Java并发之synchronized的解析

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在JDK中没有太多的对synchronzied的解析，主要是同步作用。下面通过jvm的中synchronizer.cpp的源码来分析

在这里可以下载jvm源码：http://hg.openjdk.java.net/

synchronzier.cpp的目录在：hotspot-9646293b9637\src\share\vm\runtime

synchronized同步底层使用JNI调用ObjectMonitor来时实现线程的wait(),notify(),notifyAll()等等.
看下面hotspot源码(使用c++写的)大致看一下就可以了。

wait()

// NOTE: must use heavy weight monitor to handle wait()
void ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
  if (UseBiasedLocking) {
    BiasedLocking::revoke_and_rebias(obj, false, THREAD);
    assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
  }
  if (millis < 0) {//时间小于0会抛出异常
    TEVENT (wait - throw IAX) ;
    THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
  }
  ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
  DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis);

  //调用了ObjectMonitor中的wait
  monitor->wait(millis, true, THREAD);

  /* This dummy call is in place to get around dtrace bug 6254741.  Once
     that&#39;s fixed we can uncomment the following line and remove the call */
  // DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD);
  dtrace_waited_probe(monitor, obj, THREAD);
}</span>

ObjectMonitor::wait()

// Note: a subset of changes to ObjectMonitor::wait()
// will need to be replicated in complete_exit above
void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
   Thread * const Self = THREAD ;
   assert(Self->is_Java_thread(), "Must be Java thread!");
   JavaThread *jt = (JavaThread *)THREAD;

   DeferredInitialize () ;

   // Throw IMSX or IEX.
   CHECK_OWNER();

   // check for a pending interrupt 是否有中断信号
   if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
     // post monitor waited event.  Note that this is past-tense, we are done waiting.
     if (JvmtiExport::should_post_monitor_waited()) {
        // Note: 'false' parameter is passed here because the
        // wait was not timed out due to thread interrupt.
        JvmtiExport::post_monitor_waited(jt, this, false);
     }
     TEVENT (Wait - Throw IEX) ;
     THROW(vmSymbols::java_lang_InterruptedException());
     return ;
   }
   TEVENT (Wait) ;

   assert (Self->_Stalled == 0, "invariant") ;
   Self->_Stalled = intptr_t(this) ;
   //设置线程的监视锁
   jt->set_current_waiting_monitor(this);

   // create a node to be put into the queue
   // Critically, after we reset() the event but prior to park(), we must check
   // for a pending interrupt.
   //添加一个节点放入到等待队列中
   ObjectWaiter node(Self);
   node.TState = ObjectWaiter::TS_WAIT ;
   Self->_ParkEvent->reset() ;
   OrderAccess::fence();          // ST into Event; membar ; LD interrupted-flag

   // Enter the waiting queue, which is a circular doubly linked list in this case
   // but it could be a priority queue or any data structure.
   // _WaitSetLock protects the wait queue.  Normally the wait queue is accessed only
   // by the the owner of the monitor *except* in the case where park()
   // returns because of a timeout of interrupt.  Contention is exceptionally rare
   // so we use a simple spin-lock instead of a heavier-weight blocking lock.
   //添加元素c++使用同步的方式，获取锁
   Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
   //添加节点
   AddWaiter (&node) ;
   //释放锁
   Thread::SpinRelease (&_WaitSetLock) ;

   if ((SyncFlags & 4) == 0) {
      _Responsible = NULL ;
   }
   intptr_t save = _recursions; // record the old recursion count
   //增加等待线程数
   _waiters++;                  // increment the number of waiters
   _recursions = 0;             // set the recursion level to be 1
   exit (Self) ;                    // exit the monitor
   guarantee (_owner != Self, "invariant") ;

   // As soon as the ObjectMonitor's ownership is dropped in the exit()
   // call above, another thread can enter() the ObjectMonitor, do the
   // notify(), and exit() the ObjectMonitor. If the other thread's
   // exit() call chooses this thread as the successor and the unpark()
   // call happens to occur while this thread is posting a
   // MONITOR_CONTENDED_EXIT event, then we run the risk of the event
   // handler using RawMonitors and consuming the unpark().
   //
   // To avoid the problem, we re-post the event. This does no harm
   // even if the original unpark() was not consumed because we are the
   // chosen successor for this monitor.
   if (node._notified != 0 && _succ == Self) {
      node._event->unpark();
   }

   // The thread is on the WaitSet list - now park() it.
   // On MP systems it's conceivable that a brief spin before we park
   // could be profitable.
   //
   // TODO-FIXME: change the following logic to a loop of the form
   //   while (!timeout && !interrupted && _notified == 0) park()

   int ret = OS_OK ;
   int WasNotified = 0 ;
   { // State transition wrappers
     OSThread* osthread = Self->osthread();
     OSThreadWaitState osts(osthread, true);
     {
       ThreadBlockInVM tbivm(jt);
       // Thread is in thread_blocked state and oop access is unsafe.
       jt->set_suspend_equivalent();

       if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
           // Intentionally empty
       } else
       if (node._notified == 0) {
         if (millis <= 0) {
            Self->_ParkEvent->park () ;
         } else {
            ret = Self->_ParkEvent->park (millis) ;
         }
       }

