Cannot invoke "java.lang.Object.hashCode()" because "value" is null - spring

Question

In spring app:
User.java:
#Entity
#Data
#NoArgsConstructor
public class User {
#Id #GeneratedValue
private Long id;
#Column(unique = true)
private String username;
private String about;
#OneToMany(mappedBy = "owner", cascade = CascadeType.ALL, fetch = FetchType.EAGER)
#MapKey(name = "friendId")
private Map<User, Friendship> friendships = new HashMap<>();
#OneToMany(mappedBy = "author", cascade = CascadeType.ALL, fetch = FetchType.EAGER)
#MapKey(name = "title")
private Map<String, Post> posts = new HashMap<>();
public User(String username) {
this.username = username;
}
public User addFriend(User friend){
Friendship friendship = new Friendship();
friendship.setOwner(this);
friendship.setFriend(friend);
friend.getFriendships().put(this, friendship);
getFriendships().put(friend, friendship);
return friend;
}
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (!(o instanceof User)) return false;
User user = (User) o;
return this.username.equals(user.getUsername());
}
#Override
public int hashCode() {
return this.username.hashCode();
}
#Override
public String toString() {
return "User{" +
"id=" + id +
", username='" + username + '\'' +
", about='" + about + '\'' +
", posts=" + posts +
'}';
}
}
Friendship.java:
#IdClass(Friendship.class)
#Entity
#Data
public class Friendship implements Serializable {
#Id #Column(name = "owner_id")
private Long ownerId;
#Id #Column(name = "friend_id")
private Long friendId;
#ManyToOne #MapsId("owner_id")
private User owner;
#ManyToOne #MapsId("friend_id")
private User friend;
private String level;
}
DemoApplication:
#Bean
public CommandLineRunner loadData(UserRepository userRepo){
return new CommandLineRunner() {
#Override
public void run(String... args) throws Exception {
User owner = new User("Barta");
User martin = owner.addFriend(new User("Martin"));
User milan = owner.addFriend(new User("Milan"));
userRepo.save(owner);
}
};
}
When run, the error is Cannot invoke "java.lang.Object.hashCode()" because "value" is null where I suppose the "value" is the username. But the username is set in the constructor (I can see it even in debugging), so why do I get NullPointerException?
PS:
The full error stack:
java.lang.IllegalStateException: Failed to execute CommandLineRunner
at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:794) ~[spring-boot-2.5.3.jar:2.5.3]
at org.springframework.boot.SpringApplication.callRunners(SpringApplication.java:775) ~[spring-boot-2.5.3.jar:2.5.3]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:345) ~[spring-boot-2.5.3.jar:2.5.3]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1343) ~[spring-boot-2.5.3.jar:2.5.3]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1332) ~[spring-boot-2.5.3.jar:2.5.3]
at com.example.demo.DemoApplication.main(DemoApplication.java:16) ~[classes/:na]
Caused by: java.lang.NullPointerException: Cannot invoke "java.lang.Object.hashCode()" because "value" is null
at org.hibernate.type.descriptor.java.AbstractTypeDescriptor.extractHashCode(AbstractTypeDescriptor.java:78) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.type.AbstractStandardBasicType.getHashCode(AbstractStandardBasicType.java:200) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.type.AbstractStandardBasicType.getHashCode(AbstractStandardBasicType.java:205) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.type.EntityType.getHashCode(EntityType.java:383) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.type.ComponentType.getHashCode(ComponentType.java:249) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.engine.spi.EntityKey.generateHashCode(EntityKey.java:61) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.engine.spi.EntityKey.<init>(EntityKey.java:54) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.internal.AbstractSharedSessionContract.generateEntityKey(AbstractSharedSessionContract.java:536) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.event.internal.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:172) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:135) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.event.internal.DefaultPersistEventListener.entityIsTransient(DefaultPersistEventListener.java:185) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:128) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.internal.SessionImpl$$Lambda$1219/0x00000000a219d8f8.applyEventToListener(Unknown Source) ~[na:na]
