JSP Processing

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 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.

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.

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 like  platform and vendor independence, integration, efficiency, scalability, robustness, and security.