JavaWeb分页的实现

分页的分类

分页的实现分为真分页和假分页两种。

1.真分页（物理分页）：

实现原理： SELECT * FROM xxx [WHERE...] LIMIT ?, 10;
第一个参数是开始数据的索引位置
10是要查询多少条数据，即每页显示的条数
优点： 不会造成内存溢出
缺点： 翻页的速度比较慢

2.假分页（逻辑分页）：

实现原理： 一次性将所有的数据查询出来放在内存之中，每次需要查询的时候就直接从内存之中去取出相应索引区间的数据
优点： 分页的速度比较快
缺点： 可能造成内存溢出

分页的一些术语：
-- 数据总条数： totalCount ： select count(1) from t_user;
-- 每页显示条数：pageSize
-- 总页数：totalPage
-- 当前页：currPage
-- 起始索引： startIndex

-- 通过当前页码查询第几页的数据

select * from t_user limit 0, 5; -- 页码 1
select * from t_user limit 5, 5; -- 页码 2
select * from t_user limit 10, 5; -- 页码 3

-- 公式：startIndex = (currPage - 1) * pageSize

-- 计算一共有多少页

-- 方法一：result = totalCount%pageSize，如果余数result为0，
-- totalPage = totalCount / pageSize
-- 如果余数result不为0，
-- totalPage = totalCount / pageSize + 1;
-- 方法二：totalPage = (totalCount + pageSize - 1) / pageSize

Pageing工具类

public class PaginationBean<T> {
    
    private List<T> dataList;
    
    private int currPage;
    
    private int totalPage;

    public List<T> getDataList() {
        return dataList;
    }

    public void setDataList(List<T> dataList) {
        this.dataList = dataList;
    }

    public int getCurrPage() {
        return currPage;
    }

    public void setCurrPage(int currPage) {
        this.currPage = currPage;
    }

    public int getTotalPage() {
        return totalPage;
    }

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

Servlet

@WebServlet("/showUserList")
public class ShowUserListServlet extends HttpServlet implements Servlet {
    
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        request.setCharacterEncoding("UTF-8");
        String operation = request.getParameter("operation");
        String currPageStr = request.getParameter("currPage");
        int currPage = 0;
        
        IUserService userSevice = new UserServiceImpl();
        int totalPage = userSevice.getTotalPage();
        
        if ("首页".equals(operation) || operation == null || currPageStr == null || currPageStr.length() == 0) {
            
            currPage  = 1;
        } else if ("上一页".equals(operation)) {
            
            currPage = Integer.parseInt(currPageStr) - 1;
            if (currPage <= 0) {
                currPage = 1;
            }
        } else if ("下一页".equals(operation)) {
            
            currPage = Integer.parseInt(currPageStr) + 1;
            if (currPage >= totalPage) {
                currPage = totalPage;
            }
        } else {
            
            currPage = totalPage;
        }
        
        List<TestUserBean> userList = userSevice.getUserListByCurrPage(currPage);
        
        PaginationBean<TestUserBean> pageBean = new PaginationBean<TestUserBean>();
        pageBean.setDataList(userList);
        pageBean.setCurrPage(currPage);
        pageBean.setTotalPage(totalPage);
        
        request.setAttribute("page", pageBean);
        
        request.getRequestDispatcher("/userList.jsp").forward(request, response);
    }

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

}

jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<%-- 引入JSTL --%>    
<%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
<style>
    table {
        border-collapse: collapse;
    }
</style>
</head>
<body>
    <table border="1">
        <tr>
            <th>ID</th>
            <th>姓名</th>
            <th>密码</th>
            <th>身份证号</th>
        </tr>
        <c:forEach items="${page.dataList }" var="user">
            <tr>
                <td>${user.id }</td>
                <td>${user.name }</td>
                <td>${user.pwd }</td>
                <td>${user.idCard }</td>
            </tr>
        </c:forEach>
    </table>
    <span>第${page.currPage }页/共${page.totalPage }页</span>
    <br>
    <br>
    <form action="showUserList" method="get">
        <input type="submit" name="operation" value="首页">
        <input type="submit" name="operation" value="上一页">
        <input type="submit" name="operation" value="下一页">
        <input type="submit" name="operation" value="尾页">
        
        <input type="hidden" name="currPage" value="${page.currPage }">
    </form>
</body>
</html>