JNDI Datasource definition in Tomcat 6.0

I want to define a DataSource to an Oracle database on my Tomcat 6.0.

So, in conf/server.xml (yes, I know that this DataSource will be available for all the webapps in Tomcat, but it's not a problem here), I've set this Resource:

<GlobalNamingResources>
    <Resource name="hibernate/HibernateDS" auth="Container" type="javax.sql.DataSource"
        url="jdbc:oracle:thin:@myserver:1542:foo"
        username="foo" password="bar"
        driverClassName="oracle.jdbc.OracleDriver"
        maxActive="50" maxIdle="10" validationQuery="select 1 from dual"/>

Then, in the web.xml of my application, I set a resource-ref element:

<resource-ref>
    <description>Hibernate Datasource</description>
    <res-ref-name>hibernate/HibernateDS</res-ref-name>
    <res-type>javax.sql.DataSource</res-type>
    <res-auth>Container</res-auth>
</resource-ref>

Finally, as Hibernate is used to manage the database connection, I have a webapps/mywebapp/WEB-INF/classes/hibernate.cfg.xml that creates a session-factory using the JNDI DataSource:

<hibernate-configuration>
    <session-factory>
        <property name="connection.datasource">java:comp/env/hibernate/HibernateDS</property>
        ...

However, when I start my Tomcat server, I get an error that says it could not create the

INFO  [net.sf.hibernate.util.NamingHelper] JNDI InitialContext properties:{}
INFO  [net.sf.hibernate.connection.DatasourceConnectionProvider] Using datasource: java:comp/env/hibernate/HibernateDS
INFO  [net.sf.hibernate.transaction.TransactionFactoryFactory] Transaction strategy: net.sf.hibernate.transaction.JDBCTransactionFactory
INFO  [net.sf.hibernate.transaction.TransactionManagerLookupFactory] No TransactionManagerLookup configured (in JTA environment, use of process level read-write cache is not recommended)
WARN  [net.sf.hibernate.cfg.SettingsFactory] Could not obtain connection metadata
org.apache.tomcat.dbcp.dbcp.SQLNestedException: Cannot create JDBC driver of class '' for connect URL 'null'
        at org.apache.tomcat.dbcp.dbcp.BasicDataSource.createDataSource(BasicDataSource.java:1150)
        at org.apache.tomcat.dbcp.dbcp.BasicDataSource.getConnection(BasicDataSource.java:880)
        at net.sf.hibernate.connection.DatasourceConnectionProvider.getConnection(DatasourceConnectionProvider.java:59)
        at net.sf.hibernate.cfg.SettingsFactory.buildSettings(SettingsFactory.java:84)
        at net.sf.hibernate.cfg.Configuration.buildSettings(Configuration.java:1172)
        ...
Caused by: java.lang.NullPointerException
        at sun.jdbc.odbc.JdbcOdbcDriver.getProtocol(JdbcOdbcDriver.java:507)
        at sun.jdbc.odbc.JdbcOdbcDriver.knownURL(JdbcOdbcDriver.java:476)
        at sun.jdbc.odbc.JdbcOdbcDriver.acceptsURL(JdbcOdbcDriver.java:307)
        at java.sql.DriverManager.getDriver(DriverManager.java:253)
        at org.apache.tomcat.dbcp.dbcp.BasicDataSource.createDataSource(BasicDataSource.java:1143)
        ... 11 more

Do you have any idea why Hibernate is not able to construct the session-factory? What is wrong in my configuration?