从底层分析LockSupport原理机制
知识点
LockSupport的介绍
LockSupport类是Java6(JSR166-JUC)引入的一个类，提供了基本的线程同步原语。LockSupport实际上是调用了Unsafe类里的函数，归结到Unsafe里，只有两个函数，而仅仅两个简单的接口，就为上层提供了强大的同步原语，先来解析下两个函数是做什么的。
public native void unpark(Thread jthread);
public native void park(boolean isAbsolute, long time);
线程调用park函数则等待"许可"。
线程调用unpark函数为线程提供"许可(permit)"。
注意，unpark函数可以先于park调用。比如线程B调用unpark函数，给线程A发了一个"许可"，那么当线程A调用park时，它发现已经有"许可"了，那么它会马上再继续运行。（此部分比wait/notify（notifyAll））要好很多。
park和unpark的灵活之处
unpark函数可以先于park调用，这个正是它们的灵活之处。
- 一个线程它有可能在别的线程unPark之前，或者之后，或者同时调用了park，那么因为park的特性，它可以不用担心自己的park的时序问题，否则，如果park必须要在unpark之前。
考虑一下，两个线程同步，要如何处理？
- 在Java5里是用wait/notify/notifyAll来同步的。wait/notify机制有个很蛋疼的地方是，比如线程B要用notify通知线程A，那么线程B要确保线程A已经在wait调用上等待了，否则线程A可能永远都在等待。
另外，是调用notify，还是notifyAll？
notify只会唤醒一个线程，如果错误地有两个线程在同一个对象上wait等待，那么又悲剧了。为了安全起见，貌似只能调用notifyAll了。
park/unpark模型真正解耦了线程之间的同步，线程之间不再需要一个Object或者其它变量来存储状态，不再需要关心对方的状态。
拓展延伸
HotSpot里park/unpark的实现，每个java线程都有一个Parker实例，Parker类是这样定义的：
class Parker : public os::PlatformParker {
private:
volatile int _counter ;
...
public:
void park(bool isAbsolute, jlong time);
void unpark();
...
}
class PlatformParker : public CHeapObj<mtInternal> {
protected:
pthread_mutex_t _mutex [1] ;
pthread_cond_t _cond [1] ;
...
}
-
可以看到Parker类实际上用Posix的mutex，condition来实现的。
-
在Parker类里的_counter字段，就是用来记录所谓的“许可”的。
-
当调用park时，先尝试直接能否直接拿到"许可"，即_counter>0时，如果成功，则把_counter设置为0,并返回：(和信号量的思路很像！)
void Parker::park(bool isAbsolute, jlong time) {
// Ideally we'd do something useful while spinning, such
// as calling unpackTime().
// Optional fast-path check:
// Return immediately if a permit is available.
// We depend on Atomic::xchg() having full barrier semantics
// since we are doing a lock-free update to _counter.
if (Atomic::xchg(0, &_counter) > 0) return;
如果不成功，则构造一个ThreadBlockInVM，然后检查_counter是不是>0，如果是，则把_counter设置为0，unlock mutex并返回：
ThreadBlockInVM tbivm(jt);
if (_counter > 0) { // no wait needed
_counter = 0;
status = pthread_mutex_unlock(_mutex);
否则，再判断等待的时间，然后再调用pthread_cond_wait函数等待，如果等待返回，则把_counter设置为0，unlock mutex并返回：
if (time == 0) {
status = pthread_cond_wait (_cond, _mutex) ;
}
_counter = 0 ;
status = pthread_mutex_unlock(_mutex) ;
assert_status(status == 0, status, "invariant") ;
OrderAccess::fence();
当unpark时，则简单多了，直接设置_counter为1，再unlock mutext返回。如果_counter之前的值是0，则还要调用pthread_cond_signal唤醒在park中等待的线程：
void Parker::unpark() {
int s, status ;
status = pthread_mutex_lock(_mutex);
assert (status == 0, "invariant") ;
s = _counter;
_counter = 1;
if (s < 1) {
if (WorkAroundNPTLTimedWaitHang) {
status = pthread_cond_signal (_cond) ;
assert (status == 0, "invariant") ;
status = pthread_mutex_unlock(_mutex);
assert (status == 0, "invariant") ;
} else {
status = pthread_mutex_unlock(_mutex);
assert (status == 0, "invariant") ;
status = pthread_cond_signal (_cond) ;
assert (status == 0, "invariant") ;
}
} else {
pthread_mutex_unlock(_mutex);
assert (status == 0, "invariant") ;
}
}
if (time == 0) {
status = pthread_cond_wait (_cond, _mutex) ;
}
这也就是为什么Java dos里提到，当下面三种情况下park函数会返回：
Some other thread invokes unpark with the current thread as the target; or
Some other thread interrupts the current thread; or
The call spuriously (that is, for no reason) returns.
