基于Springboot+Vue的二手交易平台

### 后端（Spring Boot）
首先，创建一个Spring Boot项目，并添加必要的依赖，如Spring Web、Spring Data JPA、Spring Security等。
#### User实体和Repository
```java
@Entity
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String username;
    private String password;
    // 省略构造函数、Getter和Setter
}
public interface UserRepository extends JpaRepository<User, Long> {
    User findByUsername(String username);
}
```
#### 用户服务
```java
@Service
public class UserService {
    @Autowired
    private UserRepository userRepository;
    public User register(User user) {
        return userRepository.save(user);
    }
    public User login(String username, String password) {
        User user = userRepository.findByUsername(username);
        if (user != null && user.getPassword().equals(password)) {
            return user;
        }
        return null;
    }
}
```
#### 控制器
```java
@RestController
@RequestMapping("/api/users")
public class UserController {
    @Autowired
    private UserService userService;
    @PostMapping("/register")
    public User register(@RequestBody User user) {
        return userService.register(user);
    }
    @GetMapping("/login")
    public User login(@RequestParam String username, @RequestParam String password) {
        return userService.login(username, password);
    }
}
```
### 前端（Vue）
使用Vue CLI创建一个Vue项目，并安装必要的依赖，如axios。
#### 登录组件
```vue
<template>
  <div>
    <h1>登录</h1>
    <form @submit.prevent="login">
      <input v-model="username" placeholder="用户名" />
      <input type="password" v-model="password" placeholder="密码" />
      <button type="submit">登录</button>
    </form>
  </div>
</template>
<script>
import axios from 'axios';
export default {
  data() {
    return {
      username: '',
      password: '',
    };
  },
  methods: {
    async login() {
      try {
        const response = await axios.get('http://localhost:8080/api/users/login', {
          params: {
            username: this.username,
            password: this.password,
          },
        });
        console.log('登录成功:', response.data);
      } catch (error) {
        console.error('登录失败:', error);
      }
    },
  },
};
</script>
```
#### 注册组件
```vue
<template>
  <div>
    <h1>注册</h1>
    <form @submit.prevent="register">
      <input v-model="user.username" placeholder="用户名" />
      <input type="password" v-model="user.password" placeholder="密码" />
      <button type="submit">注册</button>
    </form>
  </div>
</template>
<script>
import axios from 'axios';
export default {
  data() {
    return {
      user: {
        username: '',
        password: '',
      },
    };
  },
  methods: {
    async register() {
      try {
        const response = await axios.post('http://localhost:8080/api/users/register', this.user);
        console.log('注册成功:', response.data);
      } catch (error) {
        console.error('注册失败:', error);
      }
    },
  },
};
</script>
```