避免通过ResultSet的next()方法跳过行 [英] Avoiding skipping a row By next() Method of ResultSet

问题描述

这是一个简单的代码，可打印数据库中的某些行.但是当我执行此操作时，屏幕上不会打印任何内容.我发现问题是rs.next()方法正在跳过一行.那么，如何避免这种情况或重置rs.next()方法的位置?

 String searchword = Searchtf.getText();
 try {
        Class.forName("com.mysql.jdbc.Driver");

        java.sql.Connection con = DriverManager.getConnection(url, db_username, db_password);

        java.sql.Statement stat = con.createStatement();

        String searchQuery = "SELECT * FROM user WHERE Name LIKE '" + searchword + "' ";

java.sql.ResultSet rs = stat.executeQuery(searchQuery);

        if (rs.next()) {
            while (rs.next()) {
                System.out.print(rs.getString("idUser") + " ,");
                System.out.print(rs.getString("Name") + " ,");
                System.out.print(rs.getString("Email") + " ,");
                System.out.println(rs.getString("country") + " .");
            }
        } else {
            JOptionPane.showMessageDialog(null, "Not Found");
        }
  } catch (Exception ex) {
        ex.printStackTrace();
    }

解决方案

首先，停止像这样构建SQL-使用参数化SQL和PreparedStatement.您当前的代码容易受到SQL注入攻击的攻击.<​​/p>

基本上，不要连续两次调用rs.next()(首先在if中，然后在while中)...您可以通过将while循环转换为do/while来轻松地做到这一点.循环:

if (rs.next()) {
    do {
        System.out.print(rs.getString("idUser") + " ,");
        System.out.print(rs.getString("Name") + " ,");
        System.out.print(rs.getString("Email") + " ,");
        System.out.println(rs.getString("country") + " .");
    } while (rs.next());
}

或者 just 具有while循环，并带有一个单独的变量来检测您是否看到了 some 结果:

bool anyResults = false;
while (rs.next()) {
    anyResults = true;
    System.out.print(rs.getString("idUser") + " ,");
    System.out.print(rs.getString("Name") + " ,");
    System.out.print(rs.getString("Email") + " ,");
    System.out.println(rs.getString("country") + " .");
}
if (!anyResults) {
    JOptionPane.showMessageDialog(null, "Not Found");
}

(此外，您应该使用try-with-resources关闭ResultSet等，仅将堆栈跟踪打印到stdout几乎从来不是处理异常的适当方法...)

This is a simple code print some rows from a Database. but When I execute this nothing is print on screen. I figured that the problem is rs.next() method is skipping a row. So How can I avoid that or Reset the Position of rs.next() method?

 String searchword = Searchtf.getText();
 try {
        Class.forName("com.mysql.jdbc.Driver");

        java.sql.Connection con = DriverManager.getConnection(url, db_username, db_password);

        java.sql.Statement stat = con.createStatement();

        String searchQuery = "SELECT * FROM user WHERE Name LIKE '" + searchword + "' ";

java.sql.ResultSet rs = stat.executeQuery(searchQuery);

        if (rs.next()) {
            while (rs.next()) {
                System.out.print(rs.getString("idUser") + " ,");
                System.out.print(rs.getString("Name") + " ,");
                System.out.print(rs.getString("Email") + " ,");
                System.out.println(rs.getString("country") + " .");
            }
        } else {
            JOptionPane.showMessageDialog(null, "Not Found");
        }
  } catch (Exception ex) {
        ex.printStackTrace();
    }

解决方案

First, stop building SQL like that - use parameterized SQL and a PreparedStatement. Your current code is vulnerable to SQL injection attacks.

Basically, don't call rs.next() twice in a row (first in the if then in the while)... you can easily do that by converting your while loop into a do/while loop:

if (rs.next()) {
    do {
        System.out.print(rs.getString("idUser") + " ,");
        System.out.print(rs.getString("Name") + " ,");
        System.out.print(rs.getString("Email") + " ,");
        System.out.println(rs.getString("country") + " .");
    } while (rs.next());
}

Or just have the while loop, with a separate variable to detect that you've seen some results:

bool anyResults = false;
while (rs.next()) {
    anyResults = true;
    System.out.print(rs.getString("idUser") + " ,");
    System.out.print(rs.getString("Name") + " ,");
    System.out.print(rs.getString("Email") + " ,");
    System.out.println(rs.getString("country") + " .");
}
if (!anyResults) {
    JOptionPane.showMessageDialog(null, "Not Found");
}

(Additionally, you should use try-with-resources to close your ResultSet etc, and just printing a stack trace to stdout is almost never the appropriate way to handle exceptions...)

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