带有锁定对象的java oracle教程中的Bow/Bower示例

Question

在java文档的例子中，lock.unlock()方法被使用了两次，一次在impendingBow((方法和bow((方法中。我的问题是，lock.unlock()方法不应该impendingBow()方法中解决问题，为什么要在bow()方法中再次使用它？下面的代码是java文档的一个片段：

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());
}
}

}

Answer 1

lock.unlock()方法不应该impendingBow()方法可以

解决问题，为什么要在bow()方法中再次使用它？

在impendingBow中，当两次锁定尝试都成功时，将不会达到lock.unlock()。该方法返回true，使两个锁都处于锁定状态。请参阅解锁调用前的条件语句。在这种情况下(两个锁都成功(，bow()必须解锁锁。