This abstract ReentrantContextProvider helper class manages the creation, storage, and retrieval of concrete ReentrantContext instances which can be subclassed to hold cached contextual data. It supports reentrancy as every call to acquire() provides a new unique context instance that must later be returned for reuse by a call to release(ctx) (typically in a try/finally block). It has a couple of abstract implementations which store references in a queue and/or thread-local storage. The Providers can be configured to hold ReentrantContext instances in memory using hard, soft or weak references. The acquire() and release() methods are used to retrieve and return the contexts. The newContext() method remains abstract in all implementations and must be provided by the module to create a new subclass of ReentrantContext with the appropriate contextual data in it. Sample Usage: - create a subclass ReentrantContextImpl to hold the thread state:

static final class ReentrantContextImpl extends ReentrantContext {
    // specific cached data
}

- create the appropriate ReentrantContextProvider:

private static final ReentrantContextProvider<ReentrantContextImpl> contextProvider =
    new ReentrantContextProviderTL<ReentrantContextImpl>(ReentrantContextProvider.REF_WEAK)
    {
        @Override
        protected ReentrantContextImpl newContext() {
            return new ReentrantContextImpl();
        }
    };
...
void someMethod() {
    ReentrantContextImpl ctx = contextProvider.acquire();
    try {
        // use the context
    } finally {
        contextProvider.release(ctx);
    }
}