Monday 27th of April 2015 06:01:55 AM

App server developers are not restricted to using HTTP, they can transmit and recieve XML information using simple remote CORBA objects and RMI objects. The key is that by using XML, it makes these remote services or objects easier to build. And, by sticking with XML, any one of these technologies can be used in your design of your app server. You can use whatever technology is most appropriate to getting the job done, knowing that all the information flows as XML and can be processed by any part of the system. The reason Java object serialization did not achieve this is because it encodes object data to a binary format that is dependent on too many things (like the JVM version, and the existence of classes when things are deserialized, etc). XML is not limited by any of these restrictions (or problems), which makes it much easier to create systems that allow XML information to flow between different subsystems. Also by relying only on the data, large portions of the system can be replaced with better or different implementations for future-readiness.

App servers traditionally give their client apps access to information in remote databases, remote file systems, remote object repositories, remote web resources, and even other app servers. All these information sources don't even need to reside on the machine that hosts the app server. These remote resources may be on other machines on the Intranet or the Internet. Using Java and XML, RMI, JDBC, CORBA, JNDI, Servlet and Swing, you can create app servers that can integrate all kinds of remote and local information resources, and client apps that allow you to remotely or locally access this information from the app server.

In the future, with publicly available DTDs that are standardized for each vertical industry, XML based app servers will become very popular. Also when XML schema repositories become available and widely used, app servers will be able to take on a new role and provide application services that are not offered now. Companies will need to share information with other companies in related fields, and each company might have a different software system in which all their data is housed. By agreeing upon a set of DTDs or schemas (encoded in XML), these companies can exchange information with each other regardless of what systems they are using to store this information. If their app servers can exchange XML documents (based on some shared DTD or schema), then these disparate app servers can understand each other and share information. One of the uses for XML foreseen by the W3C is just this, vertical industries (like insurance and health care) creating sets of DTDs and schemas that all companies in the industry agree upon. Then these companies' app servers can talk to each other using some popular protocol (like HTTP or CORBA/IIOP) to exchange information between each other. This has the potential to save a lot of time and money in the daily business operations of these companies.

Web-based Applications

CSS Tutorials

CSS Tutorials

Welcome to the CSS Tutorials. In this section we cover Cascading Style Sheets, the powerful supplement to HTML that allows you complete control over the look of your websites. We'll show you how to create Cascading Style Sheets, and some of the cool tricks you can achieve with them.

For Beginners...

Introduction to CSS
This tutorial covers the basics: what are style sheets? How do I make a style sheet? It also shows some of the cool things you can do with style sheets.

CSS Units
A description of all the units that can be used with CSS, including lengths, percentages, colours and URL's.

For Intermediate Users...

Controlling Background Images and Colours
Learn how to use CSS properties to add background images and colour to your Web pages. Lots of neat tricks and examples are included!

Controlling Fonts with CSS
In this tutorial we take a look at how to control the fonts used in your page body text, using the various font properties that can be controlled with style sheets.

Controlling Text Appearance with CSS
This tutorial shows you how to control the layout of your text using the text properties available in CSS, such as line spacing and text alignment.

Making Lists Look Nicer with CSS
You can really go to town with HTML lists when you add a sprinkle of CSS! This tutorial shows you how to make your lists stand out from the crowd.

For Advanced Users...

CSS Positioning
This tutorial teaches you how to use CSS to position images, text, and other elements on your Web pages. Essential reading for anyone who wants to start using CSS for layout.




Figure 8-24

Figure 8-24. Collapsing margins and negative margins, in detail

There is one area of unresolved behavior, which is this: if elements overlap each other due to negative margins, which elements are "on top?" You'll note that few of the examples in this section use background colors for all elements. That's because if they did, content might get overwritten by the background color of a following element. The CSS specification does not say what should happen when elements overlap in this manner; instead, it's left up to implementors to decide.Youmay have noticed that in almost every circumstance, where we set aforeground color, we also set a background color. In general, this isa good idea. Since you don't know what styles a user may havepredefined, you don't know how your styles might interact withthem. Remember the example where links ended up being white on white?That's the sort of thing we want to avoid.

Let's explore this in a little more detail. Assume theunordered list and heading will be set to have negative margins:

LI {margin-bottom: 20px;}UL {margin-bottom: -15px;}H1 {margin-top: -18px;}

The larger of the two negative margins (-18px ) isadded to the largest positive margin (20px ),yielding (20px-18px = 2px ).Thus, there are only two pixels between the bottom of the listitem's content and the top of the paragraph's content.

You cannot, however, mix keywords with othervalues. Thus, top 75% is notvalid. If you use a keyword, you're stuck using only keywords,but percentages and lengths can be mixed together.

Not only that, but if you're using lengths or percentages, youcan give negative values, thus pushing the image out of the element,to some degree. Consider the example with the very large yin-yangsymbol for a background. At one point, we centered it, but what if weonly want part of it visible in the top left corner of the containing