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# 4. 基于Java的Web应用后端开发技术
## 任务目标
- 实现一个简单的用户登录功能。
- 使用Servlet技术处理用户的登录请求。
- 理解MVC架构中的模型、视图和控制器的作用。
- 学习如何在Servlet中处理HTTP请求和响应。
## 任务描述
使用Servlet技术来实现一个用户登录功能。用户首先需要在登录页面输入用户名和密码，然后Servlet会验证这些信息是否正确。如果登录成功，则显示欢迎信息；如果失败，则显示错误提示。
## 技术栈
- Java Servlet
- HTML
- JavaBean (可选)
- Apache Tomcat 作为Servlet容器
#### 任务步骤
1. **创建项目**
- 使用IntelliJ IDEA创建一个新的Java项目。
2. **编写登录页面 (`login.jsp`)**
3. **创建Servlet (`LoginServlet.java`)**
4. **配置Servlet (`web.xml`)**
5. **部署项目**
- 将项目导出为WAR文件。
```shell
jar cvfm my.war MANIFEST.MF -C . .
```
- 安装Tomcat。
https://tomcat.apache.org/download-90.cgi
- 将WAR文件放置在Tomcat的`webapps`目录下。
- 重启Tomcat。
6. **测试**
- 访问 `http://localhost:8080/my/login.jsp`。
- 输入用户名和密码，提交表单。
- 观察不同的情况下的响应结果。
#### 扩展任务
- **使用JavaBean**: 创建一个`User`类来封装用户名和密码，并在Servlet中使用这个类。
- **添加错误页面**: 创建一个错误页面，在登录失败时重定向到该页面。

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Manifest-Version: 1.0
Created-By: openjdk 11.0.23

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<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns="http://xmlns.jcp.org/xml/ns/javaee"
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
id="WebApp_ID" version="3.1">
<servlet>
<servlet-name>com.lk.webapp.servlets.LoginServlet</servlet-name>
<servlet-class>com.lk.webapp.servlets.LoginServlet</servlet-class>
</servlet>
<servlet>
<servlet-name>com.lk.webapp.servlets.Login2Servlet</servlet-name>
<servlet-class>com.lk.webapp.servlets.Login2Servlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>com.lk.webapp.servlets.LoginServlet</servlet-name>
<url-pattern>/LoginServlet</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>com.lk.webapp.servlets.Login2Servlet</servlet-name>
<url-pattern>/Login2Servlet</url-pattern>
</servlet-mapping>
</web-app>

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<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<title>Login Page</title>
</head>
<body>
<h1>Login</h1>
<form action="LoginServlet" method="post">
帐号: <input type="text" name="username"><br>
密码: <input type="password" name="password"><br>
<input type="submit" value="登录1">
</form>
<% if (request.getAttribute("error") != null) { %>
<p style="color:red;"><%= request.getAttribute("error") %></p>
<% } %>
</body>
</html>

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<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<title>Login Page</title>
</head>
<body>
<h1>Login</h1>
<form action="Login2Servlet" method="post">
帐号: <input type="text" name="username"><br>
密码: <input type="password" name="password"><br>
<input type="submit" value="登录2">
</form>
</body>
</html>

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examples/chapter04/src/com/lk/webapp/servlets/Login2Servlet.java Normal file
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package com.lk.webapp.servlets;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;
@WebServlet("/Login2Servlet")
public class Login2Servlet extends HttpServlet {
private static final long serialVersionUID = 1L;
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
request.setCharacterEncoding("UTF-8");
response.setContentType("text/html;charset=UTF-8");
String username = request.getParameter("username");
String password = request.getParameter("password");
// 验证用户名和密码
if ("admin".equals(username) && "123456".equals(password)) {
response.setStatus(HttpServletResponse.SC_OK);
PrintWriter writer = response.getWriter();
writer.println("<html><body>");
writer.println("<h1>欢迎, " + username + "!</h1>");
writer.println("<p>登录成功!</p>");
writer.println("</body></html>");
} else {
response.sendError(HttpServletResponse.SC_UNAUTHORIZED, "Invalid credentials");
}
}
}

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package com.lk.webapp.servlets;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
@WebServlet("/LoginServlet")
public class LoginServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
request.setCharacterEncoding("UTF-8");
response.setContentType("text/html;charset=UTF-8");
String username = request.getParameter("username");
String password = request.getParameter("password");
// 验证用户名和密码
if ("admin".equals(username) && "123456".equals(password)) {
request.setAttribute("message", "欢迎, " + username + "! 您已经成功登录.");
request.getRequestDispatcher("welcome.jsp").forward(request, response);
} else {
request.setAttribute("error", "Invalid credentials");
request.getRequestDispatcher("login.jsp").forward(request, response);
}
}
}

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<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<title>欢迎页面</title>
</head>
<body>
<h1>热烈欢迎！</h1>
<p><%= request.getAttribute("message") %></p>
</body>
</html>