用JNDI日常出错，求解答

context.xml
<Resource 
name="jdbc/stu" type="javax.sql.Datasource" auth="Container"
maxIdle="30" maxActive="40" maxWait="10000"
username="root" password="root"
driverClassName="com.mysql.jdbc.Driver"
url="jdbc:mysql://localhost:3306/stumanage?useUnicode=true&amp;characterEncoding=utf-8"
/>
web.xml
<resource-ref>
    <description>stumanageDatasource</description>
    <res-ref-name>jdbc/stu</res-ref-name>
    <res-type>javax.sql.DataSource</res-type>
    <res-auth>Container</res-auth>
  </resource-ref>
Base.java
public Connection getCon() throws NamingException, SQLException {
        Context context = new InitialContext();
        DataSource dt = (DataSource) context.lookup("java:comp/env/jdbc/stu");
        System.out.println("Datasource: " + dt);
        con = dt.getConnection();
        return con;
    }
debug
javax.naming.NamingException: Cannot create resource instance
at org.apache.naming.factory.FactoryBase.getObjectInstance(FactoryBase.java:96)
at javax.naming.spi.NamingManager.getObjectInstance(NamingManager.java:321)
at org.apache.naming.NamingContext.lookup(NamingContext.java:840)
at org.apache.naming.NamingContext.lookup(NamingContext.java:159)
at org.apache.naming.NamingContext.lookup(NamingContext.java:827)
at org.apache.naming.NamingContext.lookup(NamingContext.java:159)
at org.apache.naming.NamingContext.lookup(NamingContext.java:827)
at org.apache.naming.NamingContext.lookup(NamingContext.java:159)
at org.apache.naming.NamingContext.lookup(NamingContext.java:827)
at org.apache.naming.NamingContext.lookup(NamingContext.java:173)
at org.apache.naming.SelectorContext.lookup(SelectorContext.java:163)
at javax.naming.InitialContext.lookup(InitialContext.java:417)
at per.sl.jndi.util.Base.getCon(Base.java:18)
at per.sl.jndi.util.Base.query(Base.java:38)
at per.sl.jndi.daoImpl.ScoreDaoImpl.getScore(ScoreDaoImpl.java:22)
at per.sl.jndi.serviceImpl.ScoreServiceImpl.getScore(ScoreServiceImpl.java:19)
at org.apache.jsp.index_jsp._jspService(index_jsp.java:147)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:443)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:385)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:329)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:475)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:80)
at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:624)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342)
at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:498)
at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:796)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1366)
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Thread.java:745)
jar也放到tomcat的llib里了，不知道哪里有问题啊。