at org.hibernate.event.service.internal.EventListenerGroupImpl.fireEventOnEachListener(EventListenerGroupImpl.java:110) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.internal.SessionImpl.firePersist(SessionImpl.java:744) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.internal.SessionImpl.persist(SessionImpl.java:712) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.engine.spi.CascadingActions$7.cascade(CascadingActions.java:298) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.engine.internal.Cascade.cascadeToOne(Cascade.java:499) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.engine.internal.Cascade.cascadeAssociation(Cascade.java:423) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.engine.internal.Cascade.cascadeProperty(Cascade.java:220) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.engine.internal.Cascade.cascadeCollectionElements(Cascade.java:532) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.engine.internal.Cascade.cascadeCollection(Cascade.java:463) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.engine.internal.Cascade.cascadeAssociation(Cascade.java:426) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.engine.internal.Cascade.cascadeProperty(Cascade.java:220) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.engine.internal.Cascade.cascade(Cascade.java:153) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.event.internal.AbstractSaveEventListener.cascadeAfterSave(AbstractSaveEventListener.java:459) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.event.internal.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:293) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.event.internal.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:193) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:135) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.event.internal.DefaultPersistEventListener.entityIsTransient(DefaultPersistEventListener.java:185) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:128) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:55) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.internal.SessionImpl$$Lambda$1218/0x00000000a21505e0.accept(Unknown Source) ~[na:na]
at org.hibernate.event.service.internal.EventListenerGroupImpl.fireEventOnEachListener(EventListenerGroupImpl.java:99) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.internal.SessionImpl.firePersist(SessionImpl.java:720) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at org.hibernate.internal.SessionImpl.persist(SessionImpl.java:706) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:78) ~[na:na]
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
at java.base/java.lang.reflect.Method.invoke(Method.java:567) ~[na:na]
at org.springframework.orm.jpa.SharedEntityManagerCreator$SharedEntityManagerInvocationHandler.invoke(SharedEntityManagerCreator.java:311) ~[spring-orm-5.3.9.jar:5.3.9]
at jdk.proxy2/jdk.proxy2.$Proxy98.persist(Unknown Source) ~[na:na]
at org.springframework.data.jpa.repository.support.SimpleJpaRepository.save(SimpleJpaRepository.java:597) ~[spring-data-jpa-2.5.3.jar:2.5.3]
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:78) ~[na:na]
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
at java.base/java.lang.reflect.Method.invoke(Method.java:567) ~[na:na]
at org.springframework.data.repository.core.support.RepositoryMethodInvoker$RepositoryFragmentMethodInvoker.lambda$new$0(RepositoryMethodInvoker.java:289) ~[spring-data-commons-2.5.3.jar:2.5.3]
at org.springframework.data.repository.core.support.RepositoryMethodInvoker$RepositoryFragmentMethodInvoker$$Lambda$1217/0x00000000a214e200.invoke(Unknown Source) ~[na:na]
at org.springframework.data.repository.core.support.RepositoryMethodInvoker.doInvoke(RepositoryMethodInvoker.java:137) ~[spring-data-commons-2.5.3.jar:2.5.3]
at org.springframework.data.repository.core.support.RepositoryMethodInvoker.invoke(RepositoryMethodInvoker.java:121) ~[spring-data-commons-2.5.3.jar:2.5.3]
at org.springframework.data.repository.core.support.RepositoryComposition$RepositoryFragments.invoke(RepositoryComposition.java:529) ~[spring-data-commons-2.5.3.jar:2.5.3]
at org.springframework.data.repository.core.support.RepositoryComposition.invoke(RepositoryComposition.java:285) ~[spring-data-commons-2.5.3.jar:2.5.3]
at org.springframework.data.repository.core.support.RepositoryFactorySupport$ImplementationMethodExecutionInterceptor.invoke(RepositoryFactorySupport.java:599) ~[spring-data-commons-2.5.3.jar:2.5.3]
