JVM 如何确保 System.identityHashCode() 永远不会改变?

问题描述

Object.hashCode() 的默认实现通常是对象在内存中的分配地址的一些函数(尽管这不是由 JLS).鉴于 VM 在内存中分流对象，为什么 System.identityHashCode() 返回的值在对象的生命周期内永远不会改变?

Typically the default implementation of Object.hashCode() is some function of the allocated address of the object in memory (though this is not mandated by the JLS). Given that the VM shunts objects about in memory, why does the value returned by System.identityHashCode() never change during the object's lifetime?

如果是一次性"计算(对象的 hashCode 计算一次并存储在对象头或其他东西中)，那么这是否意味着两个对象有可能具有相同的 identityHashCode (如果它们恰好首先分配在内存中的相同地址)?

If it is a "one-shot" calculation (the object's hashCode is calculated once and stashed in the object header or something), then does that mean it is possible for two objects to have the same identityHashCode (if they happen to be first allocated at the same address in memory)?

推荐答案

现代 JVM 将值保存在对象头中.我相信该值通常仅在首次使用时计算，以便将对象分配所花费的时间保持在最低限度(有时低至十几个周期).可以编译通用的 Sun JVM，使所有对象的标识哈希码始终为 1.

Modern JVMs save the value in the object header. I believe the value is typically calculated only on first use in order to keep time spent in object allocation to a minimum (sometimes down to as low as a dozen cycles). The common Sun JVM can be compiled so that the identity hash code is always 1 for all objects.

多个对象可以具有相同的身份哈希码.这就是哈希码的本质.

Multiple objects can have the same identity hash code. That is the nature of hash codes.

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