       // were we externally suspended while we were waiting?
       if (ExitSuspendEquivalent (jt)) {
          // TODO-FIXME: add -- if succ == Self then succ = null.
          jt->java_suspend_self();
       }

     } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm


     // Node may be on the WaitSet, the EntryList (or cxq), or in transition
     // from the WaitSet to the EntryList.
     // See if we need to remove Node from the WaitSet.
     // We use double-checked locking to avoid grabbing _WaitSetLock
     // if the thread is not on the wait queue.
     //
     // Note that we don&#39;t need a fence before the fetch of TState.
     // In the worst case we&#39;ll fetch a old-stale value of TS_WAIT previously
     // written by the is thread. (perhaps the fetch might even be satisfied
     // by a look-aside into the processor&#39;s own store buffer, although given
     // the length of the code path between the prior ST and this load that&#39;s
     // highly unlikely).  If the following LD fetches a stale TS_WAIT value
     // then we&#39;ll acquire the lock and then re-fetch a fresh TState value.
     // That is, we fail toward safety.

     if (node.TState == ObjectWaiter::TS_WAIT) {
         Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
         if (node.TState == ObjectWaiter::TS_WAIT) {
            DequeueSpecificWaiter (&node) ;       // unlink from WaitSet
            assert(node._notified == 0, "invariant");
            node.TState = ObjectWaiter::TS_RUN ;
         }
         Thread::SpinRelease (&_WaitSetLock) ;
     }

     // The thread is now either on off-list (TS_RUN),
     // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
     // The Node&#39;s TState variable is stable from the perspective of this thread.
     // No other threads will asynchronously modify TState.
     guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ;
     OrderAccess::loadload() ;
     if (_succ == Self) _succ = NULL ;
     WasNotified = node._notified ;

     // Reentry phase -- reacquire the monitor.
     // re-enter contended monitor after object.wait().
     // retain OBJECT_WAIT state until re-enter successfully completes
     // Thread state is thread_in_vm and oop access is again safe,
     // although the raw address of the object may have changed.
     // (Don&#39;t cache naked oops over safepoints, of course).

     // post monitor waited event. Note that this is past-tense, we are done waiting.
     if (JvmtiExport::should_post_monitor_waited()) {
       JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
     }
     OrderAccess::fence() ;

     assert (Self->_Stalled != 0, "invariant") ;
     Self->_Stalled = 0 ;

     assert (_owner != Self, "invariant") ;
     ObjectWaiter::TStates v = node.TState ;
     if (v == ObjectWaiter::TS_RUN) {
         enter (Self) ;
     } else {
         guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
         ReenterI (Self, &node) ;
         node.wait_reenter_end(this);
     }

     // Self has reacquired the lock.
     // Lifecycle - the node representing Self must not appear on any queues.
     // Node is about to go out-of-scope, but even if it were immortal we wouldn&#39;t
     // want residual elements associated with this thread left on any lists.
     guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ;
     assert    (_owner == Self, "invariant") ;
     assert    (_succ != Self , "invariant") ;
   } // OSThreadWaitState()

   jt->set_current_waiting_monitor(NULL);

   guarantee (_recursions == 0, "invariant") ;
   _recursions = save;     // restore the old recursion count
   _waiters--;             // decrement the number of waiters

   // Verify a few postconditions
   assert (_owner == Self       , "invariant") ;
   assert (_succ  != Self       , "invariant") ;
   assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;

   if (SyncFlags & 32) {
      OrderAccess::fence() ;
   }

   // check if the notification happened
   if (!WasNotified) {
     // no, it could be timeout or Thread.interrupt() or both
     // check for interrupt event, otherwise it is timeout
     if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
       TEVENT (Wait - throw IEX from epilog) ;
       THROW(vmSymbols::java_lang_InterruptedException());
     }
   }

   // NOTE: Spurious wake up will be consider as timeout.
   // Monitor notify has precedence over thread interrupt.
}</span>

ObjectMonitor::notify()

void ObjectMonitor::notify(TRAPS) {
  CHECK_OWNER();
  if (_WaitSet == NULL) {
     TEVENT (Empty-Notify) ;
     return ;
  }
  DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);

  int Policy = Knob_MoveNotifyee ;

  Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
  ObjectWaiter * iterator = DequeueWaiter() ;
  // ...
  //省略中间代码
  //...
        ParkEvent * ev = iterator->_event ;
        //设置状态
        iterator->TState = ObjectWaiter::TS_RUN ;
        OrderAccess::fence() ;
        //释放锁
        ev->unpark() ;
     }

     if (Policy < 4) {
       iterator->wait_reenter_begin(this);
     }
  }</span>

还有notifyAll(),SimpleEnter(),SimpeExit()，可以看具体的代码。

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