Lock Objects 锁对象

   同步代码依赖一种简单的可重入锁。虽然这种锁很容易使用，但是有许多局限。许多复杂的锁形式是由java.util.concurrent.locks包支持。我们不详细解释这个包，但是了关注它最重要最基本的接口，Lock。

   Lock对象的工作机制与用同步代码的隐式锁很相似。作为隐式锁，只有一个线程能一次拥有一个锁对象。所对象也支持wait/notify机制。，通过它们关联的条件对象。

   锁对象超过瘾式锁的最大优点是它们能撤回试图获取一个锁。tryLock方法会立即撤回如果发现锁是不可用的或者时间超时（如果有时间限制）。lockInterruptibly 方法撤回，如果另外一个线程发了一个中断在获取到锁之前。

   让我们使用锁对象解决活性（Liveness）小节中的死锁问题。Alphonse和Gaston已经训练他们去通知，当一个朋友将鞠躬的时候。我们改进了模型，通过要求在互相鞠躬前我们的朋友对象必须获取参与者双方的锁。这里是改进模型的源码，Safelock。为了验证这种形式的功能，我们假设 Alphonse和Gaston这样迷恋安全鞠躬的能力，他们不可以停止相互鞠躬。

   import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.Random;

public class Safelock {
    static class Friend {
        private final String name;
        private final Lock lock = new ReentrantLock();

        public Friend(String name) {
            this.name = name;
        }

        public String getName() {
            return this.name;
        }

        public boolean impendingBow(Friend bower) {
            Boolean myLock = false;
            Boolean yourLock = false;
            try {
                myLock = lock.tryLock();
                yourLock = bower.lock.tryLock();
            } finally {
                if (! (myLock && yourLock)) {
                    if (myLock) {
                        lock.unlock();
                    }
                    if (yourLock) {
                        bower.lock.unlock();
                    }
                }
            }
            return myLock && yourLock;
        }
           
        public void bow(Friend bower) {
            if (impendingBow(bower)) {
                try {
                    System.out.format("%s: %s has"
                        + " bowed to me!%n",
                        this.name, bower.getName());
                    bower.bowBack(this);
                } finally {
                    lock.unlock();
                    bower.lock.unlock();
                }
            } else {
                System.out.format("%s: %s started"
                    + " to bow to me, but saw that"
                    + " I was already bowing to"
                    + " him.%n",
                    this.name, bower.getName());
            }
        }

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

    static class BowLoop implements Runnable {
        private Friend bower;
        private Friend bowee;

        public BowLoop(Friend bower, Friend bowee) {
            this.bower = bower;
            this.bowee = bowee;
        }
   
        public void run() {
            Random random = new Random();
            for (;;) {
                try {
                    Thread.sleep(random.nextInt(10));
                } catch (InterruptedException e) {}
                bowee.bow(bower);
            }
        }
    }
           

    public static void main(String[] args) {
        final Friend alphonse =
            new Friend("Alphonse");
        final Friend gaston =
            new Friend("Gaston");
        new Thread(new BowLoop(alphonse, gaston)).start();
        new Thread(new BowLoop(gaston, alphonse)).start();
    }
}

这里后面添加这段源码的分析。