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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！
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