在 spring 应用中:
用户.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 +
'}';
}
}
友谊.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;
}
演示应用程序:
@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);
}
};
}
运行时，错误是 Cannot invoke "java.lang.Object.hashCode()"because "value"is null 我想 "value"是 username。但是用户名是在构造函数中设置的(我什至在调试中也能看到)，为什么我会得到 NullPointerException？
附言:
完整的错误堆栈:
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
附言:
intellij 正在标记
@MapKey(name = "friendId")
有错误:
'java.lang.Long' cannot be assigned to 'com.example.demo.model.User'
正如我试图理解的那样 -> 在创建时将 id 分配为 null，这是期望的。因此，为 id 分配值(主键)的唯一方法是在持久化时(否则我不知道，当使用 @GenerateValue 时，JPA 何时将值分配给主键)
最佳答案
大多数时候我会在 @Entity 中使用 equals 和 hashCode 的 id
@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);
}
如果你真的有理由将名称作为键，我建议你阅读一些关于@NaturalId 的内容并将 hashCode 方法设为 null 保存
@Override
public int hashCode()
{
return Objects.hash(name);
}
关于spring - 无法调用 "java.lang.Object.hashCode()"，因为 "value"为空，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/69155143/