Message-Id: <200108240304.XAA09690@pacific-carrier-annex.mit.edu>
Date: Thu, 23 Aug 2001 20:04:32 PDT
From: "JDC Tech Tips" <body_-5852066593234543465@hermes.java.sun.com>
To: alexp@mit.edu
Subject: Correction to JDC Tech Tip  for August 21, 2001
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 J  D  C    T  E  C  H    T  I  P  S

                      TIPS, TECHNIQUES, AND SAMPLE CODE


This is an update to the the Java Developer Connection(sm) (JDC) 
Tech Tip for August 21, 2001 on Delivering Dynamic Images from 
JavaServer Pages(tm) (JSP(tm)) Technology. You can view that 
issue of the Tech Tips on the Web at
http://java.sun.com/jdc/JDCTechTips/2001/tt0821.html

That tip mistakenly generated binary content from a JSP page. The 
Servlet specification requires that servlets not call both 
ServletResponse.getOutputStream() and ServletResponse.getWriter(). 
Because JSP pages already call getWriter(), they must not also
call getOutputStream(). Some versions of Tomcat delay the calling 
of getWriter(), so that the tip might work for those versions.
However the operations demonstrated in the tip should be 
considered illegal. JSP pages are only meant to deliver textual 
output.

In order for this tip to work properly, the dynamic image 
generation must be moved into a servlet. Place the following 
class definition in an appropriate servlet directory for your 
web server (such as webapps\ROOT\WEB-INF\classes for Tomcat). 
Then you should be able to call it with a URL of 
http://localhost:8080/servlet/MakeImage. The core code is 
identical to that used previously in the tip, except that it's
now part of a servlet.

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.awt.*;
import java.awt.image.*;
import com.sun.image.codec.jpeg.*;
import java.util.*;

public class MakeImage extends HttpServlet {

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

     response.setContentType("image/jpeg");

     // Create image
     int width=200, height=200;
     BufferedImage image = new BufferedImage(
       width, height, BufferedImage.TYPE_INT_RGB);

     // Get drawing context
     Graphics g = image.getGraphics();

     // Fill background
     g.setColor(Color.white);
     g.fillRect(0, 0, width, height);

     // Create random polygon
     Polygon poly = new Polygon();
     Random random = new Random();
     for (int i=0; i < 20; i++) {
       poly.addPoint(random.nextInt(width),
         random.nextInt(height));
     }

     // Fill polygon
     g.setColor(Color.cyan);
     g.fillPolygon(poly);

     // Dispose context
     g.dispose();

     // Send back image
     ServletOutputStream sos =
       response.getOutputStream();
     JPEGImageEncoder encoder =
       JPEGCodec.createJPEGEncoder(sos);
     encoder.encode(image);

   }
}

In addition, if you are a Unix user running a web server on a
headless box, you might need Xvfb, which is a virtual frame buffer 
X server, to emulate a display environment. See
http://java.sun.com/products/java-media/2D/forDevelopers/java2dfaq.html#xvfb
for more information about server-side support for working with 
images.

.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .

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JDC Tech Tips 
August 23, 2001

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