Oracle官方并发教程之锁对象

原文地址，译文地址，译者：李任，校对：郑旭东

同步代码依赖于一种简单的可重入锁。这种锁使用简单，但也有诸多限制。java.util.concurrent.locks包提供了更复杂的锁。我们不会详细考察这个包，但会重点关注其最基本的接口，锁。

锁对象作用非常类似同步代码使用的隐式锁。如同隐式锁，每次只有一个线程可以获得锁对象。通过关联Condition对象，锁对象也支持wait/notify机制。

锁对象之于隐式锁最大的优势在于，它们有能力收回获得锁的尝试。如果当前锁对象不可用，或者锁请求超时（如果超时时间已指定），tryLock方法会收回获取锁的请求。如果在锁获取前，另一个线程发送了一个中断，lockInterruptibly方法也会收回获取锁的请求。

让我们使用锁对象来解决我们在活跃度中见到的死锁问题。Alphonse和Gaston已经把自己训练成能注意到朋友何时要鞠躬。我们通过要求Friend对象在双方鞠躬前必须先获得锁来模拟这次改善。下面是改善后模型的源代码，Safelock。为了展示其用途广泛，我们假设Alphonse和Gaston对于他们新发现的稳定鞠躬的能力是如此入迷，以至于他们无法不相互鞠躬。

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();

}