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我正在研究 Hibernate，我需要两个抽象类。第一个，Product是主要的，数据库上应该没有名为product. 第二个抽象类是ComPart（继承自Product），并且一些计算机部件（如 gpu 和 cpu）是从此类继承的。应该只有一个名为的表com_part，并且 CPU 和 GPU 都应该在这个表中（每个层次结构的表）。

@MappedSuperclass
public abstract class Product{

    private long pID;
    protected String manufacturer;

    @Id
    @GeneratedValue(strategy = IDENTITY)
    @Column(name = "PID", unique = true, nullable = false)
    public int getPID() {
        return pID;
    }

    @Column(name = "Manufacturer")
    public String getManufacturer() {
        return manufacturer;
    }

}


// computer part abstract class inherited from Product
@Entity
@Table(name = "com_part")
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(
    name="disc",
    discriminatorType=DiscriminatorType.STRING
)
@AttributeOverrides({
    @AttributeOverride(name="manufacturer", column=@Column(name="Manufacturer"))
})
public abstract class ComPart extends Product {

    private String platform;

    @Column(name = "PLATFORM", length = 10)
    public String getPlatform() {
        return platform;
    }
}


// Various computer hardwares inherited from ComPart
@Entity
@Table(name="com_part")
@DiscriminatorValue("CPU")
public class Processor extends comPart {
    private String socketType;
    private String chipset;

    //getters setters...
}

@Entity
@Table(name="com_part")
@DiscriminatorValue("GPU")
public class GraphicsCard extends comPart {
    private double memory;

    //getters setters...
}

我如何映射所有这些功能？上面的代码甚至不能创建会话工厂？！它会产生这些错误：

Failed to create sessionFactory object.java.lang.NullPointerException
Exception in thread "main" java.lang.ExceptionInInitializerError
    at com.bts.core.hb.HibernateUtil.<clinit>(HibernateUtil.java:35)

和

Caused by: java.lang.NullPointerException
    at org.hibernate.cfg.Configuration.processFkSecondPassInOrder(Configuration.java:1424)
    at org.hibernate.cfg.Configuration.secondPassCompile(Configuration.java:1351)
    at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1733)
    at com.bts.core.hb.HibernateUtil.<clinit>(HibernateUtil.java:32)
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