@Mapper
public interface UserMapper {
 @Select("select * from t_user")
 List<User> list();
}

[
 {
 "userId": "1",
 "username": "admin",
 "password": "admin",
 "mobileNum": null
 },
 {
 "userId": "2",
 "username": "roots",
 "password": "roots",
 "mobileNum": null
 }
]

 @Results({
   @Result(property = "userId", column = "USER_ID"),
   @Result(property = "username", column = "USERNAME"),
   @Result(property = "password", column = "PASSWORD"),
   @Result(property = "mobileNum", column = "PHONE_NUM")
 })
 @Select("select * from t_user")
 List<User> list();

[
 {
 "userId": "1",
 "username": "admin",
 "password": "admin",
 "mobileNum": "15011791234"
 },
 {
 "userId": "2",
 "username": "roots",
 "password": "roots",
 "mobileNum": "18812342017"
 }
]

 /**
  * 1.用于获取结果集的映射关系
  */
 public static String getResultsStr(Class origin) {
  StringBuilder stringBuilder = new StringBuilder();
  stringBuilder.append("@Results({\n");
  for (Field field : origin.getDeclaredFields()) {
   String property = field.getName();
   //映射关系：对象属性(驼峰)->数据库字段(下划线)
   String column = new PropertyNamingStrategy.SnakeCaseStrategy().translate(field.getName()).toUpperCase();
   stringBuilder.append(String.format("@Result(property = \"%s\", column = \"%s\"),\n", property, column));
  }
  stringBuilder.append("})");
  return stringBuilder.toString();
 }

@Mapper
public interface UserMapper {
 @SelectProvider(type = UserSqlProvider.class, method = "list222")
 List<User> list2();
}
工具类代码如下：

public class UserSqlProvider {
 public String list222() {
  return "select * from t_user ;
 }

 <dependencies>
  <dependency> <!--添加Web依赖 -->
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-web</artifactId>
  </dependency>
  <dependency> <!--添加Mybatis依赖 -->
   <groupId>org.mybatis.spring.boot</groupId>
   <artifactId>mybatis-spring-boot-starter</artifactId>
   <version>1.3.1</version>
  </dependency>
  <dependency><!--添加MySQL驱动依赖 -->
   <groupId>mysql</groupId>
   <artifactId>mysql-connector-java</artifactId>
   <scope>runtime</scope>
  </dependency>
  <dependency><!--添加Test依赖 -->
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-test</artifactId>
   <scope>test</scope>
  </dependency>
 </dependencies>

spring:
 datasource:
 #连接MySQL
 url: jdbc:mysql://localhost:3306/socks?useSSL=false
 username: root
 password: root
 driver-class-name: com.mysql.jdbc.Driver

mybatis:
 configuration:
 #配置项：开启下划线到驼峰的自动转换. 作用：将数据库字段根据驼峰规则自动注入到对象属性。
 map-underscore-to-camel-case: true

logging:
 level:
 #打印SQL信息
 com.hehe.mapper: debug

package com.hehe.mapper;
@Mapper
public interface UserMapper {
 /**
  * 方式1：使用注解编写SQL。
  */
 @Select("select * from t_user")
 List<User> list();

 /**
  * 方式2：使用注解指定某个工具类的方法来动态编写SQL.
  */
 @SelectProvider(type = UserSqlProvider.class, method = "listByUsername")
 List<User> listByUsername(String username);

 /**
  * 延伸：上述两种方式都可以附加@Results注解来指定结果集的映射关系.
  *
  * PS：如果符合下划线转驼峰的匹配项可以直接省略不写。
  */
 @Results({
   @Result(property = "userId", column = "USER_ID"),
   @Result(property = "username", column = "USERNAME"),
   @Result(property = "password", column = "PASSWORD"),
   @Result(property = "mobileNum", column = "PHONE_NUM")
 })
 @Select("select * from t_user")
 List<User> listSample();

 /**
  * 延伸：无论什么方式,如果涉及多个参数,则必须加上@Param注解,否则无法使用EL表达式获取参数。
  */
 @Select("select * from t_user where username like #{username} and password like #{password}")
 User get(@Param("username") String username, @Param("password") String password);

 @SelectProvider(type = UserSqlProvider.class, method = "getBadUser")
 User getBadUser(@Param("username") String username, @Param("password") String password);

}

package com.hehe.mapper;

/**
 * 主要用途：根据复杂的业务需求来动态生成SQL.
 * <p>
 * 目标：使用Java工具类来替代传统的XML文件.(例如：UserSqlProvider.java <-- UserMapper.xml)
 */
public class UserSqlProvider {
 /**
  * 方式1：在工具类的方法里,可以自己手工编写SQL。
  */
 public String listByUsername(String username) {
  return "select * from t_user where username =#{username}";
 }

 /**
  * 方式2：也可以根据官方提供的API来编写动态SQL。
  */
 public String getBadUser(@Param("username") String username, @Param("password") String password) {
  return new SQL() {{
   SELECT("*");
   FROM("t_user");
   if (username != null && password != null) {
    WHERE("username like #{username} and password like #{password}");
   } else {
    WHERE("1=2");
   }
  }}.toString();
 }
}

public class User {
 private String userId;
 private String username;
 private String password;
 private String mobileNum;
 //Getters & Setters
}

USE `SOCKS`;
DROP TABLE IF EXISTS `t_user`;
CREATE TABLE `t_user` (
 `USER_ID` varchar(50) ,
 `USERNAME` varchar(50) ,
 `PASSWORD` varchar(50) ,
 `PHONE_NUM` varchar(15) 
) ;

INSERT INTO `t_user` VALUES ('1', 'admin', 'admin','15011791234');
INSERT INTO `t_user` VALUES ('2', 'roots', 'roots','18812342017');

package com.hehe.controller;

@RestController
@RequestMapping("/user/*")
public class UserController {

 @SuppressWarnings("all")
 @Autowired
 UserMapper userMapper;

 @GetMapping("list")
 public List<User> list() {
  return userMapper.list();
 }

 @GetMapping("list/{username}")
 public List<User> listByUsername(@PathVariable("username") String username) {
  return userMapper.listByUsername(username);
 }

 @GetMapping("get/{username}/{password}")
 public User get(@PathVariable("username") String username, @PathVariable("password") String password) {
  return userMapper.get(username, password);
 }

 @GetMapping("get/bad/{username}/{password}")
 public User getBadUser(@PathVariable("username") String username, @PathVariable("password") String password) {
  return userMapper.getBadUser(username, password);
 }

}