while you can focus on the more interesting things. Like how appropriate your wardrobe isn’t for a trip to Hawaii.

There are no Dumb Questions

Q:

Q: I’m confused. Looking at the DD, you still don’t have anything that indicates the actual path name of the servlet! It just says the class name. This still doesn’t answer the question of how the Container uses that class name to find a specific servlet class file. Is there yet ANOTHER mapping somewhere that says that such and such a class name maps to such and such a file in such and such a location?

A:

A: You noticed. You’re right that we put only the class name (fully-qualified to include the package name) into the <servlet-class> element. That’s because the Container has a specific place it will look for all servlets for which you’ve specified a mapping in the DD.

In fact, the Container uses a sophisticated set of rules for finding a match between the URL that comes in from the client request and an actual Java class sitting somewhere on the server. But we’ll get into that in a later chapter (on Deployment). Right now, the key point to remember is that you can do this mapping.

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