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7.5 Using Servlets to Generate GIF Images

169

2. Draw into the 

Image

.

You accomplish this task by calling the 

Image

's 

getGraphics

method and then using the resultant 

Graphics

 object in the 

usual manner. For example, with JDK 1.1, you would use vari 

ous 

drawXxx

 and 

fillXxx

 methods of 

Graphics

 to draw 

images, strings, and shapes onto the 

Image

. With the Java 2 

platform, you would cast the 

Graphics

 object to 

Graphics2D

, 

then make use of Java2D's much richer set of drawing opera 

tions, coordinate transformations, font settings, and fill patterns 

to perform the drawing. Here is a simple example:

Graphics g = img.getGraphics();

g.fillRect(...);

g.drawString(...);

3. Set the 

Content Type

 response header.

As already discussed, you use the 

setContentType

 method of 

HttpServletResponse

 for this task. The MIME type for GIF 

images is 

image/gif

. 

response.setContentType("image/gif");

4. Get an output stream.

As discussed previously, if you are sending binary data, you 

should call the 

getOutputStream

 method of 

HttpServlet 

Response

 rather than the 

getWriter

 method.

OutputStream out = response.getOutputStream();

5. Send the 

Image

 in GIF format to the output stream.

Accomplishing this task yourself requires quite a bit of work. 

Fortunately, there are several existing classes that perform this 

operation. One of the most popular ones is Jef Poskanzer's 

GifEncoder

 class, available free from 

http://www.acme.com/java/

. Here is how you would use this 

class to send an 

Image

 in GIF format:

try {

new GifEncoder(img, out).encode();

} catch(IOException ioe) {

// Error message

}

Listings 7.8 and 7.9 show a servlet that reads 

message

, 

fontName

, and

fontSize

 parameters and uses them to create a GIF image showing the mes 

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