Does servlet not have any extensions?

I like to know like whenever user requests a jsp page we write hello.jsp or any html file we write hello.html or any image hello.jpeg. My question is does servlet not have any extension ? Is it called directly开发者_如何学Python called by name?

For Servlets, you have to explicitly tell the Servlet Container what URLs (either specific URLs or wildcards) map to what servlet. For example:

<servlet>
    <servlet-name>HelloWorld</servlet-name>
    <servlet-class>com.example.HelloWorld</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>HelloWorld</servlet-name>
    <url-pattern>/hello</url-pattern>
</servlet-mapping>

The above example would map the URL /hello to the servlet com.example.HelloWorld.

You can also do some wildcard mapping. For example:

<servlet-mapping>
    <servlet-name>HelloWorld</servlet-name>
    <url-pattern>*.html</url-pattern>
</servlet-mapping>

This would map requests ending in ".html" to the HelloWorld servlet. But you aren't limited to any particular extensions. You could use anything you want:

<servlet-mapping>
    <servlet-name>HelloWorld</servlet-name>
    <url-pattern>*.foo</url-pattern>
</servlet-mapping>

All of this configuration takes place in your web application's web.xml file.

No, you have it fully in your own hands. It's not necessarily called by its name, it's called by its url-pattern. You can name it to whatever you want, e.g. /pages/* will run the servlet whenever you call http://example.com/pages/foo.jsp or http://example.com/pages/foo (which in turn gives the pathinfo back by request.getPathInfo(), so that you can determine what action to take and/or where to forward the request to). Or *.page which runs the servlet whenever you call http://example.com/foo.page (which in turn gives URI back by request.getRequestURI()).

To preprocess requests (when one requests a page for view) you normally use doGet() method. To postprocess requests (after a POST form submit), you normally use doPost() method.

You can in fact create as many servlets as you want, e.g. RegisterServlet listening on /register which is backed by a register.jsp as view and a LoginServlet listening on /login and backed by a login.jsp as view, etcetera. You can hide JSPs from direct access by placing them in /WEB-INF so that users are forced to call them through the servlet.

In MVC world, there's usually means of only one servlet listening on a certain url-pattern, which is called the Front Controller. In Sun JSF for example, there's the FacesServlet which runs whenever an URL matching by default *.jsf or /faces/* is called. In Apache Struts for example, there's the ActionServlet which listens on by default *.do. They determines which action to take and/or which view (the JSP file) to display based on the URL, request parameters and/or mappings. You're however free to change those default url-patterns. You can even change the default url-pattern of the JspServlet in servlercontainer's web.xml, which by default listens on *.jsp. It's however recommended to stick to a sensible and standardizedurl-pattern.

It might be interesting to know that any other "undefinied" URL patterns are covered by a "default" servlet. Check the servletcontainer's web.xml, you'll see one servlet which listens on / and thus in fact serves everything. It also manages display of directoy listings. In Tomcat for example it's called the DefaultServlet and described here.