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There are countless great books and articles, it's taught at universities, and easy to find both skilled labour and advanced consultants .
Fully open source, managed by the tought leaders in the Java community, decision making is open yet sponsored and legally backed by Red Hat.
Pro Advanced extension points
Hibernate ORM sports several advanced extensions such as Hibernate Search providing Lucene and Elasticsearch full-text search, Hibernate Spatial for GIS extensions, Envers for versioning and temporal queries, Hibernate OGM to use NoSQL as your primary datastore, advanced caching integrations and various connection pools.
Pro Actively maintained
Hibernate was very innovative from the beginning but is still very actively developed, improving significantly every release.
Pro High Performance
ORMs are typically not easy to scale yet the Hibernate team has been very sensitive to this topic, constantly optimising the code further and adding various advanced features to avoid paying a high cost for the convenience.
Pro Extensive integration points
Most other successful projects in the Java community integrate with Hibernate, making integration often a solved problem.