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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:

技术分享

再来一张lock调用图:

技术分享

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:

  1. CANCELLED = 1    由于超时或者被打断,该线程被取消,将不会被block;
  2. 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;
  3. 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;
  4. 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;

 

Lock的实现之ReentrantLock详解