使用带字符串的select查询时出现预准备语句

Question

我编写了一个简单的JDBC程序来执行查询并尝试创建预准备语句，但结果集变为空。但是，当通过一般语句创建执行的相同查询给出输出时，也通过SQL加上。

package com.aexp.balu.jdbc;

import java.io.Reader;
import java.sql.*;

public class SimpleConnection {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        // String url="URL";
        // String uname="uname";
        // String pass="password";
        String url = "URL";
        String uname = "uname";
        String pass = "password";
        String query = "select CARD_NO from MDC.CAS_UNMCH_ROCS where CARD_NO = ?";
        String cardNumber = "3222222222222222223";

        try {

            // step1 load the driver class
            System.out.println("registering the driver");
            Class.forName("oracle.jdbc.driver.OracleDriver");

            // step2 create the connection object . We need to have the type of
            // connection
            // The DriverManager class has a method get connection which give
            // sthe instance of Connection

            System.out.println("creating the connection....");
            Connection con = DriverManager.getConnection(url, uname, pass);

            // step3 create the statement object
            System.out.println("creating the statement");

            PreparedStatement stmt = con.prepareStatement(query);

            stmt.setString(1, cardNumber);

            // step4 execute query -- the stmnt object of the method execute
            // query results an object
            // of result set
            System.out.println("Executing the query");

            ResultSet rs = stmt.executeQuery();
            System.out.println("After Executing the query");

            while (rs.next()) {
                System.out.println(" in reading result set");

                String cardnumber = rs.getString("CARD_NO");

                System.out.println(cardNumber);

            }
            // step5 close the connection object
            rs.close();
            stmt.close();
            con.close();

        } catch (Exception e) {

            e.printStackTrace();

        }

    }
}

输出如下：

registering the driver
creating the connection....
creating the statement
Executing the query
After Executing the query