java - JPA实体: How to establish relationship between Parent and multiple Child classes

Question

我有以下 Java 类:

public Class Parent {

    int someValue1;

    ChildType child;
}


public Class ChildType {

     int someValue2;
}


public Class ChildA extends ChildType {

     int id;

     String string;
}



public Class ChildB extends ChildType {

    int id;

    Integer integer;
}

我需要将 Parent、ChildA 和 ChildB 表示为实体 bean，每个 bean 在数据库中都有一个相关的表。

当我加载 Parent 时，我还需要根据关系加载 ChildA 或 ChildB。

Answer 1

如果我没猜错的话，类父类是一个与 ChildType 实体保持一对一关系的实体。 ChildType 也是一个抽象实体，有 2 个实现:ChildA 和 ChildB。

因此每个实体的 JPA 注释配置可能如下所示:

 Parent class as Entity

@Entity
@Table(name = "PARENT")
public class Parent { // better name will do the job, because parent is often called
                      // the higher level class of the same hierarchy
  @Id
  @GeneratedValue
  @Column(name = "PARENT_ID")
  private long id;

  @Column(name = "SOME_VALUE") //if you need to persist it
  private int someValue1;

  @OneToOne(optional = false, cascade = CascadeType.ALL)
  @JoinColumn(name = "FK_PARENT_ID")
  private ChildType child;

  // getters and setters
}

 ChildType class as Entity

@Entity
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public abstract class ChildType { // this one is actually called parent class

  @Id
  @GeneratedValue(strategy = GenerationType.TABLE)
  @Column(name = "CHILDTYPE_ID")
  protected Long id;

  @Column(name = "SOME_VALUE_2")
  private int someValue2; // or maybe protected. Depends if you need childs to access it

  @Column(name = "A_STRING")
  private String string; // or maybe protected. Depends if you need childs to access it
  
  // getters and setters
}

最后我们有了 ChildA 和 ChildB

如您所见，ChildType 子类上不需要有 id 字段，因为他们从 ChildType 继承这个字段!

 ChildA as Entity

@Entity
@Table(name = "CHILD_A")
public Class ChildA extends ChildType {
  
  @Column(name = "A_STRING")
  private String string;
  
  // getters and setters
}

 ChildB as Entity

@Entity
@Table(name = "CHILD_B")
public Class ChildB extends ChildType {
  
  @Column(name = "AN_Integer")
  private Integer integer;
  
  // getters and setters
}

<小时/>

有关 JPA 继承的更多信息

检查这里:

• Java Persistence/Inheritance
• Java EE tutorial Entity Inheritance