2023-08-20 13:38

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jdk源码解读-并发包-Lock-ReentrantLock

介绍

ReentrantLock 是一个互斥锁，在基本行为和机制上与synchonized一样，只不过synchonized用方法和声明访问了隐式的锁监视器，但是ReentrantLock 做了功能上的扩展。

当锁不被其他线程拥有，一个线程会成功的申请锁资源并立即返回。如果当前线程已经拥有了锁，再申请时也会立即返回。通过调用方法isHeldByCurrentThread()获取是否当前线程获得了锁，getHoldCount()得到获得几次锁资源。

此类的构造方法接受一个可选的公平参数。当设置为 true 时，在多个线程的争用下，这些锁倾向于将访问权授予等待时间最长的线程。否则此锁将无法保证任何特定访问顺序。与采用默认设置（使用不公平锁）相比，使用公平锁的程序在许多线程访问时表现为很低的总体吞吐量（即速度很慢，常常极其慢），但是在获得锁和保证锁分配的均衡性时差异较小。不过要注意的是，公平锁不能保证线程调度的公平性。因此，使用公平锁的众多线程中的一员可能获得多倍的成功机会，这种情况发生在其他活动线程没有被处理并且目前并未持有锁时。还要注意的是，未定时的 tryLock 方法并没有使用公平设置。因为即使其他线程正在等待，只要该锁是可用的，此方法就可以获得成功。

推荐使用用try-catch 块代码去调用lock(),如下：

class X {
   private final ReentrantLock lock = new ReentrantLock();
   // ...
   public void m() {
     lock.lock();  // block until condition holds
     try {
       // ... method body
     } finally {
       lock.unlock()
     }
   }
 }}

除了实现lock接口，这个类还定义了一些public和protected方法去检查锁的状态。有些方法仅仅用来监控和维护。

这个类的序列化行为与内建的锁一样：反序列化的锁是没有获取锁状态，无论当它序列化时是否获取锁。

这个锁支持最大2147483647次的重入次数，超过这个数会报错。

类关系图：

从这个图可以看到ReentrantLock类实现了接口Lock和Serializable。

public class ReentrantLock implements Lock, java.io.Serializable {

ReentrantLock类的API调用都委托给一个内部类 Sync ,而该类继承AbstractQueuedSynchronizer类；

	/**
 	 * Base of synchronization control for this lock. Subclassed
 	 * into fair and nonfair versions below. Uses AQS state to
 	 * represent the number of holds on the lock.
 	*/
        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 fair locks
	 */
	static final class FairSync extends Sync

//Sync object for Nonfair locks


static final class NonfairSync extends Sync

ReentrantLock调用lock()方法时的调用关系图

非公平锁类调用lock()方法时的调用关系：

代码解析：

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;
}

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，如果设置成功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；

acquireQueued

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。

调用前首先调用addWaiter

addWaiter

/**  * 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，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真正让node入队，同时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；同时把当前节点设为头结点，原头结点则释放。

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；

公平锁类调用lock()方法时的调用关系：

非公平锁类和公平锁类调用lock()时的区别：

非公平锁类调用lock（）时，不排队先尝试获取锁资源，修改状态，修改不成功再入队。具体实现先调用AbstractQueuedSynchronizer的方法
protected final boolean compareAndSetState(int expect, int update) ，而公平锁类是直接入队，不给插队的机会，当直接插队失败才会入队。

调用tryAcquire()时也不同，

公平锁类的tryAcquire()

/**
 * Fair version of tryAcquire.  Don't grant access unless
 * recursive call or no waiters or is first.
 */
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
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;
}

非公平锁的tryAcquire()

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();
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;
}

公平锁类先调用!hasQueuedPredecessors()检查此节点前面有没有非头节点的节点。这样保证了顺序的获得锁资源。非公平锁不调用!hasQueuedPredecessors(）直接CAS

再来看unlock():

调用流程图：

1.Reentantlock方法unlock()：

/**
     * 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);
}

unlock()调用AbstractQueuedSynchronizer的release。

AbstractQueuedSynchronizer的release(int arg):

/**
 * 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;
}

tryRelease(arg)为true则调用unparkSucessor(h)，否则直接返回false。

3.我们再来看tryRelease(arg)：

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;
}

当释放锁的thread与当前获得锁的线程不一致时，抛出异常，参数releasese是要释放的重入锁的个数，c是释放后还剩几个。如果c==0则返回true。无论返回是true还是false都会更新state的值。state为零说明锁资源已经可以竞争了，非零说明锁资源还在某个线程没有释放。如果为零，下一步应该是唤醒一个线程，使这个线程获得竞争锁的权利。

4.unparkSucessor(h):

/**
 * 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。

最后让我们回到调用lock()时，线程被park的那段代码：

/**
 * 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);
    }
}