其实分页这一篇本来我是我不想写的，但是下来打算将泛型，为了泛型能够更加的容易理解，所以分页拖到了现在才写。
分页的实现有两种方式：
1.后台获取所有数据，然后通过前端框架来分页（不推荐，效率低，浪费资源）；
2.通过后台数据库sql语句分页获取数据（在mysql和oracle中，分页的实现语句是不一样的，mysql通过limit关键字来实现分页，而oracle通过条件语句来实现；
在这里我们使用更加方便的limit来实现分页。下面我们使用一个案例来实现分页：
创建一个web项目，遍历user数据表的所有信息，以表格的形式展示，具体操作如下，工具类就不在这里写了。同时为了方便接口这里也不在创建了，直接写dao，service的实现
package com.zs.entity;
public class User {
private int uid;
private String username;
private String password;
public int getUid() {
return uid;
}
public void setUid(int uid) {
this.uid = uid;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
}
实体类
package com.zs.dao;
import com.zs.util.DBUtils;
import java.util.List;
import java.util.Map;
public class UserDao {
/**
* 获取所有的用户信息
* @return
*/
public List<Map<String,Object>> allUser() {
String sql = "select * from user ";
return DBUtils.executeQuery(sql);
}
}
dao层
package com.zs.service;
import com.zs.dao.UserDao;
import com.zs.entity.User;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
public class UserService {
public List<User> allUser() {
UserDao userDao = new UserDao();
List<Map<String, Object>> maps = userDao.allUser();
List<User> list = fengzhuang(maps);
return list;
}
private List<User> fengzhuang(List<Map<String,Object>> maps) {
List<User> list= new ArrayList<>();
for (Map<String, Object> map : maps) {
User user = new User();
user.setUid((Integer) map.get("uid"));
user.setUsername(map.get("username").toString());
user.setPassword(map.get("password").toString());
list.add(user);
}
return list;
}
}
service
package com.zs.servlet;
import com.zs.entity.User;
import com.zs.service.UserService;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.List;
@WebServlet("/home")
public class UserServlet extends HttpServlet {
@Override
protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
UserService userService = new UserService();
List<User> users = userService.allUser();
req.setAttribute("user", users);
req.getRequestDispatcher("index.jsp").forward(req, resp);
}
}
servlet
<%--
Created by IntelliJ IDEA.
User: Administrator
Date: 2019/7/3
Time: 14:18
To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<html>
<head>
<title>$Title$</title>
</head>
<body>
<table border="1" cellpadding="1">
<tr>
<th>序号</th>
<th>用户名</th>
<th>密码</th>
</tr>
<c:forEach var="p" items="${user}" varStatus="i">
<tr>
<td>${i.count}</td>
<td>${p.username}</td>
<td>${p.password}</td>
</tr>
</c:forEach>
</table>
</body>
</html>
页面
运行测试，可以看到表中数据以表格形式展示
但是这中方式，是一次性将所有的信息都读出来，如果信息量大的话，效率低而且展示数据表格会非常长，这很不方便阅读，因此就想，如何实现分页，将所有的数据分成一页一页 的，mysql使用limit关键字来实现分页，如下：
/**
* startNum 开始下标，
* num 每页的记录数
*/
select * from user limit startNum,num;
通过limit实现分页，当点击下一页时，就根据发送过来的页数信息查询下一页的数据，然后在响应给前端页面，为了实现这种方式，我们创建一个分页的工具类，通过工具类来计算页数以及传递数据，工具类如下：
package com.zs.util;
import com.zs.entity.User;
import java.util.List;
public class PageUtil {
private int firstPage;
private int proPage;
private int currentPage;
private int nextPage;
private int lastPage;
private List<User> list;
public PageUtil() {
}
/**
* 创建构造方法，通过传入当前页的值以及数据总数，计算下一页，上一页，首页，尾页以及当前页的数据等信息
* @param currentPage 当前页数
* @param countPage 总页数
* @param list 数据集合
*/
public PageUtil(int currentPage,int countPage, List<User> list) {
this.firstPage = 1;
this.currentPage = currentPage;
this.lastPage = countPage;
this.proPage = this.currentPage == 1 ? 1 : (this.currentPage - 1);
this.nextPage = this.currentPage == this.lastPage ? this.currentPage : this.currentPage + 1;
this.list = list;
}
public int getFirstPage() {
return firstPage;
}
