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();
}
}
文章转自
并发编程网-ifeve.com
