Java并发编程--AQS

概述

  抽象队列同步器(AbstractQueuedSynchronizer,简称AQS)是用来构建锁或者其他同步组件的基础框架,它使用一个整型的volatile变量(命名为state)来维护同步状态,通过内置的FIFO队列来完成资源获取线程的排队工作。

  volatile变量的读写和CAS是concurrent包得以实现的基础。CAS表示如果当前状态值等于预期值,则以原子方式将同步状态设置为给定的更新值,此操作具有volatile读和写的内存语义。AQS通过volatile的读/写和CAS所具有的volatile读和写的内存语义来实现线程之间的通信。

  高层类   Lock  同步器  阻塞队列  Executor  并发容器
  基础类 AQS  非阻塞数据结构  原子变量类
  volatile变量的读/写  CAS

  concurrent包的实现结构如上图所示,AQS、非阻塞数据结构和原子变量类等基础类都是基于volatile变量的读/写和CAS实现,而像Lock、同步器、阻塞队列、Executor和并发容器等高层类又是基于基础类实现。

AQS的域和方法

  域

1 private transient volatile Node head; //同步队列的head节点
2 private transient volatile Node tail; //同步队列的tail节点
3 private volatile int state; //同步状态

 

  方法

    AQS提供的可以修改同步状态的3个方法:

1 protected final int getState();  //获取同步状态
2 protected final void setState(int newState);  //设置同步状态
3 protected final boolean compareAndSetState(int expect, int update);  //CAS设置同步状态

 

    AQS提供的模板方法,主要有以下三类:

      1)独占式获取和释放同步状态

1 public final void acquire(int arg) //独占式获取同步状态,如果不成功会进入同步队列等待。
2 public final void acquireInterruptibly(int arg) //与acquire不同的是,能响应中断
3 public final boolean tryAcquireNanos(int arg, long nanosTimeout) //增加超时机制
4 
5 public final boolean release(int arg) //独占式释放同步状态,该方法会调用重写的tryRelease(int arg)。

 

        以上三种获取同步状态的方法都会调用自定义的tryAcquire(int arg)方法,acquire的获取失败时的入队等待机制、acquireInterruptibly的响应中断机制、tryAcquireNanos的超时机制等AQS已经实现好,这样开发人员只需要实现自己的获取同步状态机制,就可以大大降低了实现一个可靠自定义同步组件的门槛。

      2)共享式获取和释放同步状态

1 public final void acquireShared(int arg) //共享式获取同步状态,如果不成功会进入同步队列等待。与独占式不同的是,同一时刻可以有多个线程获取到同步状态。
2 public final void acquireSharedInterruptibly(int arg) //可响应中断
3 public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout) //超时机制
4 
5 public final boolean releaseShared(int arg) //共享式释放同步状态,该方法会调用重写的tryReleaseShared(int arg)。

 

        同样以上三种获取同步状态的方法会调用自定义的tryAcquireShared方法。

      3)查询同步队列中的等待线程情

1 publicfinalCollection<Thread>getQueuedThreads()
2 publicfinalbooleanhasQueuedThreads()//返回包含可能正在等待获取的线程列表,需要遍历链表。返回的只是个估计值,且是无序的。这个方法的主要是为子类提供的监视同步队列措施而设计。
3 。。。

 

    

    AQS提供的自定义方法

      以上AQS的方法都为final方法,不能被子类重写,因为它们对于任何自定义同步器应该是不需要更改的,下面为AQS提供的可以重写的方法。开发者需要根据自定义同步组件的特点,重写以下方法。这些方法的实现在内部必须是线程安全的,通常应该很短并且不被阻塞。

1 protected boolean tryAcquire(int arg) //独占式获取同步状态,此方法应该查询是否允许它在独占模式下获取对象状态,如果允许,则获取它。返回值语义:true代表获取成功,false代表获取失败。
2 protected boolean tryRelease(int arg) //独占式释放同步状态
3 
4 protected int tryAcquireShared(int arg) //共享式获取同步状态,返回值语义:负数代表获取失败、0代表获取成功但没有剩余资源、正数代表获取成功,还有剩余资源。
5 protected boolean tryReleaseShared(int arg) //共享式释放同步状态
6 
7 protected boolean isHeldExclusively() //AQS是否被当前线程所独占

 

AQS的使用

  怎么使用AQS实现自定义同步组件?

