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The 2nd category of Java applications called Java Application Servers (or app servers) and they make good use of XML. Unlike client side graphical Java apps (from the previous section) which are very standalone in their operations, app servers tie many different networked software components together in order to provide information from multiple sources to a set of client side Java apps or web browsers (maybe even running on different devices). This is shown in Figure 2. An app server is actually a conglomeration of several distributed and client/server software systems. So when you write an app server, you are actually writing many different software systems which are all networked to work together, to process information that comes from various sources, and distribute this information to a set of client apps (that you also have to write) running on different devices and platforms.

How can XML help app servers do their work? As you can see in Figure 2, in order for the app server to harvest information from such a rich variety of sources, there must be some common ground between all of these sources (each of which might be running on a different hardware and software system). This common ground is the information which flows throughout the entire system, regardless of what source the information comes from. CORBA is an example of tying disparate systems together based on the interfaces that certain remote objects implement. XML does the same thing for data. It allows these disparate systems to share information in a medium that consists only of pure information (and the structural relationships that exist inside of that information). By taking the lowest common denominator approach by using plain text to encode data, XML allows these systems to talk with each other without requiring any special binary information format converters or other service layers to translate between binary formats (for encoding data). Also, since HTTP already supports transmission of plain text, it is completely natural to move XML around using the Hyper Text Transfer Protocol through firewalls and disparate networks. This is shown in Figure 3. XML can be transmitted between systems using one of the most prevalent protocols in use today, Hypertext Transfer Protocol or HTTP 1.1 (which is the protocol of the web).

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.

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and <SPAN STYLE="vertical-align: top;">tall</SPAN> and which is<BR>larger than the surrounding text.</P>

Now we're back to our earlier example, where the middle linebox is taller than the other line boxes. However, notice how the"tall" text is aligned in Figure 8-54.

Figure 8-54

Figure 8-54. Top-aligning text

What's happened here is that the top of the "tall"text's inline box is aligned with the top of the line box.

Applies to

all elements

WARNING

Percentage values refer to the width of the parent element.