ThreadLocal usage examaple

Bar.java

/**
 * It is instantiated only once.
 * However can be initialized one or more times per thread.
 * The code prints something like this on console:
 * field threadLocal has been instantiated!
 * Thread with id = 10
 * Thread with id = 11
 * Thread with id = 12
 * */
public class Bar {
    public static void main(String[] args) throws InterruptedException {
        Foo sharedObject = new Foo();
        Thread thread1 = new Thread(sharedObject);
        Thread thread2 = new Thread(sharedObject);
        Thread thread3 = new Thread(sharedObject);
        thread1.start();
        thread2.start();
        thread3.start();

        thread1.join();
        thread2.join();
        thread3.join();
    }
}

class Foo implements Runnable{
    private ThreadLocal threadLocal = new ThreadLocal() {
        { System.out.println("field threadLocal has been instantiated!" ); }
    };

    @Override
    public void run() {
        threadLocal.set("Thread with id = " + Thread.currentThread().getId());
        try {
            Thread.sleep(200);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(threadLocal.get());
    }
}