There are three ways to generate extra space around elements. The first is to add only padding to an element; second, to add only margins; and third, to add a combination of padding and margins. Under certain circumstances, it doesn't really matter which you pick. If an element has a background, however, then it does matter which of the three options you choose. That's because the background will extend into the padding, but not the margin. Thus,
Web-based applications are similar to app servers, except for one thing: Web-based applications don't have client apps, instead they use web browsers on the client side. They generate their front ends using HTML, which is dynamically generated by the web-based app. In the Java world, Servlets are best suited for this job.
Web-based apps might themselves rely on another app server to gather information that is presented on the client web browser. Also, you can write Servlets that get information from remote or local databases, XML document repositories and even other Servlets. One good use for web-based apps is to be a wrapper around an app server, so that you can allow your customers to access at least part of the services offered by your app server via a simple web browser. So web-based apps allow you to integrate many components including app servers, and provide access to this information over the web via a simple web browser.
Web-based apps are very deployable, since they don't require special Java VMs to be installed on the client side, or any other special plug ins, if the creator of the web-based app relies solely on HTML. Unfortunately, this can restrict the level of service that can be offered by a web-based app when compared to the functionality offered by custom clients of an app server, but they are a good compromise when it comes to providing web-based access to your information. In fact, in a real world scenario, both a web-based app and app server may be used together, in order to provide your customers access to their information. In an Intranet setting, you might deploy the clients that come with the app server, and in an Internet setting it would be better to deploy a web-based app that sits on top of this app server, and gives your customers (relatively) limited access to their data over the web (via a simple web browser).
Web-based apps and app servers integrate very well, and this is another reason why Java and XML make a powerful combination for developing systems that give your customers access to their information from anywhere, using any browser over the web. In the future, you can imagine various different web-based apps servicing different kinds of clients, e.g. web browsers on desktops, web browsers on PDAs, and web browsers on all kinds of different consumer electronics devices. By keeping your information structured in a pure way (by using XML), and by allowing access to this information through app servers, you can write many different web-based apps that render this information by customizing it uniquely for each different device that is allowed access to this information. This is more a more scalable solution that storing all this information in web pages, even if these web pages are dynamically generated. So you can have one app server that stores all the data in XML format. You can write a web-based app (which sits on top of this app-server) that allows PalmPilots to access this information over the web. You can write another web-based app (that also sits on top of the same app server) that allows conventional web browsers to access this information over the web. XML and Java have the potential to make this truly platform independent and device independent computing a reality.
If you set background colors for the elements involved, as illustrated in Figure 7-4, the difference becomes a little more clear. The elements with padding have extra background, as it were, whereas those with margins do not.
In the end, deciding how to set margins and padding is up to the designer, who has to balance the various possibilities against the intended effect and pick the best alternative. In order to be able to make these choices, of course, it helps to know what properties can be used.
Copyright © 2002 O'Reilly & Associates. All rights reserved.
<length> | <percentage>