深入理解Java中的底層阻塞原理及實現

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 生產-消費模式，限流統計等等。什么 ArrayBlockingQueue、 LinkedBlockingQueue、DelayQueue 等等，都是阻塞隊列的實現啊，多簡單！

阻塞，一般有兩個特性很亮眼：1. 不耗 CPU 等待；2. 線程安全；

額，要這么說也 OK 的。畢竟，我們遇到的問題，到這里就夠解決了。但是有沒有想過，這容器的阻塞又是如何實現的呢？

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，看起來像那么回事。那么這個同步器的阻塞又是如何實現的呢？

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，這里的實現就比較簡潔了，先獲取當前線程，設置阻塞對象，阻塞，然后解除阻塞。

好吧，到底什么是真正的阻塞，我們還是不得而知！

UNSAFE.park(false, 0L); 是個什么東西？ 看起來就是這一句起到了最關鍵的作用呢！但由于這里已經是 native 代碼，我們已經無法再簡單的查看源碼了！那咋整呢？

那不行就看C/C++的源碼唄，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() 方法到底是如何實現的呢？ 其實是繼承的 os::PlatformParker 的功能，也就是平臺相關的私有實現，以 Linux 平臺實現為例（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，也就是說阻塞實現完全交由特定平臺代碼處理了（os_linux.cpp）：

// park方法的實現，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 實現，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 標準的阻塞接口）等平臺相關的方法進行阻塞了！

而 park.cpp 中，則只有 Allocate、Release 等的一些常規操作！

// 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，更多的是借助于操作系統的實現來進行阻塞的。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，即使在阻塞狀態也是可以接收中斷信號的，這為上層語言打開了方便之門。

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