Communication between sibling webapps? ( also posted to the jsp forum )

Question

Hi Folks,
I'm working with a web application ( with multiple webapps ) in which some requests to some of the webapps ( call them A, because they are all similar ( one for each of our clients ), but may have their own ( possible multiple )specific implementations of an interface that is available across the application ) are handled ( via a RequestDispatcher include( request, response ) ) by a servlet in another webapp ( I'll call it B ), that provides some specialized services ( and receives the Request it is working on ). The include is part of an custom jsp tag, which is used on some of an A's pages, and which always includes from B.
B's service may need an instance of one of those A-specific implementations, but the service doesn't know whether that is the case, or which specific implementation class to use until it receives the request. The problem is that if the .class file is in A's WEB-INF ( classes or lib ), if B's code uses Class.forName ( or similar ) it can't see the class, because its ClassLoader is a sibling of A's, and not a parent.
This has been dealt with so far dealt by putting ALL of the implementation classes in B's WEB-INF, but that is ugly, crowded, and confusing ( and possibly risky - a typo in a configuration file could have A1 using an A2 implementation ). So, it would be better to move the implementations to the appropriate A's WEB-INF.
The question is, what is the best/recommended/cleanest/least-risky ( multiple answers accepted ) way of letting B's service get the specific implementation it needs.
So far the possibilities that I've heard are ( in no particular order ):
---- 1 ----
a. Have the tag instance get the ClassLoader of the page the tag was on, and add it as an attribute to the request.
b. The service looks up ( from configuration ) the classname it has determined it needs, as it does currently.
c. Have the service get the ClassLoader from the attribute, and use it with something like Class.forName( String, boolean, ClassLoader ) to instantiate the implementation it needs, and use it.
---- 2 ----
a. Like 1, except that the tag passes an instance of some factory that was loaded by A's classloader, that will be able to load the implementation whose .class file is in A.
---- 3 ----
a. Have the tag try to determine the implementation that will be needed, instantiate it, and pass it along as in 1 or 2. ( and WHEN it will be needed - the service isn't used on all paths through the tag, though that is also the case for 1 & 2, but there the attribute value can be the same instance each time, while in this case, we probably need a new instance of the implementation )
---- 4 ----
a. Do something else ( please fill in the blank )
Any suggestions will be greatly appreciated - as well as comments on the issues that might be involved with each possibility.
( I apologize for the over use of parentheses, but that's the way I think - early LISP influence )
Thanks,
George McKinney

Answer 1

Most web application servers have a means of sharing common libraries between web applications. In Tomcat you can put the common libraries in the common or shared directory. In Weblogic you can bundle the war files in an ear file and put the APP_INF directory of the ear file. Consult the documentation for your web application server for how it's classloading works.