Why do I get this recursion stackoverflow error?

Question

I am having two objects with two many to many relations to eachother. When I try to populate one of the pivot tables it returns a stackoverflow exception. What I tried was turning on FetchType to LAZY, and I even added @JsonIgnore to both properties. Still the problem keeps occurring...

Below I will put both of my models, the controller function and the exception. Thanks alot!

User model

@Data
@Entity
@Table(name = "users")
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;

@Column(name = "username")
private String username;

@Column(name = "first_name")
private String firstName;

@Column(name = "last_name")
private String lastName;

@Column(name = "password")
private String password;

@OneToMany(mappedBy = "user", cascade = CascadeType.REMOVE, fetch = FetchType.EAGER)
@JsonIgnore
private Set<Car> cars;

@ManyToMany(cascade = CascadeType.ALL)
@JoinTable
@JsonIgnore
private Set<Group> groups;

@ManyToMany(cascade = CascadeType.ALL)
@JoinTable
@JsonIgnore
private Set<Group> groupJoinRequests;}

Group model

@Data
@Entity
@Table(name = "groups")
public class Group {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;

@Column(name = "title")
private String title;

@Column(name = "description")
private String description;

@Column(name = "image_path")
private String imagePath;

@ManyToMany(mappedBy = "groups")
@JsonIgnore
private Set<User> members;

@ManyToMany(mappedBy = "groups")
@JsonIgnore
private Set<User> groupJoinRequests;}

Controller method

    @PostMapping("/api/groups/{groupId}/requests/{userId}/accept")
public ResponseEntity<List<User>> acceptJoinRequest(@PathVariable("groupId") long groupId, @PathVariable("groupId") long userId) {
    try {
        Optional<Group> groupData = groupRepository.findById(groupId);
        Optional<User> userData = userRepository.findById(groupId);

        if (groupData.isPresent() && userData.isPresent()) {
            Group group = groupData.get();
            User user = userData.get();


            group.getGroupJoinRequests().remove(user);
            group.getMembers().add(user);
            groupRepository.save(group);
            return new ResponseEntity<>(null, HttpStatus.OK);
        } else {
            return new ResponseEntity<>(HttpStatus.NOT_FOUND);
        }
    } catch (Exception e) {
        return new ResponseEntity<>(null, HttpStatus.INTERNAL_SERVER_ERROR);
    }
}

