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服务器之家 - 编程语言 - Java教程 - Java源码解析之可重入锁ReentrantLock

Java源码解析之可重入锁ReentrantLock

2021-06-28 10:47李灿辉 Java教程

今天小编就为大家分享一篇关于Java源码解析之可重入锁ReentrantLock,小编觉得内容挺不错的,现在分享给大家,具有很好的参考价值,需要的朋友一起跟随小编来看看吧

本文基于jdk1.8进行分析。

reentrantlock是一个可重入锁,在concurrenthashmap中使用了reentrantlock。

首先看一下源码中对reentrantlock的介绍。如下图。reentrantlock是一个可重入的排他锁,它和synchronized的方法和代码有着相同的行为和语义,但有更多的功能。reentrantlock是被最后一个成功lock锁并且还没有unlock的线程拥有着。如果锁没有被别的线程拥有,那么一个线程调用lock方法,就会成功获取锁并返回。如果当前线程已经拥有该锁,那么lock方法会立刻返回。这个可以通过isheldbycurrentthread方法和getholdcount方法进行验证。除了这部分介绍外,类前面的javadoc文档很长,就不在这里全部展开。随着后面介绍源码,会一一涉及到。

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/**
 * a reentrant mutual exclusion {@link lock} with the same basic
 * behavior and semantics as the implicit monitor lock accessed using
 * {@code synchronized} methods and statements, but with extended
 * capabilities.
 * <p>a {@code reentrantlock} is <em>owned</em> by the thread last
 * successfully locking, but not yet unlocking it. a thread invoking
 * {@code lock} will return, successfully acquiring the lock, when
 * the lock is not owned by another thread. the method will return
 * immediately if the current thread already owns the lock. this can
 * be checked using methods {@link #isheldbycurrentthread}, and {@link
 * #getholdcount}.

首先看一下成员变量,如下图。reentrantlock只有一个成员变量sync,即同步器,这个同步器提供所有的机制。sync是abstractqueuedsynchronizer的子类,同时,sync有2个子类,nonfairsync和fairsync,分别是非公平锁和公平锁。sync,nonfairesync和fairsync的具体实现后面再讲。

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/** synchronizer providing all implementation mechanics **/
private final sync sync;

下面看一下构造函数。如下图。可以看到,reentrantlock默认是非公平锁,它可以通过参数,指定初始化为公平锁或非公平锁。

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/**
 * creates an instance of {@code reentrantlock}.
 * this is equivalent to using {@code reentrantlock(false)}.
 **/
public reentrantlock() {
  sync = new nonfairsync();
}
/**
 * creates an instance of {@code reentrantlock} with the
 * given fairness policy.
 * @param fair {@code true} if this lock should use a fair ordering policy
 **/
public reentrantlock(boolean fair) {
  sync = fair ? new fairsync() : new nonfairsync();
}

下面看一下reentrantlock的主要方法。首先是lock方法。如下图。lock方法的实现很简单,就是调用sync的lock方法。而sync的lock方法是个抽象的,具体实现在nonfairsync和fairsync中。这里我们先不展开讲,而是先读一下lock方法的注释,看看它的作用。lock方法的作用是获取该锁。分为3种情况。

1,如果锁没有被别的线程占有,那么当前线程就可以获取到锁并立刻返回,并把锁计数设置为1。

2,如果当前线程已经占有该锁了,那么就会把锁计数加1,立刻返回。

3,如果锁被另一个线程占有了,那么当前线程就无法再被线程调度,并且开始睡眠,直到获取到锁,在获取到到锁时,会把锁计数设置为1。

lockinterruptibly方法与lock功能类似,但lockinterruptibly方法在等待的过程中,可以响应中断。

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/**
 * acquires the lock.
 * <p>acquires the lock if it is not held by another thread and returns
 * immediately, setting the lock hold count to one.
 * <p>if the current thread already holds the lock then the hold
 * count is incremented by one and the method returns immediately.
 * <p>if the lock is held by another thread then the
 * current thread becomes disabled for thread scheduling
 * purposes and lies dormant until the lock has been acquired,
 * at which time the lock hold count is set to one.
 **/
public void lock() {
  sync.lock();
}
public void lockinterruptibly() throws interruptedexception {
  sync.acquireinterruptibly(1);
}