  自定义同步组件实例一:独占锁(使用独占式的获取与释放)。

    Mutex同步组件同一时刻只允许一个线程占用锁,不支持可重入。0表示未锁定状态,1表示锁定状态。 

 1 class Mutex implements Lock, java.io.Serializable {
 2  
 3     //静态内部类,自定义同步器
 4     private static class Sync extends AbstractQueuedSynchronizer {
 5     
 6         // 释放处于占用状态(重写isHeldExclusively)Report whether in locked state
 7         protected boolean isHeldExclusively() { 
 8             return getState() == 1; 
 9         }
10 
11         // 独占式获取锁(重写tryAcquire) Acquire the lock if state is zero
12         public boolean tryAcquire(int acquires) {
13             assert acquires == 1; // Otherwise unused
14             if (compareAndSetState(0, 1)) {    //CAS设置状态为1。
15                 setExclusiveOwnerThread(Thread.currentThread());
16                 return true;
17             }
18             return false;
19         }
20 
21         // 独占式释放锁(重写tryRelease) Release the lock by setting state to zero
22         protected boolean tryRelease(int releases) {
23             assert releases == 1; // Otherwise unused
24             if (getState() == 0) //获取状态
25                 throw new IllegalMonitorStateException();
26             setExclusiveOwnerThread(null);
27             setState(0);    //设置状态为0
28             return true;
29         }
30        
31         // Provide a Condition
32         //每个Condition都包含一个队列
33         Condition newCondition() { return new ConditionObject(); }
34 
35         // Deserialize properly
36         private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException {
37             s.defaultReadObject();
38             setState(0); // reset to unlocked state
39         }
40     }
41 
42     // The sync object does all the hard work. We just forward to it.
43     private final Sync sync = new Sync();
44     
45     //仅需要将操作代理到sync
46     public void lock()                { sync.acquire(1); }    //调用AQS的模板方法,
47     public boolean tryLock()          { return sync.tryAcquire(1); }
48     public void unlock()              { sync.release(1); }
49     public Condition newCondition()   { return sync.newCondition(); }
50     public boolean isLocked()         { return sync.isHeldExclusively(); }
51     public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); }
52     public void lockInterruptibly() throws InterruptedException { 
53         sync.acquireInterruptibly(1);
54     }
55     public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException {
56         return sync.tryAcquireNanos(1, unit.toNanos(timeout));
57     }
58 }

 

  自定义同步组件实例二:锁存器(使用共享的获取与释放方法)

    BooleanLatch可用在多个线程需要等待某个事件发生才能继续执行的情况中。初始状态state=0, 此时所有线程获取同步状态方法tryAcquireShared返回-1,即获取失败,入等待队列。直到有线程调用tryReleaseShared释放同步状态,被阻塞的状态才会进行执行。

 1 class BooleanLatch {
 2 
 3     private static class Sync extends AbstractQueuedSynchronizer {
 4         boolean isSignalled() { return getState() != 0; }
 5 
 6         protected int tryAcquireShared(int ignore) {
 7             return isSignalled()? 1 : -1;
 8         }
 9         
10         protected boolean tryReleaseShared(int ignore) {
11             setState(1);
12             return true;
13         }
14     }
15 
16     private final Sync sync = new Sync();
17     public boolean isSignalled() { return sync.isSignalled(); }
18     public void signal()         { sync.releaseShared(1); }
19     public void await() throws InterruptedException {
20         sync.acquireSharedInterruptibly(1);
21     }
22 }

 

AQS的实现原理

  同步队列

    同步队列中的节点Node用来保存获取同步状态失败的线程引用、等待状态、以及前驱和后继节点。

    AQS中两个域:head节点和tail节点,组成一个FIFO的双向队列。

      private transient volatile Node head;

      private transient volatile Node tail;

