为什么继承深度在 GAE 上的 Datanucleus JDO 中很重要？

Question

如果我有一层继承，则 App Engine 中的所有内容都会按预期保留：

Worker.java

import javax.jdo.annotations.IdGeneratorStrategy;
import javax.jdo.annotations.Inheritance;
import javax.jdo.annotations.InheritanceStrategy;
import javax.jdo.annotations.PersistenceCapable;
import javax.jdo.annotations.Persistent;
import javax.jdo.annotations.PrimaryKey;

@PersistenceCapable
@Inheritance(strategy = InheritanceStrategy.SUBCLASS_TABLE)
public abstract class Worker {
    @PrimaryKey
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
    private Key key;

    @Persistent
    private String department;
}

Employee.java

// ... imports ...

@PersistenceCapable
public class Employee extends Worker {
    @Persistent
    private int salary;

}

Intern.java

import java.util.Date;
// ... imports ...

@PersistenceCapable
public class Intern extends Worker {
    @Persistent
    private Date internshipEndDate;
}

但是，如果我添加一层额外的继承，则最高级别子类中的字段不会保留：

Human .java

@PersistenceCapable
public abstract class Human {

    @Persistent
    private String name;
}

Worker.java

@PersistenceCapable
@Inheritance(strategy = InheritanceStrategy.SUBCLASS_TABLE)
public abstract class Worker extends Human {
    @PrimaryKey
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
    private Key key;

    @Persistent
    private String department;
}

为什么可以扩展抽象类，但不能让该抽象类扩展更高级别的类？

Answer 1

并且您的主键现在不在（持久）继承树的根类中，因此 JDO 和 JPA 规范无效。

Answer 2

您还应该将以下声明移至 Human 类：

@Inheritance(strategy = InheritanceStrategy.SUBCLASS_TABLE)