hotspot解析jdk1.8Unsafe类park和unpark方法使用

public native void park(boolean isAbsolute, long time);

public static void park() {
        UNSAFE.park(false, 0L);
}

 public static void parkNanos(long nanos) {
        if (nanos > 0)
            UNSAFE.park(false, nanos);
 }

 public static void parkUntil(long deadline) {
        UNSAFE.park(true, deadline);
 }

class Parker : public os::PlatfORMParker {
private:
  volatile int _counter ;   //计数
  Parker * FreeNext ;      //指向下一个Parker
  JavaThread * AssociatedWith ; // 指向parker所属的线程。
public:
  Parker() : PlatformParker() {
    _counter       = 0 ;    //初始化为0
    FreeNext       = NULL ;
    AssociatedWith = NULL ;
  }
protected:
  ~Parker() { ShouldNotReachHere(); }
public:
  // For simplicity of interface with Java, all forms of park (indefinite,
  // relative, and absolute) are multiplexed into one call.
  void park(bool isAbsolute, jlong time);
  void unpark();
  // Lifecycle operators
  static Parker * Allocate (JavaThread * t) ;
  static void Release (Parker * e) ;private:
  static Parker * volatile FreeList ;
  static volatile int ListLock ;
};

class PlatformParker : public CHeapObj<mtInternal> {
  protected:
    enum {
        REL_INDEX = 0,
        ABS_INDEX = 1
    };
    int _cur_index;  // 条件变量数组下标，which cond is in use: -1, 0, 1
    pthread_mutex_t _mutex [1] ;  //pthread互斥锁
    pthread_cond_t  _cond  [2] ; // pthread条件变量数组,一个用于相对时间，一个用于绝对时间。
  public:       // TODO-FIXME: make dtor private
    ~PlatformParker() { guarantee (0, "invariant") ; }
  public:
    PlatformParker() {
      int status;
      status = pthread_cond_init (&_cond[REL_INDEX], os::linux::condAttr());
      assert_status(status == 0, status, "cond_init rel");
      status = pthread_cond_init (&_cond[ABS_INDEX], NULL);
      assert_status(status == 0, status, "cond_init abs");
      status = pthread_mutex_init (_mutex, NULL);
      assert_status(status == 0, status, "mutex_init");
      _cur_index = -1; // mark as unused
    }
};

 // jsR166 per-thread parker
private:
  Parker*    _parker;