    Node源码如下。

 1 static final class Node {
 2     /** Marker to indicate a node is waiting in shared mode */
 3     static final Node SHARED = new Node();    //共享方式
 4     /** Marker to indicate a node is waiting in exclusive mode */
 5     static final Node EXCLUSIVE = null;        //独占方式
 6 
 7     /** waitStatus value to indicate thread has cancelled */
 8     static final int CANCELLED =  1;    //waitStatus=1为取消状态
 9     /** waitStatus value to indicate successor's thread needs unparking */
10     static final int SIGNAL    = -1;    //后继节点的线程处于等待状态,需要被唤醒。
11     /** waitStatus value to indicate thread is waiting on condition */
12     static final int CONDITION = -2;    //当前线程在condition上等待
13     static final int PROPAGATE = -3;    //表示下一次共享式同步状态获取将会无条件的被传播下去。
14 
15     volatile int waitStatus;    //等待状态,0-初始状态
16     volatile Node prev;            //前驱节点
17     volatile Node next;            //后继节点
18     volatile Thread thread;        //获取同步的线程
19     Node nextWaiter;            
20 
21     final boolean isShared() {
22         return nextWaiter == SHARED;
23     }
24     //返回前驱节点
25     final Node predecessor() throws NullPointerException {
26         Node p = prev;
27         if (p == null)
28             throw new NullPointerException();
29         else
30             return p;
31     }
32 
33     Node() {    // Used to establish initial head or SHARED marker
34     }
35 
36     Node(Thread thread, Node mode) {     // Used by addWaiter
37         this.nextWaiter = mode;
38         this.thread = thread;
39     }
40 
41     Node(Thread thread, int waitStatus) { // Used by Condition
42         this.waitStatus = waitStatus;
43         this.thread = thread;
44     }
45 }

 

  独占式获取同步状态

    1)调用自定义的tryAcquire方法,该方法要保证线程线程安全的获取同步状态(如Mutex中的tryAcquire使用CAS更新保证原子性),如果获取成功则return。

    2)如果获取失败,构造Node,并通过addWaiter方法将节点插入队列尾部。Node.EXCLUSIVE表示节点以独占方式等待。

    3)acquireQueued方法中该节点自旋方式尝试获取同步状态。如果获取不到同步状态,则阻塞节点中的线程,被阻塞线程唤醒依靠前驱节点的出队或阻塞线程被终端来实现。

 1 //该方法对中断不敏感,也就是由于线程获取同步状态失败后进入同步队列中,后续对线程进行中断操作时,线程不会从同步队列中移出.
 2 public final void acquire(int arg) {
 3     if (!tryAcquire(arg) &&
 4         acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
 5         selfInterrupt();
 6 }
 7 
 8 //将节点添加到队尾
 9 private Node addWaiter(Node mode) {
10     Node node = new Node(Thread.currentThread(), mode);
11     // Try the fast path of enq; backup to full enq on failure
12     //快速尝试入队,如果失败则需要调用enq(node)方法入队,这样做有什么好处?有一定概率减少一次方法调用
13     //compareAndSetTail保证Node入队是线程安全的
14     Node pred = tail;
15     if (pred != null) {
16         node.prev = pred;
17         if (compareAndSetTail(pred, node)) {
18             pred.next = node;
19             return node;
20         }
21     }
22     enq(node);
23     return node;
24 }
25 
26 //初始化或自旋CAS直到入队成功
27 private Node enq(final Node node) {
28     for (;;) {
29         Node t = tail;
30         if (t == null) { // Must initialize
31             if (compareAndSetHead(new Node()))
32                 tail = head;
33         } else {
34             node.prev = t;
35             if (compareAndSetTail(t, node)) {
36                 t.next = node;
37                 return t;
38             }
39         }
40     }
41 }

 

    入同步队列之后怎么获取同步状态?阻塞机制是怎样的?

