写一个java用户注册登录系统

好的，我可以给您提供一个简单的 Java 用户注册登录系统的实现代码，代码如下：

User.java

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

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

    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;
    }

    public String getPhone() {
        return phone;
    }

    public void setPhone(String phone) {
        this.phone = phone;
    }
}

UserDAO.java

public class UserDAO {
    private List<User> userList = new ArrayList<>();

    public void addUser(User user) {
        userList.add(user);
    }

    public User getUserByUsername(String username) {
        for (User user : userList) {
            if (user.getUsername().equals(username)) {
                return user;
            }
        }
        return null;
    }

    public boolean isUsernameExist(String username) {
        for (User user : userList) {
            if (user.getUsername().equals(username)) {
                return true;
            }
        }
        return false;
    }

    public boolean isEmailExist(String email) {
        for (User user : userList) {
            if (user.getEmail().equals(email)) {
                return true;
            }
        }
        return false;
    }

    public boolean isPhoneExist(String phone) {
        for (User user : userList) {
            if (user.getPhone().equals(phone)) {
                return true;
            }
        }
        return false;
    }
}

RegisterServlet.java

public class RegisterServlet extends HttpServlet {
    private UserDAO userDAO = new UserDAO();

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String username = request.getParameter("username");
        String password = request.getParameter("password");
        String email = request.getParameter("email");
        String phone = request.getParameter("phone");

        if (userDAO.isUsernameExist(username)) {
            request.setAttribute("message", "用户名已存在");
            request.getRequestDispatcher("register.jsp").forward(request, response);
        } else if (userDAO.isEmailExist(email)) {
            request.setAttribute("message", "邮箱已存在");
            request.getRequestDispatcher("register.jsp").forward(request, response);
        } else if (userDAO.isPhoneExist(phone)) {
            request.setAttribute("message", "手机号已存在");
            request.getRequestDispatcher("register.jsp").forward(request, response);
        } else {
            User user = new User(username, password, email, phone);
            userDAO.addUser(user);
            request.getSession().setAttribute("user", user);
            response.sendRedirect("home.jsp");
        }
    }
}

LoginServlet.java

public class LoginServlet extends HttpServlet {
    private UserDAO userDAO = new UserDAO();

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String username = request.getParameter("username");
        String password = request.getParameter("password");

        User user = userDAO.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);
            response.sendRedirect("home.jsp");
        }
    }
}

这些代码实现了用户注册、登录的基本功能，包括用户信息的存储、用户信息的查询和验证等等。当用户注册成功或登录成功时，将用户信息存储到 session 中，以便后续的操作。当用户注册信息或登录信息验证不通过时，给用户相应的提示信息，让其重新填写。