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多线程基础(七)：关于HotSpot中notify方法不具备随机

多线程基础(七)：关于HotSpot中notify方法不具备随机

作者: 冬天里的懒喵 | 来源:发表于2020-09-09 14:48 被阅读0次

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    在前面关于wait/notify及notifyAll方法的时候，notify在源码的注释中说到notify选择唤醒的线程是任意的，但是依赖于具体实现的jvm。原文如下：

     * Wakes up a single thread that is waiting on this object's
 * monitor. If any threads are waiting on this object, one of them
 * is chosen to be awakened. The choice is arbitrary and occurs at
 * the discretion of the implementation. A thread waits on an object's
 * monitor by calling one of the {@code wait} methods.

    但是在很多博客或者面试中聊到notify和notifyAll的时候，很多人都说notify是随机的。那么真的是随机吗 ？我们现在来对这个情况进行实验验证。

    1.实验一

    定义50个线程，分别让其依次wait，之后再进行notify。为了对照明显，特别将wait之后加了sleep，这样线程能按id依次wait。wait之前不存在锁竞争。

    package com.dhb.notify;

import java.util.LinkedList;
import java.util.List;
import java.util.concurrent.TimeUnit;

public class NotifyTest{

    private static List<String> before = new LinkedList<>();
    private static List<String> after = new LinkedList<>();

    private static Object lock = new Object();


    public static void main(String[] args) throws InterruptedException{

        for(int i=0;i<50;i++){
            String threadName = Integer.toString(i);
            new Thread(() -> {
                synchronized (lock) {
                    String cthreadName = Thread.currentThread().getName();
                    System.out.println("Thread ["+cthreadName+"] wait.");
                    before.add(cthreadName);
                    try {
                        lock.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println("Thread ["+cthreadName+"] weak up.");
                    after.add(cthreadName);
                }
            },threadName).start();
            TimeUnit.MILLISECONDS.sleep(50);
        }

        TimeUnit.SECONDS.sleep(1);

        for(int i=0;i<50;i++){
            synchronized (lock) {
                lock.notify();
                TimeUnit.MILLISECONDS.sleep(10);
            }
        }
        TimeUnit.SECONDS.sleep(1);

        System.out.println(before.toString());
        System.out.println(after.toString());
    }
}

    上面代码的执行结果：

    Thread [0] wait.
Thread [1] wait.
Thread [2] wait.
Thread [3] wait.
Thread [4] wait.
Thread [5] wait.
Thread [6] wait.
Thread [7] wait.
Thread [8] wait.
Thread [9] wait.
Thread [10] wait.
Thread [11] wait.
Thread [12] wait.
Thread [13] wait.
Thread [14] wait.
Thread [15] wait.
Thread [16] wait.
Thread [17] wait.
Thread [18] wait.
Thread [19] wait.
Thread [20] wait.
Thread [21] wait.
Thread [22] wait.
Thread [23] wait.
Thread [24] wait.
Thread [25] wait.
Thread [26] wait.
Thread [27] wait.
Thread [28] wait.
Thread [29] wait.
Thread [30] wait.
Thread [31] wait.
Thread [32] wait.
Thread [33] wait.
Thread [34] wait.
Thread [35] wait.
Thread [36] wait.
Thread [37] wait.
Thread [38] wait.
Thread [39] wait.
Thread [40] wait.
Thread [41] wait.
Thread [42] wait.
Thread [43] wait.
Thread [44] wait.
Thread [45] wait.
Thread [46] wait.
Thread [47] wait.
Thread [48] wait.
Thread [49] wait.
Thread [0] weak up.
Thread [19] weak up.
Thread [18] weak up.
Thread [17] weak up.
Thread [16] weak up.
Thread [15] weak up.
Thread [14] weak up.
Thread [13] weak up.
Thread [12] weak up.
Thread [11] weak up.
Thread [10] weak up.
Thread [9] weak up.
Thread [8] weak up.
Thread [7] weak up.
Thread [6] weak up.
Thread [5] weak up.
Thread [4] weak up.
Thread [3] weak up.
Thread [2] weak up.
Thread [1] weak up.
Thread [44] weak up.
Thread [43] weak up.
Thread [42] weak up.
Thread [41] weak up.
Thread [40] weak up.
Thread [39] weak up.
Thread [38] weak up.
Thread [37] weak up.
Thread [36] weak up.
Thread [35] weak up.
Thread [34] weak up.
Thread [33] weak up.
Thread [32] weak up.
Thread [31] weak up.
Thread [30] weak up.
Thread [29] weak up.
Thread [28] weak up.
Thread [27] weak up.
Thread [26] weak up.
Thread [25] weak up.
Thread [24] weak up.
Thread [23] weak up.
Thread [22] weak up.
Thread [21] weak up.
Thread [20] weak up.
Thread [47] weak up.
Thread [46] weak up.
Thread [45] weak up.
Thread [49] weak up.
Thread [48] weak up.
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49]
[0, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 44, 43, 42, 41, 40, 39, 38, 37, 36, 35, 34, 33, 32, 31, 30, 29, 28, 27, 26, 25, 24, 23, 22, 21, 20, 47, 46, 45, 49, 48]

