Mysql5.1 用PerpareStatement动态传参更新数据，报错，急！！！
MYSQL 5.1 难道不支持这种动态传参吗？？？？？？？

MYSQL： 5.1.51-community  
连接器：mysql-connector-java-5.1.7-bin.jar


代码：
Class.forName("com.mysql.jdbc.Driver");
Connection con = (Connection)DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/test"
, "root", "123456");
String sql;
sql="update student set UPDATE_F = ? where ID = ?";  
System.out.println(stu.getUpdate_f());
System.out.println(stu.getStudentID());
PreparedStatement pst = con.prepareStatement(sql);
pst.setString(1, stu.getUpdate_f());
pst.setString(2, stu.getStudentID());
int num = pst.executeUpdate(sql); //本行报错

错误信息：
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 '? where ID = ?' at line 1
sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
java.lang.reflect.Constructor.newInstance(Constructor.java:513)
com.mysql.jdbc.Util.handleNewInstance(Util.java:406)
com.mysql.jdbc.Util.getInstance(Util.java:381)
com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1030)
com.mysql.jdbc.SQLError.createSQLException(SQLError.java:956)
com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3515)
com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3447)
com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1951)
com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2101)
com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2548)
com.mysql.jdbc.StatementImpl.executeUpdate(StatementImpl.java:1605)
com.mysql.jdbc.StatementImpl.executeUpdate(StatementImpl.java:1524)
sql.StudentUpateSql.updateSql(StudentUpateSql.java:27)
tutorial.UpdateAction.execute(UpdateAction.java:41)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
java.lang.reflect.Method.invoke(Method.java:597)
com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:453)
com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:292)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:255)
org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:256)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:176)
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:265)
org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:68)
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodF