java - 在java中解决死锁的替代方法

Question

我正在阅读 Java Concurrency Tutorial来自 Oracle。

还有一个例子 DeadLock

核心代码:

    public synchronized void bow(Friend bower) {
        System.out.format("%s: %s" + "  has bowed to me!%n", this.name, bower.getName());
        bower.bowBack(this);
    }

    public synchronized void bowBack(Friend bower) {
        System.out.format("%s: %s" + " has bowed back to me!%n", this.name, bower.getName());
    }

然后当教程转到 LockObject 时，它提供了一个处理死锁的解决方案 Lock Object .

现在我想知道是否有另一种方法可以在不使用 Lock 对象的情况下避免死锁？

Answer 1

避免死锁的一种方法是通过强加获取两个锁的顺序来打破对称性。例如，你可能会说，当开始鞠躬时，线程应该总是锁定名字按字母顺序排在第一位的 friend ，这样任何做鞠躬的线程都必须先获得 Alphonse 的锁，然后再获得 Gaston 的锁，永远不会反过来:

    public void bow(Friend bower) {
        Friend lock = (this.name.compareTo(bower.name)<0)? this : bower);
        synchronized (lock) {
            System.out.format("%s: %s has bowed to me!%n", this.name, bower.getName());
            bower.bowBack(this);
        }
    }
    public void bowBack(Friend bower) {
        Friend lock = (this.name.compareTo(bower.name)>0)? this : bower);
        synchronized (lock) {
            System.out.format("%s: %s has bowed back to me!%n", this.name, bower.getName());
        }
    }

无锁选项使用原子变量来指示弓是否已经在进行中，如果有则等待:

    private static AtomicBoolean bowing = new AtomicBoolean();
    public void bow(Friend bower) {
        while (!bowing.compareAndSet(false, true)) Thread.yield();

        System.out.format("%s: %s has bowed to me!%n", this.name, bower.getName());
        bower.bowBack(this);

        bowing.set(false);
    }

另一种避免死锁的方法是使用单个锁而不是两个；竞争同一个锁的两个线程不会死锁。

    private static Object lock = new Object();
    public void bow(Friend bower) {
        synchronized (lock) {
            System.out.format("%s: %s has bowed to me!%n", this.name, bower.getName());
            bower.bowBack(this);
        }
    }

    public void bowBack(Friend bower) {
        synchronized (lock) {
            System.out.format("%s: %s has bowed back to me!%n", this.name, bower.getName());
        }
    }