啥？小胖连公平锁 & 非公平锁都不知道？真的菜！

JavaFish

发布于 2020-12-17 14:31:19

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发布于 2020-12-17 14:31:19

文章被收录于专栏：狗哥的 Java 世界

公平锁 & 非公平锁

来到多线程的第十二篇，前十一篇请点文末底部的上、下一篇标签，这篇说说什么是公平锁 & 非公平锁？开篇之前，先聊聊它们的定义以及优缺点。

「公平锁」：多个线程按顺序排队获取锁，每个线程获取机会均等，但永远只有队列首位线程能获取到锁。

优点：每个线程等待一段时间后，都有执行的机会，不至于出现某个线程饿死在队列中。
缺点：是队列里面除了第一个线程，其他的线程都会阻塞，cpu 唤醒阻塞线程的开销会很大。

「非公平锁」：多个线程（不管是不是队列首位）去获取锁时会尝试直接获取锁，能获取到就执行任务，否则乖乖排队。

优点：获取锁更加灵活、吞吐量大、减少 CPU 唤醒线程的开销。
缺点：会出现某些线程永远获取不到锁，饿死在队列中，最终由 JVM 回收。

说了这么一堆，大家也可能看得不是很明白。狗哥就举个在大学跟室友一起排队买早饭的场景。是这样的，帅得一批的狗哥（也就是我啦），跟室友小钊（渣男）、小宝以及小民去买早饭。狗哥以及室友们都看做是一个线程。

「首先是公平锁」

某天狗哥起的最晚，到了饭堂。室友们都已经在排队买早饭了，作为良好市民的狗哥自然也是乖乖过去排队。这样大家都准守秩序，先到先得，很公平。

公平锁

「然后是非公平锁」

还是狗哥起的最晚，到了食堂刚好小宝买完去上徐国保老师的课了。小钊这比还在思考今晚帮哪个妹子修电脑。狗哥不讲武德，趁机插了上去队头，见到是我插队，后面的小钊、小民即使不爽也不敢说啥，只能看着我买早餐。这就是非公平锁插队成功的例子。

非公平锁插队成功

但是，偶尔也有不成功的时候。比如小钊这比思考完要帮妹子阿毛修电脑，这时我想插上去被他发现使出一记闪电五连鞭把我给赶走了，狗哥也是欺软怕硬，只能乖乖去后面排队。这就是非公平锁插队失败的例子。

非公平锁插队失败

看完了这个例子是不是对非公平锁 & 公平锁这对 CP 的理解清晰了很多？刚看例子，不讲代码的行为无耻至极。下面随狗哥来读读源码。

源码分析

「如何使用？」

ReentrantLock lock = new ReentrantLock();

上面的代码熟悉么？其实，大家平时应该有使用过 ReentrantLock 的话就已经使用过非公平锁（ReentrantLock 默认）了。源码如下所示：

想要使用公平锁，创建锁的时候直接给个 true 即可。

ReentrantLock lock = new ReentrantLock(true);

「具体是怎么实现的？」

源码可以看到 ReentrantLock 内部有一个 Sync 内部类。他继承了 AbstractQueuedSynchronizer （也就是我们常说的 AQS），在操作锁的时候大多都是通过 Sync 实现的。

public class ReentrantLock implements Lock, java.io.Serializable {

        private static final long serialVersionUID = 7373984872572414699 L;

        /** Synchronizer providing all implementation mechanics */

        private final Sync sync;

abstract static class Sync extends AbstractQueuedSynchronizer {
    ...
}

下图得知，Sync 他又有两个子类，分别是 NonfairSync（非公平锁） & FairSync（公平锁），见名知义。

「非公平锁加锁实现」

从 nonfairTryAcquire 方法内部的 compareAndSetState 方法可以看到，非公平锁获取锁主要是通过 CAS 方式，修改 state 状态为 1，并且通过 setExclusiveOwnerThread (current) 把当前持有锁的线程改为自己。

static final class NonfairSync extends Sync {
    private static final long serialVersionUID = 7316153563782823691 L;

    final void lock() {
        if (compareAndSetState(0, 1))
            setExclusiveOwnerThread(Thread.currentThread());
        else
            acquire(1);
    }

    protected final boolean tryAcquire(int acquires) {
        return nonfairTryAcquire(acquires);
    }
}

final boolean nonfairTryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();
    // 获取锁的关键代码
    if (c == 0) {
        if (compareAndSetState(0, acquires)) {
            setExclusiveOwnerThread(current);
            return true;
        }
    } else if (current == getExclusiveOwnerThread()) {
        int nextc = c + acquires;
        if (nextc < 0) // overflow
            throw new Error("Maximum lock count exceeded");
        setState(nextc);
        return true;
    }
    return false;
}

从上面我们知道 AbstractQueuedSynchronizer (AQS) 是 ReentrantLock 加解锁的核心，这样说可能有些笼统，还是刚刚的场景，狗哥给你们画几个图理解下。

