What is the difference between Unidirectional and Bidirectional JPA and Hibernate associations

Question

What is the difference between Unidirectional and Bidirectional associations   Since the table generated in the db are all the same so the only difference I found is that each side of the bidiretional assocations will have a refer to the other and the unidirectional not   This is a Unidirectional association  public class User       private int     id      private String  name       ManyToOne      JoinColumn              name    groupId       private Group   group     public class Group       private int     id      private String  name      The Bidirectional association  public class User       private int     id      private String  name       ManyToOne      JoinColumn              name    groupId       private Group   group    public class Group       private int         id      private String      name       OneToMany mappedBy  group       private List lt User gt   users      The difference is whether the group holds a reference of the user   So I wonder if this is the only difference  which is recommended

User · Answer

I m not 100  sure this is the only difference  but it is the main difference   It is also recommended to have bi-directional associations by the Hibernate docs   http   docs jboss org hibernate core 3 3 reference en html best-practices html  Specifically      Prefer bidirectional associations         Unidirectional associations are more difficult to query  In a large   application  almost all associations   must be navigable in both directions   in queries    I personally have a slight problem with this blanket recommendation -- it seems to me there are cases where a child doesn t have any practical reason to know about its parent  e g   why does an order item need to know about the order it is associated with    but I do see value in it a reasonable portion of the time as well   And since the bi-directionality doesn t really hurt anything  I don t find it too objectionable to adhere to

User · Answer

The main differenece is that bidirectional relationship provides navigational access in both directions  so that you can access the other side without explicit queries  Also it allows you to apply cascading options to both directions  Note that navigational access is not always good  especially for  quot one-to-very-many quot  and  quot many-to-very-many quot  relationships  Imagine a Group that contains thousands of Users   How would you access them  With so many Users  you usually need to apply some filtering and or pagination  so that you need to execute a query anyway  unless you use collection filtering  which looks like a hack for me   Some developers may tend to apply filtering in memory in such cases  which is obviously not good for performance  Note that having such a relationship can encourage this kind of developers to use it without considering performance implications   How would you add new Users to the Group  Fortunately  Hibernate looks at the owning side of relationship when persisting it  so you can only set User group  However  if you want to keep objects in memory consistent  you also need to add User to Group users  But it would make Hibernate to fetch all elements of Group users from the database    So  I can t agree with the recommendation from the Best Practices  You need to design bidirectional relationships carefully  considering use cases  do you need navigational access in both directions   and possible performance implications  See also   Deterring    ToMany    Relationships in JPA models Hibernate mapped collections performance problems

User · Answer

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   For  OneToMany  unidirectional associations don t perform as well as bidirectional ones  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  For  ManyToMany  the collection type makes quite a difference as Sets perform better than Lists

User · Answer

In terms of coding  a bidirectional relationship is more complex to implement because the application is responsible for keeping both sides in synch according to JPA specification 5  on page 42   Unfortunately the example given in the specification does not give more details  so it does not give an idea of the level of complexity   When not using a second level cache it is usually not a problem to do not have the relationship methods correctly implemented because the instances get discarded at the end of the transaction   When using second level cache  if anything gets corrupted because of wrongly implemented relationship handling methods  this means that other transactions will also see the corrupted elements  the second level cache is global    A correctly implemented bi-directional relationship can make queries and the code simpler  but should not be used if it does not really make sense in terms of business logic

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

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