CODEWITHSUNDEEP
hibernatespring-bootjpakotlinh2
Andre
Andre

Reputation: 364

StackOverflowError while using @ManyToMany

after trying around and googling for a similar issue for hours, I still could not fix the issue:

I am building a sprint boot application (in Kotlin) that uses JPA (so also hibernate) and H2 (don't think that it's relevant). I want to model a many-to-many relationship between classes User and Achievement (so a user can have multiple achievements and an achievement can be achieved by multiple users). Here are the model classes:

@Entity
data class User(

        @Id
        @GeneratedValue(strategy = GenerationType.AUTO)
        val id: Int = 0,

        @Column(nullable = false)
        val name: String,

        @ManyToMany(cascade = [ CascadeType.PERSIST, CascadeType.MERGE ])
        val achievements: MutableSet<Achievement> = HashSet()

)

@Entity
data class Achievement(

        @Id
        @GeneratedValue(strategy = GenerationType.AUTO)
        val id: Int = 0,

        val key: String,

        @ManyToMany(mappedBy = "achievements")
        val users: MutableSet<User> = HashSet()

)

This model seems to work fine as I see following logs:

Hibernate: create table achievement (id integer not null, key varchar(255), priority integer not null, category_id integer, primary key (id))
Hibernate: create table user (id integer not null, name varchar(255) not null, primary key (id))
Hibernate: create table user_achievements (users_id integer not null, achievements_id integer not null, primary key (users_id, achievements_id))

Then I prepopulate the data in a @Service @Transactional as follows:

val achievement = Achievement(key = "SOME_ACHIEVEMENT_KEY")
achievementRepository.save(achievement)

val user = User(name = "John Doe")
userRepository.save(user)

and add a relation:

val foundAchievement = achievementRepository.findById(achievementId)
val foundUser = userRepository.findById(userId)

foundAchievement.ifPresent { achievement ->
    foundUser.ifPresent { user ->
        user.achievements.add(achievement)
        userRepository.save(user)
    }
}

If I now try to access the data by executing GET on one of following URLs:

http://localhost:8080/achievements/2/users
http://localhost:8080/users/3/achievements

I get a java.lang.StackOverflowError and logs show me that hibernate is trying to query for achievements and users over and over again (endless loop until stack overflow).

So here a my questions:

  1. Do you have an idea why there is this error?
  2. Is my model correct?
  3. Is my way to connect the user to the achievement correct or do i need to add the user to achievement.users also (or even save the whole thing in the achievementsRepository)?

Upvotes: 2

Views: 1325

Answers (1)

Andre
Andre

Reputation: 364

Ok the issue was a combination of bidirectional relation and auto generated equals() and hashCode() by Kotlin.

Since I was using Kotlin data class, it generated hashCode() and equals() automatically for each property defined in the primary constructor. I took the @ManyToMany annotated properties out of the primary constructor the issue dissapeared:

@Entity
data class Achievement(

        @Id
        @GeneratedValue(strategy = GenerationType.AUTO)
        val id: Int = 0,

        val key: String

) {

    @ManyToMany(mappedBy = "achievements")
    val users: MutableSet<User> = HashSet()

}

@Entity
data class User(

        @Id
        @GeneratedValue(strategy = GenerationType.AUTO)
        val id: Int = 0,

        @Column(nullable = false)
        val name: String

) {

    @ManyToMany(cascade = [CascadeType.PERSIST, CascadeType.MERGE])
    val achievements: MutableSet<Achievement> = HashSet()

}

Upvotes: 6

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