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[Java教程]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:

  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;