Spring Boot+mybatis项目切换sql为传参成无参

一、项目文件结构树

bash 复制代码

src/main/java/com/example/
├── controller
│   └── UserController.java
├── service
│   ├── UserService.java
│   └── impl
│       └── UserServiceImpl.java
├── mapper
│   └── UserMapper.java
└── entity
    └── User.java

src/main/resources/
├── mapper
│   └── UserMapper.xml
└── application.properties

二、改动前

UserController.java

java 复制代码

@RestController
@RequestMapping("/users")
public class UserController {
    @Autowired
    private UserService userService;

    @GetMapping("/{id}")
    public User getUser(@PathVariable Long id) {
        return userService.getUserById(id);
    }
}

UserService.java

java 复制代码

public interface UserService {
    User getUserById(Long id);
}

UserServiceImpl.java

java 复制代码

@Service
public class UserServiceImpl implements UserService {
    @Autowired
    private UserMapper userMapper;

    @Override
    public User getUserById(Long id) {
        return userMapper.selectUserById(id);
    }
}

UserMapper.java

java 复制代码

@Mapper
public interface UserMapper {
    User selectUserById(@Param("id") Long id);
}

UserMapper.xml

xml 复制代码

<mapper namespace="com.example.mapper.UserMapper">
    <select id="selectUserById" resultType="com.example.entity.User">
        SELECT * FROM user WHERE id = #{id}
    </select>
</mapper>

三、修改后

UserController.java

java 复制代码

@RestController
@RequestMapping("/users")
public class UserController {
    @Autowired
    private UserService userService;

    @GetMapping
    public List<User> getAllUsers() {          // 改为返回列表，不再接收路径参数
        return userService.getAllUsers();
    }
}

UserService.java

java 复制代码

public interface UserService {
    List<User> getAllUsers();                  // 方法名和返回值都改变
}

UserServiceImpl.java

java 复制代码

@Service
public class UserServiceImpl implements UserService {
    @Autowired
    private UserMapper userMapper;

    @Override
    public List<User> getAllUsers() {           // 实现无参方法
        return userMapper.selectAllUsers();     // 调用 Mapper 中的无参方法
    }
}

UserMapper.java

java 复制代码

@Mapper
public interface UserMapper {
    List<User> selectAllUsers();                // 方法签名去掉参数，返回列表
}

UserMapper.xml

xml 复制代码

<mapper namespace="com.example.mapper.UserMapper">
    <select id="selectAllUsers" resultType="com.example.entity.User">
        SELECT * FROM user                     <!-- SQL 中去掉了 WHERE 条件 -->
    </select>
</mapper>

改的不对的话容易报错NullPointerException

一定仔细检查XML及对应的mapper文件等