ThreadLocals和并行类加载的影响 [英] Effect of ThreadLocals and side-by-side classloading

问题描述

假设

class A{
         private static final ThreadLocal<String> tl = new ThreadLocal<String>();
}

如果A仅在vm的一个类加载器中加载，则t1的值很明显。但是，如果将A并排加载到两个不同的类加载器中，那么t1会怎样？

If A is loaded in just one classloader on the vm, the value of t1 is obvious. But what happens to t1 if A is loaded side-by-side in two different classloaders ? Will the value be shared for a given thread ?

推荐答案

有趣的问题。正如 Tom Hawtin-大头钉所解释的，您基本上是在创建 ThreadLocal< String>（）的实例。现在让我们看一下 ThreadLocal 如何实际存储值（简化）：

Interesting question. As Tom Hawtin - tackline explained, you are basically creating to instances of ThreadLocal<String>(). Now let's have a look at how ThreadLocal actually stores the values (simplified):

public void set(T value) {
    ThreadLocalMap map = getMap(Thread.currentThread());
    map.set(this, value);
}

它需要某种映射，该映射绑定到每个线程并设置使用 this （我自己）作为键的值。这意味着，如果您有两个 ThreadLocals （由不同的类加载器创建），则它们具有不同的 this 引用，因此有效存储不同的值。

It takes some sort of a map that is bound to every thread and sets the value using this (myself) as a key. This means that if you have two ThreadLocals (created by different class loaders), they have different this reference, thus effectively storing different values.

全部-您不能例如使用 ThreadLocal 作为类加载器本地单例并创建线程本地单例的解决方法。

All in all - you cannot e.g. use ThreadLocal as a workaround to class-loader local singletons and creating thread-local ones.

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