Exemple Deadlock & Livelock

Deadlock

Cand doua sau mai multe fire de executie sunt blocate in asteptarea eliberarii unor resurse de care au nevoie. Solutie: how to prevent deadlock in Java.

private static void deadlock() {
    final String res1 = "Resource_1";
    final String res2 = "Resource_2";

    Thread t1 = new Thread(() -> {
        synchronized (res1) {
            System.out.println("[t1] acquired access to res1");
            try {
                Thread.sleep(5000);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
            synchronized (res2) {
                System.out.println("[t1] Yay! escaped deadlock."); // not happening
            }
        }
    });
    Thread t2 = new Thread(() -> {
        synchronized (res2) {
            System.out.println("[t2] acquired access to res2");
            try {
                Thread.sleep(5000);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
            synchronized (res1) {
                System.out.println("[t2] Yay! escaped deadlock.");
            }
        }
    });

    t1.start();
    t2.start();
}

Livelock

Doua sau mai multe fire de executie isi cedeaza dreptul de a rula in favoarea celorlalte astfel ajungand sa nu ruleze niciodata, iar aplicatia nu progreseaza. Solutie: schimbarea logicii.

static class Spoon {
    Diner owner;

    public Spoon (Diner firstOwner) {
        this.owner = firstOwner;
    }

    synchronized void setOwner(Diner owner) {
        this.owner = owner;
    }

    synchronized void use() {
        System.out.println(owner.name + " just ate!");
    }
}

static class Diner {
    String name;
    boolean isHungry;

    public Diner (String name) {
        this.name = name;
        this.isHungry = true;
    }

    public void eatWith (Spoon spoon, Diner spouse) {
        while (isHungry) {
            if (spoon.owner != this) {
                // wait for a while for the spoon to be released
                try {
                    Thread.sleep(3000);
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                }
            }

            // after wait, try to give the spoon to the spouse if she's hungry
            if (spouse.isHungry) {
                System.out.println("[" + name + "] Eat, baby, eat!");
                spoon.owner = spouse;
            } else {
                // finally
                spoon.use();
                isHungry = false;
                System.out.println("[" + name + "] Finally ate!!!"); // never
                spoon.owner = spouse;
            }
        }
    }
}

private static void livelock() {
    final Diner husband = new Diner("Adnan");
    final Diner wife = new Diner("Hannan");
    Spoon spoon = new Spoon(wife);
    
    try {
        new Thread(() -> husband.eatWith(spoon, wife)).start();
        Thread.sleep(1500);
        new Thread(() -> wife.eatWith(spoon, husband)).start();
    } catch (InterruptedException e) {
        throw new RuntimeException(e);
    }
}