随机产生验证码

package com;

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

public class CheckCodeServlet extends HttpServlet {
	private static final long serialVersionUID = 524849328573697679L;
	
	private static int WIDTH=60;
	private static int HEIGHT=20;
	
	
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		doPost(req,resp);
	}
	@Override
	protected void doPost(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		
		HttpSession session=req.getSession();
		
		resp.setContentType("image/jpeg");
		
		ServletOutputStream sos=resp.getOutputStream();
		
		//设置浏览器不要缓存此图片
		resp.setHeader("Pragma", "No-cache");
		
		resp.setHeader("Cache-Control", "no-cache");
		
		resp.setDateHeader("Expires", 0);
		
		
		//创建内存图像并获得其图形上下文
		BufferedImage image=new BufferedImage(WIDTH,HEIGHT,BufferedImage.TYPE_INT_RGB);
		Graphics g=image.getGraphics();
		
		
		//产生随机的验证码
		char[] rands=generateCheckCode();
		
		//产生图像
		drawBackground(g);
		drawRands(g,rands);
		
		//结束图像的绘制过程，完成图像
		g.dispose();
		
		//将图像输出到客户端
		ByteArrayOutputStream bos=new ByteArrayOutputStream();
		ImageIO.write(image, "JPEG", bos);
		byte[] buf=bos.toByteArray();
		resp.setContentLength(buf.length);
		sos.write(buf);//也可以写成bos.writeTo(sos)
		bos.close();
		sos.close();
		
		//将当前验证码存入到Session中
		session.setAttribute("check_code", new String(rands));
		
		
		//直接使用下面的代码将有问题，Session对象必须在提交响应前获得
		//req.getSession().setAttribute("check_code", new String(rands))
		
		
	}
	private void drawRands(Graphics g, char[] rands) {
		g.setColor(Color.BLACK);
		g.setFont(new Font(null,Font.ITALIC|Font.BOLD,18));
		//在不同的高度输出验证码的每个字符
		g.drawString(""+rands[0], 1, 17);
		g.drawString(""+rands[1], 16, 15);
		g.drawString(""+rands[2], 31, 18);
		g.drawString(""+rands[3], 46, 16);
		System.out.println(rands);
		
		
	}
	private void drawBackground(Graphics g) {
		// 画背景
		g.setColor(new Color(0xDCDCDC));
		g.fillRect(0, 0, WIDTH, HEIGHT);
		
		//随机产生120个干扰点
		for(int i=0; i<120; i++){
			int x=(int)(Math.random()*WIDTH);
			int y=(int)(Math.random()*HEIGHT);
			
			int red=(int)(Math.random()*255);
			int green=(int)(Math.random()*255);
			int blue=(int)(Math.random()*255);
			
			g.setColor(new Color(red,green,blue));
			g.drawOval(x, y, 1, 0);
		}
		
	}
	private char[] generateCheckCode() {
		String chars = "0123456789abcdefghijklmnopqrstuvwxyz";
		
		char [] rands = new char [4];
		
		for(int i = 0; i < 4; i++){
			int rand = (int) (Math.random()*36);
			rands[i] = chars.charAt(rand);
		}
		return rands;
	}

}