下面,详细看一下非公平锁和公平锁中对lock函数的实现。如下图。下图同时列出了公平锁和非公平锁中lock的实现逻辑。从注释和代码逻辑中,都可以看出,非公平锁进行lock时,先尝试立刻闯入(抢占),如果成功,则获取到锁,如果失败,再执行通常的获取锁的行为,即acquire(1)。

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/**
 * 非公平锁中的lock
 * performs lock. try immediate barge, backing up to normal
 * acquire on failure.
 **/
final void lock() {
  if (compareandsetstate(0, 1))
    setexclusiveownerthread(thread.currentthread());
  else
    acquire(1);
}
//公平锁中的lock
final void lock() {
  acquire(1);
}

那么,我们首先了解下,非公平锁“尝试立刻闯入”,究竟做了什么。稍后再继续讲解通常的获取锁的行为。下图是立即闯入行为compareandsetstate(0, 1)的实现。从compareandsetstate函数的注释中,可以知道,如果同步状态值与期望值相等,那么就把它的值设置为updated值。否则同步状态值与期望值不相等,则返回false。这个操作和volatile有着相同的内存语义,也就是说,这个操作对其他线程是可见的。compareandsetstate函数注释里描述的功能,是通过unsafe.compareandswapint方法实现的,而unsafe.compareandswapint是一个native方法,是用c++实现的。那么继续追问,c++底层是怎么实现的?c++底层是通过cas指令来实现的。什么是cas指令呢?来自维基百科的解释是,cas,比较和交换,compare and swap,是用用于实现多线程原子同步的指令。它将内存位置的内容和给定值比较,只有在相同的情况下,将该内存的值设置为新的给定值。这个操作是原子操作。那么继续追问,cas指令的原子性,是如何实现的呢?我们都知道指令时cpu来执行的,在多cpu系统中,内存是共享的,内存和多个cpu都挂在总线上,当一个cpu执行cas指令时,它会先将总线lock位点设置为高电平。如果别的cpu也要执行cas执行,它会发现总线lock位点已经是高电平了,则无法执行cas执行。cpu通过lock保证了指令的原子执行。

现在来看一下非公平锁的lock行为,compareandsetstate(0, 1),它期望锁状态为0,即没有别的线程占用,并把新状态设置为1,即标记为占用状态。如果成功,则非公平锁成功抢到锁,之后setexclusiveownerthread,把自己设置为排他线程。非公平锁这小子太坏了。如果抢占失败,则执行与公平锁相同的操作。

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/**
 * atomically sets synchronization state to the given updated
 * value if the current state value equals the expected value.
 * this operation has memory semantics of a {@code volatile} read
 * and write.
 * @param expect the expected value
 * @param update the new value
 * @return {@code true} if successful. false return indicates that the actual
 *     value was not equal to the expected value.
 **/
protected final boolean compareandsetstate(int expect, int update) {
  // see below for intrinsics setup to support this
  return unsafe.compareandswapint(this, stateoffset, expect, update);
}
public final native boolean compareandswapint(object var1, long var2, int var4, int var5);

