JDNI数据库连接问题

通过jndi连接自己的数据库，非全局配置，但一直报空指针异常，代码贴下来，请大神看看哪里错了
java代码，第一个是获取连接，第二个获取返回结果，代码运行在ps = conn.prepareStatement(sql)这里报异常，表名字没错，参数也没错，语句也没错，快奔溃了~

Context ctx = new InitialContext();ds = (DataSource)ctx.lookup("java:com/env/mysqlDataSource");return ds.getConnection();

PreparedStatement ps = null;        ResultSet rs = null;        if(conn==null){            conn = ConnectionInfoBean.getConnection();        }        String sql = "SELECT "+get+" FROM "+table;        System.out.println(sql);        try {            ps = conn.prepareStatement(sql);            rs = ps.executeQuery();

web.xml配置

<resource-ref>        <description>MySQL DB Connection</description>        <res-ref-name>mysqlDataSource</res-ref-name>        <res-type>javax.sql.DataSource</res-type>        <res-auth>Container</res-auth>    </resource-ref>

tomcat 配置，项目叫bynn ,所以在localhost的xml名叫bynn.xml,配置如下,名字和密码为安全起见没写

<?xml version="1.0" encoding="UTF-8"?> <Context>        <Resource name="mysqlDataSource" auth="Container"                         type="javax.sql.DataSource"                         maxActive="100" maxIdle="10" maxWait="10000"                         driverClassName="com.mysql.jdbc.Driver"                         url="jdbc:mysql://localhost:3306/bynn"                         username="" password="" />        <WatchedResource>WEB-INF/web.xml</WatchedResource>        </Context>