How are jsp pages executed,what's the principle in it?

I'm reading the steps to build a web app skeleton:

http://maestric.com/doc/java/spring/setup#web_app_skeleton

And feel it's greatly different from other languages like PHP.

How is it开发者_Go百科 executed?

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web.xml is a deployment descriptor. It does not actually work in a way, the server (servlet container) reads this file to know what your application's needs are (what filters to create, servlets, configuration params etc).

Consider CGI for a second. In a CGI you have code that is executed and outputs HTML. The equivalent of this in Java is a Servlet. You have Java code that includes HTML.

The JSP is a Servlet turned inside out. You have HTML that includes Java code. When "executing" a JSP, the server will in fact turn it inside out and transform it into a Servlet.

The servlet is then executed, not the JSP.

P.S. this is off course a simplistic description, a lot more is going on behind the scenes.