java 将休眠与泛型结合使用
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Using hibernate with generics
提问by Ryan
I am having some trouble understanding how Hibernate deals with generics and was wondering the best way to accomplish my goal.
我在理解 Hibernate 如何处理泛型时遇到了一些麻烦，并且想知道实现我的目标的最佳方法。
Given a simple generic entity:
给定一个简单的通用实体：
@Entity
public class Box<T>{
private T t;
@Id
private long id;
public void setT(T t) {
this.t = t;
}
public T getT() {
return t;
}
public void setId(long id) {
this.id = id;
}
public long getId() {
return id;
}
}
When going through hibernate initialization, I am getting the exception: ...has an unbound type and no explicit target entity. Resolve this Generic usage issue or set an explicit target attribute (eg @OneToMany(target=) or use an explicit @Type
在进行休眠初始化时，我收到异常： ...has an unbound type and no explicit target entity. Resolve this Generic usage issue or set an explicit target attribute (eg @OneToMany(target=) or use an explicit @Type
I am nearly certain this is because I haven't given hibernate a list of restrictions of what <T>can actually be. I know you can specify things such as targetEntity=String.classabove tin an annotation, but then you lose the flexibility of having generics. Can I limit the scope of what is an acceptable generic using annotations? For instance: What if I want classes ChildA, ChildB, whom inherit from an abstract class Parentto be persistable there. In addition, it should also be able to accept Strings. Can Hibernatedeal with such a thing?
我几乎可以肯定这是因为我没有给 hibernate 列出<T>实际可能存在的限制。我知道您可以在注释中指定诸如targetEntity=String.class上面t的内容，但是这样您就失去了拥有泛型的灵活性。我可以使用注释来限制可接受的泛型的范围吗？例如：如果我希望从抽象类继承的classes ChildA,可以在那里持久化ChildB，该怎么办Parent。此外，它还应该能够接受Strings。能Hibernate应付这种事吗？
采纳答案by Ryan Stewart
What you're looking for is probably Hibernate's implicit polymorphism. There's also a little-known "any" relationshipwhich gives complete flexibility, but it has its tradeoffs. You can also use an "any" in a many-to-any.
您正在寻找的可能是 Hibernate 的隐式多态性。还有一个鲜为人知的“任何”关系，它提供了完全的灵活性，但它有其权衡。您还可以在多对任何中使用“任何” 。
Edit:I've created a runnable example on Github based around your "Box" class and using an @Anymapping. You can browse it(or the Box classspecifically) or check it out and run it with
编辑：我基于您的“Box”类并使用@Any映射在 Github 上创建了一个可运行的示例。您可以浏览它（或专门的Box 类）或检查它并运行它
git clone git://github.com/zzantozz/testbed tmp
cd tmp
mvn -q compile exec:java -Dexec.mainClass=rds.hibernate.AnyMapping -pl hibernate-any
回答by Jerome Cance
I've already done that but with subclasses.
我已经这样做了，但是有子类。
Your generic class must be abstract and subclasses must define the generic parameter
你的泛型类必须是抽象的，子类必须定义泛型参数
