static void addService(Object reference, Object service, int rank) {
        synchronized (SERVICES) {
            SERVICES.put(reference, new RankedService(service, rank));
        }
    }
Sets the context class loader to use for all threads that access
this class. Used for example in an OSGi environment to avoid problems
with the default context class loader.

@param loader default context class loader,
              or <code>null</code> to automatically pick the loader
tika-core/src/main/java/org/apache/tika/config/ServiceLoader.java