LockSupport 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：

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") ;
  }
}

简而言之，是用mutex和condition保护了一个_counter的变量，当park时，这个变量置为了0，当unpark时，这个变量置为1。
值得注意的是在park函数里，调用pthread_cond_wait时，并没有用while来判断，所以posix condition里的"Spurious wakeup"一样会传递到上层Java的代码里。

  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.