[java] What is the difference between Unidirectional and Bidirectional JPA and Hibernate associations?

There are two main differences.

Accessing the association sides

The first one is related to how you will access the relationship. For a unidirectional association, you can navigate the association from one end only.

So, for a unidirectional @ManyToOne association, it means you can only access the relationship from the child side where the foreign key resides.

If you have a unidirectional @OneToMany association, it means you can only access the relationship from the parent side which manages the foreign key.

For the bidirectional @OneToMany association, you can navigate the association in both ways, either from the parent or from the child side.

You also need to use add/remove utility methods for bidirectional associations to make sure that both sides are properly synchronized.

Performance

The second aspect is related to performance.

  1. For @OneToMany, unidirectional associations don't perform as well as bidirectional ones.
  2. For @OneToOne, a bidirectional association will cause the parent to be fetched eagerly if Hibernate cannot tell whether the Proxy should be assigned or a null value.
  3. For @ManyToMany, the collection type makes quite a difference as Sets perform better than Lists.

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