(三)死磕并发之深入Hotspot源码剖析Synchronize

引言

关于源码分析如果不是功底特别深厚的小伙伴可能需要用心的去细心咀嚼，千万不要抱着看一边就能懂的心态学习，不然最终也没有任何作用。如果只是想要研究Synchronized关键字原理那么请观看我的上一篇文章：彻底理解Java并发编程之Synchronized关键字实现原理剖析。

五、Hotspot源码深度解读Synchronized关键字原理

从 monitorenter和 monitorexit这两个指令来开始阅读源码，JVM将字节码加载到内存以后，会对这两个指令进行解释执行， monitorenter, monitorexit的指令解析是通过 InterpreterRuntime.cpp中的两个方法实现：

/** JavaThread 当前获取锁的线程  BasicObjectLock 基础对象锁 **/
InterpreterRuntime::monitorenter(JavaThread* thread, BasicObjectLock* elem)
InterpreterRuntime::monitorexit(JavaThread* thread, BasicObjectLock* elem)

5.1、Hotspot中Synchronized关键字相关的源码位置

不过对于C/C++以及Hotspot源码目录不熟悉的小伙伴可以根据我提供目录找到对应的实现：

• Monitor：openjdk\hotspot\src\share\vm\runtime\objectMonitor.hpp
• MarkWord：openjdk\hotspot\src\share\vm\oops\markOop.hpp
• monitorenter|exit指令：openjdk\hotspot\src\share\vm\interpreter\interpreterRuntime.cpp
• 偏向锁：openjdk\hotspot\src\share\vm\runtime\biasedLocking.cpp

我们基于monitorenter为入口，沿着无锁态->偏向锁->轻量级锁->重量级锁的路径来分析synchronized的实现过程：

IRT_ENTRY_NO_ASYNC(void, InterpreterRuntime::monitorenter(JavaThread* thread, BasicObjectLock* elem))
#ifdef ASSERT
  thread->last_frame().interpreter_frame_verify_monitor(elem);
#endif
  ...
  if (UseBiasedLocking) {
    // Retry fast entry if bias is revoked to avoid unnecessary inflation
    ObjectSynchronizer::fast_enter(h_obj, elem->lock(), true, CHECK);
  } else {
    ObjectSynchronizer::slow_enter(h_obj, elem->lock(), CHECK);
  }
  ...
#ifdef ASSERT
  thread->last_frame().interpreter_frame_verify_monitor(elem);
#endif
IRT_END

通过观察大家会发现，如果我们启动了偏向锁，会执行 ObjectSynchronizer::fast_enter的逻辑，而如果我们没有开启偏向锁，则执行 ObjectSynchronizer::slow_enter逻辑，绕过偏向锁，直接进入轻量级锁。

5.2、开启偏向锁状态执行逻辑

ObjectSynchronizer::fast_enter的实现在 synchronizer.cpp文件中，具体实现如下：

void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock, bool attempt_rebias, TRAPS) {
 //判断是否开启了偏向锁
 if (UseBiasedLocking) { 
     //如果不处于全局安全点
    if (!SafepointSynchronize::is_at_safepoint()) {
      //通过`revoke_and_rebias`这个函数尝试获取偏向锁
      BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
      //如果是撤销与重偏向直接返回
      if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {
        return;
      }
    } else {//如果在安全点，撤销偏向锁
      assert(!attempt_rebias, "can not rebias toward VM thread");
      BiasedLocking::revoke_at_safepoint(obj);
    }
    assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 }

 slow_enter (obj, lock, THREAD) ;
}

fast_enter实现简单流程：

• 再次检查偏向锁是否已开启
• 当处于不安全点时，通过 revoke_and_rebias尝试获取偏向锁，如果成功则直接返回，如果失败则进入轻量级锁获取过程
• revoke_and_rebias这个偏向锁的获取逻辑在 biasedLocking.cpp中
• 如果偏向锁未开启，则进入 slow_enter获取轻量级锁的流程

