JavaServer Pages Overview

There are four key components to JSPsdirectives, actions, scripting elements and tag libraries. Directives are messages to the JSP containerthe server component that executes JSPsthat enable the programmer to specify page settings, to include content from other resources and to specify custom tag libraries for use in a JSP. Actions encapsulate functionality in predefined tags that programmers can embed in a JSP. Actions often are performed based on the information sent to the server as part of a particular client request. They also can create Java objects for use in JSP scriptlets. Scripting elements enable programmers to insert Java code that interacts with components in a JSP (and possibly other Web application components) to perform request processing. Scriptlets, one kind of scripting element, contain code fragments that describe the action to be performed in response to a user request. Tag libraries are part of the tag extension mechanism that enables programmers to create custom tags. Such tags enable Web page designers to manipulate JSP content without prior Java knowledge. These JSP component types are discussed in detail in subsequent sections.

In some ways, JavaServer Pages look like standard XHTML or XML documents. In fact, JSPs normally include XHTML or XML markup. Such markup is known as fixed-template data or fixed-template text. Fixed-template data often helps a programmer decide whether to use a servlet or a JSP. Programmers tend to use JSPs when most of the content sent to the client is fixed-template data and little or none of the content is generated dynamically with Java code. Programmers typically use servlets when only a small portion of the content sent to the client is fixed-template data. In fact, some servlets do not produce content. Rather, they perform a task on behalf of the client, then invoke other servlets or JSPs to provide a response. Note that in most cases servlet and JSP technologies are interchangeable. As with servlets, JSPs normally execute as part of a Web server.

Software Engineering Observation 27.1

When a JSP-enabled server receives the first request for a JSP, the JSP container translates the JSP into a Java servlet that handles the current request and future requests to the JSP. Literal text in a JSP becomes string literals in the servlet that represents the translated JSP. Any errors that occur in compiling the new servlet result in translation-time errors. The JSP container places the Java statements that implement the JSP's response in method _jspService at translation time. If the new servlet compiles properly, the JSP container invokes method _jspService to process the request. The JSP may respond directly or may invoke other Web application components to assist in processing the request. Any errors that occur during request processing are known as request-time errors.

Performance Tip 27.1

Overall, the request-response mechanism and the JSP life cycle are the same as those of a servlet. JSPs can override methods jspInit and jspDestroy (similar to servlet methods init and destroy), which the JSP container invokes when initializing and terminating a JSP, respectively. JSP programmers can define these methods using JSP declarationspart of the JSP scripting mechanism.