The Java Package Manager and Microsoft® Internet Explorer go through the following steps before a DU is installed.
If the DU is not present on the current system, the appropriate cabinet file archive is downloaded from the URL specified by the useslibrarycodebase parameter.
If the DU is present on the current system, the version number, specified in the useslibraryversion parameter, is checked to determine whether the DU is current with the version offered. This eliminates the need to download, open, and examine the contents of the archive to determine if it is current (as is the case when you use <OBJECT> tags with Internet Explorer 3.0).
Note Internet Explorer 3.x does not recognize useslibrary and its associated parameters. You must use the <APPLET> and <OBJECT> tags and their parameters to emulate this distribution.
Using DUs to distribute code enables users to run applets without constantly downloading them in a networked environment. For example, where once you had to interact with security managers every time you visited a site and encountered a signed cabinet file or code, you can now install the code into the Java Package Manager on the first visit and avoid subsequent security interactions; the code is already installed. If the DU's cabinet file is signed, all security and permissions apply the first time the cabinet is downloaded.