i am setting value of threadlocal in one class can i access the value in another class

Question

in the below code i am storing temporary threadlocal values in a threadlocal class.and set the value into another class..

List<String> spaces=new ArrayList<String>();
spaces.add(spaceKey);
ThreadLocal<List<String>> threadLocal=new ThreadLocal<List<String>>();
threadLocal.set(spaces);  
setShardHolder(threadLocal);

i have getters and setters in another class

private ThreadLocal<List<String>> shardHolder;
public ThreadLocal<List<String>> getShardHolder() {
return shardHolder;
}
public void setShardHolder(ThreadLocal<List<String>> shardHolder) {
this.shardHolder = shardHolder;
}

i am not able to access the values using the getter

Answer 1

You don't seem to have understood how ThreadLocal variables work. A ThreadLocal instance is a bit like a container for a single value. If you store something in it, it will be associated to the current thread, and you can retrieve it later, from the same thread.

But of course, if you store something in a ThreadLocal instance, and try to retrieve it from a different ThreadLocal instance, you'll always get null: they're not the same container.

So a ThreadLocal is often hidden behind static methods. Suppose you want to associate a user with the current thread, for example. You would do

public class CurrentUser {
    private static final ThreadLocal<User> CURRENT = new ThreadLocal<User>();

    public static User getCurrentUser() {
        return CURRENT.get();
    }

    public static void setCurrentUser(User user) {
        CURRENT.set(user);
    }
}

Then let's design a scenario :

public class Test
    public static void main(String[] args) {
        CurrentUser.set(new User("foo"));
        System.out.println(CurrentUser.get()); // prints foo
        new Thread(new Runnable() {
            public void run() {
                System.out.println(CurrentUser.get()); // prints null: different thread
            }
        }).start();
    }
}