Implement paging: there are two ways: traditional paging and classification based on pageHelper plug-in. The latter is recommended

First, create a pageUtil tool manually to record all kinds of information of paging, and then use the tedious way to call data

pageUtil class can be customized as required

package com.imooc.project.util;
/**
 * In order to use the traditional method for paging design, we need to establish an information class for paging to store the paging information
 * @author lqf
 *
 */

import java.util.List;

public class PageUtil {
   private int currentPageNum; //What page do you want to see now 
   
   private int pageSize=10 ; //Number of entries per page 
   
   private int totalSize; //Total number of records
   
   private int startIndex; //Index of query start record  limit ? ?  limit Use
   
   private int totalPageNum; //PageCount 
   
   private int nextPageNum; //next page 
   
   private int prePageNum; //Previous page 
   
   private List records; //Recordset of current page     List<User> records   Back to each page user Set
   
   //Number of pages used to display navigation on the page user can customize 
   private int startPageNum; // home page
   private int endPageNum ; // trailer page 
   private String url ;
   //Use the construction method. Pass the necessary two parameters: the first is the page number, the second is the total number of records to get all other data 
   
   public PageUtil(int currentPageNum,int totalrecords)
   {
       this.currentPageNum=currentPageNum;
       this.totalSize= totalrecords ; 
       /*   
        * 1   0  - 10    limit(0,10)   
        *  2  10 - 20    limit(10,10)
        *  3 20 -30     limit(20,30)  
        *   (Current page - 1) * 10 = index value of the page startIndex 
        *    (currentPageNum-1)*pageSize  
        */
       
       //Calculate index of start record 
       startIndex=(currentPageNum-1)*pageSize;
       //Calculate total pages 
       totalPageNum=totalSize%pageSize==0?totalSize/pageSize:totalSize/pageSize+1;
       //Calculate the start and end page numbers, which can be designed by itself     12..9
       if(totalPageNum>9)
       {
           startPageNum=currentPageNum-4;
            endPageNum=currentPageNum+4;

            if(startPageNum<1)
            {
                startPageNum=1;
                endPageNum=startPageNum+8;
            }
            if(endPageNum>totalPageNum)
            {
                endPageNum=totalPageNum;
                startPageNum=endPageNum-8;
            }
       }
       else
        {
            startPageNum=1;
            endPageNum=totalPageNum;

        }
   }

   public int getStartPageNum() {
        return startPageNum;
    }


    public void setStartPageNum(int startPageNum) {
        this.startPageNum = startPageNum;
    }


    public int getEndPageNum() {
        return endPageNum;
    }


    public void setEndPageNum(int endPageNum) {
        this.endPageNum = endPageNum;
    }

//Previous page
    public int getPrePageNum() {
        prePageNum=currentPageNum-1;
        
        if(prePageNum<=0)
        {
            prePageNum=1;
        }
        return prePageNum;
    }
  // next page
    public int getNextPageNum() {
        
        nextPageNum=currentPageNum+1;
        
        if(nextPageNum>totalPageNum)
        {
            nextPageNum=totalPageNum;
        }
        return nextPageNum;
    }

    public int getCurrentPageNum() {
        return currentPageNum;
    }

    public void setCurrentPageNum(int currentPageNum) {
        this.currentPageNum = currentPageNum;
    }

    public int getPageSize() {
        return pageSize;
    }

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

    public int getTotalSize() {
        return totalSize;
    }

    public void setTotalSize(int totalSize) {
        this.totalSize = totalSize;
    }

    public int getStartIndex() {
        return startIndex;
    }

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

    public int getTotalPageNum() {
        return totalPageNum;
    }

    public void setTotalPageNum(int totalPageNum) {
        this.totalPageNum = totalPageNum;
    }

    public List  getRecords() {
        return records;
    }

    public void setRecords(List  records) {
        this.records = records;
    }

    public void setPrePageNum(int prePageNum) {
        this.prePageNum = prePageNum;
    }

    public void setNextPageNum(int nextPageNum) {
        this.nextPageNum = nextPageNum;
    }


    public String getUrl() {
        return url;
    }


    public void setUrl(String url) {
        this.url = url;
    }
    
    
}

Short for web layer

    //Get current page 
        String num = req.getParameter("num");
        if(num==null)
        {
            num="1";
        }
        //call pageUtil tool 
        PageUtil page= null ; 
        try {
            page=service.getAllUsers(num);
        } catch (Exception e) {
            e.printStackTrace();
        }
        //Transfer data into the domain and forward to users.jsp Page
        req.setAttribute("page", page);
        req.getRequestDispatcher("/users.jsp").forward(req, resp);

service level

  userDao userDao= new userDao() ;
     
     public static SqlSessionFactory sqlSessionFactory = null;
    //Get tool class information 
     public PageUtil getAllUsers(String num) throws  Exception{
       //Current page
         int  currentPageNum=1 ;
        // trim() Eliminate leading and trailing spaces
         if (num != null ) {
             currentPageNum = Integer.parseInt(num);
            System.out.println(currentPageNum);
            }
        int totalRecords =userDao.totalRecords();

        PageUtil pg = new PageUtil(currentPageNum, totalRecords);

        List<User> users = userDao.getAllUsers(pg.getStartIndex(), pg.getPageSize());

        pg.setRecords(users);

        return pg;

    }

dao

public class userDao  extends BaseDao {
  
    //Get all the information
    public  int totalRecords()  throws Exception{

        // Use jdbc Template as connection layer of data
        int result = 0;
        Connection conn = this.getConnection();
        PreparedStatement ps = null;
        ResultSet rs = null;
        // count(*) Returns the number of rows found count(Column names) Returns the number of data items in a column count(*) count Set alias as count
        String sql = "select count(*)  count from person";
        ps = conn.prepareStatement(sql);
        rs = ps.executeQuery();

        if (rs.next()) {
            result = rs.getInt("count");
        }

        return result;
    }
    //Get the information of the corresponding page 
    public List<User> getAllUsers(int startIndex, int pageSize) throws  Exception{
        //Use array to get data call DBCP Template 
        QueryRunner queryRunner= new QueryRunner();
        String sql= "select * from person limit ?,?";
        List<User> users= null ; 
        //Get data
        users=queryRunner.query(this.getConnection(), sql,new BeanListHandler<User>(User.class),new Object[]{startIndex,pageSize});
        return users;
    }

}

 *************************************************

Use the mybatis pageHelper plug-in to develop

How to use the pageHelper plug-in

This is a tutorial for the developer of pageHelper on GitHub

     https://github.com/pagehelper/Mybatis-PageHelper/blob/master/wikis/zh/HowToUse.md

Configuration information of mybatis insert pagehelper plug-in

      
     <! -- configure the page helper plug-ins in the mybatis configuration file in the correct order or an error will be reported -- >
    <plugins>
        <plugin interceptor="com.github.pagehelper.PageInterceptor">
         <! -- set six database types: Oracle, mysql, MariaDB, SQLite, HSQLDB and PostgreSQL -- >   
           <property name="helperDialect" value="mysql"/>
        </plugin>
        
    </plugins>

mapper.xml

web level

pageHelper plug-in is much simpler than the traditional plug-in. After all, it is a plug-in. It is the control of the four objects of mybatis: parameterHandler, resultSetHandler, statementHandler, executor. The enhancement of the function realized by reflection dynamic agent is similar to the function of filter

However, it should be noted that the use of plug-ins will change the underlying design, so we should try to minimize the use of plug-ins