深入了解java中的底層阻塞原理及實作

　　談到阻塞，相信大家都不會陌生了。阻塞的應用場景真的多得不要不要的，比如生産-消費模式，限流統計等等。什麼 ArrayBlockingQueue, LinkedBlockingQueue, DelayQueue... 都是阻塞隊列的實作啊，多簡單！

　　阻塞，一般有兩個特性很亮眼：1. 不耗cpu的等待；2. 線程安全；

　　額，要這麼說也ok的。畢竟，我們遇到的問題，到這裡就夠解決了。但是有沒有想過，這容器的阻塞又是如何實作的呢？

好吧，翻開源碼，也很簡單了：（比如ArrayBlockingQueue的take、put....）

// ArrayBlockingQueue

    /**
     * Inserts the specified element at the tail of this queue, waiting
     * for space to become available if the queue is full.
     *
     * @throws InterruptedException {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     */
    public void put(E e) throws InterruptedException {
        checkNotNull(e);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            while (count == items.length)
                // 阻塞的點
                notFull.await();
            enqueue(e);
        } finally {
            lock.unlock();
        }
    }


    /**
     * Inserts the specified element at the tail of this queue, waiting
     * up to the specified wait time for space to become available if
     * the queue is full.
     *
     * @throws InterruptedException {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean offer(E e, long timeout, TimeUnit unit)
        throws InterruptedException {

        checkNotNull(e);
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            while (count == items.length) {
                if (nanos <= 0)
                    return false;
                // 阻塞的點
                nanos = notFull.awaitNanos(nanos);
            }
            enqueue(e);
            return true;
        } finally {
            lock.unlock();
        }
    }

    public E take() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            while (count == 0)
                // 阻塞的點
                notEmpty.await();
            return dequeue();
        } finally {
            lock.unlock();
        }
    }

看來，最終都是依賴了AbstractQueuedSynchronizer類（著名的AQS）的await方法，看起來像那麼回事。那麼這個同步器的阻塞又是如何實作的呢？

java的代碼總是好跟蹤的：

// AbstractQueuedSynchronizer.await()

/**
         * Implements interruptible condition wait.
         * <ol>
         * <li> If current thread is interrupted, throw InterruptedException.
         * <li> Save lock state returned by {@link #getState}.
         * <li> Invoke {@link #release} with saved state as argument,
         *      throwing IllegalMonitorStateException if it fails.
         * <li> Block until signalled or interrupted.
         * <li> Reacquire by invoking specialized version of
         *      {@link #acquire} with saved state as argument.
         * <li> If interrupted while blocked in step 4, throw InterruptedException.
         * </ol>
         */
        public final void await() throws InterruptedException {
            if (Thread.interrupted())
                throw new InterruptedException();
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                // 此處進行真正的阻塞
                LockSupport.park(this);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null) // clean up if cancelled
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
        }

如上，可以看到，真正的阻塞工作又轉交給了另一個工具類： LockSupport的 park 方法了，這回跟鎖扯上了關系，看起來已經越來越接近事實了：

// LockSupport.park()

/**
     * Disables the current thread for thread scheduling purposes unless the
     * permit is available.
     *
     * <p>If the permit is available then it is consumed and the call returns
     * immediately; otherwise
     * the current thread becomes disabled for thread scheduling
     * purposes and lies dormant until one of three things happens:
     *
     * <ul>
     * <li>Some other thread invokes {@link #unpark unpark} with the
     * current thread as the target; or
     *
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     *
     * <li>The call spuriously (that is, for no reason) returns.
     * </ul>
     *
     * <p>This method does <em>not</em> report which of these caused the
     * method to return. Callers should re-check the conditions which caused
     * the thread to park in the first place. Callers may also determine,
     * for example, the interrupt status of the thread upon return.
     *
     * @param blocker the synchronization object responsible for this
     *        thread parking
     * @since 1.6
     */
    public static void park(Object blocker) {
        Thread t = Thread.currentThread();
        setBlocker(t, blocker);
        UNSAFE.park(false, 0L);
        setBlocker(t, null);
    }

