In tomcat - context in server.xml (url and resource location mapping) is not working in web.xml?

When I am trying to call the mapped url in web.xml, then the browser shows the resource is not available.On the other hand,if we enter the mapped url manually via browser it works fine.

I think web.xml (container) is loaded before the context specified in the host in server.xml

Help me to find reason and suggest me the solution to overcome this.