Lock Objects

同步代码依赖于一种简单的可重入锁。这种锁易于使用,但有很多限制。 java.util.concurrent.locks软件包支持更复杂的锁定习惯用法。我们不会详细研究此软件包,而是将重点放在其最基本的interfaceLock上。

Lock对象的工作方式非常类似于同步代码使用的隐式锁。与隐式锁一样,一次只能有一个线程拥有Lock对象。 Lock对象还通过其关联的Condition对象支持wait/notify机制。

Lock对象相对于隐式锁的最大优点是,它们可以退出获取锁的try。如果没有立即或在超时到期之前没有可用的锁定,则tryLock方法将退出。如果在获取锁之前另一个线程发送了中断,则lockInterruptibly方法将退出。

让我们使用Lock对象解决在Liveness中看到的死锁问题。阿方斯(Alphonse)和加斯顿(Gaston)已经训练自己去注意朋友何时要鞠躬。我们通过要求Friend对象在进行弓箭操作之前必须为两个参与者都获得锁来对这种改进进行建模。这是改进模型Safelock的源代码。为了证明这个习语的通用性,我们假设阿方斯和加斯顿对新发现的安全鞠躬能力非常着迷,以至于他们无法停止互相鞠躬:

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();
    }
}