1、首先狗哥因为长得帅，线程狗哥一进来就 CAS 判断 state == 0 有戏，把自己设置为加锁线程，成功获取到锁并买到早饭了。

非公平锁狗哥顺利获取

2、这时小钊这逼过来想插队，但一判断 state 发现是 1，有人已经持有锁了。于是只能灰溜溜的滚回去排队了。

非公平锁渣男小钊获取失败

3、狗哥买完早饭，把 state 设置为 0 并释放锁，唤醒小钊。

非公平锁唤醒小钊

4、就在唤醒期间（小钊还没醒的时刻）烫了个原谅绿藻头的小宝不讲武德插队，先行 CAS 判断 state == 0，可以设置自己为加锁线程。这时小钊可算醒了，CAS 判断 state，发现是 1，这尼玛。狗子不是说到我了吗？怎么又被人占用了，于是又灰溜溜的去排队。

非公平锁小宝插队

以上就是非公平锁的加锁过程，上面提到非公平锁有部分线程可能会饿死，看完大家也可能理解了。小钊就是那个饿死的线程。。。

「公平锁加锁实现」

公平锁跟非公平锁加锁的逻辑差不多，唯一就是公平加锁的 if 判断中多了 hasQueuedPredecessors 是否队首的判断。

static final class FairSync extends Sync {
    private static final long serialVersionUID = -3000897897090466540 L;

    final void lock() {
        acquire(1);
    }

    protected final boolean tryAcquire(int acquires) {
        final Thread current = Thread.currentThread();
        int c = getState();
        if (c == 0) {
            // 多了 hasQueuedPredecessors 是否队首判断
            if (!hasQueuedPredecessors() &&
                compareAndSetState(0, acquires)) {
                setExclusiveOwnerThread(current);
                return true;
            }
        } else if (current == getExclusiveOwnerThread()) {
            int nextc = c + acquires;
            if (nextc < 0)
                throw new Error("Maximum lock count exceeded");
            setState(nextc);
            return true;
        }
        return false;
    }
}

这点从以下源码可以看出，就算看不懂。注释也写得很清楚。

hasQueuedPredecessors

以上还是刚刚的场景，狗哥继续给你们画几个图理解下公平锁。

1、狗哥今天起得比较早（人长得帅，还这么努力）。来到食堂就 CAS 判断 state 是不是 = 0，是就修改为 1，，一然后发现自己居然排第一，最后把自己设置为加锁线程，成功买早饭。

公平锁狗哥成功获取锁

2、小钊这比昨晚帮妹子修电脑起得比较晚，来到饭堂先判断下 state 判断状态。发现 = 1，有人占用。只能灰溜溜的去排队。

公平锁小钊滚回去排队

3、过段时间，狗哥买完早饭，将 state 设置为 0，并且把持有锁线程设置为 null，然后去唤醒队首的小钊。

（在小钊还未醒的时刻）另一位绿藻头小民，昨晚看语气助词片看的比较晚。来了就判断 state == 0，想插队。但是公平锁规定必须队首获取锁，他发现自己不是队首，没法获取锁很尴尬。

公平锁小民插队不成功

4、终于，小钊醒了。判断 state == 0，修改为 1。此时不能忘记还要看看自己是不是队首。发现是，最后把持有锁线程修改为自己，开心的买到了早餐。

公平锁小钊成功获取锁

看到这里，相信大家也彻底理解了吧？公平锁的缺点就是必须队首线程获取锁。如上例子小民都 CAS 了一遍，但因为不是队首，还是得阻塞。增加了 CPU 负担。

一个特例

针对 tryLock () 方法，它不遵守设定的公平原则。

❝例如，当有线程执行 tryLock () 方法的时候，一旦有线程释放了锁，那么这个正在 tryLock 的线程就能获取到锁，即使设置的是公平锁模式，即使在它之前已经有其他正在等待队列中等待的线程，简单地说就是 tryLock 可以插队。 ❞

看它的源码就会发现：

public boolean tryLock() {
    return sync.nonfairTryAcquire(1);
}

这里调用的就是 nonfairTryAcquire ()，表明了是不公平的，和锁本身是否是公平锁无关。

测试代码

/**
 * 描述：演示公平锁，分别展示公平和不公平的情况，非公平锁会让现在持有锁的线程优先再次获取到锁。代码借鉴自Java并发编程实战手册2.7。
 */

public class FairAndUnfair {

    public static void main(String args[]) {

        PrintQueue printQueue = new PrintQueue();

        Thread thread[] = new Thread[10];

        for (int i = 0; i < 10; i++) {
            thread[i] = new Thread(new Job(printQueue), "Thread " + i);
        }

        for (int i = 0; i < 10; i++) {
            thread[i].start();
            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

class Job implements Runnable {

    private PrintQueue printQueue;

    public Job(PrintQueue printQueue) {
        this.printQueue = printQueue;
    }

    @Override
    public void run() {
        System.out.printf("%s: Going to print a job\n", Thread.currentThread().getName());
        printQueue.printJob(new Object());
        System.out.printf("%s: The document has been printed\n", Thread.currentThread().getName());
    }
}

class PrintQueue {

    private final Lock queueLock = new ReentrantLock(false);

    public void printJob(Object document) {

        queueLock.lock();

        try {
            Long duration = (long)(Math.random() * 10000);
            System.out.printf("%s: PrintQueue: Printing a Job during %d seconds\n",
                Thread.currentThread().getName(), (duration / 1000));
            Thread.sleep(duration);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            queueLock.unlock();
        }

        queueLock.lock();

        try {
            Long duration = (long)(Math.random() * 10000);
            System.out.printf("%s: PrintQueue: Printing a Job during %d seconds\n",
                Thread.currentThread().getName(), (duration / 1000));
            Thread.sleep(duration);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            queueLock.unlock();
        }
    }
}

