从之前的用户管理系统中我们会发现，JSP主要是做界面的，但却用了JSP来对用户进行验证和分页处理，而Servlet处理页面的转跳是最快捷和最方便的，所以对用户管理系统的框架进行了改进，使用MVC模式。
MVC是一种设计模式，它强制性的使应用程序的输入、处理和输出分开。使用MVC应用程序被分为三个核心部件：模型、视图和控制器。它们各自处理自己的任务。
MVC的各个字母分别是M（model模型）、V（view视图）和C（controller控制器）
M主要由Java class来处理，也可以是Java bean，ejb等
V由JSP来处理
C由Servlet来处理
MVC提倡视图（JSP）和模型（业务逻辑，Java class等）完全分离
MVC模式简单的框架图如下所示：
经过对问题的分析，我们可以对程序进行改进：
增加控制器，将loginCl.jsp用Servlet来替代，并在控制器中去调用模型完成用户的验证，发挥Servlet在控制中的优势，可以改进为如下的框架:
实现代码如下：
视图部分：
1.登录界面（login.jsp）
<%@ page language="java" contentType="text/html; charset=gb2312"
pageEncoding="gb2312"%>
<!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=gb2312">
<title>用户登录系统</title>
</head>
<body>
<center>
用户登录系统<br>
<hr>
<form action="LoginClServlet" method="post">
用户名：<input type="text" name="username"><br>
密 码：<input type="password" name="passwd"><br>
<input type="submit" value="login">
<input type="reset" value="重置">
</form>
<hr>
</center>
</body>
</html>
2.欢迎页面（wel.jsp）
<%@ page language="java" contentType="text/html; charset=gb2312" import="java.sql.*,java.util.*,com.chongqing.model.*"
pageEncoding="gb2312"%>
<!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=gb2312">
<title>欢迎页面</title>
</head>
<body>
<center>
welcome,<%= request.getParameter("user") %><br>
<a href="login.jsp">返回重新登录</a>
<hr>
<%
//分页的功能
//从request中取出pageNow的值
int pageNow = Integer.parseInt((String)request.getAttribute("pageNow"));
//取出al的值
ArrayList<Object> al = (ArrayList<Object>)request.getAttribute("result");
//用表格显示用户的信息
%>
<table border="1">
<tr><td>用户ID</td><td>用户名</td><td>密码</td><td>邮箱</td><td>等级</td></tr>
<%
for(int i=0; i<al.size(); ++i)
{
UserBean ub = (UserBean)al.get(i);
out.println("<tr>");
out.println("<td>"+ub.getUserId()+"</td>");
out.println("<td>"+ub.getUserName()+"</td>");
out.println("<td>"+ub.getPasswd()+"</td>");
out.println("<td>"+ub.getMail()+"</td>");
out.println("<td>"+ub.getGrade()+"</td>");
out.println("</tr>");
}
%>
</table>
<%
//显示超链接
if(1 != pageNow)
{
out.println("<a href=UserClServlet?s_pageNow=1>[首页]</a>");
out.println("<a href=UserClServlet?s_pageNow="+(pageNow-1)+">[上一页]</a>");
}
//取出pageCount的值
int pageCount = Integer.parseInt((String)request.getAttribute("pageCount"));
for(int i=1; i<=pageCount; ++i){
out.println("<a href=UserClServlet?s_pageNow="+i+">["+i+"]</a>");
}
if(pageCount != pageNow)
{
out.println("<a href=UserClServlet?s_pageNow=1>[下一页]</a>");
out.println("<a href=UserClServlet?s_pageNow="+(pageNow+1)+">[尾页]</a>");
}
%>
</center>
</body>
</html>
模型部分：
1.ConnDB.java
//连接到数据库
package com.chongqing.model;
import java.sql.*;
public class ConnDB {
private Connection ct = null;
public Connection getConn(){
try {
//连接到数据库
Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver");
ct = DriverManager.getConnection("jdbc:sqlserver://127.0.0.1:1433;DatabaseName=TestServlet","sa","123456");
}
catch (Exception ex) {
ex.printStackTrace();
}
//不用关闭资源
return ct;
}
}
2.UserBean.java
// 映射到数据库中的users表，它的一个对象与users表的一条记录相对应，操作UserBean就相当于操作数据库
package com.chongqing.model;
public class UserBean {
private int userId;
private String userName;
private String passwd;
private String mail;
private int grade;
public int getUserId() {
return userId;
}
public String getUserName() {
return userName;
}
public String getPasswd() {
return passwd;
}
public String getMail() {
return mail;
}
public int getGrade() {
return grade;
}
public void setUserId(int userId) {
this.userId = userId;
}
public void setUserName(String userName) {
this.userName = userName;
}
public void setPasswd(String passwd) {
this.passwd = passwd;
}
public void setMail(String mail) {
this.mail = mail;
}
public void setGrade(int grade) {
this.grade = grade;
}
}
3.UserBeanCl.java
//业务逻辑和处理类，用来处理users表，即操作UserBean
package com.chongqing.model;
import java.sql.*;
import java.util.*;
public class UserBeanCl {
private Connection ct = null;
