文章目录
- 1.实验一
- 2.实验二
- 3.问题分析
- 4.HotSpot源码
- 5.总结
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：
代码语言：javascript复制 * 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。
代码语言：javascript复制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：
代码语言：javascript复制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
代码语言：javascript复制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：
代码语言：javascript复制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：
代码语言：javascript复制for(int i=0;i<50;i ){
synchronized (lock) {
lock.notify();
TimeUnit.MILLISECONDS.sleep(10);
}
}实验二：
代码语言：javascript复制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；
- 2.此后执行notify，假定C1被通知到，进入waitSet队列。
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。
- 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。
- 5.线程N获得锁之后，会再次notify一个线程到WaitSet队列。
- 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。
- 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。
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：
代码语言：javascript复制[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：
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：
代码语言：javascript复制// 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：
代码语言：javascript复制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。