    根据上面的执行结果，貌似weakup的顺序是随机的。那么要是我们这么快就得到了结果，哪岂不是本文的标题有问题。我们再来看看第二个实验。

    2.实验二

    package com.dhb.notify;

import java.util.LinkedList;
import java.util.List;
import java.util.concurrent.TimeUnit;

public class NotifyTest{

    private static List<String> before = new LinkedList<>();
    private static List<String> after = new LinkedList<>();

    private static Object lock = new Object();


    public static void main(String[] args) throws InterruptedException{

        for(int i=0;i<50;i++){
            String threadName = Integer.toString(i);
            new Thread(() -> {
                synchronized (lock) {
                    String cthreadName = Thread.currentThread().getName();
                    System.out.println("Thread ["+cthreadName+"] wait.");
                    before.add(cthreadName);
                    try {
                        lock.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println("Thread ["+cthreadName+"] weak up.");
                    after.add(cthreadName);
                }
            },threadName).start();
            TimeUnit.MILLISECONDS.sleep(50);
        }

        TimeUnit.SECONDS.sleep(1);

        for(int i=0;i<50;i++){
            synchronized (lock) {
                lock.notify();
            }
            TimeUnit.MILLISECONDS.sleep(10);
        }
        TimeUnit.SECONDS.sleep(1);

        System.out.println(before.toString());
        System.out.println(after.toString());
    }
}

    执行结果如下：

    Thread [0] wait.
Thread [1] wait.
Thread [2] wait.
Thread [3] wait.
Thread [4] wait.
Thread [5] wait.
Thread [6] wait.
Thread [7] wait.
Thread [8] wait.
Thread [9] wait.
Thread [10] wait.
Thread [11] wait.
Thread [12] wait.
Thread [13] wait.
Thread [14] wait.
Thread [15] wait.
Thread [16] wait.
Thread [17] wait.
Thread [18] wait.
Thread [19] wait.
Thread [20] wait.
Thread [21] wait.
Thread [22] wait.
Thread [23] wait.
Thread [24] wait.
Thread [25] wait.
Thread [26] wait.
Thread [27] wait.
Thread [28] wait.
Thread [29] wait.
Thread [30] wait.
Thread [31] wait.
Thread [32] wait.
Thread [33] wait.
Thread [34] wait.
Thread [35] wait.
Thread [36] wait.
Thread [37] wait.
Thread [38] wait.
Thread [39] wait.
Thread [40] wait.
Thread [41] wait.
Thread [42] wait.
Thread [43] wait.
Thread [44] wait.
Thread [45] wait.
Thread [46] wait.
Thread [47] wait.
Thread [48] wait.
Thread [49] wait.
Thread [0] weak up.
Thread [1] weak up.
Thread [2] weak up.
Thread [3] weak up.
Thread [4] weak up.
Thread [5] weak up.
Thread [6] weak up.
Thread [7] weak up.
Thread [8] weak up.
Thread [9] weak up.
Thread [10] weak up.
Thread [11] weak up.
Thread [12] weak up.
Thread [13] weak up.
Thread [14] weak up.
Thread [15] weak up.
Thread [16] weak up.
Thread [17] weak up.
Thread [18] weak up.
Thread [19] weak up.
Thread [20] weak up.
Thread [21] weak up.
Thread [22] weak up.
Thread [23] weak up.
Thread [24] weak up.
Thread [25] weak up.
Thread [26] weak up.
Thread [27] weak up.
Thread [28] weak up.
Thread [29] weak up.
Thread [30] weak up.
Thread [31] weak up.
Thread [32] weak up.
Thread [33] weak up.
Thread [34] weak up.
Thread [35] weak up.
Thread [36] weak up.
Thread [37] weak up.
Thread [38] weak up.
Thread [39] weak up.
Thread [40] weak up.
Thread [41] weak up.
Thread [42] weak up.
Thread [43] weak up.
Thread [44] weak up.
Thread [45] weak up.
Thread [46] weak up.
Thread [47] weak up.
Thread [48] weak up.
Thread [49] weak up.
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49]

