Java 如何解决此异常：“引用；X引用一个未知实体：Y"；？

这是我的密码：

@MappedSuperclass
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public class GenericData {
    @Id
    @Column(length = 1024)
    private String url;

    @Temporal(TemporalType.TIMESTAMP)
    private Date lastUpdated;

    @ManyToOne
    private Set<GenericData> inbounds = new HashSet<GenericData>();

    @OneToMany
    private Set<GenericData> outbounds = new HashSet<GenericData>();

    // Getters, setters and rest of class ...

} // End of class

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我缺少什么？

一个

@ManyToOne

应该存储一件东西。使用：

@ManyToOne
private GenericData inbounds;

@ManyToOne
private GenericData inbounds;