看得出來，這裡的實作就比較簡潔了，先擷取目前線程，設定阻塞對象，阻塞，然後解除阻塞。

好吧，到底什麼是真正的阻塞，我們還是不得而知！

UNSAFE.park(false, 0L); 是個什麼東西？看起來就是這一句起到了最關鍵的作用呢！但由于這裡已經是 native代碼，我們已經無法再簡單的檢視源碼了！那咋整呢？

那不行就看C/C++的源碼呗，看一下parker的定義（park.hpp）：

class Parker : public os::PlatformParker {
private:
  volatile int _counter ;
  Parker * FreeNext ;
  JavaThread * AssociatedWith ; // Current association

public:
  Parker() : PlatformParker() {
    _counter       = 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.  c中暴露出兩個方法給java調用
  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 ;

};

那 park() 方法到底是如何實作的呢？其實是繼承的 os::PlatformParker 的功能，也就是平台相關的私有實作，以 linux 平台實作為例（os_linux.hpp）：

// linux中的parker定義
class PlatformParker : public CHeapObj<mtInternal> {
  protected:
    enum {
        REL_INDEX = 0,
        ABS_INDEX = 1
    };
    int _cur_index;  // which cond is in use: -1, 0, 1
    pthread_mutex_t _mutex [1] ;
    pthread_cond_t  _cond  [2] ; // one for relative times and one for abs.

  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
    }
};

看到 park.cpp 中沒有重寫 park() 和 unpark() 方法，也就是說阻塞實作完全交由特定平台代碼處理了（os_linux.cpp）：

// park方法的實作，依賴于 _counter, _mutex[1], _cond[2]
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;

  Thread* thread = Thread::current();
  assert(thread->is_Java_thread(), "Must be JavaThread");
  JavaThread *jt = (JavaThread *)thread;

  // Optional optimization -- avoid state transitions if there's an interrupt pending.
  // Check interrupt before trying to wait
  if (Thread::is_interrupted(thread, false)) {
    return;
  }

  // Next, demultiplex/decode time arguments
  timespec absTime;
  if (time < 0 || (isAbsolute && time == 0) ) { // don't wait at all
    return;
  }
  if (time > 0) {
    unpackTime(&absTime, isAbsolute, time);
  }


  // Enter safepoint region
  // Beware of deadlocks such as 6317397.
  // The per-thread Parker:: mutex is a classic leaf-lock.
  // In particular a thread must never block on the Threads_lock while
  // holding the Parker:: mutex.  If safepoints are pending both the
  // the ThreadBlockInVM() CTOR and DTOR may grab Threads_lock.
  ThreadBlockInVM tbivm(jt);

  // Don't wait if cannot get lock since interference arises from
  // unblocking.  Also. check interrupt before trying wait
  if (Thread::is_interrupted(thread, false) || pthread_mutex_trylock(_mutex) != 0) {
    return;
  }

  int status ;
  if (_counter > 0)  { // no wait needed
    _counter = 0;
    status = pthread_mutex_unlock(_mutex);
    assert (status == 0, "invariant") ;
    // Paranoia to ensure our locked and lock-free paths interact
    // correctly with each other and Java-level accesses.
    OrderAccess::fence();
    return;
  }

#ifdef ASSERT
  // Don't catch signals while blocked; let the running threads have the signals.
  // (This allows a debugger to break into the running thread.)
  sigset_t oldsigs;
  sigset_t* allowdebug_blocked = os::Linux::allowdebug_blocked_signals();
  pthread_sigmask(SIG_BLOCK, allowdebug_blocked, &oldsigs);
#endif

