The SearchFactory object keeps track of the underlying Lucene resources for Hibernate Search, it's also a convenient way to access Lucene natively. The SearchFactory can be accessed from a FullTextSession:

FullTextSession fullTextSession = Search.createFullTextSession(regularSession);
SearchFactory searchFactory = fullTextSession.getSearchFactory();

You can always access the Lucene directories through plain Lucene, the Directory structure is in no way different with or without Hibernate Search. However there are some more convenient ways to access a given Directory. The SearchFactory keeps track of the DirectoryProviders per indexed class. One directory provider can be shared amongst several indexed classes if the classes share the same underlying index directory. While usually not the case, a given entity can have several DirectoryProviders is the index is sharded (see Section 3.2, “Index sharding”).

DirectoryProvider[] provider = searchFactory.getDirectoryProviders(Order.class);
org.apache.lucene.store.Directory directory = provider[0].getDirectory();

In this example, directory points to the lucene index storing Orders information. Note that the obtained Lucene directory must not be closed (this is Hibernate Search responsibility).

Queries in Lucene are executed on an IndexReader. Hibernate Search caches such index readers to maximize performances. Your code can access such cached / shared resources. You will just have to follow some "good citizen" rules.

DirectoryProvider orderProvider = searchFactory.getDirectoryProviders(Order.class)[0];
DirectoryProvider clientProvider = searchFactory.getDirectoryProviders(Client.class)[0];

ReaderProvider readerProvider = searchFactory.getReaderProvider();
IndexReader reader = readerProvider.openReader(orderProvider, clientProvider);

try {
    //do read-only operations on the reader
}
finally {
    readerProvider.closeReader(reader);
}

The ReaderProvider (described in Reader strategy), will open an IndexReader on top of the index(es) referenced by the directory providers. This IndexReader being shared amongst several clients, you must adhere to the following rules:

Aside from those rules, you can use the IndexReader freely, especially to do native queries. Using the shared IndexReaders will make most queries more efficient.