Jsp——分页

为什么需要分页？

因为我们要把数据库成千上万条的数据一页一页的展现给用户，而不是长短不一的展示给用户，这就是需要分页的原因。

package com.zking.servlet;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.List;

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

import com.zking.dao.NewsDao;
import com.zking.entity.News;

/**
 * 分页数据加载界面
 * @author 14916
 *
 */
public class LoadDataServlet extends HttpServlet{

	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		doPost(req, resp);
	}

	@Override
	protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		try {
//			1.设置编码
			req.setCharacterEncoding("utf-8");
			resp.setContentType("text/html;charset=utf-8");
			
//		定义当前页面变量		
		int pageIndex = 1;//默认展示第一页的数据	
//		获取前台传来的页码变量
		String myPageIndex = req.getParameter("pageIndex");
		
		
//		把从前台获取的页码myPageIndex赋给当前页码pageIndex
		if(null != myPageIndex) {
			pageIndex = Integer.parseInt(myPageIndex);
		}

		//调用模糊查询方法获取数据
		NewsDao nd = new NewsDao();
		
//		2.获取内置对象
		PrintWriter out = resp.getWriter();
		HttpSession session = req.getSession();  
		
		//获取前台传来的模糊查询关键字
		String str = req.getParameter("str");
		if(null == str) {
			str = "";
		}
		
		//调用分页的方法   
		List<News> listNews = nd.query(pageIndex,str);
//		把获取的数据集合listnews 保存到request作用域里
		req.setAttribute("listNews", listNews);
//		把当前页码pageIndex保存到 request作用域里
		req.setAttribute("pageIndex", pageIndex);
		//把模糊查询关键字保存到request作用域里面
		req.setAttribute("str", str);
		
		
//		使用转发转发到amdin.jsp里
		req.getRequestDispatcher("/admin/admin.jsp").forward(req,resp);
		} catch (Exception e) {
		e.printStackTrace();
		}
	}
}

/**
	 * 求news表里面数据可展示的总页码
	 * @return str 模糊查询字段的数据有几条
	 * @throws Exception
	 */
	public int getMaxPage(String str) throws Exception{
		con = DBhelper.getCon();
		sql = "select count(0) from news where title like '%"+str+"%'";
		ps = con.prepareStatement(sql);
		rs = ps.executeQuery();
		if(rs.next()) {
			maxPage = rs.getInt(1) / 5 ;
			if(rs.getInt(1)%5!=0) {
				maxPage ++;
			}
		}
		DBhelper.closeObj(con, ps, rs);
		return maxPage;
	}

/**
	 * 模糊查询分页的方法，每页最多展示五条数据
	 * @param pageIndex 当前页码
	 * @param str 模糊查询关键字
	 * @return
	 * @throws Exception
	 */
	public List<News> query(int pageIndex,String str) throws Exception{
		int pageSize=5;//页大小
		int start = (pageIndex-1)*pageSize+1;//每页开始数据条数
		int end = pageIndex * pageSize;//每页结束的数据条数
		listNews = new ArrayList<>();
		con = DBhelper.getCon();
		//我使用的是Oracle
		sql = "select * from (select a.*,rownum myid from news a where a.title like '%"+str+"%')b where myid between ? and ?";
		ps = con.prepareStatement(sql);
		ps.setInt(1, start);
		ps.setInt(2, end);
		rs = ps.executeQuery();
		while(rs.next()) {
			news = new News(rs.getInt("nid"),rs.getInt("sid"),rs.getString("title"),rs.getString("author"),rs.getString("summary"),rs.getString("content"),null);
			listNews.add(news);
		}
		DBhelper.closeObj(con, ps, rs);
		return listNews;
	}	
	

<a href="<%=request.getContextPath()%>/loadDataServlet.do?str=<%=str %>&pageIndex=1">首页</a>
//这里可以看到我在下一页和上一页使用了三元运算符;
<a href="<%=request.getContextPath()%>/loadDataServlet.do?str=<%=str %>&pageIndex=<%=pageIndex>1?pageIndex-1:1 %>">上一页</a>
 	当前页数:[<%=pageIndex %>/<%=maxPage %>]&nbsp; 
 	<a href="<%=request.getContextPath()%>/loadDataServlet.do?str=<%=str %>&pageIndex=<%=pageIndex<maxPage?pageIndex+1:maxPage%>">下一页</a>
 	<a href="<%=request.getContextPath()%>/loadDataServlet.do?str=<%=str %>&pageIndex=<%=maxPage%>">末页</a> </p>