One of the next features of Seven I'm working on is the ability of a
service to keep access to all download JAR files as seen be a service
during its life-time, including restarts of the service and/or container.
This should lead to:
- increased reliability of codebase annotations, especially handy when
persisting marshalled objects;
- faster startup times (also see Gregg's requirements for vhttp);
- less bandwidth usage;
- statistics about the usage of downloadable code by a service;
- if you ever want to move a service from one container to another
you can move its downloadable code dependencies with it.
This works because Seven uses specialized jar protocol handlers that are
used for all the class loaders created by PreferredClassProvider, it
also works for each protocol used in codebase annotations.
So far this work nice, there is however one spoiler and that is the
current implementation of net.jini.loader.pref.PreferredClassLoader as
this one performs always a direct check against the first URL to see
whether the there is a PREFERRED.LIST for reasons as stated by inline
comments:
* First determine if the JAR file exists by attempting to
* access it directly, without using a "jar:" URL, because
* the "jar:" URL handler can mask the distinction between
* definite lack of existence and less definitive errors.
* Unfortunately, this direct access circumvents the JAR
* file caching done by the "jar:" handler, so it ends up
* causing a duplicate request of the JAR file on first
* use. (For HTTP-protocol URLs, the initial request will
* use HEAD instead of GET.)
First can somebody shed me light how a typical jar: handler can mask the
distinction between definite lack of existence and less definite errors.
I'm not that concerned about the performance implications though the
current implementation defeats "increased reliability of codebase
annotations", I don't have a solution in mind yet, but I would like to
see a way to be able to get what I want without completely overriding
the protected method isPreferredResource(String, boolean).
--
Mark
