NPE in Hibernate when executing nested GraphQL query with collection attributes

[igdianov]

For example:
query { Human(id:"1001") { favoriteDroid { name } appearsIn, friends { name friends { name } } } }

Generates the following NPE with the stack trace:

java.lang.NullPointerException: null
	at org.hibernate.engine.internal.StatefulPersistenceContext.getLoadedCollectionOwnerOrNull(StatefulPersistenceContext.java:786) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.event.spi.AbstractCollectionEvent.getLoadedOwnerOrNull(AbstractCollectionEvent.java:58) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.event.spi.InitializeCollectionEvent.<init>(InitializeCollectionEvent.java:22) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.internal.SessionImpl.initializeCollection(SessionImpl.java:1989) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.collection.internal.AbstractPersistentCollection$4.doWork(AbstractPersistentCollection.java:570) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:252) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.collection.internal.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:566) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.collection.internal.AbstractPersistentCollection.read(AbstractPersistentCollection.java:135) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.collection.internal.PersistentSet.hashCode(PersistentSet.java:430) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at com.introproventures.graphql.jpa.query.example.model.Character.hashCode(Character.java:43) ~[classes/:na]
	at com.introproventures.graphql.jpa.query.example.model.Human.hashCode(Human.java:29) ~[classes/:na]
	at java.util.HashMap.hash(HashMap.java:338) ~[na:1.8.0_40]
	at java.util.HashMap.put(HashMap.java:611) ~[na:1.8.0_40]
	at java.util.HashSet.add(HashSet.java:219) ~[na:1.8.0_40]
	at java.util.AbstractCollection.addAll(AbstractCollection.java:344) ~[na:1.8.0_40]
	at org.hibernate.collection.internal.PersistentSet.endRead(PersistentSet.java:327) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.engine.loading.internal.CollectionLoadContext.endLoadingCollection(CollectionLoadContext.java:234) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.engine.loading.internal.CollectionLoadContext.endLoadingCollections(CollectionLoadContext.java:221) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.engine.loading.internal.CollectionLoadContext.endLoadingCollections(CollectionLoadContext.java:194) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.loader.Loader.endCollectionLoad(Loader.java:1180) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.loader.Loader.initializeEntitiesAndCollections(Loader.java:1144) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.loader.Loader.processResultSet(Loader.java:992) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.loader.Loader.doQuery(Loader.java:930) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:336) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.loader.Loader.doList(Loader.java:2617) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.loader.Loader.doList(Loader.java:2600) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.loader.Loader.listIgnoreQueryCache(Loader.java:2429) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.loader.Loader.list(Loader.java:2424) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.loader.hql.QueryLoader.list(QueryLoader.java:501) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.hql.internal.ast.QueryTranslatorImpl.list(QueryTranslatorImpl.java:371) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.engine.query.spi.HQLQueryPlan.performList(HQLQueryPlan.java:216) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.internal.SessionImpl.list(SessionImpl.java:1326) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.internal.QueryImpl.list(QueryImpl.java:87) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.jpa.internal.QueryImpl.list(QueryImpl.java:606) ~[hibernate-entitymanager-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.jpa.internal.QueryImpl.getSingleResult(QueryImpl.java:529) ~[hibernate-entitymanager-5.0.12.Final.jar:5.0.12.Final]
	at org.hibernate.jpa.criteria.compile.CriteriaQueryTypeQueryAdapter.getSingleResult(CriteriaQueryTypeQueryAdapter.java:54) ~[hibernate-entitymanager-5.0.12.Final.jar:5.0.12.Final]
	at com.introproventures.graphql.jpa.query.schema.impl.GraphQLJpaSimpleDataFetcher.get(GraphQLJpaSimpleDataFetcher.java:46) ~[classes/:na]

[igdianov]

The test entity models use Project Lombok annotations for generating boiler plate Java code.

import lombok.Data;

@Entity
@GraphQLDescription("Abstract representation of an entity in the Star Wars Universe")
@Data
public abstract class Character {

    @Id
    @GraphQLDescription("Primary Key for the Character Class")
    String id;

    @GraphQLDescription("Name of the character")
    String name;

    @GraphQLDescription("Who are the known friends to this character")
    @ManyToMany
    @JoinTable(name="character_friends",
            joinColumns=@JoinColumn(name="source_id", referencedColumnName="id"),
            inverseJoinColumns=@JoinColumn(name="friend_id", referencedColumnName="id"))
    Set<Character> friends;

    @GraphQLDescription("What Star Wars episodes does this character appear in")
    @ElementCollection(targetClass = Episode.class)
    @Enumerated(EnumType.ORDINAL)
    Set<Episode> appearsIn;

}

The root cause of the NPE in Hibernate is hashCode() method in Character class used by Hibernate to initialize loaded collection associations. It is generated by Lombok's @EqualsAndHashCode annotation which is a part of @Data annotation in Character entity class. Replacing @Data with @Getter, @Setter and @EqualsAndHashCode(exclude={"friends","appearsIn"}) fixes the NPE

import lombok.Getter;
import lombok.Setter;
import lombok.ToString;

@Entity
@GraphQLDescription("Abstract representation of an entity in the Star Wars Universe")
@Getter
@Setter
@ToString
@EqualsAndHashCode(exclude={"appearsIn","friends"})
public abstract class Character {

    @Id
    @GraphQLDescription("Primary Key for the Character Class")
    String id;

    @GraphQLDescription("Name of the character")
    String name;

    @GraphQLDescription("Who are the known friends to this character")
    @ManyToMany
    @JoinTable(name="character_friends",
            joinColumns=@JoinColumn(name="source_id", referencedColumnName="id"),
            inverseJoinColumns=@JoinColumn(name="friend_id", referencedColumnName="id"))
    Set<Character> friends;

    @GraphQLDescription("What Star Wars episodes does this character appear in")
    @ElementCollection(targetClass = Episode.class)
    @Enumerated(EnumType.ORDINAL)
    Set<Episode> appearsIn;

}