浅谈JAVA并发之ReentrantLock

结合上面的ReentrantLock类图，ReentrantLock实现了Lock接口，它的内部类Sync继承自AQS，绝大部分使用AQS的子类需要自定义的方法存在Sync中。而ReentrantLock有公平与非公平的区别，即’是否先阻塞就先获取资源’，它的主要实现就是FairSync与NonfairSync，后面会从源码角度看看它们的区别。

Sync是ReentrantLock控制同步的基础。它的子类分为了公平与非公平。使用AQS的state代表获取锁的数量

abstract static class Sync extends AbstractQueuedSynchronizer { private static final long serialVersionUID = -5179523762034025860L; /*** Performs {@link Lock#lock}. The main reason for subclassing* is to allow fast path for nonfair version.*/ abstract void lock(); ...}

我们可以看出内部类Sync是一个抽象类，继承它的子类(FairSync与NonfairSync)需要实现抽象方法lock。

下面我们先从非公平锁的角度来看看获取资源与释放资源的原理

故事就从就两个变量开始:

// 获取一个非公平的独占锁/*** public ReentrantLock() {* sync = new ReentrantLock.NonfairSync();* }*/private Lock lock = new ReentrantLock();// 获取条件变量private Condition condition = lock.newCondition();2.1 上锁(获取资源)

lock.lock()

public void lock() { sync.lock();}

static final class NonfairSync extends Sync { private static final long serialVersionUID = 7316153563782823691L; // 获取资源 final void lock() {// 若此时没有线程获取到资源，直接设置当前线程独占访问资源。if (compareAndSetState(0, 1)) setExclusiveOwnerThread(Thread.currentThread());else // AQS的方法 acquire(1); } protected final boolean tryAcquire(int acquires) {// 实现在父类Sync中return nonfairTryAcquire(acquires); }}

AQS的acquire

public final void acquire(int arg) { if (!tryAcquire(arg) &&acquireQueued(addWaiter(Node.EXCLUSIVE), arg))selfInterrupt();}

// Sync实现的非公平的tryAcquirefinal boolean nonfairTryAcquire(int acquires) { final Thread current = Thread.currentThread(); int c = getState(); // 此时若没有线程获取到资源，当前线程就直接占用该资源 if (c == 0) {if (compareAndSetState(0, acquires)) { setExclusiveOwnerThread(current); return true;} } // 若当前线程已经占用了该资源，可以再次获取该资源 ->这个行为就是可重入锁的支撑 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;}

尝试获取资源的过程是非常简单的，这里再贴一下acquire的流程

lock.unlock();

public void unlock() { // AQS的方法 sync.release(1);}

AQS的release

public final boolean release(int arg) { if (tryRelease(arg)) {Node h = head;if (h != null && h.waitStatus != 0) unparkSuccessor(h);return true; } return false;}

release的流程已经剖析过了，接下来看看tryRelease的实现

protected final boolean tryRelease(int releases) { int c = getState() - releases; // 这里可以看出若没有持有锁，就释放资源，就会报错 if (Thread.currentThread() != getExclusiveOwnerThread())throw new IllegalMonitorStateException(); boolean free = false; if (c == 0) {free = true;setExclusiveOwnerThread(null); } setState(c); return free;}

tryRelease的实现也很简单，这里再贴一下release的流程图

公平锁与非公平锁，即’是否先阻塞就先获取资源’, ReentrantLock中公平与否的控制就在tryAcquire中。下面我们看看，公平锁的tryAcquire

static final class FairSync extends Sync {private static final long serialVersionUID = -3000897897090466540L;final void lock() { acquire(1);}protected final boolean tryAcquire(int acquires) { final Thread current = Thread.currentThread(); int c = getState(); if (c == 0) {// (2.3.1)// sync queue中是否存在前驱结点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;} }

区别在代码(2.3.1)

hasQueuedPredecessors

判断当前线程的前面有无其他线程排队；若当前线程在队列头部或者队列为空返回false

public final boolean hasQueuedPredecessors() { // The correctness of this depends on head being initialized // before tail and on head.next being accurate if the current // thread is first in queue. Node t = tail; // Read fields in reverse initialization order Node h = head; Node s; return h != t &&((s = h.next) == null || s.thread != Thread.currentThread());}

结合下面的入队代码(enq), 我们分析hasQueuedPredecessors为true的情况:

1.h != t ,表示此时queue不为空; (s = h.next) == null, 表示另一个结点已经运行了下面的步骤(2)，还没来得及运行步骤(3)。简言之，就是B线程想要获取锁的同时，A线程获取锁失败刚好在入队(B入队的同时，之前占有的资源的线程，刚好释放资源)

2.h != t 且 (s = h.next) ！= null，表示此时至少有一个结点在sync queue中；s.thread != Thread.currentThread()，这个情况比较复杂，设想一下有这三个结点 A -> B C， A此时获取到资源，而B此时因为获取资源失败正在sync queue阻塞，C还没有获取资源(还没有执行tryAcquire)。

时刻一：A释放资源成功后(执行tryRelease成功)，B此时还没有成功获取资源(C执行s = h.next时，B还在sync queue中且是老二)

时刻二: C此时执行hasQueuedPredecessors，s.thread != Thread.currentThread()成立，此时s.thread表示的是B

private Node enq(final Node node) { for (;;) {Node t = tail;if (t == null) { // Must initialize if (compareAndSetHead(new Node())) // (1) 第一次初始化tail = head;} else { node.prev = t; if (compareAndSetTail(t, node)) { // (2) 设置queue的tailt.next = node; // (3)return t; }} }}

Note that 1. because cancellations due to interrupts and timeouts may occur at any time, a true return does not guarantee that some other thread will acquire before the current thread（虚假true）. 2. Likewise, it is possible for another thread to win a race to enqueue after this method has returned false, due to the queue being empty（虚假false）.

这位大佬对hasQueuedPredecessors进行详细的分析，他文中解释了虚假true以及虚假false。我这里简单解释一下:

1.虚假true, 当两个线程都执行tryAcquire，都执行到hasQueuedPredecessors，都返回true，但是只有一个线程执行compareAndSetState(0, acquires)成功

2.虚假false，当一个线程A执行doAcquireInterruptibly，发生了中断，还没有清除掉该结点时；此时，线程B执行hasQueuedPredecessors时，返回true

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