caching : Java Glossary


Caching is a general term of keeping a small subset of a larger set handy for rapid access. SQL databases keep the most popular parts of the disk-based database in RAM (Random Access Memory). DNS lookup servers keep locally copies of the names it uses most frequently. Operating systems keep copies of the most commonly used parts of the hard disk in a RAM cache.

Starting with Java version 1.3, the Java Plugin offers Applet caching. This means Applets need be downloaded only when new versions have been uploaded to the server. Otherwise they are executed from a local cache. There are special plug-in commands to control which Applets are sticky and which are downloaded fresh each time.

To speed things up even further, so that last modified dates and jar sizes don’t need to be checked from the server, you can put jar version numbers in your <PARAM NAME=cache_version tags. The cacher compares these with the version numbers in its cache to see if it needs to refresh the jar. The jars themselves have no such embedded version numbers.

To clear the in-RAM ClassLoader Applet cache in Internet Explorer, click Tools ⇒ Java Console ⇒ x.

To clear the in-RAM ClassLoader Applet cache in Opera, click Tools ⇒ Advanced ⇒ Java Console ⇒ x .

You can configure the cache size in a browser. Don’t be crazy generous. If you choose something like 9000 MB you will eventually fill up your entire hard disk with junk you never use and your machine will grind to a halt. To make matters worse, IE (Internet Explorer) cleverly hides the cached files from view that are causing all the trouble. Especially on Windows 98, to add insult to injury, having too big a cache will slow you down too. 20 to 100 MB is probably sufficient before you reap no additional benefit.

To manually clear the Applet/Jar cache, delete files in:

If you are using https: protocol, there can be no caching, since content cannot be shared.

To control caching, your server software must insert HTTP (Hypertext Transfer Protocol) headers, (not HTML (Hypertext Markup Language) meta headers embedded in the document).

Servers have no ability to invalidate intermediate caches or users’ caches. The best they do is rename something to force a refetch. Their only control is setting various time to live parameters in the headers.


book cover recommend book⇒Web Cachingto book home
by Duane Wessels 978-1-56592-536-6 paperback
publisher O’Reilly recommended 978-0-613-91260-0 hardcover
published 2001-06
The author is the creator of Squid an open-source web caching program. The book also tackles such things as why web site owners deliberately foil caching. The author has a refreshingly simple and direct way of writing.
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