Imagine having a tool that can automatically detect if you are using JPA and Hibernate properly.
Hypersistence Optimizer is that tool!
Introduction
As simple as JPA annotations might be, it’s not always obvious how efficient they are behind the scenes. In this article, I’m going to show you what is the best way to use the JPA @ManyToMany annotation when using Hibernate.
Domain Model
Assuming we have the following database tables:
A typical many-to-many database association includes two parent tables which are linked through a third one containing two Foreign Keys referencing the parent tables.
Using java.util.List
The first choice for many Java developers is to use a java.util.List for Collections that don’t entail any specific ordering.
@Entity(name = "Post")
@Table(name = "post")
public class Post {
@Id
@GeneratedValue
private Long id;
private String title;
public Post() {}
public Post(String title) {
this.title = title;
}
@ManyToMany(cascade = {
CascadeType.PERSIST,
CascadeType.MERGE
})
@JoinTable(name = "post_tag",
joinColumns = @JoinColumn(name = "post_id"),
inverseJoinColumns = @JoinColumn(name = "tag_id")
)
private List<Tag> tags = new ArrayList<>();
//Getters and setters ommitted for brevity
public void addTag(Tag tag) {
tags.add(tag);
tag.getPosts().add(this);
}
public void removeTag(Tag tag) {
tags.remove(tag);
tag.getPosts().remove(this);
}
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (!(o instanceof Post)) return false;
return id != null && id.equals(((Post) o).getId());
}
@Override
public int hashCode() {
return 31;
}
}
@Entity(name = "Tag")
@Table(name = "tag")
public class Tag {
@Id
@GeneratedValue
private Long id;
@NaturalId
private String name;
@ManyToMany(mappedBy = "tags")
private List<Post> posts = new ArrayList<>();
public Tag() {}
public Tag(String name) {
this.name = name;
}
//Getters and setters ommitted for brevity
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Tag tag = (Tag) o;
return Objects.equals(name, tag.name);
}
@Override
public int hashCode() {
return Objects.hash(name);
}
}
There are several aspects to note on the aforementioned mapping that are worth explaining:
The tags association in the Post entity only defines the PERSIST and MERGE cascade types. As explained in this article, the REMOVEentity state transition doesn’t make any sense for a @ManyToMany JPA association since it could trigger a chain deletion that would ultimately wipe both sides of the association.
As explained in this article, the add/remove utility methods are mandatory if you use bidirectional associations so that you can make sure that both sides of the association are in sync.
The Post entity uses the entity identifier for equality since it lacks any unique business key. As explained in this article, you can use the entity identifier for equality as long as you make sure that it stays consistent across all entity state transitions.
The Tag entity has a unique business key which is marked with the Hibernate-specific @NaturalId annotation. When that’s the case, the unique business key is the best candidate for equality checks.
The mappedBy attribute of the posts association in the Tag entity marks that, in this bidirectional relationship, the Post entity owns the association. This is needed since only one side can own a relationship, and changes are only propagated to the database from this particular side.
For more details about the @NaturalId annotation, check out this article.
final Long postId = doInJPA(entityManager -> {
Post post1 = new Post("JPA with Hibernate");
Post post2 = new Post("Native Hibernate");
Tag tag1 = new Tag("Java");
Tag tag2 = new Tag("Hibernate");
post1.addTag(tag1);
post1.addTag(tag2);
post2.addTag(tag1);
entityManager.persist(post1);
entityManager.persist(post2);
return post1.id;
});
When removing a Tag entity from a Post:
doInJPA(entityManager -> {
Tag tag1 = new Tag("Java");
Post post1 = entityManager.find(Post.class, postId);
post1.removeTag(tag1);
});
Hibernate generates the following SQL statements:
SELECT p.id AS id1_0_0_,
t.id AS id1_2_1_,
p.title AS title2_0_0_,
t.name AS name2_2_1_,
pt.post_id AS post_id1_1_0__,
pt.tag_id AS tag_id2_1_0__
FROM post p
INNER JOIN
post_tag pt
ON p.id = pt.post_id
INNER JOIN
tag t
ON pt.tag_id = t.id
WHERE p.id = 1
DELETE FROM post_tag
WHERE post_id = 1
INSERT INTO post_tag
( post_id, tag_id )
VALUES ( 1, 3 )
So, instead of deleting just one post_tag entry, Hibernate removes all post_tag rows associated to the given post_id and reinserts the remaining ones back afterward. This is not efficient at all because it’s extra work for the database, especially for recreating indexes associated with the underlying Foreign Keys.
For this reason, it’s not a good idea to use the java.util.List for @ManyToMany JPA associations.
Using java.util.Set
Instead of a List, we can use a Set.
The Post entity tags association will be changed as follows:
And the Tag entity will undergo the same modification:
@ManyToMany(mappedBy = "tags")
private Set<Post> posts = new HashSet<>();
If you worry about the lack of a predefined entry order, then you can use either the @OrderBy or @OrderColumn JPA annotations.
@OrderBy does the sorting in-memory, after the entries are fetched from the database while @OrderColumn materializes the element order in a dedicated column that is stored in the post_tag link table.
Now, when rerunning the previous test case, Hibernate generates the following SQL statements:
SELECT p.id AS id1_0_0_,
t.id AS id1_2_1_,
p.title AS title2_0_0_,
t.name AS name2_2_1_,
pt.post_id AS post_id1_1_0__,
pt.tag_id AS tag_id2_1_0__
FROM post p
INNER JOIN
post_tag pt
ON p.id = pt.post_id
INNER JOIN
tag t
ON pt.tag_id = t.id
WHERE p.id = 1
DELETE FROM post_tag
WHERE post_id = 1 AND tag_id = 3
Much better! There is only one DELETE statement executed which removes the associated post_tag entry.
