1. 描述

1.1 MybatisPlusInterceptor

我们在开发的过程中，经常会遇到分页操作，其分为逻辑分页和物理分页，具体可参考我的博文：逻辑分页和物理分页

如果你用的是Mybatis-Plus框架，可用MybatisPlusInterceptor按如下配置分页代码：

/**
 * @author 念兮为美
 * @datetime 2022/11/28 14:10
 * @desc mybatis plus 配置类
 */
@Configuration
public class MybatisPlusConfig {

  @Bean
  public MybatisPlusInterceptor mybatisPlusInterceptor() {
    MybatisPlusInterceptor interceptor = new MybatisPlusInterceptor();
    // 设置为使用 MYSQL 方言
    interceptor.addInnerInterceptor(new PaginationInnerInterceptor(DbType.MYSQL));
    return interceptor;
  }
}

如果你用的是低版本的spring boot，可用PaginationInterceptor按如下配置：

/**
 * @author 念兮为美
 * @datetime 2022/11/27 15:22
 * @desc mybatis plus 配置类
 */
@Configuration
public class MybatisPlusConfig {
    @Bean
    public PaginationInterceptor paginationInterceptor() {
        return new PaginationInterceptor();
    }
}

MybatisPlusInterceptor插件是核心插件，目前代理了Executor#query、Executor#update、StatementHandler#prepare方法，其属性：

private List<InnerInterceptor> interceptors = new ArrayList<>();

1.2 InnerInterceptor

注意List<InnerInterceptor>泛型中的InnerInterceptor，mybatis-plus提供的插件都将基于此接口来实现功能，目前已有的功能如下：

1. 自动分页： PaginationInnerInterceptor

2. 多租户：TenantLineInnerInterceptor

3. 动态表名：DynamicTableNameInnerInterceptor

4. 乐观锁：OptimisticLockerInnerInterceptor

5. sql性能规范： IllegalSQLInnerInterceptor

6. 防止全表更新与删除: BlockAttackInnerInterceptor

【注意】使用多个功能需要注意顺序关系，建议使用如下顺序：

1. 多租户

2. 动态表名

3. 分页，乐观锁

4. sql性能规范,防止全表更新与删除

总结：对sql进行单次改造的优先放入，不对sql进行改造的最后放入。

2. 实现

2.1 不带条件的分页查询

1. 编写查询展示类

/**
 * @author 念兮为美
 * @datetime 2023/2/2 09:58
 * @desc 用户查询结果
 */
@NoArgsConstructor
@Data
@ApiModel(description = "用户返回结果")
public class UserPageVo {

  @ApiModelProperty(name = "username", value = "用户名")
  private String username;

  @ApiModelProperty(name = "nickname", value = "昵称")
  private String nickname;

  @ApiModelProperty(name = "userType", value = "用户类型")
  private String userType;
}

2. 编写mapper类

@Mapper
public interface UserMapper extends BaseMapper<User> {

   @Select("select " +
            "username,nickname,user_type as userType " +
          "from " +
            "user " +
          "order by " +
            "create_time desc"
  )
  List<UserPageVo> findPageUsers(Page<UserPageVo> page);
}

3. 编写service类

@Service
public class UserService extends ServiceImpl<UserMapper, User> {

  @Resource private UserMapper userMapper;
  
  public Page<UserPageVo> findPageUsers() {
    Long currentPage = 4L;
    Long pageSize = 3L;
    Page<UserPageVo> page = new Page<>(currentPage, pageSize);
    List<UserPageVo> pageUsers = userMapper.findPageUsers(page);
    page.setRecords(pageUsers);
    return page;
  }
}

4. 编写controller类

@Api(tags = "用户模块")
@RestController
@RequestMapping("/user")
@Slf4j
@Validated
public class UserController {

  @Autowired private UserService userService;

  @ApiOperationSupport(author = "念兮为美")
  @ApiOperation(value = "用户查询接口")
  @GetMapping("/findPageUsers")
  public Page<UserPageVo> findPageUsers() {
    return userService.findPageUsers();
  }
}

5. 测试运行结果

JDBC Connection [com.alibaba.druid.proxy.jdbc.ConnectionProxyImpl@2ce7d43] will not be managed by Spring
==>  Preparing: SELECT COUNT(*) AS total FROM user
==> Parameters: 
<==    Columns: total
<==        Row: 24
<==      Total: 1
==>  Preparing: select username,nickname,user_type as userType from user order by create_time desc LIMIT ?,?
==> Parameters: 9(Long), 3(Long)
<==    Columns: username, nickname, userType
<==        Row: cs, null, admin
<==        Row: lin, null, admin
<==        Row: test3, null, TEST4
<==      Total: 3

