Hibernate Lazy Loading, Proxies and Inheritance

Question

I have a problem with lazy loading in hibernate when dealing with inheritance. I have one entity that references a second entity that is subclassed. I want the reference to load lazily, but this causes errors in my .equals() methods.

In the code below, if you call equals() on an instance of A, the check fails in the C.equals() function when checking if the Object o is an instance of C. It fails because the other object is actually a Hibernate proxy created by javassist, which extends B, not C.

I understand that Hibernate cannot create a proxy of type C without going to the database and thus breaking the lazy loading. Is there a way to make the getB() function in class A return the concrete B instance instead of the proxy (lazily)? I've tried using the Hibernate specific @LazyToOne(LazyToOneOption.NO_PROXY) annotation on the getB() method to no avail.

@Entity @Table(name="a")
public class A {
    private B b;

    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="b")
    public B getB() {
        return this.b;
    }

    public boolean equals(final Object o) {
        if (o == null) {
            return false;
        }

        if (!(o instanceof A)) {
            return false;
        }
        final A other = (A) o;
        return this.getB().equals(o.getB());
    }
}

@Entity @Table(name="b")
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(
    name="type",
    discriminatorType=DiscriminatorType.STRING
)
public abstract class B {
   private long id;

   public boolean equals(final Object obj) {
       if (this == obj) {
           return true;
       }
       if (obj == null) {
           return false;
       }
       if (!(obj instanceof B)) {
           return false;
       }
       final B b = (B) o;
       return this.getId().equals(b.getId());
    }
}

@Entity @DiscriminatorValue("c")
public class C extends B {
   private String s;

   public boolean equals(Object obj) {
       if (this == obj) {
           return true;
       }
       if (!super.equals(obj)) {
           return false;
       }
       if (obj == null) {
           return false;
       }
       if (!super.equals(obj)) {
           return false;
       }
       if (!(obj instanceof C)) {
           return false;
       }
       final C other = (C) o;
       if (this.getS() == null) {
           if (other.getS() != null) {
               return false;
           }
       } else if (!this.getS().equals(other.getS())) {
           return false;
       }
       return true;
    }
}

@Entity @DiscriminatorValue("d")
public class D extends B {
    // Other implementation of B
}

sutlermb · Accepted Answer · 2012-01-12 15:19:00Z

1

It turns out that I was on the right tracking trying to use the @LazyToOne(LazyToOneOption.NO_PROXY) annotation. It wasn't working for me out of the box because I hadn't yet run the Hibernate bytecode enhancer tool on it. Instructions for this can be found here:

19.1.7. Using lazy property fetching

answered Jan 12, 2012 at 15:19

sutlermb

1151 silver badge7 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Lukasz Frankowski Over a year ago

There's an option to use NO_PROXY and instrumentation. You can expose "this" outside the object and use it to check instanceof. In "this" you will always have unwrapped and already initialied object. Here is my detailed solution lifeinide.com/post/…

Gaurav · Accepted Answer · 2012-01-11 16:52:09Z

0

You might want to try changing the fetch property on getB() to FetchType.EAGER. Hope this helps.

answered Jan 11, 2012 at 16:52

Gaurav

1,5672 gold badges15 silver badges32 bronze badges

1 Comment

sutlermb Over a year ago

That would do it, but I would like this field to be lazy loaded.

JB Nizet · Accepted Answer · 2012-01-11 16:53:58Z

0

Regardless of whether the objects are entities and/or lazy-loaded, it's practically impossible to respect the contract of equals and to have a specialization of equals in a subclass which uses instanceof. Indeed, you would then be in a situation where you would have b1.equals(c1) == true, but c1.equals(b1) == false.

So, I think the superclass (B) should define equals, and make it final, because all the subclasses should use the base class equals method.

That said, your equals method in B isn't right:

if (!super.equals(obj)) {
   return false;
}

This means that the Object implementation of equals must return true to have two B instances equal. Which means that two B instances are only equals if they are the same object.

if (!(obj instanceof C)) {
    return false;
}

Why does the B class check that the other instance is an instance of C. It should check if the other instance is an instance of B.

Since in the end, two Bs are equal if they have the same ID, and since the IDs must be unique for all the inheritance tree, you're safe if you make this equals method final.

answered Jan 11, 2012 at 16:53

JB Nizet

694k94 gold badges1.3k silver badges1.3k bronze badges

5 Comments

sutlermb Over a year ago

The errors in the equals functions were type-o's / copy-paste errors. I corrected them. I am trying to use these objects both when persisted and when not persisted, so I want equals to tell me if the objects are equal, not just that the IDs are equal.

JB Nizet Over a year ago

Well, then you'll need to have a functional ID in your B entity. This functional ID can be returned by a protected abstract method: return getFunctionalId().equals(b.getFunctionalId()). But it must also be unique among the whole inheritance tree.

sutlermb Over a year ago

I just used equals() as an example. I will run into the same problem if I need to cast a.getB() to type C in an external class. The question is: can I get the concrete type of B from getB() and not the proxy.

JB Nizet Over a year ago

You can't without using proprietary Hibernate code to deproxify the reference. Hibernate recommends only accessing Bs as Bs, using polymorphic methods of B. So declare abstract methods in B, or use the visitor pattern.

sutlermb Over a year ago

OK, thanks. Do you know what is the purpose of @LazyToOne(LazyToOneOption.NO_PROXY) then? The docs seem to imply that this would do what I want, but it seems to have no effect. Do you know of any other JPA compliant ORMs that might support this functionality?

Collectives™ on Stack Overflow

Hibernate Lazy Loading, Proxies and Inheritance

3 Answers 3

1 Comment

1 Comment

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

1 Comment

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related