十二、持久层框架（MyBatis）

PageHelper是一款MyBatis的分页插件，只需要简单的配置，然后直接调用方法就可以。

1、配置PageHelper插件

在mybatis-config.xml中，配置如下

<plugins>
    <plugin>
        inteceptor="com.github.pagehelper.PageInterceptor"
    </plugin>
</plugins>

具体mybatis-config.xml如下：

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <settings>  
            <!-- 打开延迟加载的开关 -->  
            <setting name="lazyLoadingEnabled" value="true" />  
            <!-- 将积极加载改为消息加载即按需加载 -->  
            <setting name="aggressiveLazyLoading" value="false"/>  
        </settings>    
    <typeAliases>
      <package name="com.demo.pojo"/>
    </typeAliases>
    <plugins>
    <!--配置PageHelper分页插件-->
      <plugin interceptor="com.github.pagehelper.PageInterceptor">
        </plugin>
    </plugins>     
    <!--数据库连接环境配置--> 
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
            <property name="driver" value="com.mysql.jdbc.Driver"/>
            <property name="url" value="jdbc:mysql://localhost:3306/how2java?characterEncoding=UTF-8"/>
            <property name="username" value="root"/>
            <property name="password" value="admin"/>
            </dataSource>
        </environment>
    </environments>
    <!--映射文件配置-->
    <mappers>
            <!--基于xml方式映射-->
        <mapper resource="com/demo/pojo/Category.xml"/>
        <!--基于注解方式映射-->
        <mapper class="com.how2java.mapper.CategoryMapper"/>  
        <mapper class="com.how2java.mapper.ProductMapper"/>  
    </mappers>  
</configuration>

2、使用PageHelper提供的方法进行分页查询测试

package com.demo;
import java.io.IOException;
import java.io.InputSteam;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import org.apache.ibatis.io.Resouces;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import com.demo.pojo.Product;
import com.github.pagehelper.PageHelper;
import com.github.pagehelper.PageInfo;

public class TestMyBatis{
    public static void main(String[] args) throws IOException{
        String resouce="mybatis-config.xml";
        InputSteam inputStream=Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory =new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession session=sqlSessionFactory.openSession();
        
        PageHelper.offsetPage(0,5);//表示从0开始，每一页显示5条数据
        List<Product> list=session.selectList("listCategory");
        for(Product p:list){
            System.out.println(p);
        }
        PageInfo pageInfo=new PageInfo<>(p);
        System.out.println("总数："+pageInfo.getTotal());//总个数
        System.out.println(pageInfo);//PageInfo{pageNum=1,pageSize=5,size=5,startRow=1,end}
        session.commit();
        session.close();
        
    }    
}

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