Safe publication and final fields

Definition

Safe publication means handing an object to other threads so they see it fully built. Objects whose fields are final are safe to share even through a plain field, because Java guarantees other threads see the final fields set by the constructor.

Example code

An immutable object shared between threads

class Settings {
    final String host;                         // final fields are visible once the constructor finishes
    final int port;

    Settings(String host, int port) {
        this.host = host;
        this.port = port;
    }
}

class Demo {
    static Settings current;                    // shared between threads without a lock

    public static void main(String[] args) throws InterruptedException {
        Thread publisher = new Thread(() -> current = new Settings("localhost", 8080));
        publisher.start();
        publisher.join();

        // a reader can never see a half-built Settings with a null host or port 0
        System.out.println(current.host + ":" + current.port);
    }
}

// Output:
// localhost:8080