下面看一下公平锁获取锁时的行为。如下图。这部分的逻辑有些多,请阅读代码中的注释进行理解。

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/**
 * 公平锁的lock
 **/
final void lock() {
  acquire(1);
}
/**
 * acquires in exclusive mode, ignoring interrupts. implemented
 * by invoking at least once {@link #tryacquire},
 * returning on success. otherwise the thread is queued, possibly
 * repeatedly blocking and unblocking, invoking {@link
 * #tryacquire} until success. this method can be used
 * to implement method {@link lock#lock}.
 * @param arg the acquire argument. this value is conveyed to
 *    {@link #tryacquire} but is otherwise uninterpreted and
 *    can represent anything you like.
 **/
public final void acquire(int arg) {
  /**
   * acquire首先进行tryacquire()操作。如果tryacquire()成功时则获取到锁,即刻返回。
   * 如果tryacquire()false时,会执行acquirequeued(addwaiter(node.exclusive), arg)
   * 操作。如果acquirequeued(addwaiter(node.exclusive), arg)true时,则当前线程中断自己。
   * 如果acquirequeued(addwaiter(node.exclusive), arg)false,则返回。
   * 其中tryacquire()操作在nonfairsync中和fairsync中实现又有所区别。
   **/
  if (!tryacquire(arg) &&
      acquirequeued(addwaiter(node.exclusive), arg))
    selfinterrupt();
}
/**
 * nonfairsync中的tryacquire。
 * @param acquires
 * @return
 **/
protected final boolean tryacquire(int acquires) {
  return nonfairtryacquire(acquires);
}
/**
 * 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();
  //首先获取当前同步状态值
  int c = getstate();
  if (c == 0) {
    //c为0,表示目前没有线程占用锁。没有线程占用锁时,当前线程尝试抢锁,如果抢锁成功,则返回true。
    if (compareandsetstate(0, acquires)) {
      setexclusiveownerthread(current);
      return true;
    }
  }
  else if (current == getexclusiveownerthread()) {
    //c不等于0时表示锁被线程占用。如果是当前线程占用了,则将锁计数加上acquires,并返回true。
    int nextc = c + acquires;
    if (nextc < 0) // overflow
      throw new error("maximum lock count exceeded");
    setstate(nextc);
    return true;
  }
  //以上情况都不是时,返回false,表示非公平抢锁失败。
  return false;
}
/**
 * fair version of tryacquire. don't grant access unless
 * recursive call or no waiters or is first.
 * 这个是公平版本的tryacquire
 **/
protected final boolean tryacquire(int acquires) {
  final thread current = thread.currentthread();
  int c = getstate();
  if (c == 0) {
    //c=0时表示锁未被占用。这里是先判断队列中前面是否有别的线程。没有别的线程时,才进行cas操作。
    //公平锁之所以公平,正是因为这里。它发现锁未被占用时,首先判断等待队列中是否有别的线程已经在等待了。
    //而非公平锁,发现锁未被占用时,根本不管队列中的排队情况,上来就抢。
    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;
}
/**
 * acquires in exclusive uninterruptible mode for thread already in
 * queue. used by condition wait methods as well as acquire.
 * 当抢锁失败时,先执行addwaiter(node.exclusive),将当前线程加入等待队列,再执行该方法。
 * 该方法的作用是中断当前线程,并进行检查,知道当前线程是队列中的第一个线程,并且抢锁成功时,
 * 该方法返回。
 * @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);
  }
}

接下来是trylock方法。代码如下。从注释中我们可以理解到,只有当调用trylock时锁没有被别的线程占用,trylock才会获取锁。如果锁没有被另一个线程占用,那么就获取锁,并立刻返回true,并把锁计数设置为1. 甚至在锁被设置为公平排序的情况下,若果锁可用,调用trylock会立刻获取锁,而不管有没有别的线程在等待锁了。从这里我们总结出,不管可重入锁是公平锁还是非公平锁,trylock方法只会是非公平的。

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/**
   * acquires the lock only if it is not held by another thread at the time
   * of invocation.
   * <p>acquires the lock if it is not held by another thread and
   * returns immediately with the value {@code true}, setting the
   * lock hold count to one. even when this lock has been set to use a
   * fair ordering policy, a call to {@code trylock()} <em>will</em>
   * immediately acquire the lock if it is available, whether or not
   * other threads are currently waiting for the lock.
   * this "barging" behavior can be useful in certain
   * circumstances, even though it breaks fairness. if you want to honor
   * the fairness setting for this lock, then use
   * {@link #trylock(long, timeunit) trylock(0, timeunit.seconds) }
   * which is almost equivalent (it also detects interruption).
   * <p>if the current thread already holds this lock then the hold
   * count is incremented by one and the method returns {@code true}.
   * <p>if the lock is held by another thread then this method will return
   * immediately with the value {@code false}.
   * @return {@code true} if the lock was free and was acquired by the
   *     current thread, or the lock was already held by the current
   *     thread; and {@code false} otherwise
   **/
  public boolean trylock() {
    return sync.nonfairtryacquire(1);
  }
  public boolean trylock(long timeout, timeunit unit)
      throws interruptedexception {
    return sync.tryacquirenanos(1, unit.tonanos(timeout));
  }

