concurrency - 活锁的好例子？

Question

我明白什么是活锁，但我想知道是否有人有一个很好的基于代码的示例？通过基于代码，我并不是指“两个人试图在走廊里超越对方”。如果我再读一遍，我就会失去午餐。

Answer 1

这是一个非常简单的 Java 活锁示例，丈夫和妻子试图喝汤，但他们之间只有一把勺子。夫妻双方都太客气了，如果对方还没吃饭，就会把勺子递过去。

public class Livelock {
    static class Spoon {
        private Diner owner;
        public Spoon(Diner d) { owner = d; }
        public Diner getOwner() { return owner; }
        public synchronized void setOwner(Diner d) { owner = d; }
        public synchronized void use() { 
            System.out.printf("%s has eaten!", owner.name); 
        }
    }
    
    static class Diner {
        private String name;
        private boolean isHungry;
        
        public Diner(String n) { name = n; isHungry = true; }       
        public String getName() { return name; }
        public boolean isHungry() { return isHungry; }
        
        public void eatWith(Spoon spoon, Diner spouse) {
            while (isHungry) {
                // Don't have the spoon, so wait patiently for spouse.
                if (spoon.owner != this) {
                    try { Thread.sleep(1); } 
                    catch(InterruptedException e) { continue; }
                    continue;
                }                       
            
                // If spouse is hungry, insist upon passing the spoon.
                if (spouse.isHungry()) {                    
                    System.out.printf(
                        "%s: You eat first my darling %s!%n", 
                        name, spouse.getName());
                    spoon.setOwner(spouse);
                    continue;
                }
                
                // Spouse wasn't hungry, so finally eat
                spoon.use();
                isHungry = false;               
                System.out.printf(
                    "%s: I am stuffed, my darling %s!%n", 
                    name, spouse.getName());                
                spoon.setOwner(spouse);
            }
        }
    }
    
    public static void main(String[] args) {
        final Diner husband = new Diner("Bob");
        final Diner wife = new Diner("Alice");
        
        final Spoon s = new Spoon(husband);
        
        new Thread(new Runnable() { 
            public void run() { husband.eatWith(s, wife); }   
        }).start();

        new Thread(new Runnable() { 
            public void run() { wife.eatWith(s, husband); } 
        }).start();
    }
}

运行程序，你会得到:

Bob: You eat first my darling Alice!
Alice: You eat first my darling Bob!
Bob: You eat first my darling Alice!
Alice: You eat first my darling Bob!
Bob: You eat first my darling Alice!
Alice: You eat first my darling Bob!
...

如果不间断的话，这将永远持续下去。这是一个活锁，因为 Alice 和 Bob 都在无限循环中反复要求对方先走(因此live)。在死锁情况下，Alice 和 Bob 都会被卡住，等待对方先走——除了等待之外，他们不会做任何事情(因此死了)。