java - java oracle 教程中带有锁对象的 Bow/Bower 示例

Question

在 java 文档示例中，lock.unlock()方法使用了两次，一次是在 impendingBow() 方法中，一次是在 Bow() 方法中。我的问题是，不应该lock.unlock() impendingBow()中的方法方法可以解决问题，为什么在 bow() 中再次使用它方法？下面的代码是来自 java 文档的片段:

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

shouldn't lock.unlock() method in impendingBow() method do the trick, why use it again in bow() method?

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