生成验证码的servlet:CodeServlet（区分大小写）

//这个生成验证码的servlet已经将随机数（也就是这里的验证码）放入session中，以便于校验时获取
req.getSession().setAttribute(“scode”, randomString);
System.out.println(randomString);

package com.web.servlet;

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Random;

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


public class CodeServlet extends HttpServlet {
    /**
	 * 
	 */
	private static final long serialVersionUID = 1L;
	public static final int WIDTH = 120;//生成图片的宽度
    public static final int HEIGHT = 30;//生成图片的高度
    public static final int WORDS_NUMBER = 4;//验证码中字符的个数
    
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // TODO Auto-generated method stub
        this.doPost(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // TODO Auto-generated method stub
        String createTypeFlag = req.getParameter("createTypeFlag");//接收客户端传递的createTypeFlag标识
        //在内存中创建一张图片
        BufferedImage bi = new BufferedImage(WIDTH,HEIGHT,BufferedImage.TYPE_3BYTE_BGR);
        //得到图片
        Graphics g = bi.getGraphics();
        //设置图片的背景色
        setBackGround(g);
        //设置图片的边框
        setBorder(g);
        //在图片上画干扰线
        drawRandomLine(g);
        //在图片上放上随机字符
        String randomString = this.drawRandomNum((Graphics2D)g, createTypeFlag);
        
        //将随机数存在session中
        req.getSession().setAttribute("scode", randomString);
        System.out.println(randomString);
        //设置响应头通知浏览器以图片的形式打开
        resp.setContentType("image/jpeg");
        
        //设置响应头控制浏览器不要缓存
        resp.setDateHeader("expries", -1);
        resp.setHeader("Cache-Control", "no-cache");
        resp.setHeader("Pragma", "no-cache");
        
        //将图片传给浏览器
        ImageIO.write(bi, "jpg", resp.getOutputStream());
    }
    
    //设置图片背景色
    //@param g
    private void setBackGround(Graphics g) {
        //设置颜色
        g.setColor(Color.WHITE);
        //填充区域
        g.fillRect(0, 0, WIDTH, HEIGHT);
    }
    
    /*
     * 设置图片的边框
     * @param g
     * */
    
    private void setBorder(Graphics g) {
        //设置边框颜色
        g.setColor(Color.BLUE);
        //边框区域
        g.drawRect(1, 1, WIDTH - 2, HEIGHT -2);
    }
    
    /*
     * 在图片上画随机线条 
     * @param g
     * */
    private void drawRandomLine(Graphics g) {
        //设置颜色
        g.setColor(Color.GREEN);
        //设置线条个数并画线
        for ( int i = 0 ; i < 3 ; i++ ) {
            int x1 = new Random().nextInt(WIDTH);
            int y1 = new Random().nextInt(HEIGHT);
            int x2 = new Random().nextInt(WIDTH);
            int y2 = new Random().nextInt(HEIGHT);
            g.drawLine(x1, y1, x2, y2);
        }
    }
    
    /*
     * 在图片上画随机字符
     * @param g
     * @param createTypeFlag
     * @return String
     * */
    private String drawRandomNum(Graphics g,String createTypeFlag) {
        //设置颜色
        g.setColor(Color.RED);
        g.setFont(new Font("宋体",Font.BOLD,20));
        
        //数字字母的组合
        String baseNumLetter = "0123456789ABCDEFGHJKLMNOPQRSTUVWXYZ";
        String baseNum = "0123456789";
        String baseLetter = "ABCDEFGHJKLMNOPQRSTUVWXYZ";
        if ( createTypeFlag != null  && createTypeFlag.length() > 0 ) {
            if( createTypeFlag.equals("nl") ) {
                //截取数字和字母的组合
                return createRandomChar((Graphics2D) g,baseNumLetter);
            } else if ( createTypeFlag.equals("n") ) {
                //截取数字的组合
                return createRandomChar((Graphics2D) g,baseNum);
            } else if ( createTypeFlag.equals("l") ) {
                //截取字母的组合
                return createRandomChar((Graphics2D) g,baseLetter);
            }
        } else {
            //截取数字和字母的组合
            return createRandomChar((Graphics2D) g,baseNumLetter);
        }
        return "";
    }
    
    /*
     * 创建随机字符
     * @param g
     * @param baseChar
     * @return String
     * */
    private String createRandomChar(Graphics2D g , String baseChar) {
        StringBuffer b = new StringBuffer();
        int x = 5;
        String ch = "";
        for ( int i = 0 ; i < WORDS_NUMBER ; i++ ) {
            //设置字体的旋转角度
            int degree = new Random().nextInt() % 30;
            ch = baseChar.charAt(new Random().nextInt(baseChar.length())) + "";
            b.append(ch);
            
            //正向角度
            g.rotate(degree  * Math.PI / 180 , x,20);
            g.drawString(ch, x, 20);
            //反向角度
            g.rotate(-degree  * Math.PI / 180 , x,20);
            x+=30;
        }
        return b.toString();
    }
}

验证码展示：运行CodeServlet

做一个验证码提交页面:yanzhengma.jsp（这里面调用了CodeServlet，效果如下）

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
	<script>
	function changeImg()
	{
		document.getElementById("codeImg").src="CodeServlet?i="+Math.random();
	}

	</script>
</head>
<body>
	<form action="CheckCodeServlet" method="post">
		验证码：<input type="text" name="ccode">  
		<img alt="验证码看不清，换一张" src="CodeServlet" id="codeImg" onclick="changeImg()">
		<a href="#" onclick="changeImg()">看不清，换一张</a>
		<br>
		<input type="submit" value="提交">
	</form>
</body>
</html>

对验证码进行验证的servlet：CheckCodeServlet

package com.web.servlet;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class CheckCodeServlet
 */
public class CheckCodeServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;
 
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		response.setContentType("text/html;charset=utf-8");
		//客户端那边输入的验证码
		String cCode = request.getParameter("ccode");
		//生成验证码页面放入域中的验证码
		String sCode = (String) request.getSession().getAttribute("scode");
		//System.out.println(cCode);
		//System.out.println(sCode);
		if(cCode.equals(sCode))
		{
			response.getWriter().println("校验成功！");
		}else{
			response.getWriter().println("校验失败！");
		}
	}
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		doGet(request, response);
	}

}

回到yanzhengma.jsp页面，输入验证码

校验成功

校验失败