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Chapter 7 Generating the Server Response: HTTP Response Headers

7.5 Using Servlets to Generate GIF 

Images

Although servlets often generate HTML output, they certainly don't always

do so. For example, Section 11.2 (The contentType Attribute) shows a JSP

page (which gets translated into a servlet) that builds Excel spreadsheets and

returns them to the client. Here, I'll show you how to generate GIF images.

First, let me summarize the two main steps servlets have to perform in

order to build multimedia content. First, they have to set the 

Content Type

response header by using the 

setContentType

 method of 

HttpServlet 

Response

. Second, they have to send the output in the appropriate format.

This format varies among document types, of course, but in most cases you

use send binary data, not strings as with HTML documents. Consequently,

servlets will usually get the raw output stream by using the 

getOutput 

Stream

 method, rather than getting a 

PrintWriter

 by using 

getWriter

.

Putting these two points together, servlets that generate non HTML content

usually have a section of their 

doGet

 or 

doPost

 method that looks like this:

response.setContentType("type/subtype");

OutputStream out = response.getOutputStream();

Those are the two general steps required to build non HTML content.

Next, let's look at the specific steps required to generate GIF images.

1. Create an 

Image

.

You create an 

Image

 object by using the 

createImage

 method 

of the 

Component

 class. Since server side programs should not 

actually open any windows on the screen, they need to explicitly 

tell the system to create a native window system object, a pro 

cess that normally occurs automatically when a window pops 

up. The 

addNotify

 method accomplishes this task. Putting this 

all together, here is the normal process:

Frame f = new Frame();

f.addNotify();

int width = ...;

int height = ...;

Image img = f.createImage(width, height);

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