我正在开发分布式软件系统，并且试图首次使用Hibernate框架。我的一个类（它是一个映射的实体）有一个ArrayList <>，其中包含另一个类（也是一个实体）的对象。
例如：
我有一个Event类，它是一个@MappedSuperclass。
我有一个CorporateEvent类，它是一个继承自Event的实体。
我有一个Task类，它也是一个实体。
我的CorporateEvent类具有一个名为“任务”的属性，它是一个ArrayList。
如何将其正确映射到数据库，以便有一个名为“事件”的表和一个名为“任务”的表，该表具有将它们彼此连接的键？
我正在使用最新的Hibernate版本，并且数据库位于PostgreSQL服务器上。 Hibernate中的方言设置为PostgreSQL。
我们尝试使用@ElementCollection和@CollectionTable，但是我们遇到了一些奇怪的NullPointerExceptions。它可能与代码的继承方面有关。
这里是有问题的3个课程：
事件类：
@MappedSuperclass
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public abstract class Event {
@Column(name="title")
private String title;
@Column(name="location")
private String location;
@Column(name="date")
private String date;
CorporateEvent类：
@Entity
@Table(name="corporate_event")
public class CorporateEvent extends Event {
@Id
@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "id_generator")
@SequenceGenerator(name = "id_generator", sequenceName = "corporate_event_id_seq", allocationSize = 1)
@Column(name = "id", updatable = false, nullable = false)
private int id;
@ElementCollection
private ArrayList<Task> tasks;
@Column(name="expenses")
private String expenses;
任务类别：
@Entity
@Table(name="task")
public class Task {
@Id
@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "id_generator")
@SequenceGenerator(name="id_generator", sequenceName = "task_task_id_seq", allocationSize=1)
@Column(name="task_id", updatable = false, nullable = false)
private int id;
@Column(name="name")
private String taskName;
@Column(name="description")
private String description;
我希望输出的结果是CorporateEvent对象已保存到数据库中，并且其ArrayList中的Task对象每个都使用将它们连接到正确事件的CorporateEvent ID保存在Task表中。
我得到的是在尝试使用Hibernate保存CorporateEvent对象的那一行的NullPointerException。这是错误消息：
------------------------------------------------------------------------
Building Eventer 1.0-SNAPSHOT
------------------------------------------------------------------------
--- maven-resources-plugin:2.5:resources (default-resources) @ Eventer ---
[debug] execute contextualize
Using 'UTF-8' encoding to copy filtered resources.
Copying 11 resources
--- maven-compiler-plugin:3.6.1:compile (default-compile) @ Eventer ---
Nothing to compile - all classes are up to date
--- exec-maven-plugin:1.2.1:exec (default-cli) @ Eventer ---
Nov 08, 2019 5:31:30 PM org.hibernate.Version logVersion
INFO: HHH000412: Hibernate Core {5.4.8.Final}
Nov 08, 2019 5:31:30 PM org.hibernate.annotations.common.reflection.java.JavaReflectionManager <clinit>
INFO: HCANN000001: Hibernate Commons Annotations {5.1.0.Final}
Nov 08, 2019 5:31:31 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl configure
WARN: HHH10001002: Using Hibernate built-in connection pool (not for production use!)
Nov 08, 2019 5:31:31 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001005: using driver [org.postgresql.Driver] at URL [jdbc:postgresql://tek-mmmi-db0a.tek.c.sdu.dk:5432/si3_2019_group_5_db]
Nov 08, 2019 5:31:31 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001001: Connection properties: {user=si3_2019_group_5, password=****}
Nov 08, 2019 5:31:31 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001003: Autocommit mode: false
Nov 08, 2019 5:31:31 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl$PooledConnections <init>
INFO: HHH000115: Hibernate connection pool size: 1 (min=1)
Nov 08, 2019 5:31:31 PM org.hibernate.dialect.Dialect <init>
INFO: HHH000400: Using dialect: org.hibernate.dialect.PostgreSQLDialect
Nov 08, 2019 5:31:35 PM org.hibernate.cfg.AnnotationBinder bindClass
WARN: HHH000503: A class should not be annotated with both @Inheritance and @MappedSuperclass. @Inheritance will be ignored for: com.mycompany.domain.event.Event.
