派生 class 属性的空输出

Question

我能够使用本教程中的 SINGLE_TABLE 继承策略将我的模型与实体映射 https://wiki.eclipse.org/EclipseLink/UserGuide/JPA/Basic_JPA_Development/Entities/Inheritance#Avoiding_Inheritance

基础 class 初始化正常，但是当我尝试访问最派生的部分属性时，我得到空值。

由于我是 EcpliseLink/Hibernate 的初学者，我认为问题出在映射中...

基础Class


@Entity
@Table(name="event")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "EVENT_TYPE")
public abstract class Events implements Serializable {

    public Events(){

    }

     public Events(java.sql.Date date, int totalPlaces, int availablePlaces, String shortDescription,
                  String longDescription) {
        Date = date;
        TotalPlaces = totalPlaces;
        AvailablePlaces = availablePlaces;
        ShortDescription = shortDescription;
        LongDescription = longDescription;
        this.location = location;
        this.picture = picture;
    }

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    protected int Id;

    @Basic(optional=false)
    protected Date Date;

    @Basic(optional=false)
    protected String Company;

    @Basic(optional=false)
    protected int TotalPlaces;

    @Basic(optional=false)
    protected int AvailablePlaces;

    @Column(nullable = true, name="ShortDescription")
    protected String ShortDescription;

    @Column(nullable = true, name="LongDescription")
    protected String LongDescription;

    @OneToOne(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
    @PrimaryKeyJoinColumn(name="Id")
    protected Location location;

    @OneToOne(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
    @PrimaryKeyJoinColumn(name="Id")
    protected Pictures picture;

    abstract public String getCompany() {
        return "Hochschule";
    }

    public void setCompany(String company) {
        //
    }

    abstract public Double getPrice();

    public void setPrice(Double price) {
        //
    }

    abstract public void identify();

    //gtrs strs

派生部分

@Entity
@DiscriminatorValue(value="EXCURSION")
public class Excursion extends Events {

    public Excursion(){

    }

    public Excursion(java.sql.Date date, Double price, int totalPlaces, int availablePlaces, String shortDescription, String longDescription) {
        super(date, totalPlaces, availablePlaces, shortDescription, longDescription);
        this.Price = price;
    }

    @Basic(optional=false)
    private Double Price;

    @Override
    public Double getPrice() {
        System.out.println("Derived Part getPrice called ");
        return Price;
    }

    @Override
    public String getCompany(){
        return "Hochschule Ulm";
    }

    @Override
    public void setPrice(Double price) {
        Price = price;
    }

    @Override
    public void identify(){
        System.out.println("This is Excursion");
    }
}

主要


        List<Events> ls = em.createNamedQuery("Events.findAllEvents").getResultList();

        for(Events e : ls){
            e.identify();
            System.out.println(e.getId()+" "+e.getPrice());
            // getPrice() returns null for every record
        }

    }

Answer 1

好的，我自己解决了这个问题。派生部分根本没有得到初始化。当我从数据库中获取所有事件时，只有事件的基础部分被初始化。我发现这样做的方法是，分别获取所有子类，然后将它们存储在 List

List<Excursion> le = em.createNamedQuery("Excursion.findAllExcursions").getResultList();
List<CompanyExcursion> lc = em.createNamedQuery("CompanyExcursion.findAllExcursions").getResultList();

List<Events> ls = new ArrayList<>();
ls.addAll(le);
ls.addAll(lc);

派生 class 属性的空输出

Null output for derived class attributes

java

polymorphism

orm

eclipselink