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 ;   //計(jì)數(shù)
  Parker * FreeNext ;      //指向下一個(gè)Parker
  JavaThread * AssociatedWith ; // 指向parker所屬的線程。
public:
  Parker() : PlatformParker() {
    _counter       = 0 ;    //初始化為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;  // 條件變量數(shù)組下標(biāo)，which cond is in use: -1, 0, 1
    pthread_mutex_t _mutex [1] ;  //pthread互斥鎖
    pthread_cond_t  _cond  [2] ; // pthread條件變量數(shù)組,一個(gè)用于相對(duì)時(shí)間，一個(gè)用于絕對(duì)時(shí)間。
  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) {
  //原子交換，如果_counter > 0,則將_counter置為0，直接返回，否則_counter為0
  if (Atomic::xchg(0, &_counter) > 0) return;
  //獲取當(dāng)前線程
  Thread* thread = Thread::current();
  assert(thread->is_Java_thread(), "Must be JavaThread");
  //下轉(zhuǎn)型為java線程
  JavaThread *jt = (JavaThread *)thread;
  //如果當(dāng)前線程設(shè)置了中斷標(biāo)志，調(diào)用park則直接返回，所以如果在park之前調(diào)用了
  //interrupt就會(huì)直接返回
  if (Thread::is_interrupted(thread, false)) {
    return;
  }
  // 高精度絕對(duì)時(shí)間變量
  timespec absTime;
  //如果time小于0，或者isAbsolute是true并且time等于0則直接返回
  if (time < 0 || (isAbsolute && time == 0) ) { // don't wait at all
    return;
  }
  //如果time大于0，則根據(jù)是否是高精度定時(shí)計(jì)算定時(shí)時(shí)間
  if (time > 0) {
    unpackTime(&absTime, isAbsolute, time);
  }
  //進(jìn)入安全點(diǎn)避免死鎖
  ThreadBlockInVM tbivm(jt);
  //如果當(dāng)前線程設(shè)置了中斷標(biāo)志，或者獲取mutex互斥鎖失敗則直接返回
  //由于Parker是每個(gè)線程都有的，所以_counter cond mutex都是每個(gè)線程都有的，
  //不是所有線程共享的所以加鎖失敗只有兩種情況，第一unpark已經(jīng)加鎖這時(shí)只需要返回即可，
  //第二調(diào)用調(diào)用pthread_mutex_trylock出錯(cuò)。對(duì)于第一種情況就類(lèi)似是unpark先調(diào)用的情況，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，說(shuō)明unpark已經(jīng)調(diào)用完成了將_counter置為了1，
  //現(xiàn)在只需將_counter置0，解鎖，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 /* not Object.wait() */);
  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，說(shuō)明是相對(duì)時(shí)間也就是isAbsolute是fasle(否則前面就直接返回了),則直接掛起
  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
    //判斷isAbsolute是false還是true，false的話(huà)使用_cond[0]，否則用_cond[1]
    _cur_index = isAbsolute ? ABS_INDEX : REL_INDEX;
    //使用條件變量使得當(dāng)前線程掛起。
    status = os::Linux::safe_cond_timedwait (&_cond[_cur_index], _mutex, &absTime) ;
    //如果掛起失敗則銷(xiāo)毀當(dāng)前的條件變量重新初始化。
    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成功則以下代碼都是線程被喚醒后執(zhí)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
  //將_counter變量重新置為1
  _counter = 0 ;
  //解鎖
  status = pthread_mutex_unlock(_mutex) ;
  assert_status(status == 0, status, "invariant") ;
  // 使用內(nèi)存屏障使_counter對(duì)其它線程可見(jiàn)
  OrderAccess::fence();
  // 如果在park線程掛起的時(shí)候調(diào)用了stop或者suspend則還需要將線程掛起不能返回
  if (jt->handle_special_suspend_equivalent_condition()) {
    jt->java_suspend_self();
  }
}

void Parker::unpark() {
  int s, status ;
  //加互斥鎖
  status = pthread_mutex_lock(_mutex);
  assert (status == 0, "invariant") ;
  s = _counter;
  _counter = 1; //將_counter置1
  //如果_counter是0則說(shuō)明調(diào)用了park或者沒(méi)調(diào)用(初始為counter0）
  //這也說(shuō)明park和unpark調(diào)用沒(méi)有先后順序。
  if (s < 1) {
    // 說(shuō)明當(dāng)前parker對(duì)應(yīng)的線程掛起了，因?yàn)開(kāi)cur_index初始是-1，并且等待條件變量的線程被喚醒
    //后也會(huì)將_cur_index重置-1
    if (_cur_index != -1) {
       //如果設(shè)置了WorkAroundNPTLTimedWaitHang先調(diào)用signal再調(diào)用unlock，否則相反
      //這兩個(gè)先后順序都可以，在hotspot在Linux下默認(rèn)使用這種方式
      //即先調(diào)用signal再調(diào)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說(shuō)明線程沒(méi)在等待條件變量，則直接解鎖
      pthread_mutex_unlock(_mutex);
      assert (status == 0, "invariant") ;
    }
  } else {//如果_counter == 1,說(shuō)明線程調(diào)用了一次或多次unpark但是沒(méi)調(diào)用park，則直接解鎖
    pthread_mutex_unlock(_mutex);
    assert (status == 0, "invariant") ;
  }