mysql-JDBC使用占位符的插入语句插入不成功是什么原因

……public static void insert(String name,String password) throws Exception{          Connection conn = null;          PreparedStatement st = null;          ResultSet rs =null;          conn = DbUtils.getConn();        String sql="insert into User(name,password) values(?,?)";        st = conn.prepareStatement(sql);        st.setString(1,name);        st.setString(2, password);        int num = st.executeUpdate(sql);                    if(num>0){                     System.out.println("插入成功！！");             }                   DbUtils.release(conn, st, rs);    }        public static void main(String args[]) throws Exception{    ……         IUserDao.insert("hello","89790");    ……     }}

+----------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+--------------+------+-----+---------+-------+
| userID | int(11) | YES | | NULL | |
| name | varchar(255) | YES | | NULL | |
| sex | varchar(255) | YES | | NULL | |
| birthday | date | YES | | NULL | |
| password | varchar(255) | YES | | NULL | |
+----------+--------------+------+-----+---------+-------+
一直报错，但是不知道是怎么错了。求教 orz
Exception in thread "main" com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '?,?)' at line 1
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:526)
at com.mysql.jdbc.Util.handleNewInstance(Util.java:404)
at com.mysql.jdbc.Util.getInstance(Util.java:387)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:939)
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3878)
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3814)
at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2478)
at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2625)
at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2547)
at com.mysql.jdbc.StatementImpl.executeUpdateInternal(StatementImpl.java:1541)
at com.mysql.jdbc.StatementImpl.executeLargeUpdate(StatementImpl.java:2605)
at com.mysql.jdbc.StatementImpl.executeUpdate(StatementImpl.java:1469)
at dao.IUserDao.insert(IUserDao.java:107)
at dao.IUserDao.main(IUserDao.java:120)

表名没错，id不是自增的，没有primarykey。把sql写死是可以插入的，用“？”就不行了，为什么呢？