public native void unpark(Thread jthread);  
public native void park(boolean isAbsolute, long time);  
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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] ;  
   ...  
}  
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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;  
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ThreadBlockInVM tbivm(jt);  
if (_counter > 0)  { // no wait needed  
  _counter = 0;  
  status = pthread_mutex_unlock(_mutex);  
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if (time == 0) {  
 status = pthread_cond_wait (_cond, _mutex) ;  
}  
_counter = 0 ;  
status = pthread_mutex_unlock(_mutex) ;  
assert_status(status == 0, status, "invariant") ;  
OrderAccess::fence();  
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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") ;  
  }  
}
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if (time == 0) {  
  status = pthread_cond_wait (_cond, _mutex) ;  
}  
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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.
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// 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：
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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 ;  
  }  
}