ThreadLocal value access across different threads

Question

Given that a ThreadLocal variable holds different values for different threads, is it possible to access the value of one ThreadLocal variable from another thread?

Answer 1

As Peter says, this isn't possible. If you want this sort of functionality, then conceptually what you really want is just a standard Map - where most operations will be done with a key of Thread.currentThread(), but you can pass in other threads if you wish.

However, this likely isn't a great idea. For one, holding a reference to moribund threads is going to mess up GC, so you'd have to go through the extra hoop of making the key type WeakReference instead. And I'm not convinced that a Thread is a great Map key anyway.

So once you go beyond the convenience of the baked-in ThreadLocal, perhaps it's worth questioning whether using a Thread object as the key is the best option? It might be better to give each threads unique IDs (Strings or ints, if they don't already have natural keys that make more sense), and simply use these to key the map off. I realise your example is contrived, but you could do the same thing with a Map and using keys of "t1" and "t2".

It would also arguably be clearer since a Map represents how you're actually using the data structure; ThreadLocals are more like scalar variables with a bit of access-control magic than a collection, so even if it were possible to use them as you want it would likely be more confusing for other people looking at your code.