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.
Đang xem: Unidirectional là gì, nghĩa của từ unidirectional
This is a Unidirectional association
public class User { private int id; private String name;
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;
JoinColumn( name = “groupId”) private Group group;}public class Group { private int id; private String name;
OneToMany(mappedBy=”group”) private List 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?
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 “one-to-very-many” and “many-to-very-many” relationships. Imagine a Group that contains thousands of Users:
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:
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edited Oct 10 “20 at 3:32
PerkOne
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answered Mar 19 “11 at 10:40
axtavtaxtavt
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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.
Xem thêm: Chùm Ngây Là Cây Gì ? Hình Ảnh, Tác Dụng, Cách Sử Dụng Chùm Cây Chùm Ngây Chữa Bệnh Gì
Performance
The second aspect is related to performance.
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edited Oct 13 “20 at 17:02
answered Feb 8 “18 at 9:00
Vlad MihalceaVlad Mihalcea
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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.
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answered Jul 26 “13 at 19:54
Constantino CronembergerConstantino Cronemberger
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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.
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answered Mar 19 “11 at 8:03
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