//GenerateImage.java    
 /* 动态产生JPEG图像的例子   */
import java.awt.*;
import java.awt.image.*;
import com.sun.image.codec.jpeg.*;  //编码类
java.util.*;
javax.servlet.*;
javax.servlet.http.*;
java.io.*;

class GenerateImage extends HttpServlet
public void doGet(HttpServletRequest request,HttpServletResponse response)
               throws ServletException, java.io.IOException {

 response.setContentType("image/jpeg"):
 int width=200, height=200;
 BufferedImage image = BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);
 Graphics g = image.getGraphics();
 g.setColor(Color.white);
 g.fillRect(0,0,width,heitht);
 Polygon poly = new Polygon();
 Random random = new Random();
 for(int i=0;i<5;i++)
 {
 poly.addPoint(random.nextInt(width),random.nextInt(height));
 }
  g.setColor(Color.cyan);
  g.fillPolygon(poly);
  g.dispose(); //消拙Graphics 对图形没有肖毁
 
  ServletOutputStream sos =response.getOutputStream();  //二进制
  JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(sos); //创建编码器,输出到sos那里
  encoder.encode(image);
  }

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

}

http://blog.csdn.net/laiahu/archive/2007/01/23/1491681.aspx