DataNucleus可以坚持使用泛型参数化的抽象基类吗？ [英] Can DataNucleus persist an abstract base class parameterized with Generics?

问题描述

使用DataNucleus，我很高兴使用抽象基类为子类提供长ID（请注意原始类型）。

在改编来自JPA的示例时，我意识到参数化基类。目的是支持不同的ID类型，比如String。

  @PersistenceCapable 
 @Inheritance（strategy = InheritanceStrategy。 SUBCLASS_TABLE）
 @Version（strategy = VersionStrategy.VERSION_NUMBER，column =jdo_version）
 public abstract class VersionedIdEntity< P> 
实现了Serializable {
 
 static final long serialVersionUID = 1L; 
 
 @PrimaryKey 
 @Persistent（valueStrategy = IdGeneratorStrategy.INCREMENT）
 private P id; 
 
 public P getId（）{
 return id; 
 
 
 
 
 
 
 $ p泛型让我使用包装的Java类型'Long' ，所以子类声明如下： 
 
 
  public class Account extends VersionedIdEntity< Long> 
  
我切换到新的基类导致了一个错误，指出'java.lang.Object '被发现。
 $ b 
 
 org.datanucleus.exceptions.NucleusUserException：字段VersionedIdEntity.id被声明为引用类型（interface / Object ），但没有找到java.lang.Object的实现类！
 
 
错误与类型擦除有关吗？类型可在DataNucleus中持久化，例如： 
 
 
 < ;?扩展PersistableType> 
  
虽然我在其他地方找不到帮助，但问题之前可能已经提出。在这种情况下，一个简短的指针将是非常值得赞赏的。
解决方案
你的意思是JDO定义了一个实现应该支持这个吗？答：不。 
 JDO定义了有效的PK类型，什么是P？就类而言，它是Object（即（java.lang。）Object，因为它没有实际的类型）。 JPA也没有为此定义它。
 
Using DataNucleus, I have been happy using an abstract base class to provide a 'long' ID to the sub-classes (please note the primitive type).

When adapting an example from JPA I got the idea to parameterize the base class. The purpose was to support different ID types, such as String.
@PersistenceCapable
@Inheritance(strategy=InheritanceStrategy.SUBCLASS_TABLE)
@Version(strategy=VersionStrategy.VERSION_NUMBER, column="jdo_version")
public abstract class VersionedIdEntity<P>
    implements Serializable {

  static final long serialVersionUID = 1L;

  @PrimaryKey
  @Persistent(valueStrategy=IdGeneratorStrategy.INCREMENT)
  private P id;

  public P getId() {
    return id;
  }
}
Generics made me use the wrapped Java type 'Long', so sub-classes were declared like this:
public class Account extends VersionedIdEntity<Long>
My switch to the new base class caused an error stating that no implementation for 'java.lang.Object' was found.

  
org.datanucleus.exceptions.NucleusUserException: Field "VersionedIdEntity.id" is declared as a reference type (interface/Object) but no implementation classes of "java.lang.Object" have been found!
Is the error related to type erasure?

Additional questions would be (1) how JPA deals with it and (2) whether I can restrict the type P to types persistable in DataNucleus, such as:
<? extends PersistableType>
While I could not find help elsewhere, the question could have been asked before. In that event, a brief pointer would be much appreciated.
解决方案
You mean does JDO define that an implementation should support this? Answer : no. 
JDO defines the valid PK types, and what is a "P" ? it is Object as far as a class is concerned (i.e (java.lang.)Object, since it doesn't have the real type there). Neither does JPA define it for that matter.

                        
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