5.3、偏向锁获取逻辑

BiasedLocking::revoke_and_rebias 是用来获取当前偏向锁的状态(可能是偏向锁撤销后重新偏向)。这个方法的逻辑在 biasedLocking.cpp中,偏向锁获取逻辑具体实现如下：

BiasedLocking::Condition BiasedLocking::revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS) {
  assert(!SafepointSynchronize::is_at_safepoint(), "must not be called while at safepoint");
  markOop mark = obj->mark(); //获取锁对象的对象头
  //判断mark是否为可偏向状态，即mark的偏向锁标志位为1，锁标志位为 01，线程id为null
  if (mark->is_biased_anonymously() && !attempt_rebias) {
    //这个分支是进行对象的hashCode计算时会进入，在一个非全局安全点进行偏向锁撤销
    markOop biased_value       = mark;
    //创建一个非偏向的markword
    markOop unbiased_prototype = markOopDesc::prototype()->set_age(mark->age());
    //Atomic:cmpxchg_ptr是CAS操作，通过cas重新设置偏向锁状态
    markOop res_mark = (markOop) Atomic::cmpxchg_ptr(unbiased_prototype, obj->mark_addr(), mark);
    if (res_mark == biased_value) {//如果CAS成功，返回偏向锁撤销状态
      return BIAS_REVOKED;
    }
  } else if (mark->has_bias_pattern()) {//如果锁对象为可偏向状态（biased_lock:1, lock:01，不管线程id是否为空）,尝试重新偏向
    Klass* k = obj->klass(); 
    markOop prototype_header = k->prototype_header();
    //如果已经有线程对锁对象进行了全局锁定，则取消偏向锁操作
    if (!prototype_header->has_bias_pattern()) {
      markOop biased_value       = mark;
      //CAS 更新对象头markword为非偏向锁
      markOop res_mark = (markOop) Atomic::cmpxchg_ptr(prototype_header, obj->mark_addr(), mark);
      assert(!(*(obj->mark_addr()))->has_bias_pattern(), "even if we raced, should still be revoked");
      return BIAS_REVOKED; //返回偏向锁撤销状态
    } else if (prototype_header->bias_epoch() != mark->bias_epoch()) {
      //如果偏向锁过期，则进入当前分支
      if (attempt_rebias) {//如果允许尝试获取偏向锁
        assert(THREAD->is_Java_thread(), "");
        markOop biased_value       = mark;
        markOop rebiased_prototype = markOopDesc::encode((JavaThread*) THREAD, mark->age(), prototype_header->bias_epoch());
        //通过CAS 操作， 将本线程的 ThreadID 、时间错、分代年龄尝试写入对象头中
        markOop res_mark = (markOop) Atomic::cmpxchg_ptr(rebiased_prototype, obj->mark_addr(), mark);
        if (res_mark == biased_value) { //CAS成功，则返回撤销和重新偏向状态
          return BIAS_REVOKED_AND_REBIASED;
        }
      } else {//不尝试获取偏向锁，则取消偏向锁
        //通过CAS操作更新分代年龄
        markOop biased_value       = mark;
        markOop unbiased_prototype = markOopDesc::prototype()->set_age(mark->age());
        markOop res_mark = (markOop) Atomic::cmpxchg_ptr(unbiased_prototype, obj->mark_addr(), mark);
        if (res_mark == biased_value) { //如果CAS操作成功，返回偏向锁撤销状态
          return BIAS_REVOKED;
        }
      }
    }
  }
  ...//省略
}

