如何使用 Hibernate 在实体内部映射实体集合？

Question

我正在开发一个分布式软件系统，并且是第一次尝试使用 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（它也是一个实体）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。

我有一个任务类，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。

如何将其正确映射到数据库，以便有一个名为 Event 的表和一个名为 Task 的表，其中有一个将它们相互连接的键？

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。

我们已经尝试使用@ElementCollection 和@CollectionTable，但是我们得到了一些奇怪的 NullPointerExceptions。它可能与代码的继承方面有关。

这是有问题的 3 个类：

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;

企业事件类：

@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：

@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 CorporateEvent 对象被保存到数据库中，并且其 ArrayList 中的 Task 对象每个都保存在 Task 表中，并带有将它们连接到正确事件的 CorporateEvent ID。

我在尝试使用 Hibernate 保存 CorporateEvent 对象的行上得到的是 NullPointerException。这是错误信息：

------------------------------------------------------------------------
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.

它抱怨的那一行是“session.close();”在这个方法中：

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 session.close();我收到以下注释错误，我不明白，因为应该支持 ArrayList？：

（开头是一样的，所以我只是把不同的地方贴进去）

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。但是新的问题出现了。如下所示，hibernate 只插入 CorporateEvent 表，而不是任务表。

------------------------------------------------------------------------
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：

企业事件类：

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

任务类：

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

代码运行成功，但它不会将数据映射到我数据库中的任务表。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);

数据库有关于这个问题的以下表格：

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);

Answer 1

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。我会改用OneToMany 注释实现。

您的 CorporateEvent 课程：

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

你的任务类：

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