分页的后台代码实现

web层servlet代码：

package com.zys.web;

import java.io.IOException;
import java.sql.SQLException;
import java.util.List;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.zys.domain.Product;
import com.zys.service.ProductListService;
import com.zys.vo.PageBean;

public class ProductListServlet extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        
        //将请求传递到dao层，从数据库获取所有的商品信息
        ProductListService service = new ProductListService();
        
        //模拟当前是第一页
        int currentPage = 1;
        //认为每页显示12条
        int currentCount = 12;  
        
        PageBean<Product> pageBean = null;
        try {
            pageBean = service.findPageBean(currentPage,currentCount);
        } catch (SQLException e) {
            e.printStackTrace();
        }
        
        
        //将获取到的数据保存到request域
        request.setAttribute("pageBean", pageBean);
        
        //转发到对应的jsp页面
        request.getRequestDispatcher("/product_list.jsp").forward(request, response);
        
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        doGet(request, response);
    }
}

service层代码：

package com.zys.service;

import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;

import com.zys.dao.ProductListDao;
import com.zys.domain.Product;
import com.zys.vo.PageBean;

public class ProductListService {

    //分页操作
    public PageBean findPageBean(int currentPage, int currentCount) throws SQLException {
        
        ProductListDao dao = new ProductListDao();
        
        // 目的：想办法封装一个PageBean 并返回
        PageBean pageBean = new PageBean();
        //1、当前页private int currentPage; 
        pageBean.setCurrentPage(currentPage);
        //2、当前页显示的条数private int currentCount;
        pageBean.setCurrentCount(currentCount);
        //3、总条数private int totalCount;
        int totalCount = dao.getTotalCount();
        pageBean.setTotalCount(totalCount);
        //4、总页数private int totalPage;
        /*
         * 总条数  当前页显示的条数    总页数
         *  10      4               3
         *  11      4               3
         *  12      4               3
         *  13      4               4
         *  
         *  公式：总页数=Math.ceil(总条数/当前显示的条数)
         *
         */
        //ceil向上取整 floor向下取整
        //1.0*是因为两个整型相除，得到的还是整型，如果此时向上取整得不到正确的数，所以在这里先*1.0变成double型
        int totalPage = (int) Math.ceil(1.0*totalCount/currentCount);
        pageBean.setTotalPage(totalPage);
        //5、每页显示的数据private List<T> productList = new ArrayList<T>();
        /*
         *页数与limit起始索引的关系
         *例如：每页显示4条 
         *页数    起始索引    每页显示条数
         * 1    0       4
         * 2    4       4
         * 3    8       4
         * 4    12      4
         *  1页 limit 0,4
         *  2页 limit 4,4
         *  3页 limit 8,4
         * 
         *      索引index = (当前页数-1)*每页显示的条数
         * 
         */
        int index = (currentPage-1)*currentCount;
        
        List<Product> productList = dao.findProductListForPageBean(index,currentCount);
        //封装数据到pageBean
        pageBean.setProductList(productList);
        
        return pageBean;
    }


}

dao层代码：

package com.zys.dao;

import java.sql.SQLException;
import java.util.List;

import org.apache.commons.dbutils.QueryRunner;
import org.apache.commons.dbutils.handlers.BeanListHandler;
import org.apache.commons.dbutils.handlers.ScalarHandler;

import com.zys.domain.Product;
import com.zys.utils.DataSourceUtils;

public class ProductListDao {

    //获得全部的商品条数
    public int getTotalCount() throws SQLException {
        QueryRunner runner = new QueryRunner(DataSourceUtils.getDataSource());
        String sql = "select count(*) from product";
        Long query = (Long) runner.query(sql, new ScalarHandler());
        return query.intValue();
    }

    //获得分页的商品数据
    public List<Product> findProductListForPageBean(int index, int currentCount) throws SQLException {
        QueryRunner runner = new QueryRunner(DataSourceUtils.getDataSource());
        String sql = "select * from product limit ?,?";
        return runner.query(sql, new BeanListHandler<Product>(Product.class), index,currentCount);
    }

}

对于商品显示页面一般要定义的实体类变量：

image.png
为了PageBean实体类达到通用，所以在集合后面加<T>泛型
为了方便调用时使用，在实体类内提前初始化
image.png
分页：limit（起始索引，显示条数）；
image.png