5.4、偏向锁撤销逻辑

当到达一个全局安全点时，这时会根据偏向锁的状态来判断是否需要撤销偏向锁，调用 revoke_at_safepoint方法，这个方法也是在 biasedLocking.cpp中定义的，具体实现如下：

void BiasedLocking::revoke_at_safepoint(Handle h_obj) {
  assert(SafepointSynchronize::is_at_safepoint(), "must only be called while at safepoint");
  oop obj = h_obj();
  //更新撤销偏向锁计数，并返回偏向锁撤销次数和偏向次数
  HeuristicsResult heuristics = update_heuristics(obj, false);
  if (heuristics == HR_SINGLE_REVOKE) {//可偏向且未达到批量处理的阈值(下面会单独解释)
    revoke_bias(obj, false, false, NULL); //撤销偏向锁
  } else if ((heuristics == HR_BULK_REBIAS) || 
             (heuristics == HR_BULK_REVOKE)) {//如果是多次撤销或者多次偏向
    //批量撤销
    bulk_revoke_or_rebias_at_safepoint(obj, (heuristics == HR_BULK_REBIAS), false, NULL);
  }
  clean_up_cached_monitor_info();
}

偏向锁的释放，需要等待全局安全点（在这个时间点上没有正在执行的字节码），首先暂停拥有偏向锁的线程，然后检查持有偏向锁的线程是否还活着，如果线程不处于活动状态，则将对象头设置成无锁状态。如果线程仍然活着，则会升级为轻量级锁，遍历偏向对象的所记录。栈帧中的锁记录和对象头的Mark Word要么重新偏向其他线程，要么恢复到无锁，或者标记对象不适合作为偏向锁。最后唤醒暂停的线程。

JVM内部为每个类维护了一个偏向锁revoke计数器，对偏向锁撤销进行计数，当这个值达到指定阈值时，JVM会认为这个类的偏向锁有问题，需要重新偏向(rebias),对所有属于这个类的对象进行重偏向的操作成为 批量重偏向(bulk rebias)。在做bulk rebias时，会对这个类的epoch的值做递增，这个epoch会存储在对象头中的epoch字段。在判断这个对象是否获得偏向锁的条件是:markword的 biased_lock:1、lock:01、threadid和当前线程id相等、epoch字段和所属类的epoch值相同，如果epoch的值不一样，要么就是撤销偏向锁、要么就是rebias； 如果这个类的revoke计数器的值继续增加到一个阈值，那么jvm会认为这个类不适合偏向锁，就需要进行bulk revoke操作。

5.5、轻量级锁获取逻辑

轻量级锁获取是调用 ::slow_enter方法，该方法同样位于 synchronizer.cpp文件中，具体实现如下：

void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
  markOop mark = obj->mark();
  assert(!mark->has_bias_pattern(), "should not see bias pattern here");

  if (mark->is_neutral()) { //如果当前是无锁状态, markword的biase_lock:0，lock:01
    //直接把mark保存到BasicLock对象的_displaced_header字段
    lock->set_displaced_header(mark);
    //通过CAS将mark word更新为指向BasicLock对象的指针，更新成功表示获得了轻量级锁
    if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {
      TEVENT (slow_enter: release stacklock) ;
      return ;
    }
    // Fall through to inflate() ... 
  }
  //如果markword处于加锁状态、且markword中的ptr指针指向当前线程的栈帧，表示为重入操作，不需要争抢锁 
  else if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
    assert(lock != mark->locker(), "must not re-lock the same lock");
    assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");
    lock->set_displaced_header(NULL);
    return;
  }

#if 0
  // The following optimization isn't particularly useful.
  if (mark->has_monitor() && mark->monitor()->is_entered(THREAD)) {
    lock->set_displaced_header (NULL) ;
    return ;
  }
#endif
  //代码执行到这里，说明有多个线程竞争轻量级锁，轻量级锁通过`inflate`进行膨胀升级为重量级锁
  lock->set_displaced_header(markOopDesc::unused_mark());
  ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
}

