Hibernate 等于和代理 [英] Hibernate equals and proxy

问题描述

我有一个抽象 id 和 version 属性的 BaseEntity.这个类还实现了基于 PK (id) 属性的 hashcode 和 equals.

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;
}


}

现在两个Entity A和B扩展BaseEntity如下

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;

关闭会话用惰性 b 加载 a 并尝试 a1.getB().equals(b1) 给出 false但如果我与 a1.getB().getId().equals(b1.getId()) 进行比较然后给出真奇怪！我认为是因为java辅助代理对象，无论如何要解决这个问题?

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?

推荐答案

为了能够延迟加载ab关联，Hibernate在b字段中设置了b字段>a 到代理.代理是扩展 B 的类的实例，但不是 B.因此，将非代理 B 实例与代理 B 实例进行比较时，您的 equals() 方法将始终失败，因为它比较了两个对象的类:

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;

对于 Hibernate 实体，您应该将其替换为

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

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

另外，请注意

• Hibernate 建议不要使用 ID 来实现 equals() 和 hashCode()，而是使用自然标识符.使用 ID 实现它可能会导致问题，因为实体在保存并生成 ID 之前没有 ID
• 使用实体继承时，问题更严重.假设 B 是两个子实体 B1 和 B2 的超类.Hiberante 在加载之前无法知道 a.b 是哪种类型(B1 或 B2).所以 a.b 将被初始化为一个代理，它是 B 的子类，但不是 B1 或 B2 的子类.所以 hashCode() 和 equals() 方法应该在 B 中实现，但不能在 B1 和 B2 中被覆盖.如果两个 B 实例是 B 的实例并且具有相同的标识符，则应将它们视为相等.

• 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.

这篇关于Hibernate 等于和代理的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！