为何使用 <servlet-mapping> 在jsp / servlet的web.xml（部署描述符）中？

Question

<servlet>
    <servlet-name>Hello</servlet-name>
    <servlet-class>HelloWorld</servlet-class>
</servlet>

<servlet-mapping>
    <servlet-name>Hello</servlet-name>
    <url-pattern>/HelloWorld.do</url-pattern>
</servlet-mapping>

Why do we use url-pattern inside servlet-mapping tag. Why not in servlet tag itself.

It's seems just an extra tag to write.

Is it just because servlet/jsp spec writers decided to do so or has it some logical reason behind its existance ?

Answer 1

This is more likely due to the fact that servlets were intended to support multiple protocols, and not just HTTP. URL patterns are specific to HTTP alone, and therefore the mapping of the servlet to HTTP URL patterns is done in a servlet-mapping tag, instead of the servlet tag which is used for declaring the more generic properties of the servlet.

Answer 2

u may try to write another that also linked to the same servlet,and then u will know that the servlet can have more than one servlet-mapping.

<servlet>
   <servlet-name>Hello</servlet-name>    
   <servlet-class>HelloWorld</servlet-class>
</servlet>
<servlet-mapping>    
   <servlet-name>Hello</servlet-name>    
   <url-pattern>/HelloWorld.do</url-pattern>
</servlet-mapping>
<servlet-mapping>    
   <servlet-name>Hello</servlet-name>    
   <url-pattern>/HelloWorld2.do</url-pattern>
</servlet-mapping>

Answer 3

I agree with Vineet Reynold that servlet-mapping is used to provide support for different protocols available for communication in network. therfore, url-pattern tag let know the servlet the type of protocol ie. HTTP requesting the service.

Answer 4

The limitations and weaknesses of the standard servlet url pattern was a driving factor behind many of the early MVC frameworks (Struts etc). No support for regular expression matching, or even ant-style patterns. No support for url exclusion patterns etc etc.

As to why the specification is the way it is (and still continue to be), I have no idea. The only people who know for sure are the ones who wrote the specs.