Lock的实现之ReentrantLock详解
摘要
Lock在硬件层面依赖CPU指令，完全由Java代码完成，底层利用LockSupport类和Unsafe类进行操作；
虽然锁有很多实现，但是都依赖AbstractQueuedSynchronizer类，我们用ReentrantLock进行讲解；
ReentrantLock调用过程
ReentrantLock类的API调用都委托给一个内部类 Sync ,而该类继承了 AbstractQueuedSynchronizer类；
public class ReentrantLock implements Lock, java.io.Serializable { ...... abstract static class Sync extends AbstractQueuedSynchronizer { ......
而Sync又分为两个子类：公平锁和非公平锁，默认为非公平锁
/** * Sync object for non-fair locks */ static final class NonfairSync extends Sync {
/** * Sync object for fair locks */ static final class FairSync extends Sync {
Lock的调用过程如下图(其中涉及到 ReentrantLock类、Sync(抽象类)、AbstractQueuedSynchronizer类，NofairSync类，这些类将 Template方法用的淋漓尽致，相当赞)：
先来一张类依赖图：
再来一张lock调用图：
Lock API详解
自底而上来看，由被调用一步步向上分析
nofairTryAcquire
/** * Performs non-fair tryLock. tryAcquire is implemented in * subclasses, but both need nonfair try for trylock method. */ final boolean nonfairTryAcquire(int acquires) { final Thread current = Thread.currentThread(); int c = getState(); 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; }
来看这段代码，首先获取当前状态（初始化为0），当它等于0的时候，代表还没有任何线程获得该锁，然后通过CAS（底层是通过CompareAndSwapInt实现）改变state，并且设置当前线程为持有锁的线程；其他线程会直接返回false;当该线程重入的时候，state已经不等于0，这个时候并不需要CAS，因为该线程已经持有锁，然后会重新通过setState设置state的值，这里就实现了一个偏向锁的功能，即锁偏向该线程；
addWaiter
只有当锁被一个线程持有，另外一个线程请求获得该锁的时候才会进入这个方法
/** * Creates and enqueues node for current thread and given mode. * * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared * @return the new node */ private Node addWaiter(Node mode) { Node node = new Node(Thread.currentThread(), mode); // Try the fast path of enq; backup to full enq on failure Node pred = tail; if (pred != null) { node.prev = pred; if (compareAndSetTail(pred, node)) { pred.next = node; return node; } } enq(node); return node; }
首先持有该锁之外的线程进入到该方法，这里涉及到一个CLH（三个人的名字首字母：Craig, Landin, and Hagersten）队列，其实就是一个链表，
简单说下CLH队列：
CLH队列由node节点组成，mode代表每个Node有两种模式：共享模式和排他模式，并且维护了一个状态：waitStatus，可取值如下：
- CANCELLED = 1 由于超时或者被打断，该线程被取消，将不会被block；
- SIGNAL = -1 当前线程的后继节点线程通过park正处于或即将处于block状态；
- CONDITION = -2 当前线程正处于条件队列，正式因为调用了condition.await造成阻塞；
- PROPAGATE = -3 共享锁应该被传播出去
首先，new一个节点，这个时候模式为：mode为 Node.EXCLUSIVE，默认为null即排它锁；
然后：
如果该队列已经有node即tail!=null，则将新节点的前驱节点置为tail，再通过CAS将tail指向当前节点，前驱节点的后继节点指向当前节点，然后返回当前节点；
如果队列为空或者CAS失败，则通过enq入队：
/** * Inserts node into queue, initializing if necessary. See picture above. * @param node the node to insert * @return node's predecessor */ private Node enq(final Node node) { for (;;) { Node t = tail; if (t == null) { // Must initialize if (compareAndSetHead(new Node())) tail = head; } else { node.prev = t; if (compareAndSetTail(t, node)) { t.next = node; return t; } } } }
进队的时候，要么是第一个入队并且设置head节点并且循环设置tail，要么是add tail，如果CAS不成功，则会无限循环，直到设置成功，即使高并发的场景，也最终能够保证设置成功，然后返回包装好的node节点；
acquireQueued
/** * Acquires in exclusive uninterruptible mode for thread already in * queue. Used by condition wait methods as well as acquire. * * @param node the node * @param arg the acquire argument * @return {@code true} if interrupted while waiting */ final boolean acquireQueued(final Node node, int arg) { boolean failed = true; try { boolean interrupted = false; for (;;) { final Node p = node.predecessor(); if (p == head && tryAcquire(arg)) { setHead(node); p.next = null; // help GC failed = false; return interrupted; } if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt()) interrupted = true; } } finally { if (failed) cancelAcquire(node); } }
该方法的主要作用就是将已经进入虚拟队列的节点进行阻塞，我们看到，如果当前节点的前驱节点是head并且尝试获取锁的时候成功了，则直接返回，不需要阻塞；
如果前驱节点不是头节点或者获取锁的时候失败了，则进行判定是否需要阻塞：
