java - Bow/BowBack 线程锁解释

Question

关于这个https://docs.oracle.com/javase/tutorial/essential/concurrency/newlocks.html

我们有一个我无法理解的 Bow/BowBack 示例，我想要详细解释到底发生了什么

确实，我删除了一些我不明白其用途的行:

    if (! (myLock && yourLock)) {
        if (myLock) {
             lock.unlock();
        }
        if (yourLock) {
             bower.lock.unlock();
        }
    }

而且它仍然有效!

获取锁然后释放它的目的是什么，

在做这项工作之前？然后完成工作后再次释放!!

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();
    }
}

Answer 1

如果线程成功获取锁(或已经拥有锁)，

tryLock() 返回 true。您不理解的代码会尝试获取两个 friend 的锁。如果至少其中一个无法获得:

if (! (myLock && yourLock)) {

然后它释放所获得的(如果有的话):

    if (myLock) {
         lock.unlock();
    }
    if (yourLock) {
         bower.lock.unlock();
    }

因为该方法的重点是，如果两个锁都成功获得，则返回 true，如果两个锁均未成功获得，则返回 false，并且不持有任何锁。