When and how should I use a ThreadLocal variable?

Question

When should I use a ThreadLocal variable?

How is it used?

Answer 1

Try this small example, to get a feel for ThreadLocal variable:

public class Book implements Runnable {
    private static final ThreadLocal> WORDS = ThreadLocal.withInitial(ArrayList::new);

    private final String bookName; // It is also the thread's name
    private final List words;


    public Book(String bookName, List words) {
        this.bookName = bookName;
        this.words = Collections.unmodifiableList(words);
    }

    public void run() {
        WORDS.get().addAll(words);
        System.out.printf("Result %s: '%s'.%n", bookName, String.join(", ", WORDS.get()));
    }

    public static void main(String[] args) {
        Thread t1 = new Thread(new Book("BookA", Arrays.asList("wordA1", "wordA2", "wordA3")));
        Thread t2 = new Thread(new Book("BookB", Arrays.asList("wordB1", "wordB2")));
        t1.start();
        t2.start();
    }
}

Console output, if thread BookA is done first:
Result BookA: 'wordA1, wordA2, wordA3'.
Result BookB: 'wordB1, wordB2'.

Console output, if thread BookB is done first:
Result BookB: 'wordB1, wordB2'.
Result BookA: 'wordA1, wordA2, wordA3'.