ThreadLocal

Definition

A ThreadLocal gives every thread its own copy of a value. It is handy for per-thread context such as the current user or a non-thread-safe helper, but values must be removed when you are done, especially in thread pools.

Example code

Each thread sees only its own value

class Demo {
    static final ThreadLocal<String> currentUser = new ThreadLocal<>();

    public static void main(String[] args) throws InterruptedException {
        currentUser.set("main-user");

        Thread other = new Thread(() -> {
            System.out.println("other thread sees: " + currentUser.get());     // null: it has no value yet
            currentUser.set("other-user");
            System.out.println("other thread now sees: " + currentUser.get());
            currentUser.remove();                                              // clean up when done
        });
        other.start();
        other.join();

        System.out.println("main still sees: " + currentUser.get());           // unchanged
    }
}

// Output:
// other thread sees: null
// other thread now sees: other-user
// main still sees: main-user