Java从入门到入土之分页工具篇

从数据库层面分页（数据库为mySQL，数据访问层框架为mybatis）

1. 定义一个PageBean工具类，
   工具类最低需要6个私有属性，totalRecord 数据总条数；totalPage 数据总页数；pageSize 每一页几条数据；pageNow 当前页码；List list 当前页码的数据集合；startIndex sql语句查询的起点。

package com.pagehelper.test;
import java.util.List;
public class PageBean<T> {
    /*总记录条数*/
    private int totalRecord;
    /*总页数*/
    private int totalPage;
    /*每一页数据条数*/
    private int pageSize;
    /*当前页码*/
    private int pageNow;
    /*获取到的数据集合泛型为了使工具类具有通用性*/
    private List<T> list;
    /*数据库查询从那条记录开始*/
    private int startIndex;
    public PageBean(int pageNow, int pageSize, int totalRecord){
        this.totalRecord = totalRecord;
        /*三目运算，totalRecord能被pageSize整除totalPage=totalRecord/pageSize
         * 不能整除totalPage=totalRecord/pageSize+1*/
        totalPage = totalRecord%pageSize==0?totalRecord/pageSize:totalRecord/pageSize+1;
        this.pageSize = pageSize;
        this.pageNow = pageNow;
        startIndex = (pageNow-1)*pageSize;
    }
    /*固定PageSize的构造器*/
    public PageBean(int pageNow, int totalRecord){
        this.totalRecord = totalRecord;
        pageSize = 5;
        totalPage = totalRecord%pageSize==0?totalRecord/pageSize:totalRecord/pageSize+1;
        this.pageNow = pageNow;
        startIndex = (pageNow-1)*pageSize;
    }
    /*Set,Get方法*/
    public int getTotalRecord() {
        return totalRecord;
    }

    public void setTotalRecord(int totalRecord) {
        this.totalRecord = totalRecord;
    }

    public int getTotalPage() {
        return totalPage;
    }

    public void setTotalPage(int totalPage) {
        this.totalPage = totalPage;
    }

    public int getPageSize() {
        return pageSize;
    }

    public void setPageSize(int pageSize) {
        this.pageSize = pageSize;
    }

    public int getPageNow() {
        return pageNow;
    }

    public void setPageNow(int pageNow) {
        this.pageNow = pageNow;
    }

    public List<T> getList() {
        return list;
    }

    public void setList(List<T> list) {
        this.list = list;
    }

    public int getStartIndex() {
        return startIndex;
    }

    public void setStartIndex(int startIndex) {
        this.startIndex = startIndex;
    }

}

Mapper接口最低需要两个方法，一个方法查询总记录数，另一个查数据
Worker是我数据库对于的pojo类。

int getTotalRecord();

List<Worker> selectAll(@Param(value = "startIndex") int startIndex, @Param(value = "pageSize")int pageSize);

下面代码是Worker对应得resultMap和接口方法对应得实现

<mapper namespace="com.pagehelper.mapper.WorkerMapper" >
    <resultMap id="BaseResultMap" type="com.pagehelper.pojo.Worker" >
        <id column="wid" property="wid" jdbcType="INTEGER" />
        <result column="password" property="password" jdbcType="VARCHAR" />
        <result column="company" property="company" jdbcType="VARCHAR" />
        <result column="department" property="department" jdbcType="VARCHAR" />
        <result column="job" property="job" jdbcType="VARCHAR" />
        <result column="name" property="name" jdbcType="VARCHAR" />
        <result column="sex" property="sex" jdbcType="VARCHAR" />
        <result column="age" property="age" jdbcType="INTEGER" />
        <result column="tel" property="tel" jdbcType="VARCHAR" />
        <result column="email" property="email" jdbcType="VARCHAR" />
        <result column="regist_time" property="registTime" jdbcType="TIMESTAMP" />
        <result column="img" property="img" jdbcType="VARCHAR" />
    resultMap>
    <select id="getTotalRecord" resultType="int">
        SELECT COUNT(wid) FROM worker
    select>
    <select id="selectAll" resultMap="BaseResultMap">
        SELECT wid, password, company, department,job, name, sex, age, tel, email, regist_time,img FROM worker limit #{startIndex},#{pageSize}
    select>
mapper>

