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A wizard is usually created for collecting input data, so validation becomes a critical issue. You can validate the input data in two nonexclusive ways using validators and using transition event handlers. The first option involves placing validator controls in the wizard step. This guarantees that invalid input empty fields or incompatible data types is caught quickly and perhaps on the client. Figure 8-4 shows the error messages you get from the sample wizard of Figure 8-3 if you try to proceed to the next page without entering a first and a last name.

Demonstrates use of XSLT with Java servlets Compares pure J2EE user interface development with an XML approach Shows how to build multidevice and multilocale user interfaces Covers XML web publishing frameworks

Interestingly, recovery options that automatically restart a failed service can give malicious attackers additional opportunities to compromise a computer. For example, Address Space Layout Randomization (ASLR) randomly places core Windows APIs in 256 different memory address locations every time the computer starts. Many buffer overflows require that the attacker correctly guess the needed API address or else the buffer overflow attack fails. If a service is automatically restarted after a failure, the attacker has a better chance of guessing the right memory address with subsequent retries.

Figure 8-4 The wizard doesn t let you proceed to the next page if the fields don t contain valid entries.

Figure 6-8 shows the Dependencies tab. It displays all the services (and their dependencies) that this service depends on to run or conversely, have this service as a dependency. There are many other service values that you cannot see or configure in the Services console GUI (we will cover more of those later in the chapter), including permissions, privileges, and whether the service is stoppable or pausable. Several other methods are available for configuring services in addition to the Services console, including Group Policy, sc.exe, Windows Management Instrumentation (WMI), and other programmatic means.

6:

If you need to access server-side resources to validate the input data, you re better off using transition event handlers. A transition event is one of the events the wizard raises when it is about to switch to another view. The events are detailed in Table 8-4.

Creating a robust presentation layer for your J2EE application is a challenging endeavor. This is so because the vast majority of J2EE applications are webbased, also known as thin-client , applications. In this chapter, we examine some emerging challenges in J2EE user interface design and discuss ways you might use XML technology to overcome them. We begin by exploring the nature of thin-client user interface development and the challenges you are likely to face when building and maintaining an advanced presentation layer. We then examine the pure J2EE approach to building such a layer to see where the current J2EE architecture is lacking. The remainder of the chapter focuses on overcoming the limitations of the pure J2EE approach using XSLT technology. First we develop an XSLT-based presentation layer from scratch. Then we use XSLT from within a web publishing framework to discover the benefits and drawbacks of using a third party API. The goal of this chapter is not to convince you that one architecture or product is superior to another. We wish only to make you aware of the available options, see them in action, and understand the positive and negative aspects of taking each approach.

Figure 6-8

When a button is clicked to move to another step, an event is fired to the hosting page. It s up to you to decide when and how to perform critical validation. In most cases, you ll want to perform server-side validation only when the user clicks the Finish button to complete the wizard. You can be sure that whatever route the user has taken within the wizard, Finish will complete it. Any code you bind to the FinishButtonClick event is executed only once, and only when strictly necessary. By contrast, any code bound to the Previous or Next button executes when the user moves back or forward. The page posts back on both events; be aware that you don t save any round trips by omitting PreviousButtonClick and NextButtonClick event handlers.

Any unexpected service failure should be investigated for operational and security issues. While service issues are often caused by nonmalicious causes, many malware incidents (MS-Blaster worm, DDoS, and so on) have been known to cause previously unexplainable service problems.