Java™ 教程（Lock對象）

Lock對象

同步代碼依賴於簡單的可重入鎖，這種鎖易於使用，但有許多限制，java.util.concurrent.locks包支持更復雜的鎖定語法，咱們不會詳細檢查這個包，而是將重點放在其最基本的接口Lock上。

Lock對象的工做方式與同步代碼使用的隱式鎖定很是類似，與隱式鎖同樣，一次只有一個線程能夠擁有一個Lock對象，Lock對象還經過其關聯的Condition對象支持wait/notify機制。

Lock對象優於隱式鎖的最大優勢是它們可以退出獲取鎖的嘗試，若是鎖當即不可用或超時到期以前（若是指定），則tryLock方法退出，若是另外一個線程在鎖被得到以前發送中斷，則lockInterruptibly方法將退出。

讓咱們使用Lock對象來解決咱們在活性中看到的死鎖問題，Alphonse和Gaston訓練本身注意朋友何時要鞠躬。咱們經過要求咱們的Friend對象必須在繼續執行bow以前獲取兩個參與者的鎖來對此進行建模，如下是改進模型Safelock的源代碼。爲了演示這個語法的多功能性，咱們假設Alphonse和Gaston如此迷戀他們新發現的安全鞠躬能力，他們不能中止相互鞠躬：

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();
    }
}

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