The Anatomy of a JSP Page
A JSP page is simply a regular web page with JSP elements for generating the parts that differ for each request, as shown in Figure 3-2.
Figure 3-2.
Template text and JSP elements
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Everything in the page that isn't a JSP element is called template text. Template text can be any text: HTML, WML, XML, or even plain text. Since HTML is by far the most common web-page language in use today, most of the descriptions and examples in this book use HTML, but keep in mind that JSP has no dependency on HTML; it can be used with any markup language. Template text is always passed straight through to the browser.
When a JSP page request is processed, the template text and dynamic content generated by the JSP elements are merged, and the result is sent as the response to the browser.
JSP Processing
Just as a web server needs a servlet container to provide an interface to servlets, the server needs a JSP container to process JSP pages. The JSP container is responsible for intercepting requests for JSP pages. To process all JSP elements in the page, the container first turns the JSP page into a servlet (known as the [JSP page implementation class). The conversion is pretty straightforward; all template text is converted to println( ) statements similar to the ones in the handcoded servlet shown in Example 3-1, and all JSP elements are converted to Java code that implements the corresponding dynamic behavior. The container then compiles the servlet class.
Converting the JSP page to a servlet and compiling the servlet form the translation phase. The JSP container initiates the translation phase for a page automatically when it receives the first request for the page. Since the translation phase takes a bit of time, the first user to request a JSP page notices a slight delay. The translation phase can also be initiated explicitly; this is referred to as precompilation of a JSP page. Precompiling a JSP page is a way to avoid hitting the first user with this delay. It is discussed in more detail in Chapter 16.
The JSP container is also responsible for invoking the JSP page implementation class (the generated servlet) to process each request and generate the response. This is called the request processing phase. The two phases are illustrated in Figure 3-3.
Figure 3-3.
JSP page translation and processing phases
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As long as the JSP page remains unchanged, any subsequent request goes straight to the request processing phase (i.e., the container simply executes the class file). When the JSP page is modified, it goes through the translation phase again before entering the request processing phase.
The JSP container is often implemented as a servlet configured to handle all requests for JSP pages. In fact, these two containers--a servlet container and a JSP container--are often combined in one package under the name web container.
So in a way, a JSP page is really just another way to write a servlet without having to be a Java programming wiz. Except for the translation phase, a JSP page is handled exactly like a regular servlet: it's loaded once and called repeatedly, until the server is shut down. By virtue of being an automatically generated servlet, a JSP page inherits all the advantages of a servlet described in Chapter 2: platform and vendor independence, integration, efficiency, scalability, robustness, and security.
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