给出使用 javaweb和MySQL实现简易的注册登录功能的代码

这里提供一个简单的注册登录功能的示例代码，使用JavaWeb和MySQL实现：

1. 创建数据库表

首先创建一个名为“user”的MySQL数据库表，包含以下字段：

CREATE TABLE `user` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `username` varchar(50) NOT NULL,
  `password` varchar(50) NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `username_UNIQUE` (`username`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

2. 创建JavaBean类

创建一个名为“User”的JavaBean类，包含以下属性和方法：

public class User {
    private int id;
    private String username;
    private String password;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }
}

3. 编写DAO层代码

创建一个名为“UserDAO”的类，用于操作数据库。包含以下方法：

public class UserDAO {
    // 数据库连接信息
    private static final String DRIVER = "com.mysql.jdbc.Driver";
    private static final String URL = "jdbc:mysql://localhost:3306/test";
    private static final String USERNAME = "root";
    private static final String PASSWORD = "123456";

    // 获取数据库连接
    public Connection getConnection() throws SQLException, ClassNotFoundException {
        Class.forName(DRIVER);
        return DriverManager.getConnection(URL, USERNAME, PASSWORD);
    }

    // 关闭数据库连接
    public void closeConnection(Connection conn, Statement stmt, ResultSet rs) throws SQLException {
        if (rs != null) {
            rs.close();
        }
        if (stmt != null) {
            stmt.close();
        }
        if (conn != null) {
            conn.close();
        }
    }

    // 根据用户名查询用户信息
    public User getUserByUsername(String username) throws SQLException, ClassNotFoundException {
        User user = null;
        Connection conn = null;
        PreparedStatement stmt = null;
        ResultSet rs = null;
        try {
            conn = getConnection();
            stmt = conn.prepareStatement("SELECT * FROM user WHERE username = ?");
            stmt.setString(1, username);
            rs = stmt.executeQuery();
            if (rs.next()) {
                user = new User();
                user.setId(rs.getInt("id"));
                user.setUsername(rs.getString("username"));
                user.setPassword(rs.getString("password"));
            }
        } finally {
            closeConnection(conn, stmt, rs);
        }
        return user;
    }

    // 添加用户信息
    public void addUser(User user) throws SQLException, ClassNotFoundException {
        Connection conn = null;
        PreparedStatement stmt = null;
        try {
            conn = getConnection();
            stmt = conn.prepareStatement("INSERT INTO user (username, password) VALUES (?, ?)");
            stmt.setString(1, user.getUsername());
            stmt.setString(2, user.getPassword());
            stmt.executeUpdate();
        } finally {
            closeConnection(conn, stmt, null);
        }
    }
}

4. 编写Servlet代码

创建一个名为“RegisterServlet”的Servlet类，用于处理用户注册请求。包含以下代码：

public class RegisterServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request, response);
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String username = request.getParameter("username");
        String password = request.getParameter("password");
        UserDAO dao = new UserDAO();
        try {
            // 判断用户名是否已存在
            if (dao.getUserByUsername(username) != null) {
                request.setAttribute("message", "用户名已存在！");
                request.getRequestDispatcher("/register.jsp").forward(request, response);
            } else {
                // 添加用户信息
                User user = new User();
                user.setUsername(username);
                user.setPassword(password);
                dao.addUser(user);
                request.setAttribute("message", "注册成功！");
                request.getRequestDispatcher("/login.jsp").forward(request, response);
            }
        } catch (SQLException | ClassNotFoundException e) {
            e.printStackTrace();
            request.setAttribute("message", "注册失败，请稍后重试！");
            request.getRequestDispatcher("/register.jsp").forward(request, response);
        }
    }
}

创建一个名为“LoginServlet”的Servlet类，用于处理用户登录请求。包含以下代码：

public class LoginServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request, response);
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String username = request.getParameter("username");
        String password = request.getParameter("password");
        UserDAO dao = new UserDAO();
        try {
            // 根据用户名获取用户信息
            User user = dao.getUserByUsername(username);
            if (user == null) {
                request.setAttribute("message", "用户名不存在！");
                request.getRequestDispatcher("/login.jsp").forward(request, response);
            } else if (!user.getPassword().equals(password)) {
                request.setAttribute("message", "密码错误！");
                request.getRequestDispatcher("/login.jsp").forward(request, response);
            } else {
                request.getSession().setAttribute("user", user);
                request.getRequestDispatcher("/index.jsp").forward(request, response);
            }
        } catch (SQLException | ClassNotFoundException e) {
            e.printStackTrace();
            request.setAttribute("message", "登录失败，请稍后重试！");
            request.getRequestDispatcher("/login.jsp").forward(request, response);
        }
    }
}

5. 编写JSP页面

创建一个名为“register.jsp”的JSP页面，用于用户注册。包含以下代码：

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>用户注册</title>
</head>
<body>
<h1>用户注册</h1>
<% String message = request.getAttribute("message") == null ? "" : request.getAttribute("message").toString(); %>
<% if (!message.equals("")) { %>
    <p><%= message %></p>
<% } %>
<form action="register" method="post">
    <p>
        用户名：<input type="text" name="username" required>
    </p>
    <p>
        密码：<input type="password" name="password" required>
    </p>
    <p>
        <input type="submit" value="注册">
    </p>
</form>
<a href="login.jsp">已有账号？去登录</a>
</body>
</html>

创建一个名为“login.jsp”的JSP页面，用于用户登录。包含以下代码：

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>用户登录</title>
</head>
<body>
<h1>用户登录</h1>
<% String message = request.getAttribute("message") == null ? "" : request.getAttribute("message").toString(); %>
<% if (!message.equals("")) { %>
    <p><%= message %></p>
<% } %>
<form action="login" method="post">
    <p>
        用户名：<input type="text" name="username" required>
    </p>
    <p>
        密码：<input type="password" name="password" required>
    </p>
    <p>
        <input type="submit" value="登录">
    </p>
</form>
<a href="register.jsp">没有账号？去注册</a>
</body>
</html>

创建一个名为“index.jsp”的JSP页面，用于用户登录后的首页。包含以下代码：

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>首页</title>
</head>
<body>
<h1>欢迎 <%= ((User) session.getAttribute("user")).getUsername() %> </h1>
<a href="logout">退出登录</a>
</body>
</html>

6. 部署项目

将以上代码保存到对应的文件中，并将JavaBean类和DAO类的.class文件放到WEB-INF/classes目录下。将MySQL驱动文件放到WEB-INF/lib目录下。将所有文件打包成.war文件，并部署到Tomcat或其他JavaWeb服务器上。

7. 测试功能

访问http://localhost:8080/login.jsp，进行用户登录；访问http://localhost:8080/register.jsp，进行用户注册。如果用户名已存在，则注册失败；如果用户名不存在或密码错误，则登录失败；登录成功后跳转到http://localhost:8080/index.jsp页面，显示欢迎信息和退出登录按钮。点击退出登录按钮，退出登录并返回到登录页面。