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深入理解Java中的底层阻塞原理及实现

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原文出处: 等你归去来

谈到阻塞,相信大家都不会陌生了。阻塞的应用场景真的多得不要不要的,比如 生产-消费模式,限流统计等等。什么 ArrayBlockingQueue、 LinkedBlockingQueue、DelayQueue 等等,都是阻塞队列的实现啊,多简单!

阻塞,一般有两个特性很亮眼:1. 不耗 CPU 等待;2. 线程安全;

额,要这么说也 OK 的。毕竟,我们遇到的问题,到这里就够解决了。但是有没有想过,这容器的阻塞又是如何实现的呢?

好吧,翻开源码,也很简单了:(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)
            // 阻塞的点
            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;
            // 阻塞的点
            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)
            // 阻塞的点
            notEmpty.await();
        return dequeue();
    } finally {
        lock.unlock();
    }
}

看来,最终都是依赖了 AbstractQueuedSynchronizer 类(著名的AQS)的 await 方法,看起来像那么回事。那么这个同步器的阻塞又是如何实现的呢?

Java的代码总是好跟踪的:

// 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)) {
        // 此处进行真正的阻塞
        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,真正的阻塞工作又转交给了另一个工具类: LockSupport park 方法了,这回跟锁扯上了关系,看起来已经越来越接近事实了:

// 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);
}

看得出来,这里的实现就比较简洁了,先获取当前线程,设置阻塞对象,阻塞,然后解除阻塞。

好吧,到底什么是真正的阻塞,我们还是不得而知!

UNSAFE.park(false, 0L); 是个什么东西? 看起来就是这一句起到了最关键的作用呢!但由于这里已经是 native 代码,我们已经无法再简单的查看源码了!那咋整呢?

那不行就看C/C++的源码呗,看一下 parker 的定义( 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中暴露出两个方法给java调用
  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 ;

};

park() 方法到底是如何实现的呢? 其实是继承的 os::PlatformParker 的功能,也就是平台相关的私有实现,以 Linux 平台实现为例( os_linux.hpp ):

// Linux中的parker定义
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 中没有重写 park() 和 unpark() 方法,也就是说阻塞实现完全交由特定平台代码处理了(os_linux.cpp):

// park方法的实现,依赖于 _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 实现,相对简单些
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") ;
  }
}

从上面代码可以看出,阻塞主要借助于三个变量, _cond、_mutex、_counter , 调用 Linux 系统的 pthread_cond_wait、pthread_mutex_lock、pthread_mutex_unlock (一组 POSIX 标准的阻塞接口)等平台相关的方法进行阻塞了!

而 park.cpp 中,则只有  Allocate、Release 等的一些常规操作!

// 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);
}

综上源码,在进行阻塞的时候,底层并没有(并不一定)要用 while 死循环来阻塞,更多的是借助于操作系统的实现来进行阻塞的。当然,这也更符合大家的猜想!

从上的代码我们也发现一点,底层在做许多事的时候,都不忘考虑线程中断,也就是说,即使在阻塞状态也是可以接收中断信号的,这为上层语言打开了方便之门。

如果要细说阻塞,其实还远没完,不过再往操作系统层面如何实现,就得再下点功夫,去翻翻资料了,把底线压在操作系统层面,大多数情况下也够用了!

作者:ImportNew
原文地址:深入理解Java中的底层阻塞原理及实现, 感谢原作者分享。

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