Lock对象

同步代码依赖于简单的可重入锁，这种锁易于使用，但有许多限制，java.util.concurrent.locks包支持更复杂的锁定语法，我们不会详细检查这个包，而是将重点放在其最基本的接口Lock上。

Lock对象的工作方式与同步代码使用的隐式锁定非常相似，与隐式锁一样，一次只有一个线程可以拥有一个Lock对象，Lock对象还通过其关联的Condition对象支持wait/notify机制。

Lock对象优于隐式锁的最大优点是它们能够退出获取锁的尝试，如果锁立即不可用或超时到期之前（如果指定），则tryLock方法退出，如果另一个线程在锁被获得之前发送中断，则lockInterruptibly方法将退出。

让我们使用Lock对象来解决我们在活性中看到的死锁问题，Alphonse和Gaston训练自己注意朋友什么时候要鞠躬。我们通过要求我们的Friend对象必须在继续执行bow之前获取两个参与者的锁来对此进行建模，以下是改进模型Safelock的源代码。为了演示这个语法的多功能性，我们假设Alphonse和Gaston如此迷恋他们新发现的安全鞠躬能力，他们不能停止相互鞠躬：

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();     } }

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