jsp+ajax实现验证码

1.验证码无非就是点击一个链接可以产生或更改验证码，并且输入验证码可以完成验证
2.前台界面
`

<h1>验证码</h1>
<a href="javascript:void(0)" onclick="setVerifyCode(this)">看不清,换一张</a>

<img alt="" src=""id="img1">
<span id="sp1">${sessionScope.verifyCode }</span>
<button onclick="getVerifyCodeValue()">获取验证码值</button>
<%=request.getSession().getAttribute("verifyCode") %>
<input type="text" name="verifyCode" id="verifyCode">
<button onclick="check()">检验</button>
<span id="sp3"></span>
    <span id="sp4">${verifyCode }</span>

`
3.ajax接受前台发送的请求并转发给servlet

	//响应目标函数
	var xmlrequest;
	function createXMLHttpRequest()
	{
		if(window.ActiveXObject)
		{
			xmlrequest=new ActionXObject("Miscrosoft.XMLHTTP");
			
		}
		else
		{
			xmlrequest=new XMLHttpRequest();
			
		}

	}
	function setVerifyCode(o)
	{
		createXMLHttpRequest();
		if(xmlrequest)
		{
			var req="/xiaoneinew/verifyCode2";
		//	window.alert(req);
			xmlrequest.open("get",req,true);
			//指定处理结果的函数
			xmlrequest.onreadystatechange = getVerifyCode
			//发送请求
			xmlrequest.send();
		}
	}
	function getVerifyCode()
	{
		//window.alert("0");
		if(xmlrequest.readyState==4)
		{
			//window.alert("1");
			//成功
			if(xmlrequest.status==200)
			{
				var responseText=xmlrequest.responseText;
				console.log(responseText);
				//window.alert(responseText);
				//$("#sp1").val(responseText);
				//document.getElementById("sp1").innerHTML=responseText;
				
				//document.getElementById("img1").src="/xiaoneinew/verifyCode";
				$("[id='img1']").attr("src","/xiaoneinew/verifyCode2");
				$("#sp3").html("${sesseionScope.verifyCode }");
				
				console.log(1);
			}
		}
	}

4.servlet产生验证码，并把图片输出到浏览器

package com.xiaonei.servlet;

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.Random;

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import org.apache.taglibs.standard.tag.el.core.OutTag;

import com.mysql.cj.util.Base64Decoder;




@WebServlet("/verifyCode2")
public class VerifyCode2 extends HttpServlet {
	private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public VerifyCode2() {
        super();
       
    }

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		//设置响应类型
		response.setContentType("image/JPEG");
		//定义图片宽高
		int width=80;
		int height=50;
		//创建一个缓冲区图片
		BufferedImage img=new BufferedImage(width, height, BufferedImage.TYPE_INT_BGR);
		//使用缓冲区图片创建，图像对象用于绘制验证码
		Graphics g=img.getGraphics();
		//设置画笔颜色，后续画矩阵，相当于设置图形背景颜色为黄色
		g.setColor(new Color(255,255,0));
		//设置图片填充位置
		g.fillRect(0, 0, width, height);
		//设置字体样式
		g.setFont(new Font("微软雅黑",Font.ITALIC,18));
		String srand="";
		Random random=new Random();
		for(int i=0;i<4;i++)
		{
			int y=10+random.nextInt(40);
			String rand=String.valueOf(random.nextInt(10));
			srand+=rand;
			System.out.println(rand);
			//随机变换颜色
			g.setColor(new Color(random.nextInt(255),random.nextInt(255),random.nextInt(255)));
			//将随机数画入图形
			g.drawString(rand, 5+i*width/4, y);
		}
		System.out.println("1:"+srand);
		produceLine(g,random,width,height);
		try
		{
			//释放图形资源
			g.dispose();
			
			 //保存到session中
	        HttpSession session = request.getSession();
	        session.setAttribute("verifyCode",srand);
	       
