1. 准备工作

在开始实现登录注册代码前，我们需先准备好以下工具：

JDK
Eclipse
MySQL（或其他数据库）

在这个攻略中，我们将采用MySQL数据库，并利用Java的JDBC（Java Database Connectivity）驱动程序来连接数据库。

2. 登录功能的实现

2.1 登录页面的设计

首先，我们需要设计登录页面的图形界面，例如下面这个示例：

<!DOCTYPE html>
<html>
<head>
    <title>Login Page</title>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
    <h1>Login</h1>
    <form method="POST" action="login.do">
        <input type="text" name="username" placeholder="Username" required><br>
        <input type="password" name="password" placeholder="Password" required><br>
        <input type="submit" value="Login">
    </form>
</body>
</html>

2.2 登录代码的实现

接下来，我们将通过Java代码来实现登录功能，首先需要创建一个User类来存储用户信息：

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

    public User(int id, String username, String password) {
        this.id = id;
        this.username = username;
        this.password = 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;
    }
}

我们在UserDao类中实现登录的具体细节，此处仅提示相关方法的用法：

public class UserDao {
    private Connection conn;

    public UserDao() {
        try {
            Class.forName("com.mysql.jdbc.Driver");
            String url = "jdbc:mysql://localhost:3306/mydb";
            String user = "root";
            String password = "password";
            conn = DriverManager.getConnection(url, user, password);
        } catch (ClassNotFoundException | SQLException e) {
            e.printStackTrace();
        }
    }

    public User login(String username, String password) {
        String sql = "SELECT * FROM user WHERE username = ? AND password = ?";
        try {
            PreparedStatement pstmt = conn.prepareStatement(sql);
            pstmt.setString(1, username);
            pstmt.setString(2, password);
            ResultSet rs = pstmt.executeQuery();
            if (rs.next()) {
                int id = rs.getInt("id");
                return new User(id, username, password);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return null;
    }
}

以上代码块只是示例，在实际应用中，我们需要根据实际情况进行修改和适配。

2.3 登录信息的校验

在前端页面中，我们通过POST请求将登录信息传递至后台，后台接收到登录信息后，需要先进行校验，以确保用户输入了合法的用户名和密码。

@WebServlet("/login.do")
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 userDao = new UserDao();
        User user = userDao.login(username, password);

        if (user != null) {
            request.getSession().setAttribute("user", user);
            response.sendRedirect("home.jsp");
        } else {
            response.sendRedirect("login.jsp");
        }
    }
}

2.4 登录成功后的跳转

如果用户输入了正确的用户名和密码，则应该将用户信息保存至Session中，并跳转至首页：

<% User user = (User) session.getAttribute("user"); %>

<html>
<head>
    <title>Home Page</title>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
    <h1>Welcome, <%= user.getUsername() %>!</h1>
</body>
</html>

3. 注册功能的实现

3.1 注册页面的设计

和登录功能一样，我们需要先设计注册页面的图形界面，例如下面这个示例：

<!DOCTYPE html>
<html>
<head>
    <title>Registration Page</title>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
    <h1>Registration</h1>
    <form method="POST" action="register.do">
        <input type="text" name="username" placeholder="Username" required><br>
        <input type="password" name="password" placeholder="Password" required><br>
        <input type="password" name="confirm" placeholder="Confirm Password" required><br>
        <input type="email" name="email" placeholder="Email Address" required><br>
        <input type="submit" value="Register">
    </form>
</body>
</html>

3.2 注册代码的实现

我们需要在后台实现注册功能，依旧需要先创建一个User类来存储用户信息，此处只展示相关代码：

public class User {
    private String username;
    private String password;
    private String email;

    public User(String username, String password, String email) {
        this.username = username;
        this.password = password;
        this.email = email;
    }

    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;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }
}

在UserDao类中实现注册的具体细节，如果不存在相同的用户名，则向数据库中插入新的用户信息。以下代码块只是示例，在实际应用中，我们需要根据实际情况进行修改和适配：

public class UserDao {
    private Connection conn;

    public UserDao() {
        try {
            Class.forName("com.mysql.jdbc.Driver");
            String url = "jdbc:mysql://localhost:3306/mydb";
            String user = "root";
            String password = "password";
            conn = DriverManager.getConnection(url, user, password);
        } catch (ClassNotFoundException | SQLException e) {
            e.printStackTrace();
        }
    }

    public boolean register(User user) {
        String sql = "SELECT * FROM user WHERE username = ?";
        try {
            PreparedStatement pstmt = conn.prepareStatement(sql);
            pstmt.setString(1, user.getUsername());
            ResultSet rs = pstmt.executeQuery();
            if (rs.next()) {
                return false;
            } else {
                sql = "INSERT INTO user(username, password, email) VALUES (?, ?, ?)";
                pstmt = conn.prepareStatement(sql);
                pstmt.setString(1, user.getUsername());
                pstmt.setString(2, user.getPassword());
                pstmt.setString(3, user.getEmail());
                pstmt.executeUpdate();
                return true;
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return false;
    }
}

3.3 注册信息的校验

注册功能中，同样需要校验用户输入信息的合法性，并在前台进行反馈。以下代码块还涉及到了一些JavaBean的内容，读者可以参考JavaBean的相关资料来学习，不作过多解释：

@WebServlet("/register.do")
public class RegisterServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    private boolean validate(String field, String value, Map<String, String> errorMsgs) {
        String regex = "\\w+";
        boolean result = true;
        if (value == null || value.trim().equals("")) {
            errorMsgs.put(field, field + " cannot be empty");
            result = false;
        } else if (!value.matches(regex)) {
            errorMsgs.put(field, field + " is invalid");
            result = false;
        }
        return result;
    }

    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");
        String confirm = request.getParameter("confirm");
        String email = request.getParameter("email");

        Map<String, String> errorMsgs = new HashMap<>();
        if (!validate("username", username, errorMsgs)) {
            request.setAttribute("errorMsgs", errorMsgs);
            request.getRequestDispatcher("register.jsp").forward(request, response);;
            return;
        }
        if (!validate("password", password, errorMsgs)) {
            request.setAttribute("errorMsgs", errorMsgs);
            request.getRequestDispatcher("register.jsp").forward(request, response);;
            return;
        }
        if (!password.equals(confirm)) {
            errorMsgs.put("confirm", "The two passwords do not match");
            request.setAttribute("errorMsgs", errorMsgs);
            request.getRequestDispatcher("register.jsp").forward(request, response);;
            return;
        }
        if (!validate("email", email, errorMsgs)) {
            request.setAttribute("errorMsgs", errorMsgs);
            request.getRequestDispatcher("register.jsp").forward(request, response);;
            return;
        }

        UserDao userDao = new UserDao();
        boolean success = userDao.register(new User(username, password, email));
        if (success) {
            request.getRequestDispatcher("login.jsp").forward(request, response);
        } else {
            request.setAttribute("errorMsgs", "The username already exists.");
            request.getRequestDispatcher("register.jsp").forward(request, response);;
        }
    }
}

3.4 注册成功后的跳转

如果用户输入信息合法且成功保存至数据库，则跳转至登录页面，供用户登录：

<html>
<head>
    <title>Login Page</title>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
    <h1>Welcome to My Website</h1>
    <a href="login.jsp">Click here to login</a>
    <a href="register.jsp">Click here to register</a>
</body>
</html>

至此，Java简易登录注册功能实现代码的攻略完成。