/** * Checks and updates status for a node that failed to acquire. * Returns true if thread should block. This is the main signal * control in all acquire loops. Requires that pred == node.prev. * * @param pred node's predecessor holding status * @param node the node * @return {@code true} if thread should block */ private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) { int ws = pred.waitStatus; if (ws == Node.SIGNAL) /* * This node has already set status asking a release * to signal it, so it can safely park. */ return true; if (ws > 0) { /* * Predecessor was cancelled. Skip over predecessors and * indicate retry. */ do { node.prev = pred = pred.prev; } while (pred.waitStatus > 0); pred.next = node; } else { /* * waitStatus must be 0 or PROPAGATE. Indicate that we * need a signal, but don't park yet. Caller will need to * retry to make sure it cannot acquire before parking. */ compareAndSetWaitStatus(pred, ws, Node.SIGNAL); } return false; }
这段代码对该节点的前驱节点的状态进行判断，如果前驱节点已经处于signal状态，则返回true，表明当前节点可以进入阻塞状态；
否则，将前驱节点状态CAS置为signal状态，然后通过上层的for循环进入parkAndCheckInterrupt代码块park：
/** * Convenience method to park and then check if interrupted * * @return {@code true} if interrupted */ private final boolean parkAndCheckInterrupt() { LockSupport.park(this); return Thread.interrupted(); }
这个时候将该线程交给操作系统内核进行阻塞；
总体来讲，acquireQueued就是依靠前驱节点的状态来决定当前线程是否应该处于阻塞状态，如果前驱节点处于cancel状态，则丢弃这些节点，重新构建队列；
Unlock API详解
流程类似lock api相关类的流程，这里讲主要的代码，unlock相对的比较简单
首先 ReentrantLock 调用 Sync的release接口也就是AbstractQueuedSynchronizer的release接口
/** * Releases in exclusive mode. Implemented by unblocking one or * more threads if {@link #tryRelease} returns true. * This method can be used to implement method {@link Lock#unlock}. * * @param arg the release argument. This value is conveyed to * {@link #tryRelease} but is otherwise uninterpreted and * can represent anything you like. * @return the value returned from {@link #tryRelease} */ public final boolean release(int arg) { if (tryRelease(arg)) { Node h = head; if (h != null && h.waitStatus != 0) unparkSuccessor(h); return true; } return false; }
这个时候会先调用Sync的tryRelease，如果返回true，则释放锁成功
protected final boolean tryRelease(int releases) { int c = getState() - releases; if (Thread.currentThread() != getExclusiveOwnerThread()) throw new IllegalMonitorStateException(); boolean free = false; if (c == 0) { free = true; setExclusiveOwnerThread(null); } setState(c); return free; }
这个接口的作用很简单，如果不是获得锁的线程调用直接抛出异常，否则，如果当前state-releases==0也就是lock已经完全释放，返回true，清除资源；
这个返回free之后，release拿到head节点，进入以下代码：
/** * Wakes up node's successor, if one exists. * * @param node the node */ private void unparkSuccessor(Node node) { /* * If status is negative (i.e., possibly needing signal) try * to clear in anticipation of signalling. It is OK if this * fails or if status is changed by waiting thread. */ int ws = node.waitStatus; if (ws < 0) compareAndSetWaitStatus(node, ws, 0); /* * Thread to unpark is held in successor, which is normally * just the next node. But if cancelled or apparently null, * traverse backwards from tail to find the actual * non-cancelled successor. */ Node s = node.next; if (s == null || s.waitStatus > 0) { s = null; for (Node t = tail; t != null && t != node; t = t.prev) if (t.waitStatus <= 0) s = t; } if (s != null) LockSupport.unpark(s.thread); }
这个作用即：当头结点的状态小于0，则将头结点的状态CAS为0，然后通过链表获取下一个节点，如果下一个节点为null或者不符合要求的状态，则从队尾遍历整个链表，直到遍历到离head节点最近的一个节点并且
等待状态符合预期，则将头结点的后继节点置为该节点；
对刚刚筛出来的符合要求的节点唤醒，也就是该节点获得 争夺 锁的权利；
这就是非公平锁的特点：在队列一直等待的线程不一定比后来的线程先获得锁，至此，unlock 已经解释完成；