    这个代码线程的weak up顺序就是与wait顺序一致的。

    3.问题分析

    为什么上述两个代码的执行结果会如此不同，代码貌似都相差不多。那么问题出在哪呢？先看看各位读者能否自行发现问题所在。
    这也是我在之前为了弄这个例子所犯的问题。
    对，问题就出在这个sleep上，这个sleep是在synchronized里面还是外面sleep，区别非常大。我们先看看代码。
    实验一：

    for(int i=0;i<50;i++){
    synchronized (lock) {
        lock.notify();
        TimeUnit.MILLISECONDS.sleep(10);
    }
}

    实验二：

    for(int i=0;i<50;i++){
    synchronized (lock) {
        lock.notify();
    }
    TimeUnit.MILLISECONDS.sleep(10);
}

    就是上述这几行代码，可能造成的结果就不一样。为什么实验一中的结果会出现随机呢？那是因为，当我们执行notify之后，由于sleep在symchronized内部，因此没有释放锁。那么被通知到的线程，就会进入waitSet队列，之后当通知线程进入循环之后，可能会同时竞争synchronized，由于synchronized的BLOCK到RUNNING状态是非公平的。这个通知线程可能再次又获得锁，进行第二次notify。
    整个过程复盘如下：

    • 1.初始状态，用N表示通知线程，在notify没执行之前，状态如下；


      执行notify之前

    • 2.此后执行notify，假定C1被通知到，进入waitSet队列。


      执行notify

    • 3.执行完notify之后会进行sleep，此时仍然由N持有锁，在这之后，sleep结束，N将释放锁。N还会继续执行。当N再次进入循环的时候，此时，N就会进入BLOCK对synchronized的资源进行竞争。那么需要注意的是，这个时候，之前处于BLOCK状态的线程不一定就会执行，因为这是在并发条件下进行的。很大概率的情况下，都会出现同时位于BLOCK队列的情况。


      执行notify之后

    • 4.那么由于synchronized实际上不是公平锁，其锁竞争的机制具有随机性，那么此时有可能线程N再次获得锁。就是下图这种情况。


      N再次抢到锁

    • 5.线程N获得锁之后，会再次notify一个线程到WaitSet队列。


      再次调用notify

    • 6.与第三步类似，可能此时Nsleep完成之后进入了BLOCK而C1、C2也在WaitSet队列等待。


      N sleep完毕之后重新竞争

    • 7.那么此时，有一种情况就会出现，那就是C2抢到了锁。这样C2就会在C1之前被weak up。


      C2抢到锁，weakup

    通过上述分析，就很容易得出实验一种唤醒次序随机的原因了。我们再来看看实验一的weak up输出：

    [0, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 44, 43, 42, 41, 40, 39, 38, 37, 36, 35, 34, 33, 32, 31, 30, 29, 28, 27, 26, 25, 24, 23, 22, 21, 20, 47, 46, 45, 49, 48]

    根据这个输出，可以看到第二位为19，也就是说，当线程编号为19的线程weakup的时候，已经调用了20次notify。
    对于实验二，则由于在每次notify之后，释放锁之后，再进入sleep，因此通知线程不会和WaitSet中的线程竞争锁。那么实验二中实际上得到的顺序，就是notify的顺序。
    通过实验二不难看出，notify实际上并不是随机的，而是顺序。

    4.HotSpot源码

    为了进一步证明上述问题，我们可以对HotSpot源码进行分析。
    hotspot源码
    点zip就能下载zip格式的源码：

    openJDK下载

    在此之后，我们知道，synchronized的wait和notify是位于ObjectMonitor.cpp中。
    我们来看看这个具体notify的方法：

    // Consider:
// If the lock is cool (cxq == null && succ == null) and we're on an MP system
// then instead of transferring a thread from the WaitSet to the EntryList
// we might just dequeue a thread from the WaitSet and directly unpark() it.

void ObjectMonitor::notify(TRAPS) {
  CHECK_OWNER();
  if (_WaitSet == NULL) {
     TEVENT (Empty-Notify) ;
     return ;
  }
  DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);

  int Policy = Knob_MoveNotifyee ;

  Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
  //重点再这里，是调用的DequeueWaiter方法
  ObjectWaiter * iterator = DequeueWaiter() ;
  if (iterator != NULL) {
     TEVENT (Notify1 - Transfer) ;
     guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
     guarantee (iterator->_notified == 0, "invariant") ;
     if (Policy != 4) {
        iterator->TState = ObjectWaiter::TS_ENTER ;
     }
     iterator->_notified = 1 ;
     Thread * Self = THREAD;
     iterator->_notifier_tid = Self->osthread()->thread_id();

     ObjectWaiter * List = _EntryList ;
     if (List != NULL) {
        assert (List->_prev == NULL, "invariant") ;
        assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
        assert (List != iterator, "invariant") ;
     }

     if (Policy == 0) {       // prepend to EntryList
         if (List == NULL) {
             iterator->_next = iterator->_prev = NULL ;
             _EntryList = iterator ;
         } else {
             List->_prev = iterator ;
             iterator->_next = List ;
             iterator->_prev = NULL ;
             _EntryList = iterator ;
        }
     } else
     if (Policy == 1) {      // append to EntryList
         if (List == NULL) {
             iterator->_next = iterator->_prev = NULL ;
             _EntryList = iterator ;
         } else {
            // CONSIDER:  finding the tail currently requires a linear-time walk of
            // the EntryList.  We can make tail access constant-time by converting to
            // a CDLL instead of using our current DLL.
            ObjectWaiter * Tail ;
            for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
            assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
            Tail->_next = iterator ;
            iterator->_prev = Tail ;
            iterator->_next = NULL ;
        }
     } else
     if (Policy == 2) {      // prepend to cxq
         // prepend to cxq
         if (List == NULL) {
             iterator->_next = iterator->_prev = NULL ;
             _EntryList = iterator ;
         } else {
            iterator->TState = ObjectWaiter::TS_CXQ ;
            for (;;) {
                ObjectWaiter * Front = _cxq ;
                iterator->_next = Front ;
                if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
                    break ;
                }
            }
         }
     } else
     if (Policy == 3) {      // append to cxq
        iterator->TState = ObjectWaiter::TS_CXQ ;
        for (;;) {
            ObjectWaiter * Tail ;
            Tail = _cxq ;
            if (Tail == NULL) {
                iterator->_next = NULL ;
                if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
                   break ;
                }
            } else {
                while (Tail->_next != NULL) Tail = Tail->_next ;
                Tail->_next = iterator ;
                iterator->_prev = Tail ;
                iterator->_next = NULL ;
                break ;
            }
        }
     } else {
        ParkEvent * ev = iterator->_event ;
        iterator->TState = ObjectWaiter::TS_RUN ;
        OrderAccess::fence() ;
        ev->unpark() ;
     }

     if (Policy < 4) {
       iterator->wait_reenter_begin(this);
     }

     // _WaitSetLock protects the wait queue, not the EntryList.  We could
     // move the add-to-EntryList operation, above, outside the critical section
     // protected by _WaitSetLock.  In practice that's not useful.  With the
     // exception of  wait() timeouts and interrupts the monitor owner
     // is the only thread that grabs _WaitSetLock.  There's almost no contention
     // on _WaitSetLock so it's not profitable to reduce the length of the
     // critical section.
  }

  Thread::SpinRelease (&_WaitSetLock) ;

  if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) {
     ObjectMonitor::_sync_Notifications->inc() ;
  }
}

    notify过程调用的是DequeueWaiter方法：

    inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
  // dequeue the very first waiter
  ObjectWaiter* waiter = _WaitSet;
  if (waiter) {//判断_WaitSet是否为空，否则，将第一个元素调用Dequeue方法。
    DequeueSpecificWaiter(waiter);
  }
  return waiter;
}

    实际上是将_WaitSet中的第一个元素进行出队操作。
    这也说明了notify是个顺序操作。具有公平性。

    5.总结

    经过上问两个实验分析以及查看源码可以说明：

    • 1.在HotSpot中，notify是顺序执行的，从等待队列中将队首元素出队。至于其他jvm暂时也没接触到，但是对于HotSpot确实是这样的。因此下次在有面试官问notify和notifyAll的区别的时候，希望不再是回答随机性。
    • 2.synchronized是非公平锁，本文就是最好的证明。虽然没有对源码进行分析。由于篇幅限制此处就不一一展开了。
    • 3.最终要的一点是，不要迷信权威，要敢于质疑，如果得到一个结论要反复问为什么。不行我们可以看源码。源码是不会撒谎的，我们也可以通过实验证明。

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