MyBatis分页插件PageHelper的使用与原理

select * 
from tableName ...... 
limit (pageNum - 1) * pageSize, pageSize

package com.bjpowernode.mybatis.mapper;
import com.bjpowernode.mybatis.pojo.Car;
import org.apache.ibatis.annotations.Param;
import java.util.List;
public interface CarMapper {
    /**
     * 分页查询
     * @param startIndex 起始下标
     * @param pageSize  每页显示的记录条数
     * @return
     */
    List<Car> selectAllByPage(@Param("startIndex") int startIndex, @Param("pageSize") int pageSize);
}

<?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.bjpowernode.mybatis.mapper.CarMapper">
  <select id="selectAllByPage" resultType="Car" >
    select * from t_car limit #{startInde},#{pageSize}
  </select>
</mapper>

package com.bjpowernode.mybatis.test;
import com.bjpowernode.mybatis.mapper.CarMapper;
import com.bjpowernode.mybatis.pojo.Car;
import com.bjpowernode.mybatis.utils.SqlSessionUtil;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
import java.util.List;
public class CarMapperTest {
    @Test
    public void testSelectAllByPage(){
        // 应该是从前端获取到页码
        int pageNum = 2;
        // 应该是从前端获取到每页显示的条数
        int pageSize = 2;
        // 起始下标
        int startIndex = (pageNum-1)*pageSize;
        SqlSession sqlSession = SqlSessionUtil.openSession();
        CarMapper mapper = sqlSession.getMapper(CarMapper.class);
        List<Car> cars = mapper.selectAllByPage(startIndex, pageSize);
        cars.forEach(car -> System.out.println(car));
        sqlSession.close();
    }
}

<dependency>
     <groupId>com.github.pagehelper</groupId>
     <artifactId>pagehelper</artifactId>
     <version>5.3.1</version>
</dependency>

<!--mybatis分页的拦截器-->
<plugins>
    <plugin interceptor="com.github.pagehelper.PageInterceptor"></plugin>
</plugins>

package com.bjpowernode.mybatis.mapper;
import com.bjpowernode.mybatis.pojo.Car;
import org.apache.ibatis.annotations.Param;
import java.util.List;
public interface CarMapper {
    /**
     * 查询所有的Car，通过分页查询插件PageHelper完成
     * @return
     */
    List<Car> selectAll();
}

<?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.bjpowernode.mybatis.mapper.CarMapper"> 
  <select id="selectAll" resultType="Car">
      select * from t_car
  </select>
</mapper>

package com.bjpowernode.mybatis.test;
import com.bjpowernode.mybatis.mapper.CarMapper;
import com.bjpowernode.mybatis.pojo.Car;
import com.bjpowernode.mybatis.utils.SqlSessionUtil;
import com.github.pagehelper.PageHelper;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
import java.util.List;
public class CarMapperTest {
    @Test
    public void testSelectAll(){
        SqlSession sqlSession = SqlSessionUtil.openSession();
        CarMapper mapper = sqlSession.getMapper(CarMapper.class);
        // 在执行DQL语句之前，开启分页
        int pageNum = 2; // 第二页
        int pageSize = 3; // 每页显示的条数
        PageHelper.startPage(pageNum,pageSize);
        List<Car> cars = mapper.selectAll();
        cars.forEach(car -> System.out.println(car));
        sqlSession.close();
    }
}

package com.bjpowernode.mybatis.test;
import com.bjpowernode.mybatis.mapper.CarMapper;
import com.bjpowernode.mybatis.pojo.Car;
import com.bjpowernode.mybatis.utils.SqlSessionUtil;
import com.github.pagehelper.PageHelper;
import com.github.pagehelper.PageInfo;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
import java.util.List;
public class CarMapperTest {
    @Test
    public void testSelectAll(){
        SqlSession sqlSession = SqlSessionUtil.openSession();
        CarMapper mapper = sqlSession.getMapper(CarMapper.class);
        // 1. 在执行DQL语句之前，开启分页
        int pageNum = 1; // 第一页
        int pageSize = 3; // 每页显示的条数
        PageHelper.startPage(pageNum,pageSize);
        // 2. 执行SQL语句
        List<Car> cars = mapper.selectAll();
        // 3. 执行DQL之后，封装PageInfo对象，new PageInfo对象
        // 第二个参数就是分页导航的卡片个数
        PageInfo<Car> carPageInfo = new PageInfo<>(cars, 3);
        System.out.println(carPageInfo);
        sqlSession.close();
    }
}