 1 //自旋方式尝试获取同步状态
 2 final boolean acquireQueued(final Node node, int arg) {
 3     boolean failed = true;
 4     try {
 5         boolean interrupted = false;
 6         for (;;) {
 7             final Node p = node.predecessor();    //获取当前节点的前驱节点
 8             if (p == head && tryAcquire(arg)) {    //如果前驱节点是head节点则尝试获取同步状态
 9                 setHead(node);
10                 p.next = null; // help GC
11                 failed = false;
12                 return interrupted;
13             }
14             if (shouldParkAfterFailedAcquire(p, node) &&
15                 parkAndCheckInterrupt())    //判断当前线程是否应该被阻塞,如果是则阻塞直到被唤醒继续循环,如果不是则再次尝试获取同步状态。
16                 interrupted = true;
17         }
18     } finally {
19         if (failed)
20             cancelAcquire(node);
21     }
22 }
23 
24 //判断当前线程是否应该被阻塞,如果线程应该被阻塞则返回true。检查和更新获取同步状态失败Node的前驱节点的waitStatus。
25 //其中pred为node的前驱节点
26 private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
27     int ws = pred.waitStatus;
28     if (ws == Node.SIGNAL)
29         /*
30          * This node has already set status asking a release
31          * to signal it, so it can safely park.
32          */
33         //前驱节点已经设置为SIGNAL状态,在前驱节点的线程释放同步状态会唤醒当前Node的线程。
34         return true;
35     if (ws > 0) {
36         /*
37          * Predecessor was cancelled. Skip over predecessors and
38          * indicate retry.
39          */
40          //前驱节点是cancelled状态,跳过被取消的Node,直到向前找到waitStatus > 0的Node作为当前节点的前驱,然后重试获取同步状态。
41         do {
42             node.prev = pred = pred.prev;
43         } while (pred.waitStatus > 0);
44         pred.next = node;
45     } else {
46         /* waitStatus = 0是初始状态。
47          * waitStatus must be 0 or PROPAGATE.  Indicate that we
48          * need a signal, but don't park yet.  Caller will need to
49          * retry to make sure it cannot acquire before parking.
50          */
51         //将前驱节点的等待状态改为SIGNAL。
52         compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
53     }
54     return false;
55 }
56 
57 //阻塞线程
58 private final boolean parkAndCheckInterrupt() {
59     LockSupport.park(this);    //使用LockSupport阻塞当前线程
60     return Thread.interrupted();
61 }

 

    在获取同步状态时,同步器维护一个同步队列,获取状态失败的线程都会被加入到队列中并在队列中进行自旋;移出队列(或停止自旋)的条件是前驱节点为头节点且成功获取了同步状态。

  独占式释放同步状态

    在释放同步状态时,同步器调用tryRelease(int arg)方法释放同步状态,然后唤醒头节点的后继节点。

 1 public final boolean release(int arg) {
 2     if (tryRelease(arg)) {
 3         Node h = head;
 4         if (h != null && h.waitStatus != 0)
 5             unparkSuccessor(h);    //唤醒后继节点线程
 6         return true;
 7     }
 8     return false;
 9 }
10 
11 //唤醒后继节点
12 private void unparkSuccessor(Node node) {
13     /*
14      * If status is negative (i.e., possibly needing signal) try
15      * to clear in anticipation of signalling.  It is OK if this
16      * fails or if status is changed by waiting thread.
17      */
18      //将当前节点的waitStatus改为0-原始状态,目的是什么?
19     int ws = node.waitStatus;
20     if (ws < 0)
21         compareAndSetWaitStatus(node, ws, 0);
22 
23     /*
24      * Thread to unpark is held in successor, which is normally
25      * just the next node.  But if cancelled or apparently null,
26      * traverse backwards from tail to find the actual
27      * non-cancelled successor.
28      */
29      //如果后继节点为null或被取消,则从tail向前找到正常的后继节点
30     Node s = node.next;
31     if (s == null || s.waitStatus > 0) {
32         s = null;
33         for (Node t = tail; t != null && t != node; t = t.prev)
34             if (t.waitStatus <= 0)
35                 s = t;
36     }
37     if (s != null)
38         LockSupport.unpark(s.thread);    //唤醒后继节点
39 }