If you enjoyed this article, I bet you are going to love my Book and Video Courses as well.
Conclusion
Using JPA and Hibernate is very convenient since it can boost developer productivity. However, this does not mean that you have to sacrifice application performance.
By choosing the right mappings and data access pattern, you can make the difference between an application that barely crawls and one that runs at warp speed.
So, when using the @ManyToMany annotation, always use a java.util.Set and avoid the java.util.List.
15 Comments on “The best way to use the @ManyToMany annotation with JPA and Hibernate”
Hello Vlad,
Thank you for making this awesome blog. I’m learning a lot. I have a question. My project has a User who has a need for two sets. It’s a small app for rating beers. The user has a Set for BeersDrank and a Set for WishListedBeers. I’m new to Database management, but I’m thinking, these should be two Many-to-Many tables (between User and Beer classes). This would also imply, that I need two sets of AddBeer and RemoveBeer (add/remove BeerDrank and add/remove BeerWisher). Does this sound ok to you? Your input would be invaluable. Thank you in advance.
Hello Vlad! Great Article and thank you for your time.
I am using this as a reference while implementing Spring Security login with Roles. So my ManyToMany relationship is between users and roles. When registering a user I set the roles set using setter for it. So my code looks like, user.setRoles(roles). I pass a set with the USER_ROLE in it. The user is saved but it does not populate the users_roles table. I am unsure why. When I try to use the addRole() function I get a null error when it tries to update the user on the role side of the relationship, at role.getUsers().add(this). Any advice?
Probably you didn’t set cascading property or synchronized both sides of the association. Check out my High-Performance Java Persistence book for more details about getting the most out of JPA and Hibernate.
I was implementing your code (first one) using Hibernate alone, no JPA, but cascade = {CascadeType.PERSIST, CascadeType.MERGE} is not enough. I was getting java.lang.IllegalStateException: org.hibernate.TransientObjectException: object references an unsaved transient instance exception. I had to give CascaseType.ALL to get success.
I’m curious if unidirectional @ManyToMany relationship with Set on the owner side would perform nice like the bidirectional with Sets on both sides? So, with just a single delete which would remove the entity.
Ok, thanks! Definitely gonna go through your book to remove any doubts!
Hi Vlad,
I encountered an infinite recursion issue by following exactly your guide. Adding post1 containing tag1 is okay; adding post2 also containing tag1 infinite recursion appear: post2 entity contains tag1 entity, the tag1 entity contains post2 entity, so on and so on… finally it cause stackoverflow error. Any thoughts on it?
Based on my book, High-Performance Java Persistence, this workshop teaches you various data access performance optimizations from JDBC, to JPA, Hibernate and jOOQ for the major rational database systems (e.g. Oracle, SQL Server, MySQL and PostgreSQL).
Hello Vlad,
Thank you for making this awesome blog. I’m learning a lot. I have a question. My project has a User who has a need for two sets. It’s a small app for rating beers. The user has a Set for BeersDrank and a Set for WishListedBeers. I’m new to Database management, but I’m thinking, these should be two Many-to-Many tables (between User and Beer classes). This would also imply, that I need two sets of AddBeer and RemoveBeer (add/remove BeerDrank and add/remove BeerWisher). Does this sound ok to you? Your input would be invaluable. Thank you in advance.
Yes, sounds ok. Two many-to-many associations would do it. The synchronization methods are only needed for bidirectional associations.
Thank-You this helped me
Hello vladmihalcea, I have a issue, when I use the findby(attrib) the rows of the jointables manytomany are deleted
Are you interested in my consulting services?
Hello Vlad! Great Article and thank you for your time.
I am using this as a reference while implementing Spring Security login with Roles. So my ManyToMany relationship is between users and roles. When registering a user I set the roles set using setter for it. So my code looks like, user.setRoles(roles). I pass a set with the USER_ROLE in it. The user is saved but it does not populate the users_roles table. I am unsure why. When I try to use the addRole() function I get a null error when it tries to update the user on the role side of the relationship, at role.getUsers().add(this). Any advice?
Thank you again for your time,
Philip
Probably you didn’t set cascading property or synchronized both sides of the association. Check out my High-Performance Java Persistence book for more details about getting the most out of JPA and Hibernate.
I was implementing your code (first one) using Hibernate alone, no JPA, but cascade = {CascadeType.PERSIST, CascadeType.MERGE} is not enough. I was getting java.lang.IllegalStateException: org.hibernate.TransientObjectException: object references an unsaved transient instance exception. I had to give CascaseType.ALL to get success.
Sounds like a bug in Hibernate. You should open a Jira issue for it.
Hi Vlad,
I’m curious if unidirectional @ManyToMany relationship with Set on the owner side would perform nice like the bidirectional with Sets on both sides? So, with just a single delete which would remove the entity.
Yes, it should work just fine.
Ok, thanks! Definitely gonna go through your book to remove any doubts!
Hi Vlad,
I encountered an infinite recursion issue by following exactly your guide. Adding post1 containing tag1 is okay; adding post2 also containing tag1 infinite recursion appear: post2 entity contains tag1 entity, the tag1 entity contains post2 entity, so on and so on… finally it cause stackoverflow error. Any thoughts on it?
All the code is on GitHub, in my high-performance-java-persistence repository. Send me a Pull Request which proves the infinite recursion issue.
Thank you very much for your post, it helped me a lot.