	        
	        System.out.println("verifyCode:"+session.getAttribute("verifyCode"));
			ServletOutputStream sos = response.getOutputStream();
	        ImageIO.write(img,"jpg",sos);
	       
	        
	        //禁止图像缓存。
	        response.setHeader("Pragma", "no-cache");
	        response.setHeader("Cache-Control", "no-cache");
	        response.setDateHeader("Expires", 0);
	       
	        //关闭输入流
	        sos.close();
		}
		catch(Exception e)
		{
			e.printStackTrace();
		}
	}
	
	//随机生成30条线条
	protected void  produceLine(Graphics g,Random random,int width,int height)
	{
		int x=random.nextInt(width);
		int y=random.nextInt(height);
		int x1=random.nextInt(10);
		int y1=random.nextInt(10);
		g.drawLine(x, y, x1, y1);
	}
   
	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		doGet(request, response);
	}

}

4.ajax回调函数接受servlet传递的字符串，并调用修改页面的验证码

function getVerifyCode()
	{
		//window.alert("0");
		if(xmlrequest.readyState==4)
		{
			//window.alert("1");
			//成功
			if(xmlrequest.status==200)
			{
				var responseText=xmlrequest.responseText;
				console.log(responseText);
				//window.alert(responseText);
				//$("#sp1").val(responseText);
				//document.getElementById("sp1").innerHTML=responseText;
				
				//document.getElementById("img1").src="/xiaoneinew/verifyCode";
				$("[id='img1']").attr("src","/xiaoneinew/verifyCode2");
				$("#sp3").html("${sesseionScope.verifyCode }");
				
				console.log(1);
			}
		}
	}

5.点击更换验证码，这样就产生了验证码，并且可以更换
6.那么如何验证验证码那，我的想法最初是servlet里不是更新了session吗，调用一个js函数把输入的内容与当前session里的验证码值作比较不就行了，试了很多遍都不行，因为ajax修改session的值需要刷新页面才能获取到最新的session，想了很多方法都不能实现
7.最后参考了一些网上的代码，明白了，可以通过ajax传递数据给一个servlet,把输入的验证码作为一个参数传递给servlet，在servlet里面可以获取session，然后servlet输入出一个验证成否的信息，从而完成了验证码的验证

调用的ajax

function check()
	{
		createXMLHttpRequest();
		if(xmlrequest)
		{
		    var verifyCode=$("#verifyCode").val();
		    var req="/xiaoneinew/checkVerifyCode?verifyCode="+verifyCode;
		   // window.alert(req);
		    xmlrequest.open("get",req,true);
			//指定处理结果的函数
			xmlrequest.onreadystatechange = getResult;
			//发送请求
			xmlrequest.send();
		    
		}
	}
	

调用的servlet

package com.xiaonei.servlet;

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.Random;

import javax.imageio.ImageIO;
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 org.apache.taglibs.standard.tag.el.core.OutTag;




@WebServlet("/checkVerifyCode")
public class CheckVerifyCode extends HttpServlet {
	private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public CheckVerifyCode() {
        super();
       
    }

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		PrintWriter pw=response.getWriter();
		String verifyCode=request.getParameter("verifyCode");
		System.out.println("输入的验证码"+verifyCode);
		String trueVerifyCode=(String) request.getSession().getAttribute("verifyCode");
		System.out.println("真实的验证码:"+trueVerifyCode);
		if(verifyCode.equals(trueVerifyCode))
		{
			pw.println(true);
		}
		else
		{
			pw.println(false);
		}
		
		
	
		
	
		
	}
	
	
	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		doGet(request, response);
	}

}

ajax回调函数

function getResult()
	{
		//window.alert("0");
		if(xmlrequest.readyState==4)
		{
			//window.alert("1");
			//成功
			if(xmlrequest.status==200)
			{
				var responseText=xmlrequest.responseText;
				
				window.alert(responseText);
				
			}
		}
	}