我们可以通过改变 new ReentrantLock (false) 中的参数来设置公平 / 非公平锁。以上代码在公平的情况下的输出：

Thread 0: Going to print a job
Thread 0: PrintQueue: Printing a Job during 5 seconds
Thread 1: Going to print a job
Thread 2: Going to print a job
Thread 3: Going to print a job
Thread 4: Going to print a job
Thread 5: Going to print a job
Thread 6: Going to print a job
Thread 7: Going to print a job
Thread 8: Going to print a job
Thread 9: Going to print a job
Thread 1: PrintQueue: Printing a Job during 3 seconds
Thread 2: PrintQueue: Printing a Job during 4 seconds
Thread 3: PrintQueue: Printing a Job during 3 seconds
Thread 4: PrintQueue: Printing a Job during 9 seconds
Thread 5: PrintQueue: Printing a Job during 5 seconds
Thread 6: PrintQueue: Printing a Job during 7 seconds
Thread 7: PrintQueue: Printing a Job during 3 seconds
Thread 8: PrintQueue: Printing a Job during 9 seconds
Thread 9: PrintQueue: Printing a Job during 5 seconds
Thread 0: PrintQueue: Printing a Job during 8 seconds
Thread 0: The document has been printed
Thread 1: PrintQueue: Printing a Job during 1 seconds
Thread 1: The document has been printed
Thread 2: PrintQueue: Printing a Job during 8 seconds
Thread 2: The document has been printed
Thread 3: PrintQueue: Printing a Job during 2 seconds
Thread 3: The document has been printed
Thread 4: PrintQueue: Printing a Job during 0 seconds
Thread 4: The document has been printed
Thread 5: PrintQueue: Printing a Job during 7 seconds
Thread 5: The document has been printed
Thread 6: PrintQueue: Printing a Job during 3 seconds
Thread 6: The document has been printed
Thread 7: PrintQueue: Printing a Job during 9 seconds
Thread 7: The document has been printed
Thread 8: PrintQueue: Printing a Job during 5 seconds
Thread 8: The document has been printed
Thread 9: PrintQueue: Printing a Job during 9 seconds
Thread 9: The document has been printed

可以看出，线程直接获取锁的顺序是完全公平的，先到先得。而以上代码在非公平的情况下的输出是这样的：

Thread 0: Going to print a job
Thread 0: PrintQueue: Printing a Job during 6 seconds
Thread 1: Going to print a job
Thread 2: Going to print a job
Thread 3: Going to print a job
Thread 4: Going to print a job
Thread 5: Going to print a job
Thread 6: Going to print a job
Thread 7: Going to print a job
Thread 8: Going to print a job
Thread 9: Going to print a job
Thread 0: PrintQueue: Printing a Job during 8 seconds
Thread 0: The document has been printed
Thread 1: PrintQueue: Printing a Job during 9 seconds
Thread 1: PrintQueue: Printing a Job during 8 seconds
Thread 1: The document has been printed
Thread 2: PrintQueue: Printing a Job during 6 seconds
Thread 2: PrintQueue: Printing a Job during 4 seconds
Thread 2: The document has been printed
Thread 3: PrintQueue: Printing a Job during 9 seconds
Thread 3: PrintQueue: Printing a Job during 8 seconds
Thread 3: The document has been printed
Thread 4: PrintQueue: Printing a Job during 4 seconds
Thread 4: PrintQueue: Printing a Job during 2 seconds
Thread 4: The document has been printed
Thread 5: PrintQueue: Printing a Job during 2 seconds
Thread 5: PrintQueue: Printing a Job during 5 seconds
Thread 5: The document has been printed
Thread 6: PrintQueue: Printing a Job during 2 seconds
Thread 6: PrintQueue: Printing a Job during 6 seconds
Thread 6: The document has been printed
Thread 7: PrintQueue: Printing a Job during 6 seconds
Thread 7: PrintQueue: Printing a Job during 4 seconds
Thread 7: The document has been printed
Thread 8: PrintQueue: Printing a Job during 3 seconds
Thread 8: PrintQueue: Printing a Job during 6 seconds
Thread 8: The document has been printed
Thread 9: PrintQueue: Printing a Job during 3 seconds
Thread 9: PrintQueue: Printing a Job during 5 seconds
Thread 9: The document has been printed

巨人的肩膀

https://kaiwu.lagou.com/course/courseInfo.htm?courseId=16#/detail/pc?id=262
https://mp.weixin.qq.com/s/Si6XvLd5NxkTUAi_IUncFg

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