JDBC遇到问题

异常详情：

java.sql.SQLException: No value specified for parameter 1
	at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:129)
	at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:122)
	at com.mysql.cj.jdbc.ClientPreparedStatement.execute(ClientPreparedStatement.java:396)
	at com.zy.jdbc.dao.impl.StudentDaoImpl.update(StudentDaoImpl.java:69)

源码：

preparedStatement.setString(1,student.getName());
preparedStatement = connection.prepareStatement(sql);
preparedStatement.setInt(2,student.getAge());
preparedStatement.setDate(3,new Date(student.getBir().getTime()));
preparedStatement.setString(4,student.getId());
preparedStatement.execute();

解决：应该先执行connection.prepareStatement(sql)，再给参数赋值