It is sometimes useful to index an entity even if this entity is not inserted or updated to the database. This is for example the case when you want to build your index for the first time. FullTextSession.index() allows you to do so.

FullTextSession fullTextSession = Search.getFullTextSession(session);
Transaction tx = fullTextSession.beginTransaction();
for (Customer customer : customers) {
    fullTextSession.index(customer);
}
tx.commit(); //index are written at commit time    

For maximum efficiency, Hibernate Search batches index operations and executes them at commit time. If you expect to index a lot of data, however, you need to be careful about memory consumption since all documents are kept in a queue until the transaction commit. You can potentially face an OutOfMemoryException. To avoid this exception, you can use fullTextSession.flushToIndexes(). Every time fullTextSession.flushToIndexes() is called (or if the transaction is committed), the batch queue is processed (freeing memory) applying all index changes. Be aware that once flushed changes cannot be rolled back.

Other parameters which also can affect indexing time and memory consumption are:

These parameters are Lucene specific and Hibernate Search is just passing these parameters through - see Section 3.8, “Tuning Lucene indexing performance” for more details.

fullTextSession.setFlushMode(FlushMode.MANUAL);
fullTextSession.setCacheMode(CacheMode.IGNORE);
transaction = fullTextSession.beginTransaction();
//Scrollable results will avoid loading too many objects in memory
ScrollableResults results = fullTextSession.createCriteria( Email.class )
    .setFetchSize(BATCH_SIZE)
    .scroll( ScrollMode.FORWARD_ONLY );
int index = 0;
while( results.next() ) {
    index++;
    fullTextSession.index( results.get(0) ); //index each element
    if (index % BATCH_SIZE == 0) {
        fullTextSession.flushToIndexes(); //apply changes to indexes
        fullTextSession.clear(); //clear since the queue is processed
    }
}
transaction.commit();

Try to use a batch size that guarantees that your application will not run out of memory.