 

  共享式获取同步状态

    1)首先调用自定义方法tryAcquireShared尝试获取同步状态,至少调用一次tryAcquireShared方法,如果返回值>=0,则获取成功,return;否则执行步骤2),

    2)当获取失败时,为当前线程以共享方式创建Node并插入同步队列。

    3)入队后,以自旋方式尝试获取同步状态,如果前驱节点为head节点,则尝试获取同步状态,获取失败,则阻塞线程。

 1 //共享式获取同步状态,忽略异常。
 2 //注意:实现自定义方法tryAcquireShared时,要遵循AQS定义的返回值语义,负数代表获取失败、0代表获取成功但没有剩余资源、正数代表获取成功,还有剩余资源。
 3 public final void acquireShared(int arg) {
 4     if (tryAcquireShared(arg) < 0)    //
 5         doAcquireShared(arg);
 6 }
 7 
 8 private void doAcquireShared(int arg) {
 9     final Node node = addWaiter(Node.SHARED);    //为当前线程以共享方式创建Node并插入同步队列尾部。
10     boolean failed = true;
11     try {
12         boolean interrupted = false;
13         for (;;) {
14             final Node p = node.predecessor();
15             if (p == head) {    //如果前驱节点为head节点,则尝试获取同步状态
16                 int r = tryAcquireShared(arg);
17                 if (r >= 0) {    //获取成功
18                     setHeadAndPropagate(node, r);    //设置node为head节点,还有剩余资源则继续唤醒后继的node
19                     p.next = null; // help GC
20                     if (interrupted)    //如果等待过程中被中断过,则中断当前线程
21                         selfInterrupt();    //Thread.currentThread().interrupt();
22                     failed = false;
23                     return;
24                 }
25             }
26             //shouldParkAfterFailedAcquire方法判断当前线程是否应该被阻塞,如果是则调用parkAndCheckInterrupt阻塞当前线程
27             if (shouldParkAfterFailedAcquire(p, node) &&
28                 parkAndCheckInterrupt())
29                 interrupted = true;
30         }
31     } finally {
32         if (failed)
33             cancelAcquire(node);
34     }
35 }
36 
37 //获取成功后,设置node为head节点,
38 private void setHeadAndPropagate(Node node, int propagate) {
39     Node h = head; // Record old head for check below
40     setHead(node);    //设置node为head节点,因此node出队
41     //如果还有剩余资源,尝试唤醒node节点的后继节点
42     /*
43      * Try to signal next queued node if:
44      *   Propagation was indicated by caller,
45      *     or was recorded (as h.waitStatus) by a previous operation
46      *     (note: this uses sign-check of waitStatus because
47      *      PROPAGATE status may transition to SIGNAL.)
48      * and
49      *   The next node is waiting in shared mode,
50      *     or we don't know, because it appears null
51      *
52      * The conservatism in both of these checks may cause
53      * unnecessary wake-ups, but only when there are multiple
54      * racing acquires/releases, so most need signals now or soon
55      * anyway.
56      */
57     if (propagate > 0 || h == null || h.waitStatus < 0) {
58         Node s = node.next;
59         if (s == null || s.isShared())
60             doReleaseShared();
61     }
62 }
63 
64 //设置node为head节点,因此node出队
65 private void setHead(Node node) {
66     head = node;
67     node.thread = null;
68     node.prev = null;
69 }

 

