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javaclassloaderthread-local

Effect of ThreadLocals and side-by-side classloading

Assuming 

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

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 ?

Solution

  • 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);
}

    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.

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