异常oracle.jdbc.driver.ScrollableResultSet.getCachedDatumValueAt

严重: Servlet.service() for servlet checkParam threw exception
java.lang.NullPointerException
at oracle.jdbc.driver.ScrollableResultSet.getCachedDatumValueAt(ScrollableResultSet.java:2586)
at oracle.jdbc.driver.ScrollableResultSet.getOracleObject(ScrollableResultSet.java:659)
at oracle.jdbc.driver.ScrollableResultSet.getString(ScrollableResultSet.java:680)
at org.apache.tomcat.dbcp.dbcp.DelegatingResultSet.getString(DelegatingResultSet.java:174)
重复连接数据库后，报以上错。我用的jndi数据源！
java中db部分代码：
   //初始化查找命名空间
   Context ctx = new InitialContext(); 
   //InitialContext ctx = new InitialContext();亦可 
   //找到DataSource,对名称进行定位java:comp/env是必须加的,后面跟你的DataSource名
   DataSource ds = (DataSource)ctx.lookup("java:comp/env/jdbc/dxlbs");
   //取出连接
    conn = ds.getConnection();
context.xml文件中：
                <?xml version="1.0" encoding="UTF-8"?> 
                   <Context>
                   <Resource name="jdbc/dxlbs" 
                    auth="Container" 
                    type="javax.sql.DataSource"
                    maxActive="1000" 
                    maxIdle="300" 
                    maxWait="10000"
                    username="SHDX" 
                    password="SHDX" 
                    driverClassName="oracle.jdbc.driver.OracleDriver"
                    url="jdbc:oracle:thin:@192.168.1.8:1521:QMap"/>
                    </Context>