MSSQL的Tomcat数据池连接问题

DatabaseConn.java:
import java.sql.*;
import javax.naming.*;
import javax.sql.DataSource;
//一个用于查找数据源的工具类。
public class DatabaseConn {
        public  static  synchronized Connection getConnection() throws Exception
        {
                try
                {
                        Context initCtx = new javax.naming.InitialContext();
            Context envCtx = (Context) initCtx.lookup("java:comp/env");
            DataSource ds = (DataSource)envCtx.lookup("jdbc/store"); ;
                        return ds.getConnection();//－－－－－－－－－－－第27行
                }
                catch(SQLException e)
                {
                        throw e;
                }
                catch(NamingException e)
                {
                     throw e;
                }
        }}common/lib下三个包已经放进去，其中msbase.jar包大小是296k.server.xml配置如下
<Context path="/store" reloadable="true" docBase="E:\soft\eclipse\workspace\store" debug="0"   crossContext="true">   
<Resource name="jdbc/store" auth="Container" type="javax.sql.DataSource"/>   
     <ResourceParams name="jdbc/store">
  <parameter> <name>factory</name>      <value>org.apache.commons.dbcp.BasicDataSourceFactory</value> </parameter>   
<parameter> <name>driverClassName</name> <value>com.microsoft.jdbc.sqlserver.SQLServerDriver  </value>    </parameter>      
<parameter> <name>url</name> <value>
jdbc:microsoft:sqlserver://127.0.0.1:1433;DatabaseName=store      </value>  </parameter>               
<parameter> <name>username</name> <value>sys      </value> </parameter>
     <parameter> <name>password</name> <value>sys      </value> </parameter>
   <parameter> <name>maxActive</name> <value>20      </value>  </parameter>
     <parameter> <name>maxIdle</name> <value>10      </value> </parameter>
<parameter> <name>maxWait</name> <value>-1      </value> </parameter>
     </ResourceParams>     </Context>web.xml配置
<resource-ref> <description>DB Connection</description> <res-ref-name>jdbc/store</res-ref-name> <res-type>javax.sql.DataSource</res-type> <res-auth>Container</res-auth> </resource-ref>