Glassfish jdbc/database lookup failed

I hope I am not asking a duplicate question just because I was unable to find an answer. I am getting this error:

javax.naming.NamingException: Lookup failed for 'jdbc/osclassDB' in SerialContext

This is what I did: I set up a JDBC Connection Pool and a JDBC Resource pointing to that pool (both in Glassfish).

Then I told my web.xml that there is a JDBC Resource:

<resource-ref>
    <res-ref-name>jdbc/osclassDB</res-ref-name>
    <res-type>javax.sql.DataSource</res-type>
    <res-auth>Container</res-auth>
    <res-sharing-scope>Shareable</res-sharing-scope>                
</resource-ref>

And then I tried using that resource in a Servlet:

  Connection connection = null;

  try {        
     InitialContext initialContext = new InitialContext();
     //Context dbContext = (Context) initialContext.lookup("java:comp/env");

     DataSource dataSource = (DataSource) initialContext.lookup("jdbc/osclassDB");
     connection = dataSource.getConnection();

     if (connection == null) {
         throw new SQLException("Error establishing connection!");
     }
     // some queries here
  } 
  // catch and finally close connection

But when I call the Servlet it throws me the NamingException and tells me that the Lookup failed for 'jdbc/osclassDB' in SerialContext

What am I doing wrong here? is it the web.xml? did I miss something? Thank you for your help!

Solved the problem:

1st by adding a sun-web.xml that links the resource reference from the web.xml to an actual jndi-name (the one I setup on Glassfish). Normally, that shouldn't be necessary (says Oracle) but I did it anyways [Edit: as it turns out, it is really not necessary! ]

2nd I left out the "jdbc". In the servlet, the web.xml and the sun-web.xml it is now just called "osclassDB" (my resource name) instead of "jdbc/osclassDB"

now it looks like this:

web.xml

<resource-ref>
    <res-ref-name>osclassDB</res-ref-name>
    <res-type>javax.sql.DataSource</res-type>
    <res-auth>Container</res-auth>
    <res-sharing-scope>Shareable</res-sharing-scope>                
</resource-ref>

sun-web.xml

<resource-ref>
    <res-ref-name>osclassDB</res-ref-name>
    <jndi-name>osclassDB</jndi-name>  
</resource-ref>

in the servlet

Context dbContext = (Context) initialContext.lookup("java:comp/env");
DataSource dataSource = (DataSource) dbContext.lookup("osclassDB");
connection = dataSource.getConnection();

[Edit:] the sun-web.xml is really not necessary