使用Servlet作为控制器

1、使用Servlet作为表现层的三个劣势

  • 开发效率低,所有的HTML标签都需使用页面输出流完成;
  • 不利于团队协作开发, 美工人员无法参与Servlet界面的开发;
  • 程序可维护性差,即使修改一个按钮的标题,都必须重新编辑Java代码,并重新编译。

2、Servlet仅作为控制器使用

在标准的MVC模式中,Servlet仅作为控制器使用。Java EE应用架构正式遵循MVC模式的, 对于遵循MVC模式的Java EE应用而言,JSP仅作为表现层(View)技术, 其作用有两点:

  • 负责收集用户请求参数;
  • 将应用的处理结果、状态数据呈现给用户。

3、示例,一个简单的登录验证(Servlet作为控制器的MVC应用):

1)登录页面login.jsp

<%--
  Created by IntelliJ IDEA.
  User: Chuan
  Date: 2017/9/1
  Time: 20:27
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" errorPage="" %>
<html>
<head>
    <title>登录页面</title>
</head>
<body>
<%--输出出错提示--%>
<span style="color:red; font-weight: bold">
    <%
        if (request.getAttribute("err") != null)
        {
            out.println(request.getAttribute("err") + "<br/>");
        }
    %>
</span>
请输入用户名和密码:
<%--登录表单,该表单提交到一个Servlet--%>
<form id="login" method="post" action="login">
    用户名:<input type="text" name="username" /><br/>
    密&nbsp;&nbsp码:<input type="password" name="pass" /><br/>
    <input type="submit" value="登录">
</form>
</body>
</html>

2)控制器Servlet代码LoginServlet.java

import javax.servlet.RequestDispatcher;
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 javax.servlet.http.HttpSession;
import java.io.IOException;
import java.sql.ResultSet;

@WebServlet(name = "login", urlPatterns = {"/login"})
public class LoginServlet extends HttpServlet
{
    //响应客户端请求的方法
    public void service(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException
    {
        String errMsg = "";
        //Servlet本身并不输出响应到客户端,因此必须将请求转发到视图页面
        RequestDispatcher rd;
        
        //设置请求的编码方式
        request.setCharacterEncoding("UTF-8");
        //获取请求参数
        String username = request.getParameter("username");
        String pass = request.getParameter("pass");
        try
        {
            //Servlet本身并不执行任何的业务逻辑处理,它调用JavaBean处理用户请求
            DbDao dd = new DbDao("com.mysql.jdbc.Driver",
                    "jdbc:mysql://localhost:3306/您的数据库名字", "您的用户名", "您的密码");
            //查询结果集
            ResultSet rs = dd.query("select pass from user_inf"
            + " where name = ?", username);
            if (rs.next())
            {
                //用户名和密码匹配
                if (rs.getString("pass").equals(pass))
                {
                    //获取session对象
                    HttpSession session = request.getSession(true);
                    //设置session属性,跟踪会话状态
                    session.setAttribute("name", username);
                    //获取转发对象
                    rd = request.getRequestDispatcher("/welcom.jsp");
                    rd.forward(request, response);
                }
                else
                {
                    //用户名和密码不匹配
                    errMsg += "您的用户名密码不符合, 请重新输入";
                }
            }
            else
            {
                //用户名不存在时
                errMsg += "您的用户名不存在,请先注册";
            }
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }
        if(errMsg != null && !errMsg.equals(""))
        {
            rd = request.getRequestDispatcher("/login.jsp");
            request.setAttribute("err", errMsg);
            rd.forward(request, response);
        }
    }
}

3)本应用中的Modle: DbDao.java

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;

public class DbDao
{
    private Connection conn;
    private String driver;
    private String url;
    private String username;
    private String pass;

    public DbDao()
    {
    }

    public DbDao(String driver, String url, String username, String pass)
    {
        this.driver = driver;
        this.url = url;
        this.username = username;
        this.pass = pass;
    }

    public String getDriver() {
        return driver;
    }

    public void setDriver(String driver) {
        this.driver = driver;
    }

    public String getUrl() {
        return url;
    }

    public void setUrl(String url) {
        this.url = url;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPass() {
        return pass;
    }

    public void setPass(String pass) {
        this.pass = pass;
    }

    //获取数据库连接
    public Connection getConnection() throws Exception
    {
        if (conn == null)
        {
            Class.forName(this.driver);
            conn = DriverManager.getConnection(url, username, this.pass);
        }
        return conn;
    }

    //插入记录
    public boolean insert(String sql, Object... args) throws Exception
    {
        PreparedStatement pstmt = getConnection().prepareCall(sql);
        for (int i = 0; i < args.length; i++)
        {
            pstmt.setObject(i + 1, args[i]);
        }
        if (pstmt.executeUpdate() != 1)
        {
            return false;
        }
        return true;
    }

    //执行查询
    public ResultSet query(String sql, Object... args) throws Exception
    {
        PreparedStatement pstmt = getConnection().prepareStatement(sql);
        for (int i = 0; i < args.length; i++)
        {

            pstmt.setObject(i + 1, args[i]);
        }
        return pstmt.executeQuery();
    }

    //执行修改
    public void modify(String sql, Object... args) throws Exception
    {
        PreparedStatement pstmt = getConnection().prepareStatement(sql);
        for (int i = 0; i < args.length; i++)
        {
            pstmt.setObject(i + 1, args[i]);
        }
        pstmt.executeUpdate();
        pstmt.close();
    }

    //关闭连接
    public void closeConn() throws Exception
    {
        if(conn != null && !conn.isClosed())
        {
            conn.close();
        }
    }

}

4)登录成功欢迎页面welcome.jsp

<%--
  Created by IntelliJ IDEA.
  User: Chuan
  Date: 2017/9/1
  Time: 21:23
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>欢迎页面</title>
</head>
<body>
<h3>欢迎登陆</h3>
<%=session.getAttribute("name")%>,欢迎登录
</body>
</html>

5)数据表


user_inf.PNG

6)演示结果


登录页面.PNG
欢迎页面.PNG

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