使用servlet和JSP创建样本登录页面? [英] Create a sample login page using servlet and JSP?
问题描述
我开发了一个示例登录页面，用于验证用户名和密码.如果用户提供了正确的凭据，则该页面将导航到其他页面，否则将消息从servlet返回到同一登录页面.
I developed a sample login page for validating username and password. If the user gives correct credentials the page will navigate to some other page, else return a message from the servlet to the same login page.
我在这里附上了样品code:
<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
pageEncoding="ISO-8859-1"%>`
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org /TR/html4/loose.dtd">`
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Insert title here</title>`
</head>
<body>
<form action="Login" method="post">
<input type="text" name="Name"/><br>
<input type="password" name=Pass><br><br><br>
<input type="submit" value="submit"/><br>
<%if(request.getAttribute("message")!=(""))
out.println(request.getAttribute("message"));
else
out.println("");%>
<h1>Hi This is JSP sample code </h1>
<font color="blue" size="25">
<marquee>Please login to Work with JSP</marquee></font>
<%java.text.DateFormat df = new java.text.SimpleDateFormat("MM/dd/yyyy"); %>
<h1>Current Date: <%= df.format(new java.util.Date()) %> </h1>
</form>
</body>
</html>
和servlet代码，Login.java:
And servlet code, Login.java:
package com.controller;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class Login extends HttpServlet {
private static final long serialVersionUID = 1L;
public Login() {
super();
}
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String username = request.getParameter("Name");
String password = request.getParameter("Pass");
response.setContentType("text/html");
if(username.equals("sugan") && password.equals("raj")) {
response.getWriter().println("<html><body><Marquee>Welcome to JSP!!!</marquee></body></html>");
}
else {
String message = "OOps!!! Invalid Username/Password";
request.setAttribute("message", message);
request.getRequestDispatcher("/FirstJSP.jsp").forward(request, response);
}
}
}
它工作正常，但是在加载登录页面时，它会自动显示Null.然后，如果用户提供了错误的凭据，它将显示实际消息.如何在运行时禁用Null消息?
It works fine, but while loading the login page it automatically displays Null. Then if the user gives the wrong credentials, it displays the actual message. How do I disable the Null message during run time?
推荐答案
您正在使用==将消息与空字符串进行比较.
You're comparing the message with the empty string using ==.
首先，您的比较是错误的，因为该消息将为null(而不是空字符串).
First, your comparison is wrong because the message will be null (and not the empty string).
第二，这是错误的，因为必须将对象与equals()而不是与==进行比较.
Second, it's wrong because Objects must be compared with equals() and not with ==.
第三，这是错误的，因为您应该避免在JSP中使用scriptlet，而应使用JSP EL，JSTL和其他自定义标签:
Third, it's wrong because you should avoid scriptlets in JSP, and use the JSP EL, the JSTL, and other custom tags instead:
<c:id test="${!empty message}">
<c:out value="${message}"/>
</c:if>
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