how to lookup datasource which is created in glassfish with some jndi/dbname name.

I am trying to get datasource writing code in a servlet like this

Datasource datasource = (Datasource)initialContext.lookup("jdbc/customer");
Connection con = datasource.getConnection();
but it is throwing NameNotFoudExcepion for this context .

how can access datasource created outside of tomcat that is in Glassfish
in our servlets....