在Spring中使用抽象类作为实例变量

Question

My issue is that I have a Spring model

@Entity
public class PetOwner {
    @ManyToOne
    @JoinColumn(name = "pet_id", nullable = true)
    private Pet pet;

with Pet being an abstract class with 2 subclasses Dog and Cat.

@Entity
@Inheritance(strategy= InheritanceType.JOINED)
public abstract class Pet {
    @OneToMany(fetch = FetchType.LAZY)
    protected Set<PetOwner> owners;
}

@Entity
public class Cat extends Pet 

@Entity
public class Dog extends Pet 

When I create a new PetOwner in a Struts-form and try to bind its pet to either a Dog or Cat object, I get an error on submitting the form:

Error creating bean with name 'de.model.Pet': Instantiation of bean failed; 
nested exception is org.springframework.beans.BeanInstantiationException: 
Could not instantiate bean class [de.model.Pet]: Is it an abstract class?; 
nested exception is java.lang.InstantiationException

How can I tell Spring to initialize the de.model.Dog or de.model.Cat based on the class pet-object?

FYI: I have a PetDAO and PetService, but I am not sure whether they influence this problem.

I appreciate every help!

Answer 1

Personally, I would turn Pet to an Interface and make Cat and Dog implement it to avoid this kind of problems. Sorry for off-topic!

Anyhow, if you want some member of your class to be dynamically initialized\\populated, try the Lookup Method Injection. Read pp. 3.3.4.1 here .

If the class that contains the dynamic member was created in scope=singletone (the default for spring bean container) every time you will access the field that has a lookup method assigned, you will get an appropriate object according to the business logic implemented inside the lookup method.

Also, I found this example in Spring documentation - I think it is very clear. Take a look at "3.4.6.1 Lookup method injection"

When you configure the PetOwner class assign a lookup method to its Pet member - it will be called whenever you need a new instance of the Pet bean.

Good luck!