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2.8 An Example Using Servlet Initialization and Page Modification Dates

45

Figure 2 8

Output of LotteryNumbers servlet.

with Java dates. The time is automatically converted to a date in GMT appro 

priate for the 

Last Modified

 header. More importantly, if the server receives

a conditional 

GET

 request (one specifying that the client only wants pages

marked 

If Modified Since

 a particular date), the system compares the

specified date to that returned by 

getLastModified

, only returning the page

if it has been changed after the specified date. Browsers frequently make these

conditional requests for pages stored in their caches, so supporting conditional

requests helps your users as well as reducing server load. Since the 

Last Mod 

ified

 and 

If Modified Since

 headers use only whole seconds, the 

get 

LastModified

 method should round times down to the nearest second. 

Figures 2 9 and 2 10 show the result of requests for the same servlet with

two slightly different 

If Modified Since

 dates. To set the request headers

and see the response headers, I used 

WebClient

, a Java application shown in

Section 2.10 (WebClient: Talking to Web Servers Interactively) that lets you

interactively set up HTTP requests, submit them, and see the results.

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

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