private PreparedStatement ps = null;
private ResultSet rs = null;
private int pageCount = 0; //一共有多少页（通过计算获得）
private int pageSize = 3; //每页显示3条记录
//验证用户
public boolean checkUser(String u,String p){
boolean flag = false;
try {
//得到连接
ConnDB cd = new ConnDB();
ct = cd.getConn();
ps = ct.prepareStatement("select * from users where username = ? and passwd = ?");
ps.setString(1,u);
ps.setString(2,p);
rs = ps.executeQuery();
if(rs.next()){
flag = true;
}
}
catch (Exception ex) {
ex.printStackTrace();
}
finally {
this.close();
}
return flag;
}
//分页显示结果
//不能返回ResultSet，因为关闭数据库资源的时候ResultSet也将消失，这里是返回ArrayList
public ArrayList<Object> getResultByPage(int pageNow){
ArrayList<Object> al = new ArrayList<Object>();
try {
int rowCount = 0; //一共有多少行记录（通过查表获得）
//连接数据库
ConnDB cd = new ConnDB();
ct = cd.getConn();
ps = ct.prepareStatement("select count(*) from users");
rs = ps.executeQuery();
if(rs.next()){
rowCount = rs.getInt(1);//获得rowCount的值
}
//计算pageCount的值
if(rowCount%pageSize == 0){
pageCount = rowCount/pageSize;
}else{
pageCount = rowCount/pageSize + 1;
}
ps = ct.prepareStatement("select top "+pageSize+" * from users where userID not in (select top "+pageSize*(pageNow-1)+" userID from users)");
rs = ps.executeQuery();
while(rs.next()){
//将rs中的每条记录封装到UserBean ub中
UserBean ub = new UserBean();
ub.setUserId(rs.getInt(1));
ub.setUserName(rs.getString(2));
ub.setPasswd(rs.getString(3));
ub.setMail(rs.getString(4));
ub.setGrade(rs.getInt(5));
al.add(ub); //将ub添加到ArrayList中
}
}
catch (Exception ex) {
ex.printStackTrace();
}
finally {
this.close();
}
return al;
}
//返回pageCount
public int getPageCount(){
return this.pageCount;
}
//关闭资源
public void close(){
try {
if(null != rs){
rs.close();
rs = null;
}
if(null != ps){
ps.close();
ps = null;
}
if(null != ct){
ct.close();
ct = null;
}
} catch (SQLException e) {
e.printStackTrace();
}
}
}
控制器模块：
1.验证用户
//这是控制器，主要完成对用户身份的验证
//控制器本身是不会去完成业务逻辑，它主要是通过调用模型来完成数据的处理
package com.chongqing.controller;
import java.io.IOException;
import java.util.ArrayList;
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 com.chongqing.model.UserBeanCl;
@WebServlet("/LoginClServlet")
public class LoginClServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
//接收用户名和密码，完成对用户的验证
String u = request.getParameter("username");
String p = request.getParameter("passwd");
//到数据库中进行验证
UserBeanCl ubc = new UserBeanCl();
if(ubc.checkUser(u,p)){
//合法，转到欢迎页面
//在转跳到wel.jsp页面时，就把要显示的数据给wel.jsp准备好
ArrayList<Object> al = (ArrayList<Object>)ubc.getResultByPage(1);
int pageCount = ubc.getPageCount();
request.setAttribute("result", al);
request.setAttribute("pageCount", pageCount+"");
request.setAttribute("pageNow", "1");
//response.sendRedirect("wel.jsp?user="+u);
//因为sendRedirect方法效率不高，所以软件公司常常是使用转发方法
//这种方法效率高，同时request的对象在下一个页面还可以使用
request.getRequestDispatcher("wel.jsp").forward(request, response);
}else{
//不合法，转到登录页面
request.getRequestDispatcher("login.jsp").forward(request, response);
}
}
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
this.doGet(request, response);
}
}
2.分页功能
//这个是控制器，将进行分页处理
package com.chongqing.controller;
import java.io.IOException;
import java.util.ArrayList;
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 com.chongqing.model.UserBeanCl;
@WebServlet("/UserClServlet")
public class UserClServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
int pageNow = Integer.parseInt(request.getParameter("s_pageNow"));
UserBeanCl ubc = new UserBeanCl();
ArrayList<Object> al = (ArrayList<Object>)ubc.getResultByPage(pageNow);
int pageCount = ubc.getPageCount();
request.setAttribute("result", al);
request.setAttribute("pageCount", pageCount+"");
request.setAttribute("pageNow", pageNow+"");
request.getRequestDispatcher("wel.jsp").forward(request, response);
}
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
this.doGet(request, response);
}
}