2.2 带条件的分页查询

public class UserService extends ServiceImpl<UserMapper, User> {

  @Resource private UserMapper userMapper;

  public Page<User> findPageUsers() {
    Long currentPage = 1L;
    Long pageSize = 3L;
    Page<User> page = new Page<>(currentPage, pageSize);
    QueryWrapper<User> queryWrapper = new QueryWrapper<>();
    queryWrapper.eq("username", "test5");
    Page<User> userPage = userMapper.selectPage(page, queryWrapper);
    return userPage;
  }

运行结果：

JDBC Connection [com.alibaba.druid.proxy.jdbc.ConnectionProxyImpl@475454ae] will not be managed by Spring
==>  Preparing: SELECT COUNT(*) AS total FROM user WHERE deleted = false AND (username = ?)
==> Parameters: test2(String)
<==    Columns: total
<==        Row: 1
<==      Total: 1
==>  Preparing: SELECT username,nickname,user_type FROM user WHERE deleted=false AND (username = ?) LIMIT ?
==> Parameters: test2(String), 3(Long)
<==    Columns: username, nickname, user_type
<==        Row:  test2, null,TEST6
<==      Total: 1

2.3 简述Page类

简单分页模型, 有如下几个主要属性

/**
 * 查询数据列表
 */
protected List<T> records = Collections.emptyList();

/**
 * 总数
 */
protected long total = 0;

/**
 * 每页显示条数，默认 10
 */
protected long size = 10;

/**
 * 当前页
 */
protected long current = 1;

3. 注意事项

在编写mapper.xml中的SQL语句时，或者使用@select注解编写SQL语句时，语句末尾不能使用 ; 结尾，原因是在做分页的时候会在编写的SQL语句后面拼接上limit语句, 导致出现SQL语法错误(SQLSyntaxErrorException)，如下所示：

@Select("select " +
            "username,nickname,user_type as userType " +
          "from " +
            "user " +
          "order by " +
            "create_time desc;"
  )
  List<UserPageVo> findPageUsers(Page<UserPageVo> page);

运行结果：

JDBC Connection [com.alibaba.druid.proxy.jdbc.ConnectionProxyImpl@498d7c57] will not be managed by Spring
==>  Preparing: SELECT COUNT(*) AS total FROM user
==> Parameters: 
<==    Columns: total
<==        Row: 24
<==      Total: 1
==>  Preparing: select username,nickname,user_type as userType from user order by create_time desc; LIMIT ?,?
Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6f5c4fa1]
org.springframework.jdbc.UncategorizedSQLException: 
### Error querying database.  Cause: java.sql.SQLException: sql injection violation, dbType mysql, , druid-version 1.2.11, syntax error: not supported.pos 89, line 1, column 85, token LIMIT : select username,nickname,user_type as userType from user order by create_time desc; LIMIT ?,?
### The error may exist in com/cloud/lowcode/mapper/UserMapper.java (best guess)
### The error may involve com.cloud.lowcode.mapper.UserMapper.findPageUsers
### The error occurred while executing a query
### SQL: select username,nickname,user_type as userType from user order by create_time desc; LIMIT ?,?
### Cause: java.sql.SQLException: sql injection violation, dbType mysql, , druid-version 1.2.11, syntax error: not supported.pos 89, line 1, column 85, token LIMIT : select username,nickname,user_type as userType from user order by create_time desc; LIMIT ?,?
; uncategorized SQLException; SQL state [null]; error code [0]; sql injection violation, dbType mysql, , druid-version 1.2.11, syntax error: not supported.pos 89, line 1, column 85, token LIMIT : select username,nickname,user_type as userType from user order by create_time desc; LIMIT ?,?; nested exception is java.sql.SQLException: sql injection violation, dbType mysql, , druid-version 1.2.11, syntax error: not supported.pos 89, line 1, column 85, token LIMIT : select username,nickname,user_type as userType from user order by create_time desc; LIMIT ?,?
	at org.mybatis.spring.MyBatisExceptionTranslator.translateExceptionIfPossible(MyBatisExceptionTranslator.java:92)