void Parker::park(bool isAbsolute, jlong time) {
  //原子交换，如果_counter > 0,则将_counter置为0，直接返回，否则_counter为0
  if (Atomic::xchg(0, &_counter) > 0) return;
  //获取当前线程
  Thread* thread = Thread::current();
  assert(thread->is_Java_thread(), "Must be JavaThread");
  //下转型为java线程
  JavaThread *jt = (JavaThread *)thread;
  //如果当前线程设置了中断标志，调用park则直接返回，所以如果在park之前调用了
  //interrupt就会直接返回
  if (Thread::is_interrupted(thread, false)) {
    return;
  }
  // 高精度绝对时间变量
  timespec absTime;
  //如果time小于0，或者isAbsolute是true并且time等于0则直接返回
  if (time < 0 || (isAbsolute && time == 0) ) { // don't wait at all
    return;
  }
  //如果time大于0，则根据是否是高精度定时计算定时时间
  if (time > 0) {
    unpackTime(&absTime, isAbsolute, time);
  }
  //进入安全点避免死锁
  ThreadBlockInVM tbivm(jt);
  //如果当前线程设置了中断标志，或者获取mutex互斥锁失败则直接返回
  //由于Parker是每个线程都有的，所以_counter cond mutex都是每个线程都有的，
  //不是所有线程共享的所以加锁失败只有两种情况，第一unpark已经加锁这时只需要返回即可，
  //第二调用调用pthread_mutex_trylock出错。对于第一种情况就类似是unpark先调用的情况，所以
  //直接返回。
  if (Thread::is_interrupted(thread, false) || pthread_mutex_trylock(_mutex) != 0) {
    return;
  }
  int status ;
  //如果_counter大于0，说明unpark已经调用完成了将_counter置为了1，
  //现在只需将_counter置0，解锁，返回
  if (_counter > 0)  { // no wait needed
    _counter = 0;
    status = pthread_mutex_unlock(_mutex);
    assert (status == 0, "invariant");
    OrderAccess::fence();
    return;
  }
  OSThreadWaitState osts(thread->osthread(), false );
  jt->set_suspend_equivalent();
  // cleared by handle_special_suspend_equivalent_condition() or java_suspend_self()
  assert(_cur_index == -1, "invariant");
  //如果time等于0，说明是相对时间也就是isAbsolute是fasle(否则前面就直接返回了),则直接挂起
  if (time == 0) {
    _cur_index = REL_INDEX; // arbitrary choice when not timed
    status = pthread_cond_wait (&_cond[_cur_index], _mutex) ;
  } else { //如果time非0
    //判断isAbsolute是false还是true，false的话使用_cond[0]，否则用_cond[1]
    _cur_index = isAbsolute ? ABS_INDEX : REL_INDEX;
    //使用条件变量使得当前线程挂起。
    status = os::Linux::safe_cond_timedwait (&_cond[_cur_index], _mutex, &absTime) ;
    //如果挂起失败则销毁当前的条件变量重新初始化。
    if (status != 0 && WorkAroundNPTLTimedWaitHang) {
      pthread_cond_destroy (&_cond[_cur_index]) ;
      pthread_cond_init    (&_cond[_cur_index], isAbsolute ? NULL : os::Linux::condAttr());
    }
  }
  //如果pthread_cond_wait成功则以下代码都是线程被唤醒后执行的。
  _cur_index = -1;
  assert_status(status == 0 || status == EINTR ||
                status == ETIME || status == ETIMEDOUT,
                status, "cond_timedwait");
#ifdef ASSERT
  pthread_sigmask(SIG_SETMASK, &oldsigs, NULL);
#endif
  //将_counter变量重新置为1
  _counter = 0 ;
  //解锁
  status = pthread_mutex_unlock(_mutex) ;
  assert_status(status == 0, status, "invariant") ;
  // 使用内存屏障使_counter对其它线程可见
  OrderAccess::fence();
  // 如果在park线程挂起的时候调用了stop或者suspend则还需要将线程挂起不能返回
  if (jt->handle_special_suspend_equivalent_condition()) {
    jt->java_suspend_self();
  }
}

void Parker::unpark() {
  int s, status ;
  //加互斥锁
  status = pthread_mutex_lock(_mutex);
  assert (status == 0, "invariant") ;
  s = _counter;
  _counter = 1; //将_counter置1
  //如果_counter是0则说明调用了park或者没调用(初始为counter0）
  //这也说明park和unpark调用没有先后顺序。
  if (s < 1) {
    // 说明当前parker对应的线程挂起了，因为_cur_index初始是-1，并且等待条件变量的线程被唤醒
    //后也会将_cur_index重置-1
    if (_cur_index != -1) {
       //如果设置了WorkAroundNPTLTimedWaitHang先调用signal再调用unlock，否则相反
      //这两个先后顺序都可以，在hotspot在Linux下默认使用这种方式
      //即先调用signal再调用unlock
      if (WorkAroundNPTLTimedWaitHang) {
        status = pthread_cond_signal (&_cond[_cur_index]);
        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[_cur_index]);
        assert (status == 0, "invariant");
      }
    } else { //如果_cur_index == -1说明线程没在等待条件变量，则直接解锁
      pthread_mutex_unlock(_mutex);
      assert (status == 0, "invariant") ;
    }
  } else {//如果_counter == 1,说明线程调用了一次或多次unpark但是没调用park，则直接解锁
    pthread_mutex_unlock(_mutex);
    assert (status == 0, "invariant") ;
  }