Exception thrown

java.lang.StackOverflowError: null
at com.mysql.cj.jdbc.ConnectionImpl.prepareStatement(ConnectionImpl.java:1575) ~[mysql-connector-java-8.0.19.jar:8.0.19]
at com.zaxxer.hikari.pool.ProxyConnection.prepareStatement(ProxyConnection.java:316) ~[HikariCP-3.4.2.jar:na]
at com.zaxxer.hikari.pool.HikariProxyConnection.prepareStatement(HikariProxyConnection.java) ~[HikariCP-3.4.2.jar:na]
at org.hibernate.engine.jdbc.internal.StatementPreparerImpl$5.doPrepare(StatementPreparerImpl.java:149) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.engine.jdbc.internal.StatementPreparerImpl$StatementPreparationTemplate.prepareStatement(StatementPreparerImpl.java:176) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.engine.jdbc.internal.StatementPreparerImpl.prepareQueryStatement(StatementPreparerImpl.java:151) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.internal.AbstractLoadPlanBasedLoader.prepareQueryStatement(AbstractLoadPlanBasedLoader.java:198) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.internal.AbstractLoadPlanBasedLoader.executeQueryStatement(AbstractLoadPlanBasedLoader.java:162) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.internal.AbstractLoadPlanBasedLoader.executeLoad(AbstractLoadPlanBasedLoader.java:104) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.collection.plan.AbstractLoadPlanBasedCollectionInitializer.initialize(AbstractLoadPlanBasedCollectionInitializer.java:87) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.persister.collection.AbstractCollectionPersister.initialize(AbstractCollectionPersister.java:710) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.event.internal.DefaultInitializeCollectionEventListener.onInitializeCollection(DefaultInitializeCollectionEventListener.java:76) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.event.service.internal.EventListenerGroupImpl.fireEventOnEachListener(EventListenerGroupImpl.java:102) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.internal.SessionImpl.initializeCollection(SessionImpl.java:2153) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.AbstractPersistentCollection$4.doWork(AbstractPersistentCollection.java:589) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:264) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:585) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.AbstractPersistentCollection.read(AbstractPersistentCollection.java:149) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.PersistentSet.hashCode(PersistentSet.java:458) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at com.bram.fun4.careventapp.models.Group.hashCode(Group.java:10) ~[classes/:na]
at java.base/java.util.HashMap.hash(HashMap.java:339) ~[na:na]
at java.base/java.util.HashMap.put(HashMap.java:607) ~[na:na]
at java.base/java.util.HashSet.add(HashSet.java:220) ~[na:na]
at java.base/java.util.AbstractCollection.addAll(AbstractCollection.java:352) ~[na:na]
at org.hibernate.collection.internal.PersistentSet.endRead(PersistentSet.java:355) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.engine.loading.internal.CollectionLoadContext.endLoadingCollection(CollectionLoadContext.java:239) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.engine.loading.internal.CollectionLoadContext.endLoadingCollections(CollectionLoadContext.java:224) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.engine.loading.internal.CollectionLoadContext.endLoadingCollections(CollectionLoadContext.java:198) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.process.internal.CollectionReferenceInitializerImpl.endLoading(CollectionReferenceInitializerImpl.java:154) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.process.internal.AbstractRowReader.finishLoadingCollections(AbstractRowReader.java:267) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.process.internal.AbstractRowReader.finishUp(AbstractRowReader.java:218) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.process.internal.ResultSetProcessorImpl.extractResults(ResultSetProcessorImpl.java:137) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.internal.AbstractLoadPlanBasedLoader.executeLoad(AbstractLoadPlanBasedLoader.java:105) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.collection.plan.AbstractLoadPlanBasedCollectionInitializer.initialize(AbstractLoadPlanBasedCollectionInitializer.java:87) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.persister.collection.AbstractCollectionPersister.initialize(AbstractCollectionPersister.java:710) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.event.internal.DefaultInitializeCollectionEventListener.onInitializeCollection(DefaultInitializeCollectionEventListener.java:76) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.event.service.internal.EventListenerGroupImpl.fireEventOnEachListener(EventListenerGroupImpl.java:102) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.internal.SessionImpl.initializeCollection(SessionImpl.java:2153) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.AbstractPersistentCollection$4.doWork(AbstractPersistentCollection.java:589) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:264) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:585) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.AbstractPersistentCollection.read(AbstractPersistentCollection.java:149) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.PersistentSet.hashCode(PersistentSet.java:458) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at com.bram.fun4.careventapp.models.User.hashCode(User.java:14) ~[classes/:na]
at java.base/java.util.HashMap.hash(HashMap.java:339) ~[na:na]
at java.base/java.util.HashMap.put(HashMap.java:607) ~[na:na]
at java.base/java.util.HashSet.add(HashSet.java:220) ~[na:na]
at java.base/java.util.AbstractCollection.addAll(AbstractCollection.java:352) ~[na:na]
