ThreadLocal; Isn't it the same as creating a copy of a variable each time?

Question

I'm still confused about the concept of ThreadLocal. I read the JavaDoc and other related questions posted here, but the jargon used and all this did not help me.

I kind of understand the idea of ​​ThreadLocal, that is, each thread has its own copy of the variable. So ... how does this distinguish it from ... creating a new variable every time?

For example, using DateFormatter as an example:

public void convertDate(String date) 
{ 
    // Contruct new date formatter for every invocation of the method.
    DateFormatter df = new SimpleDateFormatter(...);
    ....
}

public void convertDate(String date) 
{ 
    // Getting date formatter from threadlocal.
    DateFormatter df = threadLocal.get();
    ....
}

How is the first different from the second if everything that the second does just returns a new copy of the variable?

Thanks.

Answer 1

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Answer 2

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Answer 3

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Note: local stream objects are stored on a map attached to the stream itself. If the thread dies, all objects local to this thread are GCed. Here ThreadLocal may be simpler than most caches.