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JSP Scripting Chapter Elements

Topics in This Chapter • The purpose of JSP • How JSP pages are invoked • Using JSP expressions to insert dynamic results directly into the output page • Using JSP scriptlets to insert Java code into the method that handles requests for the page • Using JSP declarations to add methods and field declarations to the servlet that corresponds to the JSP page • Predefined variables that can be used within expressions and scriptlets
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Chapter

avaServer Pages (JSP) technology enables you to mix regular, static HTML with dynamically generated content from servlets. You simply write the regular HTML in the normal manner, using familiar Web-page-building tools. You then enclose the code for the dynamic parts in special tags, most of which start with . For example, here is a section of a JSP page that results in “Thanks for ordering Core Web Programming” for a URL of http://host/OrderConfirmation.jsp?title=Core+Web+Programming:
Thanks for ordering

J

Separating the static HTML from the dynamic content provides a number of benefits over servlets alone, and the approach used in JavaServer Pages offers several advantages over competing technologies such as ASP, PHP, or ColdFusion. Section 1.4 (The Advantages of JSP) gives some details on these advantages, but they basically boil down to two facts: that JSP is widely supported and thus doesn’t lock you into a particular operating system or Web server and that JSP gives you full access to servlet and Java technology for the dynamic part, rather than requiring you to use an unfamiliar and weaker special-purpose language. The process of making JavaServer Pages accessible on the Web is much simpler than that for servlets. Assuming you have a Web server that supports JSP, you give your file a .jsp extension and simply install it in any place you

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could put a normal Web page: no compiling, no packages, and no user CLASSPATH settings. However, although your personal environment doesn’t need any special settings, the server still has to be set up with access to the servlet and JSP class files and the Java compiler. For details, see your server’s documentation or Section 1.5 (Installation and Setup). Although what you write often looks more like a regular HTML file than a servlet, behind the scenes, the JSP page is automatically converted to a normal servlet, with the static HTML simply being printed to the output stream associated with the servlet’s service method. This translation is normally done the first time the page is requested. To ensure that the first real user doesn’t get a momentary delay when the JSP page is translated into a servlet and compiled, developers can simply request the page themselves after first installing it. Many Web servers also let you define aliases so that a URL that appears to reference an HTML file really points to a servlet or JSP page. Depending on how your server is set up, you can even look at the source code for servlets generated from your JSP pages. With Tomcat 3.0, you need to change the isWorkDirPersistent attribute from false to true in install_dir/server.xml. After that, the code can be found in install_dir/work/port-number. With the JSWDK 1.0.1, you need to change the workDirIsPersistent attribute from false to true in install_dir/webserver.xml. After that, the code can be found in install_dir/work/%3Aport-number%2F. With the Java Web Server, 2.0 the default setting is to save source code for automatically generated servlets. They can be found in install_dir/tmpdir/default/pagecompile/jsp/_JSP. One warning about the automatic translation process is in order. If you make an error in the dynamic portion of your JSP page, the system may not be able to properly translate it into a servlet. If your page has such a fatal translation-time error, the server will present an HTML error page describing the problem to the client. Internet Explorer 5, however, typically replaces server-generated error messages with a canned page that it considers friendlier. You will need to turn off this “feature” when debugging JSP pages. To do so with Internet Explorer 5, go to the Tools menu, select Internet Options, choose the Advanced tab, and make sure “Show friendly HTTP error messages” box is not checked.
Core Warning

When debugging JSP pages, be sure to turn off Internet Explorer’s “friendly” HTTP error messages.
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10.1

Scripting Elements

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Aside from the regular HTML, there are three main types of JSP constructs that you embed in a page: scripting elements, directives, and actions. Scripting elements let you specify Java code that will become part of the resultant servlet, directives let you control the overall structure of the servlet, and actions let you specify existing components that should be used and otherwise control the behavior of the JSP engine. To simplify the scripting elements, you have access to a number of predefined variables, such as request in the code snippet just shown (see Section 10.5 for more details). Scripting elements are covered in this chapter, and directives and actions are explained in the following chapters. You can also refer to Appendix (Servlet and JSP Quick Reference) for a thumbnail guide summarizing JSP syntax. This book covers versions 1.0 and 1.1 of the JavaServer Pages specification. JSP changed dramatically from version 0.92 to version 1.0, and although these changes are very much for the better, you should note that newer JSP pages are almost totally incompatible with the early 0.92 JSP engines, and older JSP pages are equally incompatible with 1.0 JSP engines. The changes from version 1.0 to 1.1 are much less dramatic: the main additions in version 1.1 are the ability to portably define new tags and the use of the servlet 2.2 specification for the underlying servlets. JSP 1.1 pages that do not use custom tags or explicitly call 2.2-specific statements are compatible with JSP 1.0 engines, and JSP 1.0 pages are totally upward compatible with JSP 1.1 engines.

10.1 Scripting Elements
JSP scripting elements let you insert code into the servlet that will be generated from the JSP page. There are three forms: 1. Expressions of the form , which are evaluated and inserted into the servlet’s output 2. Scriptlets of the form , which are inserted into the servlet’s _jspService method (called by service) 3. Declarations of the form , which are inserted into the body of the servlet class, outside of any existing methods Each of these scripting elements is described in more detail in the following sections.
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Template Text
In many cases, a large percentage of your JSP page just consists of static HTML, known as template text. In almost all respects, this HTML looks just like normal HTML, follows all the same syntax rules, and is simply “passed through” to the client by the servlet created to handle the page. Not only does the HTML look normal, it can be created by whatever tools you already are using for building Web pages. For example, I used Allaire’s HomeSite for most of the JSP pages in this book. There are two minor exceptions to the “template text is passed straight through” rule. First, if you want to have