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如何使用Hibernate在实体内部映射实体集合



我正在开发一个分布式软件系统，第一次尝试使用Hibernate框架。我的一个类(它是一个映射实体(有一个ArrayList<>包含另一个类(也是实体(的对象。

例如：

我有一个Event类，它是一个@MappedSuperclass。

我有一个CorporateEvent类，它是一个实体，继承自Event。

我有一个Task类，它也是一个实体。

我的CorporateEvent类有一个名为"tasks"的属性，它是一个ArrayList。

我如何将其正确映射到数据库，以便有一个名为Event的表和一个称为Task的表，后者有一个键将它们相互连接？

我使用的是最新的Hibernate版本，数据库位于PostgreSQL服务器上。Hibernate中的方言设置为PostgreSQL。

我们已经尝试过使用@ElementCollection和@CollectionTable，但我们得到了一些奇怪的NullPointerException。这可能与代码的继承方面有关。

以下是有问题的3个类别：

事件类别：

@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;

公司事件类别：

@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;

我希望输出是将一个CorporateEvent对象保存到数据库中，并且其ArrayList中的Task对象都保存在Task表中，并使用一个将它们连接到正确事件的CorporateEvent ID。

相反，我在尝试使用Hibernate保存CorporateEvent对象的行上得到了一个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.

它抱怨的行在这个方法中是"session.close((；"：

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session.close((；我得到了以下注释错误，我不理解，因为应该支持ArrayList？：

(开头是一样的，所以我只是在中粘贴差异

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：现在它有点工作了。但一个新的问题出现了。正如您在下面看到的，hibernate只插入到CorporateEvent表中，而不是任务表中。

------------------------------------------------------------------------
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
------------------------------------------------------------------------

以下是我在您的帮助下所做的更改：

公司事件类别：

@OneToMany(cascade=CascadeType.ALL, mappedBy="corporateEvent")
private List<Task> tasks;

任务类别：

@ManyToOne
@JoinColumn(name="corporate_id")
private CorporateEvent corporateEvent;

代码运行成功，但不会将数据映射到数据库中的Task表。这是测试(主(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);

关于这个问题，数据库有以下表格：

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);

您的错误可能是由于您使用了@ElementCollection注释而没有使用@Embeddable注释的类。我会使用OneToMany注释实现。

您的公司事件类别：

@OneToMany(mappedBy="corporate_event")
private List<Task> tasks;

您的任务类别：

@ManyToOne
@JoinColumn(name="corporate_event_id")
private CorporateEvent corporateEvent;

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