Question

I have one BaseEntity which abstracts id and version property. this class also implements hashcode and equals based on PK (id) property.

BaseEntity{

    Long id;
    Long version; 

public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + ((id == null) ? 0 : id.hashCode());
    return result;
}

public boolean equals(Object obj) {
    if (this == obj)
        return true;
    if (obj == null)
        return false;
    if (getClass() != obj.getClass())
        return false;
    BaseEntity other = (BaseEntity) obj;
    if (id == null) {
        if (other.id != null)
            return false;
    } else if (!id.equals(other.id))
        return false;
    return true;
}


}

now two Entity A and B extends BaseEntity as below

A extends BaseEntity{
    `B b`
     B getB(){return b;)
     void setB(B b){this.b=b;}
}

B extends BaseEntity{
}

object b1;
object a1;
a1.set(b1);
session.save(a1) //cascade save;

close session
load a with lazy b and try a1.getB().equals(b1) gives false
but if i compare with a1.getB().getId().equals(b1.getId()) then gives true strange!!
i think it is because of java assist proxy object, anyway to resolve this?

Answer 1

To be able to lazy-load the a.b association, Hibernate sets the b field in a to a proxy. The proxy is an instance of a class that extends B, but is not B. So, your equals() method will always fail when comparing a non-proxy B instance to a proxy B instance, because it compares the classes of both objects:

if (getClass() != obj.getClass())
    return false;

In the case of Hibernate entities, you should replace this with

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

Also, note that

• Hibernate recommends not implementing equals() and hashCode() by using the ID, but rather by using a natural identifier. Implementing it with IDs can cause problems because entities don’t have an ID until they’re saved and the ID is generated
• When using entity inheritance, the problem is even worse. Suppose B is a superclass of two sub-entities B1 and B2. Hiberante can’t know which type (B1 or B2) is a.b before loading it. So a.b will be initialized to a proxy which is a subclass of B, but is not a subclass of B1 or B2. So the hashCode() and equals() methods should be implemented in B, but not be overridden in B1 and B2. Two B instances should be considered equal if they’re instances of B, and have the same identifier.