//generateimage.java    
 /* 動態產生jpeg圖像的例子   */
import java.awt.*;
import java.awt.image.*;
import com.sun.image.codec.jpeg.*;  //編碼類
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 對圖形沒有肖毀
 
  servletoutputstream sos =response.getoutputstream();  //二進制
  jpegimageencoder encoder = jpegcodec.createjpegencoder(sos); //創建編碼器,輸出到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