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186) ~[spring-aop-5.3.9.jar:5.3.9]
at org.springframework.data.repository.core.support.QueryExecutorMethodInterceptor.doInvoke(QueryExecutorMethodInterceptor.java:163) ~[spring-data-commons-2.5.3.jar:2.5.3]
at org.springframework.data.repository.core.support.QueryExecutorMethodInterceptor.invoke(QueryExecutorMethodInterceptor.java:138) ~[spring-data-commons-2.5.3.jar:2.5.3]
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186) ~[spring-aop-5.3.9.jar:5.3.9]
at org.springframework.data.projection.DefaultMethodInvokingMethodInterceptor.invoke(DefaultMethodInvokingMethodInterceptor.java:80) ~[spring-data-commons-2.5.3.jar:2.5.3]
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186) ~[spring-aop-5.3.9.jar:5.3.9]
at org.springframework.transaction.interceptor.TransactionInterceptor$1.proceedWithInvocation(TransactionInterceptor.java:123) ~[spring-tx-5.3.9.jar:5.3.9]
at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:388) ~[spring-tx-5.3.9.jar:5.3.9]
at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:119) ~[spring-tx-5.3.9.jar:5.3.9]
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186) ~[spring-aop-5.3.9.jar:5.3.9]
at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.invoke(PersistenceExceptionTranslationInterceptor.java:137) ~[spring-tx-5.3.9.jar:5.3.9]
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186) ~[spring-aop-5.3.9.jar:5.3.9]
at org.springframework.data.jpa.repository.support.CrudMethodMetadataPostProcessor$CrudMethodMetadataPopulatingMethodInterceptor.invoke(CrudMethodMetadataPostProcessor.java:174) ~[spring-data-jpa-2.5.3.jar:2.5.3]
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186) ~[spring-aop-5.3.9.jar:5.3.9]
at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:97) ~[spring-aop-5.3.9.jar:5.3.9]
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186) ~[spring-aop-5.3.9.jar:5.3.9]
at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:215) ~[spring-aop-5.3.9.jar:5.3.9]
at jdk.proxy2/jdk.proxy2.$Proxy103.save(Unknown Source) ~[na:na]
at com.example.demo.DemoApplication$1.run(DemoApplication.java:27) ~[classes/:na]
at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:791) ~[spring-boot-2.5.3.jar:2.5.3]
... 5 common frames omitted
2021-09-13 15:55:23.677 INFO 3218 --- [ main] j.LocalContainerEntityManagerFactoryBean : Closing JPA EntityManagerFactory for persistence unit 'default'
2021-09-13 15:55:23.682 INFO 3218 --- [ main] com.zaxxer.hikari.HikariDataSource : HikariPool-1 - Shutdown initiated...
2021-09-13 15:55:23.694 INFO 3218 --- [ main] com.zaxxer.hikari.HikariDataSource : HikariPool-1 - Shutdown completed.
Process finished with exit code
PSS:
the intellij is marking the
#MapKey(name = "friendId")
with error:
'java.lang.Long' cannot be assigned to 'com.example.demo.model.User'
As I am trying to understand -> the id is assign null when creating which is expecting. So the only way the id will be assign a value (an primary key) is in time of persisting (otherwise I have no idea, when will JPA assign values to primary keys, when #GenerateValue is used)

Answer 1

Most of the time I would use the id for equals and hashCode in an #Entity
#Override
public boolean equals(Object o)
{
if (this == o)
{
return true;
}
if (o == null || getClass() != o.getClass())
{
return false;
}
User user = (User) o;
return Objects.equals(id, disclaimer.id);
}
#Override
public int hashCode()
{
return Objects.hash(id);
}
If you really have reasons to have the name as key, I would suggest you to read something about #NaturalId and make the hashCode Method null save
#Override
public int hashCode()
{
return Objects.hash(name);
}

Answer 2

What happens if you try the following code?
User barta = new User("Barta");
User martin = new User("Martin");
User milan = new User("Milan");
userRepo.save(barta);
userRepo.save(martin);
userRepo.save(milan);
User martin = barta.addFriend(martin);
User milan = owner.addFriend(martin);
userRepo.save(martin);
userRepo.save(milan);
Can you tell which line exception is thrown eventually?

Answer 3

Looking at the stack trace it seems that the exception raises when processing on cascade. So probably it fails when getting Friendship hashcode instead of User. Friedship hashcode (generated by Project Lombok) is made with hashcode of all properties. One of these properties is the "owner" User. So maybe this cycle is the culprit.
I'd try to persist "martin" and "milan" before saving "owner".