2.3 学习总结 之 分页查询

说在前面

昨天	完成了servlet的两次抽取的学习和实践
今天	学习分页查询展示数据

一、相关知识：sql分页

1、格式：select  * from 表  limit  ？，？；

2、参数含义：第一个，开始索引，默认值为0，必须是正数。

　　　　　　  第二个，每页显示个数。

3、例子：select  * from products  limit  0，5； //第一页，每页显示5条商品记录。

　　　　  select  * from products  limit  5，5； //第二页，每页显示5条商品记录。

......

 二、功能实现

1、添加PageBean.java，提供构造方法，统一算法的编写

package com.itheima.vo;

import java.util.ArrayList;
import java.util.List;

import com.itheima.domain.Product;

public class PageBean<T> {
    
    //当前页
    private int currentPage;
    //当前页显示的条数（固定值）
    private int currentCount;
    //总条数
    private int totalCount;
    //总页数
    private int totalPage;
    //每页显示的数据
    private List<T> productList = new ArrayList<T>();
    
    
    public int getCurrentPage() {
        return currentPage;
    }
    public void setCurrentPage(int currentPage) {
        this.currentPage = currentPage;
    }
    public int getCurrentCount() {
        return currentCount;
    }
    public void setCurrentCount(int currentCount) {
        this.currentCount = currentCount;
    }
    public int getTotalCount() {
        return totalCount;
    }
    public void setTotalCount(int totalCount) {
        this.totalCount = totalCount;
    }
    public int getTotalPage() {
        return totalPage;
    }
    public void setTotalPage(int totalPage) {
        this.totalPage = totalPage;
    }
    public List<T> getProductList() {
        return productList;
    }
    public void setProductList(List<T> productList) {
        this.productList = productList;
    }
    
}

2、编写 servlet 获取相关参数

ProductService service = new ProductService();
        
        //模拟当前是第一页
        String currentPageStr = request.getParameter("currentPage");
        if(currentPageStr==null) currentPageStr="1";
        int currentPage = Integer.parseInt(currentPageStr);
        //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();
        }

3、编写 service 封装好 PageBean实体

//分页操作
    public PageBean findPageBean(int currentPage,int currentCount) throws SQLException  {
        
        ProductDao dao = new ProductDao();
        
        //目的：就是想办法封装一个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)
         * 
         */
        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        其实索引        每页显示条数
         * 1        0            4
         * 2        4            4
         * 3        8            4
         * 4        12            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);
        pageBean.setProductList(productList);
        
        return pageBean;
    }

4、编写Dao，查询数据

//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);
    }

5、将数据存到request域中，返回界面

request.setAttribute("pageBean", pageBean);
        
request.getRequestDispatcher("/product_list.jsp").forward(request, response);

6、分页信息管理

1）显示每一页

<!-- 显示每一页 -->
            <c:forEach begin="1" end="${pageBean.totalPage }" var="page">
                <!-- 判断是否是当前页 -->
                <c:if test="${page==pageBean.currentPage }">
                    <li class="active"><a href="javascript:void(0);">${page }</a></li>
                </c:if>
                <c:if test="${page!=pageBean.currentPage }">
                    <li><a href="${pageContext.request.contextPath}/pagingDisplay?cid=${cid}&currentPage=${page }">${page }</a></li>
                </c:if>
            </c:forEach>

2）上一页

<!-- 上一页 -->
            <c:if test="${pageBean.currentPage==1 }">
                <li class="disabled">
                    <a href="javascript:void(0);" aria-label="Previous">
                        <span aria-hidden="true">&laquo;</span>
                    </a>
                </li>
            </c:if>
            <c:if test="${pageBean.currentPage!=1 }">
                <li>
                    <a href="${pageContext.request.contextPath}/pagingDisplay?cid=${cid}&currentPage=${pageBean.currentPage-1 }" aria-label="Previous">
                        <span aria-hidden="true">&laquo;</span>
                    </a>
                </li>
            </c:if>

3）下一页

            <!-- 下一页 -->
            <c:if test="${pageBean.currentPage==pageBean.totalPage }">
                <li class="disabled">
                    <a href="javascript:void(0);" aria-label="Next"> 
                        <span aria-hidden="true">&raquo;</span>
                    </a>
                </li>
            </c:if>
            <c:if test="${pageBean.currentPage!=pageBean.totalPage }">
                <li>
                    <a href="${pageContext.request.contextPath}/pagingDisplay?cid=${cid}&currentPage=${pageBean.currentPage+1 }" aria-label="Next"> 
                        <span aria-hidden="true">&raquo;</span>
                    </a>
                </li>
            </c:if>