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主要用到知识：1.Servlet　　2.request从客户端得到参数　　3.response向客户端响应

　　　　　　4.字符编码解决乱码问题　　　　5.jsp的使用　　6.验证码

　　　　　　7.Cookie的使用　　　　　　8.Session的使用　　9.具体功能登录时自动补全用户名

　　　　　　10.jsp页面的用户信息校验，如果为null，则跳转登录。

使用一个VerifyCodeServlet专门来生成验证码

login.jsp页面：

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%><%String path = request.getContextPath();String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";%>              
       
       
       
       
       
               
登录
    <%        String uname="";        //读名为username的cookie        Cookie[] cookies=request.getCookies();        if(cookies!=null){            for(Cookie c:cookies){                //如果为空显示：""                //如果不为显示：Cookie值                if("username".equals(c.getName())){                    uname=c.getValue();                }            }        }     %>    <%        String message="";//一个空字符串        String msg=(String)request.getAttribute("msg");//获取错误信息        if(msg!=null){            message=msg;        }     %>    <%=message %>    
   
            用户名：
    
    
        密   码：
    
    
        验证码：
                    
    
    
                
    看不清，换一张        
        
   
  

LoginServlet.java:

public class LoginServlet extends HttpServlet {    public void doPost(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {            /*             * 校验验证码             * 1.从session中获取正确的验证码             * 2.从表单中获取用户输入的验证码             * 4.如果相同，向下运行，否则保存错误信息到request域，转发到login.jsp             */            HttpSession session1 = request.getSession();            String vcode1 = (String) session1.getAttribute("session_vcode");            String vcode2 = request.getParameter("verifyCode");            System.out.println("1:"+vcode1+" 2:"+vcode2);            if(!vcode1.equalsIgnoreCase(vcode2)){//不相等----不区分大小写                request.setAttribute("msg", "验证码错误！");                RequestDispatcher rd=request.getRequestDispatcher("/login.jsp");                rd.forward(request, response);                return;//错误结束，就不向下执行            }            request.setCharacterEncoding("utf-8");            String username = request.getParameter("username");            String password = request.getParameter("password");            System.out.println("username:"+username+", password:"+password);            if(username!="" && password!=""){                if("xjs".equals(username) && "0309".equals(password)){                    //设置Cookie                    response.setContentType("text/html;charset=utf-8");                    Cookie cookie=new Cookie("username",username);                    cookie.setMaxAge(60*60);                    response.addCookie(cookie);//把它显示到用户名文本框中                    //登录成功                    HttpSession session=session1;                    session.setAttribute("username", username);                    session.setAttribute("password", password);                    //重定向到succ1.jsp                    response.sendRedirect("/XJS_Session/succ1.jsp");                }                    else{                    System.out.println("username:"+username+", password:"+password);                    //输入的用户名和密码不存在                    request.setAttribute("msg", "用户名和密码不存在！！！");                    //用转发                    RequestDispatcher rd=request.getRequestDispatcher("/login.jsp");                    rd.forward(request, response);                }            }else{                request.setAttribute("msg", "用户名或密码不能为空！！！");                //用转发---如果用重定向的话显示错误信息就不能使用request                RequestDispatcher rd=request.getRequestDispatcher("/login.jsp");                rd.forward(request, response);            }    }}

=============================验证码===========================

VerifyCode.java:

