JDBC和MySQL Statement和ResultSet错误

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英文:

JDBC and MySQL Statement and ResultSet error

问题

我正在尝试运行以下代码:

Scanner input = new Scanner(System.in);
ResultSet rs = null;
System.out.print("用户名:");
String username = input.next();
System.out.print("\n密码:");
String pword = input.next();
String stmt = "SELECT * FROM accounts WHERE username = '" + username + "' AND password = '" + pword + "' ";
Statement s = con.createStatement(); // con = Connection对象
rs = s.executeQuery(stmt);
if (rs.absolute(1)) {
    System.out.println("登录成功");
} else {
    System.out.println("无效登录");
}

然而,我遇到了以下错误:

java.sql.SQLException: ResultSet.TYPE_FORWARD_ONLY 类型的结果集不允许进行此操作
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:129)
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:97)
    at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:122)
    at com.mysql.cj.jdbc.result.ResultSetImpl.absolute(ResultSetImpl.java:391)
    at practice.JDBCtester.login(JDBCtester.java:36)
    at practice.JDBCtester.main(JDBCtester.java:17)

有人能解释一下我的代码存在的问题吗?

英文:

I am trying to run the following code:

Scanner input = new Scanner(System.in);
ResultSet rs = null;
System.out.print("Username: ");
String username = input.next();
System.out.print("\nPassword: ");
String pword = input.next();
String stmt = "SELECT * FROM accounts WHERE username = '"+username+"' AND password = '"+pword+"' ";
Statement s = con.createStatement();      // con = Connection object
rs = s.executeQuery(stmt);
if(rs.absolute(1)) {
	System.out.println("LOGIN SUCCESSFUL");
}
else {
	System.out.println("INVALID LOGIN");
}

However, I end up with the following error:

java.sql.SQLException: Operation not allowed for a result set of type ResultSet.TYPE_FORWARD_ONLY.
	at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:129)
	at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:97)
	at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:122)
	at com.mysql.cj.jdbc.result.ResultSetImpl.absolute(ResultSetImpl.java:391)
	at practice.JDBCtester.login(JDBCtester.java:36)
	at practice.JDBCtester.main(JDBCtester.java:17)

Could someone explain the problem with my code?

答案1

得分: 3

不使用absolute(),您可以验证是否至少返回了一行。例如,您可以执行以下操作:

rs = s.executeQuery(stmt);
if (rs.next()) {
  System.out.println("登录成功");
}
else {
  System.out.println("无效登录");
}

absolute()是一个高级方法,适用于读取、重新读取和更新特殊类型的结果集。您的结果集是一个简单的结果集。

英文:

Instead of using absolute() you can just verify at least one row was returned. For example you can do:

rs = s.executeQuery(stmt);
if (rs.next()) {
  System.out.println("LOGIN SUCCESSFUL");
}
else {
  System.out.println("INVALID LOGIN");
}

absolute() is an advanced method useful for reading, re-reading, and updating special types of result sets. Your result set is a simple one.

答案2

得分: 1

如在 ResultSet.absolute(int) 中记录:

> 抛出:
>   SQLException - 如果发生数据库访问错误;如果在关闭的结果集上调用此方法或结果集类型为 TYPE_FORWARD_ONLY,则会调用此方法

在大多数情况下,默认的结果集类型是 TYPE_FORWARD_ONLY,因此不支持 absolute 方法。

而是使用 ResultSet.next() 来检查是否至少有一行。

正如我之前评论的,您当前的代码非常不安全。不要将值连接到查询字符串中(尤其不要使用从用户获取的值),因为这会使您的代码容易受到 SQL 注入攻击。而是使用带有参数占位符的准备好的语句。还不要存储明文密码,而是使用适当的密码哈希算法。

英文:

As documented in the ResultSet.absolute(int):

> Throws:
>   SQLException - if a database access error occurs; this method is called on a closed result set or the result set type is TYPE_FORWARD_ONLY

In most situations, the default result set type is TYPE_FORWARD_ONLY, so the absolute method is not supported.

Instead use ResultSet.next() to check if there is at least one row.

As I also commented, your current code is highly insecure. Don't concatenate values into a query string (especially not values obtained from a user), as this makes your code vulnerable to SQL injection. Instead use a prepared statement with parameter placeholders. Also don't store plaintext passwords, but use an appropriate password hashing algorithm.

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  • 本文由 发表于 2020年7月22日 00:07:16
  • 转载请务必保留本文链接:https://go.coder-hub.com/63018535.html
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