Nov 08, 2019 5:31:35 PM org.hibernate.boot.internal.InFlightMetadataCollectorImpl addIdentifierGenerator
WARN: HHH000069: Duplicate generator name id_generator
Exception in thread "main" java.lang.NullPointerException
at com.mycompany.repositories.EventRepository.saveCorporateEvent(EventRepository.java:51)
at com.mycompany.eventer.Eventer.main(Eventer.java:42)
------------------------------------------------------------------------
BUILD FAILURE
------------------------------------------------------------------------
Total time: 7.007s
Finished at: Fri Nov 08 17:31:35 CET 2019
Final Memory: 9M/100M
------------------------------------------------------------------------
Failed to execute goal org.codehaus.mojo:exec-maven-plugin:1.2.1:exec (default-cli) on project Eventer: Command execution failed. Process exited with an error: 1 (Exit value: 1) -> [Help 1]
To see the full stack trace of the errors, re-run Maven with the -e switch.
Re-run Maven using the -X switch to enable full debug logging.
它抱怨的行是“ session.close（）;”。在这种方法中：
public int saveCorporateEvent(CorporateEvent corporateEvent) {
try {
session = ConnectRepository.factory.getCurrentSession();
session.beginTransaction();
session.save(corporateEvent);
session.getTransaction().commit();
return corporateEvent.getId();
} catch (Exception e) {
e.printStackTrace();
} finally {
session.close();
}
return 0;
}
如果我删除了session.close（）;我收到以下我不理解的注释错误，因为应该支持ArrayList？
（开始是一样的，所以我只粘贴其中的区别）
Exception in thread "main" java.lang.ExceptionInInitializerError
at com.mycompany.repositories.EventRepository.saveCorporateEvent(EventRepository.java:42)
at com.mycompany.eventer.Eventer.main(Eventer.java:42)
Caused by: org.hibernate.AnnotationException: java.util.ArrayList collection type not supported for property: com.mycompany.domain.event.CorporateEvent.tasks
at org.hibernate.cfg.annotations.CollectionBinder.getCollectionBinder(CollectionBinder.java:317)
at org.hibernate.cfg.AnnotationBinder.processElementAnnotations(AnnotationBinder.java:1939)
at org.hibernate.cfg.AnnotationBinder.processIdPropertiesIfNotAlready(AnnotationBinder.java:975)
at org.hibernate.cfg.AnnotationBinder.bindClass(AnnotationBinder.java:802)
at org.hibernate.boot.model.source.internal.annotations.AnnotationMetadataSourceProcessorImpl.processEntityHierarchies(AnnotationMetadataSourceProcessorImpl.java:254)
at org.hibernate.boot.model.process.spi.MetadataBuildingProcess$1.processEntityHierarchies(MetadataBuildingProcess.java:230)
at org.hibernate.boot.model.process.spi.MetadataBuildingProcess.complete(MetadataBuildingProcess.java:273)
at org.hibernate.boot.model.process.spi.MetadataBuildingProcess.build(MetadataBuildingProcess.java:83)
at org.hibernate.boot.internal.MetadataBuilderImpl.build(MetadataBuilderImpl.java:473)
at org.hibernate.boot.internal.MetadataBuilderImpl.build(MetadataBuilderImpl.java:84)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:689)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:724)
at com.mycompany.repositories.ConnectRepository.<clinit>(ConnectRepository.java:27)
... 2 more
------------------------------------------------------------------------
BUILD FAILURE
------------------------------------------------------------------------
Total time: 11.877s
Finished at: Fri Nov 08 17:44:27 CET 2019
Final Memory: 19M/208M
------------------------------------------------------------------------
Failed to execute goal org.codehaus.mojo:exec-maven-plugin:1.2.1:exec (default-cli) on project Eventer: Command execution failed. Process exited with an error: 1 (Exit value: 1) -> [Help 1]
To see the full stack trace of the errors, re-run Maven with the -e switch.
Re-run Maven using the -X switch to enable full debug logging.
更新： 现在有点奏效了。但是出现了一个新问题。如下所示，休眠仅插入CorporateEvent表中，而不插入任务表中。
------------------------------------------------------------------------
Building Eventer 1.0-SNAPSHOT
------------------------------------------------------------------------
--- maven-resources-plugin:2.5:resources (default-resources) @ Eventer ---
[debug] execute contextualize
Using 'UTF-8' encoding to copy filtered resources.
Copying 11 resources
--- maven-compiler-plugin:3.6.1:compile (default-compile) @ Eventer ---
Nothing to compile - all classes are up to date
--- exec-maven-plugin:1.2.1:exec (default-cli) @ Eventer ---
Nov 10, 2019 12:15:21 PM org.hibernate.Version logVersion
INFO: HHH000412: Hibernate Core {5.4.8.Final}
Nov 10, 2019 12:15:21 PM org.hibernate.annotations.common.reflection.java.JavaReflectionManager <clinit>
INFO: HCANN000001: Hibernate Commons Annotations {5.1.0.Final}
Nov 10, 2019 12:15:22 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl configure
WARN: HHH10001002: Using Hibernate built-in connection pool (not for production use!)
