expiry header : Java Glossary

by Roedy Green ©1996-2009 Canadian Mind Products
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expiry header

In the header of every HTTP document served is an expiry field. It tells caching software on the net how long this document will remain fresh. If it has a long expiry date, there is a better chance it can be found in a cache somewhere. Then it can be retrieved rapidly without pestering your server. On the other hand, if you change the document, all the old copies is cache won’t be flushed until the expiry date has passed.

If you are sitting at a browser, sometimes hitting Shift-Reload or Ctrl-Reload will force getting a fresh copy.

If you always rename images or zip files when you change them, you can safely give them a very long expiry date, e.g. three months.

Unfortunately you can’t specify the expiry date in your HTML. Your ISP needs to apply the expiry date headers based on some sort of rule such as the Apache rule scheme.

When reading files from a website programmatically, you can force a fresh copy rather a faster cached copy with URLConnection.setUseCaches( false ).

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