在spring用jndi得到tomcat中的datasource的问题

今天想把项目中配置在spring中的数据源移到tomcat中来配置，遇见了一些问题，请各位指教，小弟感谢了。

目的：从spring中得到tomcat中配置的datasource。

我的做法：

在tomcat的context.xml文件中GlobalNamingResources标签中加入如下代码：

<Resource
          name="dataSource"
          type="javax.sql.DataSource"
          password="54mwr"
          driverClassName="org.postgresql.Driver"
          maxIdle="3"
          maxWait="5000"
          username="alemis"
          url="jdbc:postgresql://localhost:5432/alemis_dev"
          maxActive="10"/>

在tomcat的context.xml文件中Host标签中加入如下代码：

 <Context path="/pro" privileged="true"
         docBase="E:/work2/alemis2/target/pro">
 </Context>

在项目的web.xml文件的最后面加了如下代码:

<resource-ref>
     <res-ref-name>dataSource</res-ref-name>
     <res-type>javax.sql.DataSource</res-type>
     <res-auth>Container</res-auth>
 </resource-ref>

然后在spring的application.xml文件中如下引用：

<bean id="dataSource" class="org.springframework.jndi.JndiObjectFactoryBean">
  <property name="jndiName">
   <value>java:comp/env/dataSource</value>
  </property> 
 </bean>

我启动tomcat的时候出现了如下错误：

2008-12-09 15:34:29,252 WARN Thread-1 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:780)
        at org.apache.tomcat.dbcp.dbcp.BasicDataSource.getConnection(BasicDataSource.java:540)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
        at java.lang.reflect.Method.invoke(Unknown Source)
        at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:310)
        at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:198)
        at $Proxy0.getConnection(Unknown Source)
        at org.springframework.orm.hibernate3.LocalDataSourceConnectionProvider.getConnection(LocalDataSourceConnectionProvider.java:82)
        at org.hibernate.cfg.SettingsFactory.buildSettings(SettingsFactory.java:84)
        at org.hibernate.cfg.Configuration.buildSettings(Configuration.java:2073)
        at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1298)
        at org.springframework.orm.hibernate3.LocalSessionFactoryBean.newSessionFactory(LocalSessionFactoryBean.java:814)
        at org.springframework.orm.hibernate3.LocalSessionFactoryBean.buildSessionFactory(LocalSessionFactoryBean.java:732)
        at org.springframework.orm.hibernate3.AbstractSessionFactoryBean.afterPropertiesSet(AbstractSessionFactoryBean.java:211)
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1367