  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");
  if (time == 0) {
    _cur_index = REL_INDEX; // arbitrary choice when not timed
    status = pthread_cond_wait (&_cond[_cur_index], _mutex) ;
  } else {
    _cur_index = isAbsolute ? ABS_INDEX : REL_INDEX;
    status = os::Linux::safe_cond_timedwait (&_cond[_cur_index], _mutex, &absTime) ;
    if (status != 0 && WorkAroundNPTLTimedWaitHang) {
      pthread_cond_destroy (&_cond[_cur_index]) ;
      pthread_cond_init    (&_cond[_cur_index], isAbsolute ? NULL : os::Linux::condAttr());
    }
  }
  _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

  _counter = 0 ;
  status = pthread_mutex_unlock(_mutex) ;
  assert_status(status == 0, status, "invariant") ;
  // Paranoia to ensure our locked and lock-free paths interact
  // correctly with each other and Java-level accesses.
  OrderAccess::fence();

  // If externally suspended while waiting, re-suspend
  if (jt->handle_special_suspend_equivalent_condition()) {
    jt->java_suspend_self();
  }
}

// unpark 實作，相對簡單些
void Parker::unpark() {
  int s, status ;
  status = pthread_mutex_lock(_mutex);
  assert (status == 0, "invariant") ;
  s = _counter;
  _counter = 1;
  if (s < 1) {
    // thread might be parked
    if (_cur_index != -1) {
      // thread is definitely parked
      if (WorkAroundNPTLTimedWaitHang) {
        status = pthread_cond_signal (&_cond[_cur_index]);
        assert (status == 0, "invariant");
        status = pthread_mutex_unlock(_mutex);
        assert (status == 0, "invariant");
      } else {
        // must capture correct index before unlocking
        int index = _cur_index;
        status = pthread_mutex_unlock(_mutex);
        assert (status == 0, "invariant");
        status = pthread_cond_signal (&_cond[index]);
        assert (status == 0, "invariant");
      }
    } else {
      pthread_mutex_unlock(_mutex);
      assert (status == 0, "invariant") ;
    }
  } else {
    pthread_mutex_unlock(_mutex);
    assert (status == 0, "invariant") ;
  }
}

從上面代碼可以看出，阻塞主要借助于三個變量，_cond, _mutex, _counter, 調用linux系統的 pthread_cond_wait, pthread_mutex_lock, pthread_mutex_unlock （一組POSIX标準的阻塞接口）等平台相關的方法進行阻塞了！

而 park.cpp中，則隻有  Allocate、Release 等的一些正常操作！

// 6399321 As a temporary measure we copied & modified the ParkEvent::
// allocate() and release() code for use by Parkers.  The Parker:: forms
// will eventually be removed as we consolide and shift over to ParkEvents
// for both builtin synchronization and JSR166 operations.

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.
  // 8028280: using concurrent free list without memory management can leak
  // pretty badly it turns out.
  Thread::SpinAcquire(&ListLock, "ParkerFreeListAllocate");
  {
    p = FreeList;
    if (p != NULL) {
      FreeList = p->FreeNext;
    }
  }
  Thread::SpinRelease(&ListLock);

  if (p != NULL) {
    guarantee (p->AssociatedWith == NULL, "invariant") ;
  } else {
    // Do this the hard way -- materialize a new Parker..
    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 ;

  Thread::SpinAcquire(&ListLock, "ParkerFreeListRelease");
  {
    p->FreeNext = FreeList;
    FreeList = p;
  }
  Thread::SpinRelease(&ListLock);
}

　　綜上源碼，在進行阻塞的時候，底層并沒有（并不一定）要用while死循環來阻塞，更多的是借助于作業系統的實作來進行阻塞的。當然，這也更符合大家的猜想！

從上的代碼我們也發現一點，底層在做許多事的時候，都不忘考慮線程中斷，也就是說，即使在阻塞狀态也是可以接收中斷信号的，這為上層語言打開了友善之門。

　如果要細說阻塞，其實還遠沒完，不過再往作業系統層面如何實作，就得再下點功夫，去翻翻資料了，把底線壓在作業系統層面，大多數情況下也夠用了！

不要害怕今日的苦，你要相信明天，更苦！

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