繼承 class 上的 JPA/Hibernate 關系返回 null

Question

我有一個用於實體的分層 class 系統，它們都很好，直到我引入關系。 下面是一些示例代碼：

基類：

@MappedSuperclass
public class BaseClass
{

    // Some fields
    // Ids are MySQL autoincrementing unsigned BIGINT
    protected Long id;
    // ... other fields

    @JsonCreator
    public BaseClass (
        // ...fields
    )
    {

        // assign fields
    }

    public BaseClass () { /** Default copy constructor */ }

    public BaseClass update (BaseClass value)
    {
        // repeat bellow line for all fields
        this.field1 = (value.field1 != null) ? value.field1 : this.field1;
        // finally, for chaining
        return this;
    }

    // Getters and Setters for all fields

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id", updatable = false)
    @JsonProperty("id")
    public Long getId ()
    {
        return this.id;
    }

    public BaseClass setId (Long value)
    {
        this.id = value != null ? value : this.id;
        return this;
    }

    @Column(name = "field1")
    @JsonProperty("field1")
    public String getField1 ()
    {
        return this.field1;
    }

    public BaseClass setField1 (Field1Type value)
    {
        this.field1 = value;
        return this;
    }

}

繼承類：

@Entity
@Table(name = "`table_name`")
@DynamicInsert
@DynamicUpdate
public class InheritedClass extends BaseClass implements Serializable
{
    /**
     * the value is for demonstration only, it's randomly generated per serializable entity
     */
    private static final long serialVersionUID = 1L;

    private List<RelatedClass1> relatedClass1Field;
    private RelatedClass2 relatedClass2Field;

    public InheritedClass () { /** Default copy constructor */ }

    public InheritedClass (BaseClass value)
    {
        super.update(value);
    }

    // inheritedClassField is the field in the many-to-one end of the relationship in RelatedClass1
    @Transient // javax.persistence, not bean
    @JsonProperty("related_class1_field")
    @OneToMany(
        targetEntity=RelatedClass1.class
        , fetch=FetchType.EAGER
        , cascade = { CascadeType.ALL }
        , mappedBy = "inheritedClassField"
    )
    public List<RelatedClass1> getRelatedClass1Field ()
    {
        return this.relatedClass1Field;
    }

    public InheritedClass setRelatedClass1Field (List<RelatedClass1> value)
    {
        this.relatedClass1Field = value;
        return this;
    }

    // inheritedClassField is the field in the many-to-one end of the relationship in RelatedClass1
    @Transient // javax.persistence, not bean
    @JsonProperty("related_class2_field")
    @ManyToOne(
        targetEntity=RelatedClass2.class
        , fetch=FetchType.EAGER
        , cascade = { CascadeType.ALL }
    )
    @JoinColumn(
        name = "related_id"
        , referencedColumnName = "id"
    )
    public RelatedClass2 getRelatedClass2Field ()
    {
        return this.relatedClass2Field;
    }

    public InheritedClass setRelatedClass2Field (RelatedClass2 value)
    {
        this.relatedClass2Field = value;
        return this;
    }

}

當我嘗試訪問InheritedClass的實例時， relatedClass1Field和relatedClass2Field是null ，但是它們已填充到數據庫中。

但

如果我通過字段訪問策略定義關系，它們會返回正確的值：

    @Access(AccessType.FIELD)
    @OneToMany(
        targetEntity=RelatedClass1.class
        , fetch=FetchType.EAGER
        , cascade = { CascadeType.ALL }
        , mappedBy = "inheritedClassField"
    )
    private List<RelatedClass1> relatedClass1Field;

    @ManyToOne(
        targetEntity=RelatedClass2.class
        , fetch=FetchType.EAGER
        , cascade = { CascadeType.ALL }
    )
    @JoinColumn(
        name = "related_id"
        , referencedColumnName = "id"
    )
    private RelatedClass2 relatedClass2Field;

    
    @JsonProperty("related_class1_field")
    public List<RelatedClass1> getRelatedClass1Field ()
    {
        return this.relatedClass1Field;
    }

    public InheritedClass setRelatedClass1Field (List<RelatedClass1> value)
    {
        this.relatedClass1Field = value;
        return this;
    }


    @JsonProperty("related_class2_field")
    public RelatedClass2 getRelatedClass2Field ()
    {
        return this.relatedClass2Field;
    }

    public InheritedClass setRelatedClass2Field (RelatedClass2 value)
    {
        this.relatedClass2Field = value;
        return this;
    }

Answer 1

您不應該在這些方法上使用@Transient注釋：

@Transient
@OneToMany(
   targetEntity=RelatedClass1.class
   , fetch=FetchType.EAGER
   , cascade = { CascadeType.ALL }
   , mappedBy = "inheritedClassField"
)
public List<RelatedClass1> getRelatedClass1Field ()
// ...

@Transient
@ManyToOne(
   targetEntity=RelatedClass2.class
   , fetch=FetchType.EAGER
   , cascade = { CascadeType.ALL }
)
@JoinColumn(
   name = "related_id"
   , referencedColumnName = "id"
)
public RelatedClass2 getRelatedClass2Field ()
// ...

根據文檔， @Transient注釋指定屬性或字段不是持久的。

和簡單的例子。 假設您有以下實體：

@Entity
@Table(name = "TST_PATIENT")
public class Person {

    private Long id;
    // ...
    private Date dateOfBirth;

    @Id
    @Column(name = "P_ID")
    public Long getId() {
       return id;
    }
    
    @Column(name = "P_DOB")
    public Date getDateOfBirth() {
       return dateOfBirth;
    }
    
    @Transient
    public long getAge() {
       return ChronoUnit.YEARS.between(
          LocalDateTime.ofInstant( Instant.ofEpochMilli( dateOfBirth.getTime()), ZoneOffset.UTC),
          LocalDateTime.now()
       );
    }

    // setters omitted for brevity
}

這里我們使用@Transient注釋，因為實際上我們在TST_PATIENT表中沒有age列（它只是根據另一個持久化字段計算得出）並且我們要指示 hibernate 將此屬性排除在實體持久性 state 的一部分之外。