public void setFirstPage(int firstPage) {
this.firstPage = firstPage;
}
public int getProPage() {
return proPage;
}
public void setProPage(int proPage) {
this.proPage = proPage;
}
public int getCurrentPage() {
return currentPage;
}
public void setCurrentPage(int currentPage) {
this.currentPage = currentPage;
}
public int getNextPage() {
return nextPage;
}
public void setNextPage(int nextPage) {
this.nextPage = nextPage;
}
public int getLastPage() {
return lastPage;
}
public void setLastPage(int lastPage) {
this.lastPage = lastPage;
}
public List<User> getList() {
return list;
}
public void setList(List<User> list) {
this.list = list;
}
}
分页工具类
然后修改dao层，service层，servlet以及页面如下：
package com.zs.dao;
import com.zs.util.DBUtils;
import java.util.List;
import java.util.Map;
public class UserDao {
/**
* 获取所有的用户信息
* @return
*/
public List<Map<String,Object>> allUser(int startNum) {
String sql = "select * from user limit ?,3 ";
return DBUtils.executeQuery(sql,startNum);
}
/**
* 获取数据的记录数，方便计算总页数
* @return
*/
public Map<String,Object> countUser() {
String sql = "select count(uid) as num from user";
return DBUtils.executeQuery(sql).get(0);
}
}
dao
package com.zs.service;
import com.zs.dao.UserDao;
import com.zs.entity.User;
import com.zs.util.PageUtil;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
public class UserService {
public PageUtil allUser(int currentPage) {
UserDao userDao = new UserDao();
Map<String, Object> map = userDao.countUser();
/*获取总记录数*/
int num =Integer.parseInt(map.get("num").toString()) ;
/*获取当前页数据, (currentPage-1)*10计算开始下标*/
List<Map<String, Object>> maps = userDao.allUser((currentPage-1)*3);
List<User> list = fengzhuang(maps);
PageUtil page = new PageUtil(currentPage, (num+2)/3, list);
return page;
}
private List<User> fengzhuang(List<Map<String,Object>> maps) {
List<User> list= new ArrayList<>();
for (Map<String, Object> map : maps) {
User user = new User();
user.setUid((Integer) map.get("uid"));
user.setUsername(map.get("username").toString());
user.setPassword(map.get("password").toString());
list.add(user);
}
return list;
}
}
service
package com.zs.servlet;
import com.zs.entity.User;
import com.zs.service.UserService;
import com.zs.util.PageUtil;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.List;
@WebServlet("/home")
public class UserServlet extends HttpServlet {
@Override
protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
Integer currentPage = 1;
try {
currentPage = Integer.parseInt(req.getParameter("page"));
} catch (Exception e) {
System.out.println("第一次访问");
}
UserService userService = new UserService();
PageUtil page = userService.allUser(currentPage);
req.setAttribute("page", page);
req.getRequestDispatcher("index.jsp").forward(req, resp);
}
}
servlet
<%--
Created by IntelliJ IDEA.
User: Administrator
Date: 2019/7/3
Time: 14:18
To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<html>
<head>
<title>$Title$</title>
</head>
<body>
<table border="1" cellpadding="1">
<tr>
<th>序号</th>
<th>用户名</th>
<th>密码</th>
</tr>
<c:forEach var="p" items="${page.list}" varStatus="i">
<tr>
<td>${i.count}</td>
<td>${p.username}</td>
<td>${p.password}</td>
</tr>
</c:forEach>
</table>
<%--分页工具--%>
<div>
<a href="<c:url value='/home?page=${page.firstPage}'/>">首页</a>
<a href="<c:url value='/home?page=${page.proPage}'/>">上一页</a>
第${page.currentPage}/${page.lastPage}页
<a href="<c:url value='/home?page=${page.nextPage}'/>">下一页</a>
<a href="<c:url value='/home?page=${page.lastPage}'/>">尾页</a>
</div>
</body>
</html>
前端页面
然后运行测试：
