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164

Chapter 7 Generating the Server Response: HTTP Response Headers

that the document has finished loading by monitoring the number of bytes

received, and reconnects on the same socket for further transactions. Persis 

tent connections of this type became standard in HTTP 1.1, and compliant

servers are supposed to use persistent connections unless the client explicitly

instructs them not to (either by a  

Connection: close

  request header or

indirectly by sending a request that specifies 

HTTP/1.0

 instead of 

HTTP/1.1

and does not also stipulate  

Connection: keep alive

 ).

Servlets can take advantage of persistent connections if the servlets are

embedded in servers that support them. The server should handle most of

the process, but it has no way to determine how large the returned document

is. So the servlet needs to set the 

Content Length

 response header by

means of 

response.setContentLength

. A servlet can determine the size of

the returned document by buffering the output by means of a 

ByteArray 

OutputStream

, retrieving the number of bytes with the byte stream's 

size

method, then sending the buffered output to the client by passing the serv 

let's output stream to the byte stream's 

writeTo

 method. 

Using persistent connections is likely to pay off only for servlets that load a

large number of small objects, where those objects are also servlet generated

and would thus not otherwise take advantage of the server's support for per 

sistent connections. Even so, the advantage gained varies greatly from Web

server to Web server and even from Web browser to Web browser. For exam 

ple, the default configuration for Sun's Java Web Server is to permit only five

connections on a single HTTP socket: a value that is too low for many appli 

cations. Those who use this server can raise the limit by going to the adminis 

tration console, selecting  Web Service  then  Service Tuning,  then entering

a value in the  Connection Persistence  window. 

Listing 7.6 shows a servlet that generates a page with 100 

IMG

 tags (see

Figure 7 4 for the result). Each of the 

IMG

 tags refers to another servlet

(

ImageRetriever

, shown in Listing 7.7) that reads a GIF file from the server

system and returns it to the client. Both the original servlet and the 

Image 

Retriever

 servlet use persistent connections unless instructed not to do so

by means of a parameter in the form data named 

usePersistence

 with a

value of 

no

. With Netscape 4.7 and a 28.8K dialup connection to talk to the

Solaris version of Java Web Server 2.0 (with the connection limit raised above

100), the use of persistent connections reduced the average download time

between 15 and 20 percent. 

Second edition of this book: www.coreservlets.com; Sequel: www.moreservlets.com.

Servlet and JSP training courses by book's author: courses.coreservlets.com.