Captcha生成及使用

@Controller
@RequestMapping("/PictureCheckCode")
public class ServletController {

    @RequestMapping
    public void pictureGenerate( HttpServletRequest req, HttpServletResponse res ) {
        try {
             MakePicture mp=new MakePicture() ;  
             String str=mp.drawPicture(60, 20,res.getOutputStream() );  
             req.getSession().setAttribute("pic", str);  
             res.getOutputStream().print(str);
        }
        catch(Exception ex) {
            ex.printStackTrace();
        }
    }
}

<form th:action="@{/login}" method="post">
            <div><label> User Name : <input type="text" name="username"/> </label></div>
            <div><label> Password: <input type="password" name="password"/> </label></div>
            <div><label> Verify: <img src="PictureCheckCode"  onclick="this.src='PictureCheckCode?d='+new Date().getTime()"/> </label></div>
            <div><input type="submit" value="Sign In"/></div>
</form>

package hello;

import java.awt.Graphics;
import java.awt.Font;
import java.awt.Color;
import java.awt.image.BufferedImage; 
import javax.imageio.ImageIO;  
import java.util.Random;
import java.io.OutputStream; 
import java.io.IOException;  
public class MakePicture  //产生识别验证图像
{   
	private char charTable[]={
		'a','A','b','B','c','C','d','D' ,'e','E' ,
		'f','F','g','G','h','H','i','I','j','J' ,
		'0','1','2','3','4','5','6','7','8','9'
	}; 
	public String drawPicture(int width,int height,OutputStream os)
	{
		if(width<=0)
			width=100 ;
		if(height<=0) 
			height=60 ;
			
		BufferedImage image=new BufferedImage(width, height,BufferedImage.TYPE_INT_RGB) ;
		Graphics g=image.getGraphics()  ;
		g.setColor(Color.LIGHT_GRAY) ; 
		g.fillRect(0, 0, width, height)  ;
		g.setColor(new Color(0x5265fd)) ;
		g.drawRect(0, 0, width, height)  ;  
		String str ="" ;
		for(int x=0;x<4;x++)
		{
		  str+=charTable[(int) (Math.random()*charTable.length)];
		}
		
		g.drawString(str.substring(0, 1), 0, 15);
		g.drawString(str.substring(1, 2), 15, 17);
		g.drawString(str.substring(2, 3), 35, 19);
		g.drawString(str.substring(3, 4), 50, 16);
		Random rand=new Random() ;
		for(int i=0;i<10;i++)
		{
		  int x=rand.nextInt(width)  ;
		  int y=rand.nextInt(height)  ;	
		  g.drawOval(x, y, 1, 1) ;
 		}
		g.dispose()  ;
		try {
			ImageIO.write(image, "JPEG",os) ;
		} catch (IOException e) {
			
			e.printStackTrace();
			return "" ;
		}
		
	   return str ;
	}
}