实现验证码

前端验证码发送servlet请求代码：

<img src="checkCode" alt="" id="codeImg"
style="width: 80px; height: 30px; border: 1px solid black;" /> 
<a href="javascript:;" onclick="document.getElementById('codeImg').src = 'checkCode?'+(new Date()).getTime()">换一张</a>
<span id="codemsg" style="color: red; font-size: 20px;"></span>

后台处理，并返回给前端处理结果

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.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;

/**
 * Servlet implementation class checkservlet
 */
@WebServlet("/checkCode")
public class checkservlet extends HttpServlet {
	 @Override
	protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		
				String codes="abcdefghijklmnopqrstuvwxyzABCDEFJHIJKLMNOPQRSTUVWXYZ0123456789";	
				BufferedImage img=new BufferedImage(80,30,BufferedImage.TYPE_3BYTE_BGR);
				Graphics g=img.getGraphics();
				//填充图片
				g.setColor(new Color(255,255,255));
				g.fillRect(0, 0, 80, 30);
				//画入五个随机数
				Random rm=new Random();
				StringBuffer sb=new StringBuffer();
				for(int i=0;i<5;i++){
					int index=rm.nextInt(codes.length());
					char code=codes.charAt(index);
					g.setColor(new Color(rm.nextInt(256),rm.nextInt(256),rm.nextInt(256)));
					g.setFont(new Font("宋体", Font.BOLD, 25));
					g.drawString(code+"", 2+15*i, 22);
					sb.append(code);
				}
				
				//画干扰线
				for(int i=0;i<10;i++){
					g.setColor(new Color(rm.nextInt(256),rm.nextInt(256),rm.nextInt(256)));
					g.drawLine(rm.nextInt(100), rm.nextInt(50), rm.nextInt(100), rm.nextInt(50));
				}
				
				//将验证码图片回写给浏览器
				response.setContentType("image/jpeg;charset=utf-8");
				OutputStream out=response.getOutputStream();
				ImageIO.write(img, "jpeg", out);
				
				//将随机字符串保存在session中
				request.getSession().setAttribute("code", sb.toString());
			
	}

}