An AggregatedServiceLoader that will detect services defined in the class path or in the module path.

This implementation only works when running Hibernate ORM on Java 9 and above.

When retrieving services from providers in the module path, the service loader internally uses a map from classloader to service catalog. Since the aggregated class loader is artificial and unknown from the service loader, it will never match any service from the module path.

To work around this problem, we try to get services from a service loader bound to the aggregated class loader first, then we try a service loader bound to each individual class loader.

This could result in duplicates, so we take specific care to avoid using the same service provider twice. See getAll().

Note that, in the worst case, the service retrieved from each individual class loader could load duplicate versions of classes already loaded from another class loader. For example in an aggregated class loader made up of individual class loader A, B, C: it is possible that class C1 was already loaded from A, but then we load service S1 from B, and this service will also need class C1 but won't find it in class loader B, so it will load its own version of that class.

We assume that this situation will never occur in practice because class loaders are structure in a hierarchy that prevents one class to be loaded twice.