使用數據庫進行登錄驗證

Question

當輸入的用戶存在於數據庫中時，結果集的if部分會運行，但如果不存在，則else部分不會。爲什麼？

這是我的代碼。

String thisname=request.getParameter("username"); 
String thispwd=request.getParameter("password"); 

//Query for selecting user name and password from database. 
String query = "select * from login where username=?"; 
pstmt = (PreparedStatement) con.prepareStatement(query); 
pstmt.setString(1, thisname); 
res = pstmt.executeQuery(); 

while(res.next()) 
{ 
    System.out.println("hhh"); 

    if ((thisname.equals(res.getString("username"))) && (thispwd.equals(res.getString("password")))) 
    { 
     System.out.println("hello");            
    }    
    else 
    { 
     System.out.println("in"); 
     session.invalidate(); 
     response.sendRedirect("index.html"); 
    }  
} 

Answer 1

@ user1069186：這是因爲如果輸入的用戶不存在，那麼在這種情況下res.next()總是返回false和控制絕不會進去一會兒。所以在你的代碼中沒有使用else部分。

嘗試使用下面的修改後的代碼：

String thisname=request.getParameter("username"); 
String thispwd=request.getParameter("password"); 

//Query for selecting user name and password from database. 
String query = "select * from login where username=?"; 
pstmt = (PreparedStatement) con.prepareStatement(query); 
pstmt.setString(1, thisname); 
res = pstmt.executeQuery(); 

if(res.next()) 
{ 
    System.out.println("hhh"); 
    if ((thisname.equals(res.getString("username"))) && (thispwd.equals(res.getString("password")))) 
    { 
     System.out.println("hello");            
    }    
}else { 
    System.out.println("in"); 
    session.invalidate(); 
    response.sendRedirect("index.html"); 
} 

讓我知道如何去。

Answer 2

@SuneetBansal正確。否則使用這種方法。

String thisname=request.getParameter("username"); 
    String thispwd=request.getParameter("password"); 

    //Query for selecting user name and password from database. 
    String query = "select * from login where username=?"; 
    pstmt = (PreparedStatement) con.prepareStatement(query); 
    pstmt.setString(1, thisname); 
    res = pstmt.executeQuery(); 

    while (res.next()) 
    { 
     thisname= res.getString("username"); 
     thispwd= res.getString("password"); 
     break; 
    } 
res.close(); 
pstmt.close();