at org.hibernate.collection.internal.PersistentSet.endRead(PersistentSet.java:355) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.engine.loading.internal.CollectionLoadContext.endLoadingCollection(CollectionLoadContext.java:239) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.engine.loading.internal.CollectionLoadContext.endLoadingCollections(CollectionLoadContext.java:224) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.engine.loading.internal.CollectionLoadContext.endLoadingCollections(CollectionLoadContext.java:198) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.process.internal.CollectionReferenceInitializerImpl.endLoading(CollectionReferenceInitializerImpl.java:154) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.process.internal.AbstractRowReader.finishLoadingCollections(AbstractRowReader.java:267) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.process.internal.AbstractRowReader.finishUp(AbstractRowReader.java:218) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.process.internal.ResultSetProcessorImpl.extractResults(ResultSetProcessorImpl.java:137) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.internal.AbstractLoadPlanBasedLoader.executeLoad(AbstractLoadPlanBasedLoader.java:105) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.collection.plan.AbstractLoadPlanBasedCollectionInitializer.initialize(AbstractLoadPlanBasedCollectionInitializer.java:87) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.persister.collection.AbstractCollectionPersister.initialize(AbstractCollectionPersister.java:710) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.event.internal.DefaultInitializeCollectionEventListener.onInitializeCollection(DefaultInitializeCollectionEventListener.java:76) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.event.service.internal.EventListenerGroupImpl.fireEventOnEachListener(EventListenerGroupImpl.java:102) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.internal.SessionImpl.initializeCollection(SessionImpl.java:2153) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.AbstractPersistentCollection$4.doWork(AbstractPersistentCollection.java:589) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:264) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:585) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.AbstractPersistentCollection.read(AbstractPersistentCollection.java:149) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.PersistentSet.hashCode(PersistentSet.java:458) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at com.bram.fun4.careventapp.models.Group.hashCode(Group.java:10) ~[classes/:na]
at java.base/java.util.HashMap.hash(HashMap.java:339) ~[na:na]
at java.base/java.util.HashMap.put(HashMap.java:607) ~[na:na]
at java.base/java.util.HashSet.add(HashSet.java:220) ~[na:na]
at java.base/java.util.AbstractCollection.addAll(AbstractCollection.java:352) ~[na:na]
at org.hibernate.collection.internal.PersistentSet.endRead(PersistentSet.java:355) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.engine.loading.internal.CollectionLoadContext.endLoadingCollection(CollectionLoadContext.java:239) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.engine.loading.internal.CollectionLoadContext.endLoadingCollections(CollectionLoadContext.java:224) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.engine.loading.internal.CollectionLoadContext.endLoadingCollections(CollectionLoadContext.java:198) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.process.internal.CollectionReferenceInitializerImpl.endLoading(CollectionReferenceInitializerImpl.java:154) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.process.internal.AbstractRowReader.finishLoadingCollections(AbstractRowReader.java:267) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.process.internal.AbstractRowReader.finishUp(AbstractRowReader.java:218) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.process.internal.ResultSetProcessorImpl.extractResults(ResultSetProcessorImpl.java:137) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.internal.AbstractLoadPlanBasedLoader.executeLoad(AbstractLoadPlanBasedLoader.java:105) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.collection.plan.AbstractLoadPlanBasedCollectionInitializer.initialize(AbstractLoadPlanBasedCollectionInitializer.java:87) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.persister.collection.AbstractCollectionPersister.initialize(AbstractCollectionPersister.java:710) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.event.internal.DefaultInitializeCollectionEventListener.onInitializeCollection(DefaultInitializeCollectionEventListener.java:76) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.event.service.internal.EventListenerGroupImpl.fireEventOnEachListener(EventListenerGroupImpl.java:102) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.internal.SessionImpl.initializeCollection(SessionImpl.java:2153) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.AbstractPersistentCollection$4.doWork(AbstractPersistentCollection.java:589) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:264) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:585) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.AbstractPersistentCollection.read(AbstractPersistentCollection.java:149) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.PersistentSet.hashCode(PersistentSet.java:458) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at com.bram.fun4.careventapp.models.User.hashCode(User.java:14) ~[classes/:na]
at java.base/java.util.HashMap.hash(HashMap.java:339) ~[na:na]
at java.base/java.util.HashMap.put(HashMap.java:607) ~[na:na]
at java.base/java.util.HashSet.add(HashSet.java:220) ~[na:na]
at java.base/java.util.AbstractCollection.addAll(AbstractCollection.java:352) ~[na:na]
at org.hibernate.collection.internal.PersistentSet.endRead(PersistentSet.java:355) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.engine.loading.internal.CollectionLoadContext.endLoadingCollection(CollectionLoadContext.java:239) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.engine.loading.internal.CollectionLoadContext.endLoadingCollections(CollectionLoadContext.java:224) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.engine.loading.internal.CollectionLoadContext.endLoadingCollections(CollectionLoadContext.java:198) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.process.internal.CollectionReferenceInitializerImpl.endLoading(CollectionReferenceInitializerImpl.java:154) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.process.internal.AbstractRowReader.finishLoadingCollections(AbstractRowReader.java:267) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.process.internal.AbstractRowReader.finishUp(AbstractRowReader.java:218) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.process.internal.ResultSetProcessorImpl.extractResults(ResultSetProcessorImpl.java:137) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.internal.AbstractLoadPlanBasedLoader.executeLoad(AbstractLoadPlanBasedLoader.java:105) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.collection.plan.AbstractLoadPlanBasedCollectionInitializer.initialize(AbstractLoadPlanBasedCollectionInitializer.java:87) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.persister.collection.AbstractCollectionPersister.initialize(AbstractCollectionPersister.java:710) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.event.internal.DefaultInitializeCollectionEventListener.onInitializeCollection(DefaultInitializeCollectionEventListener.java:76) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.event.service.internal.EventListenerGroupImpl.fireEventOnEachListener(EventListenerGroupImpl.java:102) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.internal.SessionImpl.initializeCollection(SessionImpl.java:2153) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.AbstractPersistentCollection$4.doWork(AbstractPersistentCollection.java:589) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:264) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:585) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.AbstractPersistentCollection.read(AbstractPersistentCollection.java:149) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.PersistentSet.hashCode(PersistentSet.java:458) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at com.bram.fun4.careventapp.models.Group.hashCode(Group.java:10) ~[classes/:na]
at java.base/java.util.HashMap.hash(HashMap.java:339) ~[na:na]
at java.base/java.util.HashMap.put(HashMap.java:607) ~[na:na]
at java.base/java.util.HashSet.add(HashSet.java:220) ~[na:na]
at java.base/java.util.AbstractCollection.addAll(AbstractCollection.java:352) ~[na:na]
at org.hibernate.collection.internal.PersistentSet.endRead(PersistentSet.java:355) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.engine.loading.internal.CollectionLoadContext.endLoadingCollection(CollectionLoadContext.java:239) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.engine.loading.internal.CollectionLoadContext.endLoadingCollections(CollectionLoadContext.java:224) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.engine.loading.internal.CollectionLoadContext.endLoadingCollections(CollectionLoadContext.java:198) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.process.internal.CollectionReferenceInitializerImpl.endLoading(CollectionReferenceInitializerImpl.java:154) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.process.internal.AbstractRowReader.finishLoadingCollections(AbstractRowReader.java:267) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.process.internal.AbstractRowReader.finishUp(AbstractRowReader.java:218) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.process.internal.ResultSetProcessorImpl.extractResults(ResultSetProcessorImpl.java:137) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.internal.AbstractLoadPlanBasedLoader.executeLoad(AbstractLoadPlanBasedLoader.java:105) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.collection.plan.AbstractLoadPlanBasedCollectionInitializer.initialize(AbstractLoadPlanBasedCollectionInitializer.java:87) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.persister.collection.AbstractCollectionPersister.initialize(AbstractCollectionPersister.java:710) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.event.internal.DefaultInitializeCollectionEventListener.onInitializeCollection(DefaultInitializeCollectionEventListener.java:76) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.event.service.internal.EventListenerGroupImpl.fireEventOnEachListener(EventListenerGroupImpl.java:102) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.internal.SessionImpl.initializeCollection(SessionImpl.java:2153) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.AbstractPersistentCollection$4.doWork(AbstractPersistentCollection.java:589) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:264) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:585) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.AbstractPersistentCollection.read(AbstractPersistentCollection.java:149) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.collection.internal.PersistentSet.hashCode(PersistentSet.java:458) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at com.bram.fun4.careventapp.models.User.hashCode(User.java:14) ~[classes/:na]
at java.base/java.util.HashMap.hash(HashMap.java:339) ~[na:na]
at java.base/java.util.HashMap.put(HashMap.java:607) ~[na:na]
at java.base/java.util.HashSet.add(HashSet.java:220) ~[na:na]
at java.base/java.util.AbstractCollection.addAll(AbstractCollection.java:352) ~[na:na]
at org.hibernate.collection.internal.PersistentSet.endRead(PersistentSet.java:355) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.engine.loading.internal.CollectionLoadContext.endLoadingCollection(CollectionLoadContext.java:239) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.engine.loading.internal.CollectionLoadContext.endLoadingCollections(CollectionLoadContext.java:224) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.engine.loading.internal.CollectionLoadContext.endLoadingCollections(CollectionLoadContext.java:198) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.process.internal.CollectionReferenceInitializerImpl.endLoading(CollectionReferenceInitializerImpl.java:154) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.process.internal.AbstractRowReader.finishLoadingCollections(AbstractRowReader.java:267) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.process.internal.AbstractRowReader.finishUp(AbstractRowReader.java:218) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.process.internal.ResultSetProcessorImpl.extractResults(ResultSetProcessorImpl.java:137) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.plan.exec.internal.AbstractLoadPlanBasedLoader.executeLoad(AbstractLoadPlanBasedLoader.java:105) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.loader.collection.plan.AbstractLoadPlanBasedCollectionInitializer.initialize(AbstractLoadPlanBasedCollectionInitializer.java:87) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.persister.collection.AbstractCollectionPersister.initialize(AbstractCollectionPersister.java:710) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]
at org.hibernate.event.internal.DefaultInitializeCollectionEventListener.onInitializeCollection(DefaultInitializeCollectionEventListener.java:76) ~[hibernate-core-5.4.12.Final.jar:5.4.12.Final]

