Context Path + servlet path + path info = request uri

                        |-- Context Path --|-- Servlet Path -|--Path Info--|
http://www.myserver.com     /mywebapp        /helloServlet      /hello
                        |-------- Request URI  ----------------------------|

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Remember the following three points:
1. Request URI = context path + servlet path + path info.
2. Context paths and servlet paths start with a / but do not end with it.
3. HttpServletRequest provides three methods getContextPath(),
    getServletPath() and getPathInfo() to retrieve the context path,
    the servlet path, and the path info, respectively, associated with a request.

Identifying the servlet path
To match a request URI with a servlet, the servlet container follows a simple algorithm.
Once it identifies the context path, if any, it evaluates the remaining part of the
request URI with the servlet mappings specified in the deployment descriptor, in the
following order. If it finds a match at any step, it does not take the next step.

1 The container tries to match the request URI to a servlet mapping. If it finds a
match, the complete request URI (except the context path) is the servlet path. In
this case, the path info is null.
2 It tries to recursively match the longest path by stepping down the request URI
path tree a directory at a time, using the / character as a path separator, and determining
if there is a match with a servlet. If there is a match, the matching part
of the request URI is the servlet path and the remaining part is the path info.
3 If the last node of the request URI contains an extension (.jsp, for example),
the servlet container tries to match it to a servlet that handles requests for the
specified extension. In this case, the complete request URI is the servlet path
and the path info is null.
4 If the container is still unable to find a match, it will forward the request to the
default servlet. If there is no default servlet, it will send an error message indicating
the servlet was not found.



<servlet-mapping>
    <servlet-name>RedServlet</servlet-name>
    <url-pattern>/red/*</url-pattern>
</servlet-mapping>
<servlet-mapping>
    <servlet-name>RedServlet</servlet-name>
    <url-pattern>/red/red/*</url-pattern>
</servlet-mapping>
<servlet-mapping>
    <servlet-name>RedBlueServlet</servlet-name>
    <url-pattern>/red/blue/*</url-pattern>
</servlet-mapping>
<servlet-mapping>
    <servlet-name>BlueServlet</servlet-name>
    <url-pattern>/blue/</url-pattern>
</servlet-mapping>
<servlet-mapping>
    <servlet-name>GreenServlet</servlet-name>
    <url-pattern>/green</url-pattern>
</servlet-mapping>
<servlet-mapping>
    <servlet-name>ColorServlet</servlet-name>
    <url-pattern>*.col</url-pattern>
</servlet-mapping>

Request URI                Servlet Used            Servlet Path        Path Info
/colorapp/red                RedServlet              /red                 null
/colorapp/red/               RedServlet              /red                 /
/colorapp/red/aaa            RedServlet              /red                 /aaa
/colorapp/red/blue/aa        RedBlueServlet          /red/blue            /aa
/colorapp/red/red/aaa        RedServlet              /red/red             /aaa
/colorapp/aa.col             ColorServlet            /aa.col              null
/colorapp/hello/aa.col       ColorServlet            /hello/aa.col        null
/colorapp/red/aa.col         RedServlet              /red                 /aa.col
/colorapp/blue               NONE(Error message)                         
/colorapp/hello/blue/        NONE(Error message)                         
/colorapp/blue/mydir         NONE(Error message)    
/colorapp/blue/dir/aa.col    ColorServlet            /blue/dir/aa.col     null 
/colorapp/green              GreenServlet            /green               null

解释一下:
这里的三个错误，都是错在blue上，注意blue的mapping url是/blue/而不是/blue或者/blue/*这是造成错误的主要原因