Happens-before

Definition

"Happens-before" is the rule that says when one thread is guaranteed to see what another thread wrote. Starting a thread, joining it, releasing a lock and writing a volatile field all create such a guarantee. Without one, a thread may see old values.

Example code

start() and join() make writes visible

class Demo {
    static int result;                       // a plain field: no volatile, no lock

    public static void main(String[] args) throws InterruptedException {
        result = 10;                          // written before start(): the new thread is guaranteed to see it

        Thread worker = new Thread(() -> {
            System.out.println("worker sees " + result);
            result = result * 2;              // written by the worker
        });
        worker.start();
        worker.join();                         // join() makes the worker's writes visible to main

        System.out.println("main sees " + result);
    }
}

// Output:
// worker sees 10
// main sees 20