Service 层调用代码：

@Override
    public PageBean<Worker> getAll(int pageNow) {
        /*先调用获取总记录数的方法getTotalRecord()*/
        int totalRecord = workerMapper.getTotalRecord();
        /*实例化一个工具类PageBean 传入参数pageNow，totalRecord，我这里用的PageSize固定的构造器*/
        PageBean<Worker> pageBean = new PageBean<>(pageNow,totalRecord);
        /*调用查询数据的方法selectAll(StartIndex,PageSize）,参数使用PageBean的属性，查询的结果封装到PageBean的List中*/
        pageBean.setList(workerMapper.selectAll(pageBean.getStartIndex(), pageBean.getPageSize()));
        /*返回PageBean对象*/
        return pageBean;
    }

因为建的java项目，没有写Controller和页面，所以用测试方法展示查询结果

package com.pagehelper.test;

import com.pagehelper.mapper.WorkerMapper;
import com.pagehelper.pojo.Worker;
import com.pagehelper.service.impl.WorkerServiceImpl;
import com.pagehelper.util.PageBean;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import java.io.IOException;
import java.io.InputStream;

public class Test1 {

    public static void main(String[] args) throws IOException {
        /*获取，加载配置文件*/
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        /*创建SqlSessionFactory 会话工厂*/
        SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        /*生产SQLSession*/
        SqlSession session = sessionFactory.openSession();
        /*获得WorkerMapper接口的实现*/
        WorkerMapper mapper = session.getMapper(WorkerMapper.class);
        /*Controller层调用Service层方法，实例化WorkerService*/
        WorkerServiceImpl workerService = new WorkerServiceImpl(mapper);
        /*调用WorkerService方法，返回PageBean，封装了分页相关数据
        * Controller拿到数据之后用request或module将PageBean传递到前端页面，页面通过EL表达式可以直接获取数据库数据，当前页码，总页码等相关参数*/
        PageBean<Worker> pageBean = workerService.getAll(1);
        /*打印总记录数TotalRecord*/
        System.out.println("总记录数："+pageBean.getTotalRecord());
        /*打印总页数TotalPage*/
        System.out.println("总页数:"+pageBean.getTotalPage());
        /*打印从数据库获取的数据*/
        for (Worker worker:pageBean.getList()) {
            System.out.println(worker);
        }
    }
}

运行结果

log4j:ERROR Could not find value for key log4j.appender.LogFIle
log4j:ERROR Could not instantiate appender named "LogFIle".
2019-07-24 09:26:41  [ main:1 ] - [ DEBUG ]  ==>  Preparing: SELECT COUNT(wid) FROM worker 
2019-07-24 09:26:41  [ main:56 ] - [ DEBUG ]  ==> Parameters: 
2019-07-24 09:26:41  [ main:123 ] - [ DEBUG ]  <==      Total: 1
2019-07-24 09:26:41  [ main:131 ] - [ DEBUG ]  ==>  Preparing: SELECT wid, password, company, department,job, name, sex, age, tel, email, regist_time,img FROM worker limit ?,? 
2019-07-24 09:26:41  [ main:132 ] - [ DEBUG ]  ==> Parameters: 0(Integer), 5(Integer)
2019-07-24 09:26:41  [ main:148 ] - [ DEBUG ]  <==      Total: 5
总记录数：29
总页数:6
Worker{wid=100001, password='admin', company='软帝', department='开发部', job='超级管理员', name='超级管理员', sex='男', age=18, tel='123', email='[email protected]', registTime=Sat Jul 20 11:33:18 CST 2019, img='default.jpg'}
Worker{wid=100002, password='123', company='软帝集团', department='开发部', job='职员', name='管理员', sex='男', age=122, tel='123', email='[email protected]', registTime=Sat Jul 20 16:00:59 CST 2019, img='default.jpg'}
Worker{wid=100003, password='admin', company='软帝', department='开发部', job='管理员', name='管理员', sex='男', age=18, tel='123', email='[email protected]', registTime=Sat Jul 20 11:33:20 CST 2019, img='default.jpg'}
Worker{wid=100004, password='123', company='软帝', department='开发部', job='BOSS', name='123', sex='男', age=18, tel='123', email='123', registTime=Sat Jul 20 15:10:33 CST 2019, img='default.jpg'}
Worker{wid=100005, password='123', company='软帝集团', department='人事部', job='经理', name='萨达', sex='男', age=18, tel='123', email='123', registTime=Sat Jul 20 15:10:50 CST 2019, img='default.jpg'}

Process finished with exit code 0