  共享式释放同步状态

 1 public final boolean releaseShared(int arg) {
 2     if (tryReleaseShared(arg)) {
 3         doReleaseShared();
 4         return true;
 5     }
 6     return false;
 7 }
 8 
 9 //唤醒后继节点线程并确保被传播
10 private void doReleaseShared() {
11     /*
12      * Ensure that a release propagates, even if there are other
13      * in-progress acquires/releases.  This proceeds in the usual
14      * way of trying to unparkSuccessor of head if it needs
15      * signal. But if it does not, status is set to PROPAGATE to
16      * ensure that upon release, propagation continues.
17      * Additionally, we must loop in case a new node is added
18      * while we are doing this. Also, unlike other uses of
19      * unparkSuccessor, we need to know if CAS to reset status
20      * fails, if so rechecking.
21      */
22     for (;;) {
23         Node h = head;
24         if (h != null && h != tail) {
25             int ws = h.waitStatus;
26             if (ws == Node.SIGNAL) {
27                 if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
28                     continue;            // loop to recheck cases
29                 unparkSuccessor(h);    //唤醒后继节点
30             }
31             else if (ws == 0 &&
32                      !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
33                 continue;                // loop on failed CAS
34         }
35         if (h == head)                   // loop if head changed
36             break;
37     }
38 }

 

  独占式可中断获取

    可响应中断获取与普通获取的区别:当线程被中断时,会立即返回,并抛出InterruptedException,执行cancelAcquire方法取消获取同步状态;而普通获取只是将中断标志位置为true,但线程依旧会阻塞在等待队列中。

 1 public final void acquireInterruptibly(int arg)
 2         throws InterruptedException {
 3     if (Thread.interrupted())
 4         throw new InterruptedException();    //抛出中断异常
 5     if (!tryAcquire(arg))
 6         doAcquireInterruptibly(arg);
 7 }
 8 
 9 private void doAcquireInterruptibly(int arg)
10     throws InterruptedException {
11     final Node node = addWaiter(Node.EXCLUSIVE);
12     boolean failed = true;
13     try {
14         for (;;) {
15             final Node p = node.predecessor();
16             if (p == head && tryAcquire(arg)) {
17                 setHead(node);
18                 p.next = null; // help GC
19                 failed = false;
20                 return;
21             }
22             if (shouldParkAfterFailedAcquire(p, node) &&
23                 parkAndCheckInterrupt())
24                 throw new InterruptedException();    //唯一的区别,抛出中断异常。而普通的获取操作,只是将中断标志位置为true。
25         }
26     } finally {
27         if (failed)
28             cancelAcquire(node);
29     }
30 }

 

    如何取消线程?

 1 private void cancelAcquire(Node node) {
 2     // Ignore if node doesn't exist
 3     if (node == null)
 4         return;
 5 
 6     node.thread = null;    //将线程置为null
 7 
 8     // Skip cancelled predecessors
 9     Node pred = node.prev;
10     while (pred.waitStatus > 0)    //即waitStatus=1,为cancelled状态。跳过状态为取消的前驱节点
11         node.prev = pred = pred.prev;
12 
13     // predNext is the apparent node to unsplice. CASes below will
14     // fail if not, in which case, we lost race vs another cancel
15     // or signal, so no further action is necessary.
16     Node predNext = pred.next;
17 
18     // Can use unconditional write instead of CAS here.
19     // After this atomic step, other Nodes can skip past us.
20     // Before, we are free of interference from other threads.
21     node.waitStatus = Node.CANCELLED;    //当前节点置为cancelled状态
22 
23     // If we are the tail, remove ourselves.
24     if (node == tail && compareAndSetTail(node, pred)) {    //如果当前节点为tail节点,删除该节点
25         compareAndSetNext(pred, predNext, null);
26     } else {
27         // If successor needs signal, try to set pred's next-link
28         // so it will get one. Otherwise wake it up to propagate.
29         //
30         int ws;
31         if (pred != head &&
32             ((ws = pred.waitStatus) == Node.SIGNAL ||
33              (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
34             pred.thread != null) {
35             //如果前驱节点waitStatus为SIGNAL或者CAS更新为SIGNAL成功,则pred释放同步状态时会通知后继节点
36             //并且pred.thread不为null,则cas将pred的后继节点置为node.next,
37             Node next = node.next;
38             if (next != null && next.waitStatus <= 0)
39                 compareAndSetNext(pred, predNext, next);
40         } else {
41             unparkSuccessor(node);    //唤醒后继节点
42         }
43 
44         node.next = node; // help GC next指向自己,node从等待队列中移除。
45     }
46 }

 

