Tomcat virtual/physical path mapping

In my web2.0 application, users can upload files to server. I want to build mapping from virtual path to physical path. It means that the resources accessed are located in a separate directory instead of tomcat sub directory.

Context container can be used to achieve that goal. Official document is here http://tomcat.apache.org/tomcat-6.0-doc/config/context.html.
In my case, I modified configuration file server.xml and insert following text into Host element:

<context crosscontext="true" docbase="E:\\temp\imgService" path="/imgService"></context>

Actually, at first, I tried to modify context.xml instead of service.xml because it is less invasive. However, it didn't work and I don't know why.