Why ThreadLocal variable need to static?

Question

I have read many articles on why ThreadLocal variable need to be static(although not necessary), but I didn't get the idea why it should be static.

I have read it here and many other links but didn't get the idea.

I have done something like this

public class ThreadLocalDemo{

    public static void main(String[]args)throws Exception{
        SharedRersource r1= new SharedRersource();
        Thread t1= new Thread(r1);
        Thread t2= new Thread(r1);
        t1.start();
        t2.start();
        t1.join();
        t2.join();
        System.out.println("Main thread Exiting...");
        }

    }

class SharedRersource implements Runnable{


        private ThreadLocal<Integer> threadId = new ThreadLocal(){
            protected Integer initialValue(){
                return (int)(Math.random()*100);
                }
        };
        public void run(){

            try{
                Thread.sleep(2000);
            }
            catch(InterruptedException e){
                e.printStackTrace();
                }
                System.out.println(threadId.get());

            }
        };

Here thread t1 and t2 is having private copy of threadId than why It should be static

Please give a better understanding to me. Thanks

Answer 1

Answer to this question lies into ThreadLocal implementation .

Think ThreadLocal as a container

ThreadLocal is a container that maintain a ThreadLocalMap internally , This ThreadLocalMap is the key why threadlocal need to be static(although not necessary ,but suggestion is keep it static).

Because we want single container per class , not container per instance .If we have container per instance we will be having as many container as instance and that will created memory leak .

More detail here

1. http://www.0xcafefeed.com/2004/06/of-non-static-threadlocals-and-memory/
2. https://www.appneta.com/blog/introduction-to-javas-threadlocal-storage/