  独占式可超时获取

    在响应中断的基础上增加了超时机制

 1 public final boolean tryAcquireNanos(int arg, long nanosTimeout)
 2         throws InterruptedException {
 3     if (Thread.interrupted())
 4         throw new InterruptedException();
 5     return tryAcquire(arg) || doAcquireNanos(arg, nanosTimeout);
 6 }
 7 
 8 private boolean doAcquireNanos(int arg, long nanosTimeout)
 9     throws InterruptedException {
10     long lastTime = System.nanoTime();
11     final Node node = addWaiter(Node.EXCLUSIVE);
12     boolean failed = true;
13     try {
14         for (;;) {
15             final Node p = node.predecessor();
16             if (p == head && tryAcquire(arg)) {
17                 setHead(node);
18                 p.next = null; // help GC
19                 failed = false;
20                 return true;
21             }
22             if (nanosTimeout <= 0)    //判断是否超时,nanosTimeout<=0表示超时,如果超时则return false.
23                 return false;
24             if (shouldParkAfterFailedAcquire(p, node) &&
25                 nanosTimeout > spinForTimeoutThreshold)
26                 LockSupport.parkNanos(this, nanosTimeout);    //如果还剩余的时间nanosTimeout>阈值(spinForTimeoutThreshold=1000纳秒),则阻塞当前线程nanosTimeout纳秒。当nanosTimeout<1000纳秒时,则不阻塞当前线程,而是进入快速的自旋过程。原因在于,非常短的超时等待无法做到十分精确,如果这时再进行超时等待,相反会让nanosTimeout的超时从整体上表现得反而不精确。因此,在超时非常短的场景下,同步器会进入无条件的快速自旋。
27             long now = System.nanoTime();    //当前时间
28             //lastTime表示上次唤醒时间,now - lastTime表示已经睡眠的时间
29             nanosTimeout -= now - lastTime;    //nanosTimeout表示还剩余的时间,nanosTimeout>0表示表示超时时间未到
30             lastTime = now;
31             if (Thread.interrupted())
32                 throw new InterruptedException();    //抛异常
33         }
34     } finally {
35         if (failed)
36             cancelAcquire(node);
37     }
38 }

 

  总结,三种方式获取同步状态方式的对比,主要区别在于获取同步状态失败时的处理逻辑:

 

    acquire方法直接阻塞线程,不响应中断,只是将中断标记置为true,但线程依旧会阻塞在等待队列中。

    acquireInterruptibly方法直接阻塞线程,响应中断,当线程被中断时,会立即返回,并抛出InterruptedException。

    tryAcquireNanos方法将线程阻塞nanosTimeout秒,如何超时还未获取到同步状态,则返回。同时支持响应中断。

1 public final void acquire(int arg)    //独占式获取同步状态,如果不成功会进入同步队列等待。
2 public final void acquireInterruptibly(int arg)    //与acquire不同的是,能响应中断
3 public final boolean tryAcquireNanos(int arg, long nanosTimeout)    //增加超时机制

 

LockSupport

  在阻塞和唤醒线程时,使用了LockSupport类。LockSupport提供了一系列阻塞和唤醒线程的公共方法。底层使用unsafe提供的方法实现。

 1 void park()    //阻塞当前线程,只有调用unpark或中断才能从park方法中返回
 2 void parkNanos(long nanos)    //超时阻塞当前线程,超时则返回
 3 void parkUntil(long deadline)    //截止时间阻塞当前线程,直到deadline
 4 
 5 void unpark(Thread thread)    //唤醒处于阻塞状态的线程
 6 
 7 //jdk1.6中增加了以下方法,blocker表示当前线程要等待的对象。线程dump信息比使用park方法要多,方便问题排查和监控。
 8 void park(Object blocker)
 9 void parkNanos(Object blocker, long nanos)
10 void parkUntil(Object blocker, long deadline)

 

 

参考资料:

  《Java并发编程的艺术》

  《Java并发之AQS详解》http://www.cnblogs.com/waterystone/p/4920797.html

posted @ 2017-11-03 16:44  在周末  阅读(2817)  评论(0编辑  收藏