Thread Context Class Loading in Virgo

A number of popular open source libraries, particularly persistence providers, use thread context class loading to load application types. This is an issue in OSGi once an application is divided into more than one bundle. Each bundle has its own class loader and no particular one of these bundle class loaders is necessarily suitable for loading all the application classes that libraries need to load.

Virgo overcomes this problem with its notion of a scoped application. A scoped application consists of either a PAR archive file containing the application artefacts (bundles, property files, plans, etc.) or a scoped plan referring to the application  artefacts. The scope limits the visibility of application packages and services so that scoped applications do not interfere with each another. But Virgo also creates an additional synthetic context bundle for each scope and it's this bundle's class loader which is used for thread context class loading when the application calls out to libraries.

The synthetic context bundle is very simple. It's a bundle which uses Virgo's import-bundle manifest header to import each of the bundles in the scope. Thus all the packages exported by the application are available for thread context class loading by libraries. There is a minor restriction implied by this approach: no two bundles in a scoped application may export the same package, which would be pretty unusual.

The following example should help make things clear:

An Example Scoped Application

Here a scoped application contains two bundles A and B. A calls B which calls out to a library bundle L. If L uses the thread context class loader to load application types, it will have access to all the packages exported by bundles A and B. The synthetic context bundle, which imports bundles A and B, provides the thread context class loader for the scoped application.

This simple solution enables a variety of existing open source libraries, once converted to OSGi bundles, to function correctly in their use of thread context class loading.