Tomcat declares database resources in two places

• As a resource reference in your application's "WEB-INF/web.xml" file. This provides a references to your database that does not contain information specific to your servers, so that the application can easily be ported to another server environment.
• As a resource in your application's context element. This will direct Tomcat to create the resource when the application launches. Alternatively, if you want the database to be available to multiple applications, you can provide this information in Tomcat's server.xml file.