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154

Chapter 7 Generating the Server Response: HTTP Response Headers

WWW Authenticate

This header is always included with a 401 (

Unauthorized

) status code. 

It tells the browser what authorization type and realm the client should 

supply in its 

Authorization

 header. Frequently, servlets let pass 

word protected Web pages be handled by the Web server's specialized 

mechanisms (e.g., 

.htaccess

) rather than handling them directly. For 

an example of servlets dealing directly with this header, see Section 4.5 

(Restricting Access to Web Pages).

7.3 Persistent Servlet State and 

Auto Reloading Pages

Here is an example that lets you ask for a list of some large, randomly chosen

prime numbers. This computation may take some time for very large num 

bers (e.g., 150 digits), so the servlet immediately returns initial results but

then keeps calculating, using a low priority thread so that it won't degrade

Web server performance. If the calculations are not complete, the servlet

instructs the browser to ask for a new page in a few seconds by sending it a

Refresh

 header. 

In addition to illustrating the value of HTTP response headers, this exam 

ple shows two other valuable servlet capabilities. First, it shows that the same

servlet can handle multiple simultaneous connections, each with its own

thread. So, while one thread is finishing a calculation for one client, another

client can connect and still see partial results. 

Second, this example shows how easy it is for servlets to maintain state

between requests, something that is cumbersome to implement in tradi 

tional CGI and many CGI alternatives. Only a single instance of the servlet

is created, and each request simply results in a new thread calling the serv 

let's 

service

 method (which calls 

doGet

 or 

doPost

). So, shared data simply

has to be placed in a regular instance variable (field) of the servlet. Thus,

the servlet can access the appropriate ongoing calculation when the

browser reloads the page and can keep a list of the N most recently

requested results, returning them immediately if a new request specifies

the same parameters as a recent one. Of course, the normal rules that

require authors to synchronize multithreaded access to shared data still

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

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