Visibility with volatile

Definition

Without volatile, a thread may keep a stale copy of a field in a CPU cache or a register and never notice that another thread changed it. A write to a volatile field happens-before every later read of it, so the reader sees the update and everything written before it.

Example code

A flag publishes the data written before it

class Demo {
    static int data;
    static volatile boolean ready;            // the volatile flag publishes everything written before it

    public static void main(String[] args) throws InterruptedException {
        Thread reader = new Thread(() -> {
            while (!ready) { }                 // wait for the flag
            System.out.println("reader sees data = " + data);   // guaranteed to be 42
        });
        reader.start();

        Thread.sleep(50);
        data = 42;                             // 1. write the data
        ready = true;                          // 2. then set the volatile flag
        reader.join();
    }
}

// Output:
// reader sees data = 42