关于RowBounds分页原理、RowBounds的坑记录

select * from user where id>0 limit 0,10

public class RowBounds {
  /* 默认offset是0**/
  public static final int NO_ROW_OFFSET = 0;
  /* 默认Limit是int的最大值，因此它使用的是逻辑分页**/
  public static final int NO_ROW_LIMIT = Integer.MAX_VALUE;
  public static final RowBounds DEFAULT = new RowBounds();

  private int offset;
  private int limit;

  public RowBounds() {
    this.offset = NO_ROW_OFFSET;
    this.limit = NO_ROW_LIMIT;
  }

  public RowBounds(int offset, int limit) {
    this.offset = offset;
    this.limit = limit;
  }

  public int getOffset() {
    return offset;
  }

  public int getLimit() {
    return limit;
  }

}

private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping)
      throws SQLException {
  DefaultResultContext<Object> resultContext = new DefaultResultContext<Object>();
//跳过RowBounds设置的offset值
   skipRows(rsw.getResultSet(), rowBounds);
//判断数据是否小于limit，如果小于limit的话就不断的循环取值
   while (shouldProcessMoreRows(resultContext, rowBounds) && rsw.getResultSet().next()) {
     ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, null);
     Object rowValue = getRowValue(rsw, discriminatedResultMap);
     storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
   }
}

private boolean shouldProcessMoreRows(ResultContext<?> context, RowBounds rowBounds) throws SQLException {
    //判断数据是否小于limit，小于返回true
    return !context.isStopped() && context.getResultCount() < rowBounds.getLimit();
}

  //跳过不需要的行,应该就是rowbounds设置的limit和offset
  private void skipRows(ResultSet rs, RowBounds rowBounds) throws SQLException {
    if (rs.getType() != ResultSet.TYPE_FORWARD_ONLY) {
      if (rowBounds.getOffset() != RowBounds.NO_ROW_OFFSET) {
        rs.absolute(rowBounds.getOffset());
      }
    } else {
      //跳过RowBounds中设置的offset条数据，只能逐条滚动到指定位置
      for (int i = 0; i < rowBounds.getOffset(); i++) {
        rs.next();
      }
    }
}

<dependency>
    <groupId>org.mybatis</groupId>
    <artifactId>mybatis</artifactId>
    <version>3.5.6</version>
</dependency>

import org.apache.ibatis.session.RowBounds;

public class UserServiceImpl implements UserService {
    @Autowired
    private UserDao userDao;
    
    @Override
    public Map<String, Object> queryUserList(String currentPage, String pageSize) {
        //查询数据总条数
        int total = userDao.queryCountUser();
        //返回结果集
        Map<String,Object> resultMap = new HashMap<String,Object>();
        
        resultMap.put("total", total);
        //总页数
        int totalpage = (total + Integer.parseInt(pageSize) - 1) / Integer.parseInt(pageSize);
        resultMap.put("totalpage", totalpage);
        
        //数据的起始行
        int offset = (Integer.parseInt(currentPage)-1)*Integer.parseInt(pageSize);
        RowBounds rowbounds = new RowBounds(offset, Integer.parseInt(pageSize));
        //用户数据集合
        List<Map<String, Object>> userList = userDao.queryUserList(rowbounds);
        
        resultMap.put("userList", userList);
        
        return resultMap;
    }

}

import org.apache.ibatis.session.RowBounds;

public interface UserDao {
    
    public int queryCountUser();       //查询用户总数
    public List<Map<String, Object>> queryUserList(RowBounds rowbounds);    //查询用户列表
}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.test.mapper.UserDao">
    
    <!-- 查询用户总数 -->
    <select id="queryCountUser" resultType="java.lang.Integer">
        select count(1) from user
    </select>
    
    <!-- 查询用户列表 -->
    <select id="queryUserList" resultType="java.util.Map">
        select * from user
    </select>
    
</mapper>

select * from user where id >0 limit 3,10