The Best Way To Implement A Bidirectional @ManyToMany Association

Description: This application is a proof of concept of how it is correct to implement the bidirectional @ManyToMany association from the performance perspective.

Key points:

• choose an owning and a mappedBy side
• materialize the relationships collections via Set not List
• use helper methods on the owner of the relationship to keep both sides of the association in sync
• on the owner of the relationship use CascadeType.PERSIST and CascadeType.MERGE, but avoid CascadeType.REMOVE/ALL
• on the owner of the relationship set up join table
• @ManyToMany is lazy by default; keep it this way!
• as entities identifiers, use assigned identifiers (business key, natural key (@NaturalId)) and/or database-generated identifiers and override (on both sides) properly the equals() and hashCode() methods as here
• if toString() need to be overridden, then pay attention to involve only for the basic attributes fetched when the entity is loaded from the database

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