package com.xjs.commons;import java.awt.BasicStroke;import java.awt.Color;import java.awt.Font;import java.awt.Graphics2D;import java.awt.image.BufferedImage;import java.io.IOException;import java.io.OutputStream;import java.util.Random;import javax.imageio.ImageIO;public class VerifyCode {    private int w = 70;    private int h = 35;     private Random r = new Random();     // {"宋体", "华文楷体", "黑体", "华文新魏", "华文隶书", "微软雅黑", "楷体_GB2312"}    private String[] fontNames  = {"宋体", "华文楷体", "黑体", "微软雅黑", "楷体_GB2312"};    // 可选字符    private String codes  = "23456789abcdefghjkmnopqrstuvwxyzABCDEFGHJKMNPQRSTUVWXYZ";    // 背景色    private Color bgColor  = new Color(255, 255, 255);    // 验证码上的文本    private String text ;    // 生成随机的颜色    private Color randomColor () {        int red = r.nextInt(150);        int green = r.nextInt(150);        int blue = r.nextInt(150);        return new Color(red, green, blue);    }    // 生成随机的字体    private Font randomFont () {        int index = r.nextInt(fontNames.length);        String fontName = fontNames[index];//生成随机的字体名称        int style = r.nextInt(4);//生成随机的样式, 0(无样式), 1(粗体), 2(斜体), 3(粗体+斜体)        int size = r.nextInt(5) + 24; //生成随机字号, 24 ~ 28        return new Font(fontName, style, size);    }    // 画干扰线    private void drawLine (BufferedImage image) {        int num  = 3;//一共画3条        Graphics2D g2 = (Graphics2D)image.getGraphics();        for(int i = 0; i < num; i++) {//生成两个点的坐标，即4个值            int x1 = r.nextInt(w);            int y1 = r.nextInt(h);            int x2 = r.nextInt(w);            int y2 = r.nextInt(h);            g2.setStroke(new BasicStroke(1.5F));            g2.setColor(Color.BLUE); //干扰线是蓝色            g2.drawLine(x1, y1, x2, y2);//画线        }    }    // 随机生成一个字符    private char randomChar () {        int index = r.nextInt(codes.length());        return codes.charAt(index);    }    // 创建BufferedImage    private BufferedImage createImage () {        BufferedImage image = new BufferedImage(w, h, BufferedImage.TYPE_INT_RGB);        Graphics2D g2 = (Graphics2D)image.getGraphics();        g2.setColor(this.bgColor);        g2.fillRect(0, 0, w, h);         return image;    }    // 调用这个方法得到验证码    public BufferedImage getImage () {        BufferedImage image = createImage();//创建图片缓冲区        Graphics2D g2 = (Graphics2D)image.getGraphics();//得到绘制环境        StringBuilder sb = new StringBuilder();//用来装载生成的验证码文本        // 向图片中画4个字符        for(int i = 0; i < 4; i++)  {//循环四次，每次生成一个字符            String s = randomChar() + "";//随机生成一个字母            sb.append(s); //把字母添加到sb中            float x = i * 1.0F * w / 4; //设置当前字符的x轴坐标            g2.setFont(randomFont()); //设置随机字体            g2.setColor(randomColor()); //设置随机颜色            g2.drawString(s, x, h-5); //画图        }        this.text = sb.toString(); //把生成的字符串赋给了this.text        drawLine(image); //添加干扰线        return image;    }    // 返回验证码图片上的文本    public String getText () {        return text;    }    // 保存图片到指定的输出流    public static void output (BufferedImage image, OutputStream out)                throws IOException {        ImageIO.write(image, "JPEG", out);    }}

VerifyCodeServlet.java:

import java.awt.image.BufferedImage;import java.io.IOException;import java.io.PrintWriter;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import com.xjs.commons.VerifyCode;public class VerifyCodeServlet extends HttpServlet {    public void doGet(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {        /**         * 1.生成图片         * 2.保存图片上的文本到session中         * 3.把图片响应到客户端         */        VerifyCode vc=new VerifyCode();        BufferedImage bi=vc.getImage();        request.getSession().setAttribute("session_vcode", vc.getText());        //参数输出流，ServletOutputStream  getOutputStream();返回输入流---跟getWriter()方法不能同时使用，跟JavaSE中的输出流类似        VerifyCode.output(bi, response.getOutputStream());    }}

succ1.jsp和succ2.jsp：

   <%           if(session.getAttribute("username")==null&&session.getAttribute("password")==null){               //重定向到login.jsp               response.sendRedirect("/XJS_Session/login.jsp");               return;//别的就不执行了           }    %>    <%        String username=(String)session.getAttribute("username");        String password=(String)session.getAttribute("password");        out.print("用户名："+username);        out.print("密码："+password);     %>  

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