相关的实现代码在：
http://hg.openjdk.java.net/build-infra/jdk7/hotspot/file/52c4a1ae6adc/src/share/vm/runtime/park.hpp http://hg.openjdk.java.net/build-infra/jdk7/hotspot/file/52c4a1ae6adc/src/share/vm/runtime/park.cpp http://hg.openjdk.java.net/build-infra/jdk7/hotspot/file/52c4a1ae6adc/src/os/linux/vm/os_linux.hpp http://hg.openjdk.java.net/build-infra/jdk7/hotspot/file/52c4a1ae6adc/src/os/linux/vm/os_linux.cpp
其它的一些东东： Parker类在分配内存时，使用了一个技巧，重载了new函数来实现了cache line对齐。
// We use placement-new to force ParkEvent instances to be
// aligned on 256-byte address boundaries. This ensures that the least
// significant byte of a ParkEvent address is always 0.
void * operator new (size_t sz) ;
Parker里使用了一个无锁的队列在分配释放Parker实例：
volatile int Parker::ListLock = 0 ;
Parker * volatile Parker::FreeList = NULL ;
Parker * Parker::Allocate (JavaThread * t) {
guarantee (t != NULL, "invariant") ;
Parker * p ;
// Start by trying to recycle an existing but unassociated
// Parker from the global free list.
for (;;) {
p = FreeList ;
if (p == NULL) break ;
// 1: Detach
// Tantamount to p = Swap (&FreeList, NULL)
if (Atomic::cmpxchg_ptr (NULL, &FreeList, p) != p) {
continue ;
}
// We've detached the list. The list in-hand is now
// local to this thread. This thread can operate on the
// list without risk of interference from other threads.
// 2: Extract -- pop the 1st element from the list.
Parker * List = p->FreeNext ;
if (List == NULL) break ;
for (;;) {
// 3: Try to reattach the residual list
guarantee (List != NULL, "invariant") ;
Parker * Arv = (Parker *) Atomic::cmpxchg_ptr (List, &FreeList, NULL) ;
if (Arv == NULL) break ;
// New nodes arrived. Try to detach the recent arrivals.
if (Atomic::cmpxchg_ptr (NULL, &FreeList, Arv) != Arv) {
continue ;
}
guarantee (Arv != NULL, "invariant") ;
// 4: Merge Arv into List
Parker * Tail = List ;
while (Tail->FreeNext != NULL) Tail = Tail->FreeNext ;
Tail->FreeNext = Arv ;
}
break ;
}
if (p != NULL) {
guarantee (p->AssociatedWith == NULL, "invariant") ;
} else {
// Do this the hard way -- materialize a new Parker..
// In rare cases an allocating thread might detach
// a long list -- installing null into FreeList --and
// then stall. Another thread calling Allocate() would see
// FreeList == null and then invoke the ctor. In this case we
// end up with more Parkers in circulation than we need, but
// the race is rare and the outcome is benign.
// Ideally, the # of extant Parkers is equal to the
// maximum # of threads that existed at any one time.
// Because of the race mentioned above, segments of the
// freelist can be transiently inaccessible. At worst
// we may end up with the # of Parkers in circulation
// slightly above the ideal.
p = new Parker() ;
}
p->AssociatedWith = t ; // Associate p with t
p->FreeNext = NULL ;
return p ;
}
void Parker::Release (Parker * p) {
if (p == NULL) return ;
guarantee (p->AssociatedWith != NULL, "invariant") ;
guarantee (p->FreeNext == NULL , "invariant") ;
p->AssociatedWith = NULL ;
for (;;) {
// Push p onto FreeList
Parker * List = FreeList ;
p->FreeNext = List ;
if (Atomic::cmpxchg_ptr (p, &FreeList, List) == List) break ;
}
}
