使用PreparedStatement登录在Java中不起作用[重复]

Question

此问题已经在这里有了答案：

MySQLSyntaxErrorException near “?” when trying to execute PreparedStatement（2个答案）

You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near '?' at line 1 [duplicate]（2个答案）

关闭4分钟前。

public static void main(String[] args)
{
    Scanner sc = new Scanner(System.in);
    System.out.print("Enter username: ");
    String un = sc.nextLine();
    System.out.print("Enter password: ");
    String up = sc.next();
    sc.close();
    try
    {
    Class.forName("com.mysql.jdbc.Driver");
    Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/myDB",
    "root", "pass");
    String q = "select * from users where username = ? and password = ?";       
    PreparedStatement ps = con.prepareStatement(q);
    ps.setString(1, un);
    ps.setString(2, up);
    ResultSet rs = ps.executeQuery(q);
    if(rs.next())
        System.out.println("Welcome "+un);
    else
        System.out.println("Invalid username or password");
    }
    catch(Exception e)
    { System.out.println(e.getMessage()); }

}

输出为：

Enter username: Raja
Enter password: 111
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 '? and password = ?' at line 1

为什么？应该可以，但是显示错误

Answer 1

ResultSet rs = ps.executeQuery(q);

您必须使用无参数版本，否则，您将尝试执行字符串的内容，而不是已准备好的语句。

ResultSet rs = ps.executeQuery();