Nov 10, 2019 12:15:22 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001005: using driver [org.postgresql.Driver] at URL [jdbc:postgresql://tek-mmmi-db0a.tek.c.sdu.dk:5432/si3_2019_group_5_db]
Nov 10, 2019 12:15:22 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001001: Connection properties: {user=si3_2019_group_5, password=****}
Nov 10, 2019 12:15:22 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001003: Autocommit mode: false
Nov 10, 2019 12:15:22 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl$PooledConnections <init>
INFO: HHH000115: Hibernate connection pool size: 1 (min=1)
Nov 10, 2019 12:15:22 PM org.hibernate.dialect.Dialect <init>
INFO: HHH000400: Using dialect: org.hibernate.dialect.PostgreSQLDialect
Nov 10, 2019 12:15:26 PM org.hibernate.cfg.AnnotationBinder bindClass
WARN: HHH000503: A class should not be annotated with both @Inheritance and @MappedSuperclass. @Inheritance will be ignored for: com.mycompany.domain.event.Event.
Nov 10, 2019 12:15:26 PM org.hibernate.boot.internal.InFlightMetadataCollectorImpl addIdentifierGenerator
WARN: HHH000069: Duplicate generator name id_generator
Nov 10, 2019 12:15:26 PM org.hibernate.boot.internal.InFlightMetadataCollectorImpl addIdentifierGenerator
WARN: HHH000069: Duplicate generator name id_generator
Hibernate: select nextval ('corporate_event_id_seq')
Hibernate: insert into corporate_event (date, description, location, max_participants, title, expenses, corporate_id) values (?, ?, ?, ?, ?, ?, ?)
------------------------------------------------------------------------
BUILD SUCCESS
------------------------------------------------------------------------
Total time: 9.780s
Finished at: Sun Nov 10 12:15:28 CET 2019
Final Memory: 9M/100M
------------------------------------------------------------------------
这些是我在您的帮助下所做的更改：
CorporateEvent类：
@OneToMany(cascade=CascadeType.ALL, mappedBy="corporateEvent")
private List<Task> tasks;
任务类：
@ManyToOne
@JoinColumn(name="corporate_id")
private CorporateEvent corporateEvent;
代码成功运行，但不会将数据映射到数据库中的Task表。这是测试（主要）代码：
public static void main(String[] args) {
ArrayList<Task> arrTasks = new ArrayList<>();
arrTasks.add(new Task("task1", "taskDes1"));
arrTasks.add(new Task("task2", "taskDes2"));
arrTasks.add(new Task("task3", "taskDes3"));
User user = new User("alex", "tholle", "sdu", "software engineering", "at@gmail.com", 70111213, "alexuser", "alexpass", new Date(), true, "student", "e2R213");
Meetup meetup = new Meetup(user.getUserName(), "meetup at the pub", "old irish", new Date().toString(), "det bliver fed", 10);
Task task = new Task("Cleaning", "Do some cleaing please");
CorporateEvent corporateEvent = new CorporateEvent(null, arrTasks, "Tinderbox", "forest", new Date().toString(), "edm music festival", 5000);
for (Task t : arrTasks) {
t.setCorporateEvent(corporateEvent);
}
EventRepository eventRepo = new EventRepository();
eventRepo.saveCorporateEvent(corporateEvent);
数据库具有有关此问题的下表：
create table task
(
task_id serial not null
constraint task_pk
primary key,
name varchar,
description varchar(1000),
corporate_id integer
constraint corporate_id
references corporate_event
on update cascade on delete cascade
);
alter table task
owner to si3_2019_group_5;
create unique index task_task_id_uindex
on task (task_id);
create table corporate_event
(
title varchar,
date varchar(1000),
location varchar,
description varchar,
max_participants integer,
expenses varchar(2000),
corporate_id serial not null
constraint corporate_event_pk
primary key,
tasks varchar(4000)
);
alter table corporate_event
owner to si3_2019_group_5;
create unique index corporate_event_id_uindex
on corporate_event (corporate_id);
答案 0 :(得分：2)
您的错误很可能是由于您使用@ElementCollection批注而没有使用@Embeddable批注的类。我会改用OneToMany注释实现。
您的CorporateEvent类：
@OneToMany(mappedBy="corporate_event")
private List<Task> tasks;
您的任务班：
@ManyToOne
@JoinColumn(name="corporate_event_id")
private CorporateEvent corporateEvent;