深入理解java中的底层阻塞原理及实现

　　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...  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的等待；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的 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 ;

};

那 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！