1、一共需要2个常用java文件（RandomCode.java和RandomCodeCtrl.java）：

（a、）RandomCode.java是个普通的java文件；内容如下：

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.util.Random;
import javax.imageio.ImageIO;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class RandomCode {
   /**
    * 随机取得一个字体
    * @param Random random  随机数
    * @return Font 返回一个新字体
    */ 
  private Font getsFont(Random random){
   return new Font("Fixedsys",Font.CENTER_BASELINE,18);
  }
   /**
    * 返回一个随机颜色
    * @param int fc  随机数
    * @param int bc  随机数
    * @param Random random  随机数
    * @return Color 返回一个新颜色
    */ 
  private Color getRandColor(int fc,int bc,Random random){
        if(fc>255) fc=255;
        if(bc>255) bc=255;
        int r=fc+random.nextInt(bc-fc-6);
        int g=fc+random.nextInt(bc-fc-4);
        int b=fc+random.nextInt(bc-fc-8);
        return new Color(r,g,b);
    }
   /**
    * 生成随机数图片
    */ 
  public void getRandcode(HttpServletRequest request,HttpServletResponse response)throws Exception{
   System.setProperty("java.awt.headless","true");
   HttpSession session = request.getSession();
   int width=80, height=22;//设置图片大小
   BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
   Graphics g = image.getGraphics();
   Random random = new Random();
   g.fillRect(1, 1, width, height);//设定边框
   g.setFont(new Font("Times New Roman",Font.ROMAN_BASELINE,18));
   g.setColor(getRandColor(111,133,random));
   //产生随机线
   for (int i=0;i<11;i++){
    int x = random.nextInt(width);
    int y = random.nextInt(height);
    int xl = random.nextInt(13);
    int yl = random.nextInt(15);
    g.drawLine(x,y,x+xl,y+yl);
   }
   //产生随机点
   g.setColor(getRandColor(130,150,random));
   //产生5个随机数
   String sRand="";
   for (int i=0;i<5;i++){
       g.setFont(getsFont(random));
       g.setColor(new Color(random.nextInt(101),random.nextInt(111),random.nextInt(121)));
       String rand=String.valueOf(getRandomString(random.nextInt(36)));
       sRand+=rand;
       g.translate(random.nextInt(3),random.nextInt(3));
       g.drawString(rand,13*i,16);
   }
   session.removeAttribute("Rand");
   session.setAttribute("Rand",sRand);
   g.dispose();
   ImageIO.write(image, "JPEG", response.getOutputStream());
  }
  
  private String getRandomString(int num){
   String randstring = "0123456788ABCDEFGHIJKLMNOPQRSTUVWXYZ";
   return String.valueOf(randstring.charAt(num));
  }
 
 }

（b、） RandomCodeCtrl.java是个servlet的java文件；内容如下：

import java.io.IOException;

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

public class RandomCodeCtrl extends HttpServlet {

private static final long serialVersionUID = 1L;

protected void doGet(HttpServletRequest req,
      HttpServletResponse resp) throws ServletException, IOException {
  resp.setContentType("image/jpeg");
  resp.setHeader("Pragma","No-cache");
  resp.setHeader("Cache-Control","no-cache");
  resp.setDateHeader("Expires", 0);
  RandomCode rc = new RandomCode();
  try{
   rc.getRandcode(req,resp);
  }catch(Exception e){
   System.err.println(e);
  }
 }

public void doPost(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {
  doGet(request, response);
 }

}

2、前台页面调用；举例：

<img src="http://127.0.0.1:8080/RandomCodeCtrl" />

3、验证输入信息和随机生成的图片显示的内容相同：

在RandomCode.java代码中随机生成的图片之前，就把随机生成的图片显示的内容放在session中；所以只需判断session中getAttribute("Rand")的值和用户页面输入的验证码值相等即可。