Database diagram

Answer 1

@Data
@Entity
@Table(name = "users")
public class User {
  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  private long id;
  // .... instance variables


  @ManyToMany(mappedBy="members",fetch=FetchType.LAZY)
  private Set<Group> groups;

  @ManyToMany(mappedBy="groupJoinRequests",fetch=FetchType.LAZY)
  private Set<Group> groupJoinRequests;
  @Override 
  public int hashCode() { 
  return Objects.hashCode(id); } 
  @Override 
  public boolean equals(Object obj) { if (this == obj) { return true; } 
  if (obj == null || getClass() != obj.getClass()) { return false; } 
  Long objId = ((User) obj).id; 
  return objId==null? false: Objects.equals(id, objId); }
  }

Group

    @Data
    @Entity
    @Table(name = "groups")
    public class Group {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;

    // ... instance variables
    @ManyToMany(cascade = CascadeType.ALL)
    @JsonIgnore
    @JoinTable(name = "users_group", 
    joinColumns = { @JoinColumn(name = "groups_id") }, 
    inverseJoinColumns = { @JoinColumn(name = "members_id") })
    private Set<User> members;

    @ManyToMany(cascade = CascadeType.ALL)
    @JsonIgnore
    @JoinTable(name = "users_group_join_requests", 
    joinColumns = { @JoinColumn(name = "group_join_requests_id") }, 
    inverseJoinColumns = { @JoinColumn(name = "user_id") })
    private Set<User> groupJoinRequests;

  @Override 
  public int hashCode() { 
  return Objects.hashCode(id); } 
  @Override 
  public boolean equals(Object obj) { if (this == obj) { return true; } 
  if (obj == null || getClass() != obj.getClass()) { return false; } 
  Long objId = ((Group) obj).id; 
  return objId==null? false: Objects.equals(id, objId); }
    }