接下来是释放锁的方法unlock。代码如下。unlock方式的实现,是以参数1来调用sync.release方法。而release方法是如何实现的呢?release方法首先会调用tryrelease方法,如果tryrelease成功,则唤醒后继者线程。而tryrelease的实现过程十分清晰,首先获取锁状态,锁状态减去参数(放锁次数),得到新状态。然后判断持有锁的线程是否为当前线程,如果不是当前线程,则抛出illegalmonitorstateexception。然后判断,如果新状态为0,说明放锁成功,则把持有锁的线程设置为null,并返回true。如果新状态不为0,则返回false。从tryrelease的返回值来看,它返回的true或false,指的是否成功的释放了该锁。成功的释放该锁的意思是彻底释放锁,别的线程就可以获取锁了。这里要认识到,即便tryrelease返回false,它也只是说明了锁没有完全释放,本次调用的这个释放次数值,依然是释放成功的。

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/**
 * attempts to release this lock.
 * <p>if the current thread is the holder of this lock then the hold
 * count is decremented. if the hold count is now zero then the lock
 * is released. if the current thread is not the holder of this
 * lock then {@link illegalmonitorstateexception} is thrown.
 * @throws illegalmonitorstateexception if the current thread does not
 *     hold this lock
 **/
public void unlock() {
  sync.release(1);
}
/**
 * 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;
}
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;
  }
/**
 * 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);
}

接下来是newcondition方法。关于condition这里不展开介绍,只是了解下该方法的作用。如下图。该方法返回一个和这个锁实例一起使用的condition实例。返回的condition实例支持和object的监控方法例如wait-notify和notifyall相同的用法。

  • 1,如果没有获取锁,调用condition的await,signal,signalall方法的任何一个时,会抛出illegalmonitorstateexception异常。
  • 2,调用condition的await方法时,锁也会释放,在await返回之前,锁会被重新获取,并且锁计数会恢复到调用await方法时的值。
  • 3,如果一个线程在等待的过程中被中断了,那么等待就会结束,并抛出interruptedexception异常,线程的中断标志位会被清理。
  • 4,等待的线程以fifo的顺序被唤醒。
  • 5,从await方法返回的线程们的获取到锁的顺序,和线程最开始获取锁的顺序相同,这是未指定情况下的默认实现。但是,公平锁更钟爱那些已经等待了最长时间的线程。
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/**
 * returns a {@link condition} instance for use with this
 * {@link lock} instance.
 * <p>the returned {@link condition} instance supports the same
 * usages as do the {@link object} monitor methods ({@link
 * object#wait() wait}, {@link object#notify notify}, and {@link
 * object#notifyall notifyall}) when used with the built-in
 * monitor lock.
 * <ul>
 * <li>if this lock is not held when any of the {@link condition}
 * {@linkplain condition#await() waiting} or {@linkplain
 * condition#signal signalling} methods are called, then an {@link
 * illegalmonitorstateexception} is thrown.
 * <li>when the condition {@linkplain condition#await() waiting}
 * methods are called the lock is released and, before they
 * return, the lock is reacquired and the lock hold count restored
 * to what it was when the method was called.
 * <li>if a thread is {@linkplain thread#interrupt interrupted}
 * while waiting then the wait will terminate, an {@link
 * interruptedexception} will be thrown, and the thread's
 * interrupted status will be cleared.
 * <li> waiting threads are signalled in fifo order.
 * <li>the ordering of lock reacquisition for threads returning
 * from waiting methods is the same as for threads initially
 * acquiring the lock, which is in the default case not specified,
 * but for <em>fair</em> locks favors those threads that have been
 * waiting the longest.
 * </ul>
 * @return the condition object
 **/
public condition newcondition() {
  return sync.newcondition();
}

可重入锁还有一些其他的方法,这里就不一一介绍了。this is the end.

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原文链接:https://blog.csdn.net/li_canhui/article/details/85006114

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