Is there any use in setting a ThreadSafe object to ThreadLocal?

Question

I recently saw a piece of code that used a ThreadLocal object and contained a ConcurrentHashMap inside it.

Is there any logic / benefit to this, or is it redundant?

Answer 1

If the only link to the parallel hash file is in ThreadLocal , the hash map obviously refers to only one thread. In that case, I would say that it is completely redundant.

However, it is easy to imagine that someone is "sharing" a locally stored hash file with other streams:

 ThreadLocal<ConcurrentHashMap<String, String>> tl = ... // ... final ConcurrentHashMap<String, String> props = tl.get(); EventQueue.invokeLater(new Runnable() { public void run() { props.add(key.getText(), val.getText()); } }); 

Answer 2

Either he used ThreadLocal incorrectly, or ConcurrentHashMap erroneously. The probability that the combination makes sense is close to 0.

Answer 3

In addition to what @aioobe said, consider the InheritableThreadLocal case, in which the local value is passed from the thread to each child thread that it creates.

And as @pst says, there is nothing to prevent the use of the same value in different (non-inherited) ThreadLocal s.

In short, you need to conduct a thorough analysis of the flow locales, how to initialize them, and how to use them before you can safely conclude that they should not be thread safe.