使用Java编写一个简单的具有登录功能的web项目

我们可以使用Spring Boot框架来编写一个简单的具有登录功能的web项目。项目结构如下：

- src
  - main
    - java
      - com.example.demo
        - controller
          - LoginController.java
          - UserController.java
        - model
          - User.java
        - repository
          - UserRepository.java
        - service
          - UserService.java
        - App.java
    - resources
      - application.properties
      - templates
        - login.html
        - user.html
  - test
    - java
      - com.example.demo
        - ApplicationTests.java
  - pom.xml

1. 建立一个Spring Boot应用程序

在pom.xml文件中添加以下依赖项：

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-thymeleaf</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-data-jpa</artifactId>
    </dependency>
    <dependency>
          <groupId>mysql</groupId>
          <artifactId>mysql-connector-java</artifactId>
          <version>8.0.25</version>
    </dependency>
</dependencies>

2. 创建实体类

在model包下创建User.java实体类：

package com.example.demo.model;

import javax.persistence.*;

@Entity
@Table(name = "user")
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String username;
    private String password;

    public User() {}

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

    public Long getId() {
        return id;
    }

    public void setId(Long 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. 创建Repository

在repository包中创建一个UserRepository.java接口，并继承JpaRepository：

package com.example.demo.repository;

import com.example.demo.model.User;
import org.springframework.data.jpa.repository.JpaRepository;

public interface UserRepository extends JpaRepository<User, Long> {
    User findByUsername(String username);
}

4. 创建Service

在service包中创建UserService.java类：

package com.example.demo.service;

import com.example.demo.model.User;
import com.example.demo.repository.UserRepository;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class UserService {
    @Autowired
    private UserRepository userRepository;

    public User getUserByUsername(String username) {
        return userRepository.findByUsername(username);
    }
}

5. 创建Controller

在controller包中创建LoginController.java控制器，实现登录功能：

package com.example.demo.controller;

import com.example.demo.model.User;
import com.example.demo.service.UserService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.*;

@Controller
public class LoginController {
    @Autowired
    private UserService userService;

    @GetMapping("/")
    public String login(Model model) {
        model.addAttribute("user", new User());
        return "login";
    }

    @PostMapping("/login")
    public String login(@ModelAttribute User user, Model model) {
        User u = userService.getUserByUsername(user.getUsername());
        if (u == null || !u.getPassword().equals(user.getPassword())) {
            model.addAttribute("errorMsg", "Invalid username or password");
            return "login";
        }
        return "redirect:/user/" + u.getId();
    }
}

controller包下创建UserController.java控制器，实现用户页面：

package com.example.demo.controller;

import com.example.demo.model.User;
import com.example.demo.service.UserService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.*;

@Controller
public class UserController {
    @Autowired
    private UserService userService;

    @GetMapping("/user/{id}")
    public String user(@PathVariable Long id, Model model) {
        User user = userService.getUserById(id);
        model.addAttribute("user", user);
        return "user";
    }
}

6. 创建HTML页面

在templates包中创建login.html页面：

<!DOCTYPE html>
<html xmlns:th="http://www.thymeleaf.org">
<head>
    <title>Login Page</title>
    <meta charset="UTF-8">
    <style type="text/css">
        #login-form {
            border: 2px solid #eee;
            padding: 10px;
            margin: 50px auto;
            max-width: 350px;
        }
        label {
            display: block;
            margin-bottom: 5px;
        }
        input[type="text"],input[type="password"] {
            display: block;
            width: 100%;
            margin-bottom: 20px;
            padding: 5px;
        }
        input[type="submit"] {
            background-color: #4CAF50;
            color: white;
            padding: 10px 20px;
            border: none;
            cursor: pointer;
            width: 100%;
        }
        input[type="submit"]:hover {
            background-color: #45a049;
        }
        .form-error {
            color: red;
            margin-top: 10px;
        }
    </style>
</head>
<body>
    <div id="login-form">
        <h2>Login Form</h2>
        <form method="post" th:object="${user}" th:action="@{/login}">
            <label for="username">Username:</label>
            <input type="text" id="username" name="username" th:field="*{username}"/>
            <label for="password">Password:</label>
            <input type="password" id="password" name="password" th:field="*{password}"/>
            <input type="submit" value="Login"/>
            <div th:if="${errorMsg}">
                <p class="form-error" th:text="${errorMsg}"></p>
            </div>
        </form>
    </div>
</body>
</html>

在templates包中创建user.html页面：

<!DOCTYPE html>
<html xmlns:th="http://www.thymeleaf.org">
<head>
    <title>User Page</title>
    <meta charset="UTF-8">
</head>
<body>
    <h1>Hello, <span th:text="${user.username}"></span>!</h1>
</body>
</html>

7. 运行程序

在App.java文件中添加@SpringBootApplication注解，并通过运行该文件来启动应用程序。

打开http://localhost:8080/可以看到登录页面。

输入正确的用户名和密码，会跳转到用户页面，可以看到用户的欢迎消息。

到此为止，我们已经用Java编写了一个简单的具有登录功能的web项目。