You probably know a little bit about servlets already. Essentially, they are Java classes that run on the web tier, offering a high-performance, portable alternative to CGI scripts. Java servlets are great for extracting data from a database and then generating XHTML for the browser. They are also good for validating HTTP POST or GET requests from browsers, allowing people to fill out job applications or order books online. But more powerful techniques are required when you create web applications instead of simple web sites.
Hypertext Transfer Protocol (HTTP)
browser compatibility problems
HTTP is a fairly simple protocol that enables a client to communicate with a server. Web browsers almost always use HTTP to communicate with web servers, although they may use other protocols such as HTTPS for secure connections or even FTP for file downloads. HTTP is a request/response protocol, and the browser must initiate the request. Each time you click on a hyperlink, your browser issues a new request to a web server. The server processes the request and sends a response, thus finishing the exchange.
This request/response cycle is easy to understand but makes it tedious to develop an application that maintains state information as the user moves through a complex web application. For example, as a user adds items to a shopping cart, a servlet must store that data somewhere while waiting for the client to make another request. When that request arrives, the servlet has to associate the cart with that particular client, since the servlet could be dealing with hundreds or thousands of concurrent clients. Other than establishing a timeout period, the servlet has no idea when the client abandons the cart, deciding to shop on a competitor’s site instead. The HTTP protocol makes it impossible for the server to initiate a conversation with the client, so the servlet cannot periodically ping the client as it can with a “normal” client/server application.
HTML itself can be another hindrance to web application development. It was not designed to compete with feature-rich GUI toolkits, yet customers are increasingly demanding that applications of all sorts become “web enabled.” This presents a significant challenge because HTML offers only a small set of primitive GUI components. Sophisticated HTML generation is not the subject of this book, but we will see how to use XSLT to separate complex HTML generation code from underlying programming logic and servlet code. As HTML grows ever more complex, the benefits of a clean separation become increasingly obvious.
As you probably well know, browsers are not entirely compatible with one another. As a web application developer, this generally means that you have to test on a wide variety of platforms. XSLT offers support in this area because you can write reusable stylesheets for the consistent parts of HTML and import or include browser-specific stylesheet fragments to work around browser incompatibilities. Of course, the underlying XML data and programming logic is shared across all browsers, even though you may have multiple stylesheets.
Finally, we have the issue of concurrency. In the servlet model, a single servlet instance must handle multiple concurrent requests. Although you can explicitly synchronize access to a servlet, this often results in performance degradation as individual client requests queue up, waiting for their turn. Processing requests in parallel will be an important part of our XSLT-based servlet designs in later chapters.
The difference between a “web site” and a "web application” is subjective. Although some of the technologies are the same, web applications tend to be far more interactive and more difficult to create than typical web sites. For example, a web site is mostly read-only, with occasional forms for submitting information. For this, simple technologies such as HTML combined with JavaServer Pages (JSPs) can do the job. A web application, on the other hand, is typically a custom application intended to perform a specific business or technical function. They are often written as replacements for existing systems in an effort to enable browser-based access. When replacing existing systems, developers are typically asked to duplicate all of the existing functionality, using a web browser and HTML. This is difficult at best because of HTML’s limited support for sophisticated GUI components. Most of the screens in a web application are dynamically generated and customized on a per-user basis, while many pages on a typical web site are static.
Java, XML, and XSLT are suitable for web applications because of the high degree of modularity they offer. While one programmer develops the back-end data access code, a graphic designer can be working on the HTML user interface. Yet another servlet expert can be working on the web tier, while someone else is defining and creating the XML data. Programmers and graphic designers will typically work together to define the XSLT stylesheets, although the current lack of interactive tools may make this more of a programming task.
Another reason XML is suitable for web applications is its unique ability to interoperate with back-end business systems and databases. Once an XML layer has been added to your data tier, the web tier can extract that data in XML form regardless of which operating system or hardware platform is used. XSLT can then convert that XML into HTML without a great deal of custom coding, resulting in less work for your development team.
While web sites typically deliver HTML to browsers, web applications may be asked to interoperate with applications other than browsers. It is typical to provide feature-rich Swing GUI clients for use within a company, while remote workers access the system via an XHTML interface through a web browser. An XML approach is key in this environment because the raw XML can be sent to the Swing client, while XSLT can be used to generate the XHTML views from the same XML data.
If your XML is not in the correct format, XSLT can also be used to transform it into another variant of XML. For example, a client application may expect to see:
But the XML data on the web tier deals with the data as:
In this case, XSLT can be used to transform the XML into the simplified format that the client expects.
Sending raw XML data to clients is a good approach because it interoperates with any operating system, hardware platform, or programming language. Allowing Visual Basic clients to extract XML data from a web application allows existing client software to be salvaged while enabling remote access to enterprise data using a more portable solution such as Java. But defining a custom XML format is tedious because it requires you to manually write code that encodes and decodes messages between the client and the web application.
Simple Object Access Protocol (SOAP) is a standardized protocol for exchanging data using XML messages. SOAP was originally introduced by Microsoft but has been submitted to the W3C for standardization and is endorsed by many companies. SOAP is fairly simple, allowing vendors to quickly create tools that simplify data exchange between web applications and any type of client.
Since SOAP messages are implemented using XML, they can be created and updated using XSLT stylesheets. This means that data can be extracted from a relational database as XML, transformed with XSLT into a standard SOAP message, and then delivered to a client application written in any language. For more information on SOAP standardization efforts, visit http://www.w3.org/TR/SOAP.
Cell phones, personal digital assistants (PDAs), and other handheld devices seem to be the next big thing. From a marketing perspective, it is not entirely clear how the business model of the Web will translate to the world of wireless. It is also unclear which technologies will be used for this new generation of devices. One currently popular technology is Wireless Application Protocol (WAP), which uses an XML markup language called Wireless Markup Language (WML) to render pages. Other languages have been proposed, such as Compact HTML (CHTML), but perhaps the most promising prospect is XHTML Basic. XHTML Basic is backed by the W3C and is primarily based on several XHTML modules. Its designers had the luxury of coming after WML, so they could incorporate many WML concepts and build on that experience.
Because of the uncertainties in the wireless arena, an XML and XSLT approach is the safest available today. Encoding your data in XML enables flexibility to support any markup language or protocol on the client, hopefully without rewriting major pieces of Java code. Instead, new XSLT stylesheets are written to support new devices and protocols. An added benefit of XSLT is its ability to support both traditional browser clients and newer wireless clients from the same underlying XML data and Java business logic.