简单整理轻量级锁的获取逻辑：

• mark->is_neutral()方法, is_neutral这个方法是在 markOop.hpp中定义，如果 biased_lock:0且lock:01表示无锁状态
• 如果mark处于无锁状态，则进入下一步骤，否则执行最后一个步骤
• 把mark保存到BasicLock对象的displacedheader字段
• 通过CAS尝试将markword更新为指向BasicLock对象的指针，如果更新成功，表示竞争到锁，则执行同步代码，否则执行下一步骤
• 如果当前mark处于加锁状态，且mark中的ptr指针指向当前线程的栈帧，则执行同步代码，否则说明有多个线程竞争轻量级锁，轻量级锁需要膨胀升级为重量级锁

5.6、轻量级锁释放逻辑

轻量级锁的释放是通过 monitorexit调用，具体实现如下：

IRT_ENTRY_NO_ASYNC(void, InterpreterRuntime::monitorexit(JavaThread* thread, BasicObjectLock* elem))
#ifdef ASSERT
  thread->last_frame().interpreter_frame_verify_monitor(elem);
#endif
  Handle h_obj(thread, elem->obj());
  assert(Universe::heap()->is_in_reserved_or_null(h_obj()),
         "must be NULL or an object");
  if (elem == NULL || h_obj()->is_unlocked()) {
    THROW(vmSymbols::java_lang_IllegalMonitorStateException());
  }
  ObjectSynchronizer::slow_exit(h_obj(), elem->lock(), thread);
  // Free entry. This must be done here, since a pending exception might be installed on
  // exit. If it is not cleared, the exception handling code will try to unlock the monitor again.
  elem->set_obj(NULL);
#ifdef ASSERT
  thread->last_frame().interpreter_frame_verify_monitor(elem);
#endif
IRT_END

这段代码中主要是通过 ObjectSynchronizer::slow_exit来执行，具体实现如下：

void ObjectSynchronizer::slow_exit(oop object, BasicLock* lock, TRAPS) {
  fast_exit (object, lock, THREAD) ;
}

ObjectSynchronizer::fast_exit的代码如下：

void ObjectSynchronizer::fast_exit(oop object, BasicLock* lock, TRAPS) {
  assert(!object->mark()->has_bias_pattern(), "should not see bias pattern here");
  // if displaced header is null, the previous enter is recursive enter, no-op
  markOop dhw = lock->displaced_header(); //获取锁对象中的对象头
  markOop mark ;
  if (dhw == NULL) { 
     // Recursive stack-lock.
     // Diagnostics -- Could be: stack-locked, inflating, inflated.
     mark = object->mark() ;
     assert (!mark->is_neutral(), "invariant") ;
     if (mark->has_locker() && mark != markOopDesc::INFLATING()) {
        assert(THREAD->is_lock_owned((address)mark->locker()), "invariant") ;
     }
     if (mark->has_monitor()) {
        ObjectMonitor * m = mark->monitor() ;
        assert(((oop)(m->object()))->mark() == mark, "invariant") ;
        assert(m->is_entered(THREAD), "invariant") ;
     }
     return ;
  }

  mark = object->mark() ; //获取线程栈帧中锁记录(LockRecord)中的markword

  // If the object is stack-locked by the current thread, try to
  // swing the displaced header from the box back to the mark.
  if (mark == (markOop) lock) {
     assert (dhw->is_neutral(), "invariant") ;
     //通过CAS尝试将Displaced Mark Word替换回对象头，如果成功，表示锁释放成功。
     if ((markOop) Atomic::cmpxchg_ptr (dhw, object->mark_addr(), mark) == mark) {
        TEVENT (fast_exit: release stacklock) ;
        return;
     }
  }
  //锁膨胀，调用重量级锁的释放锁方法
  ObjectSynchronizer::inflate(THREAD, object)->exit (true, THREAD) ;
}

轻量级锁的释放也比较简单，就是将当前线程栈帧中锁记录空间中的Mark Word替换到锁对象的对象头中，如果成功表示锁释放成功。否则，锁膨胀成重量级锁，实现重量级锁的释放锁逻辑。

5.7、锁膨胀过程分析

重量级锁是通过对象内部的监视器(monitor)来实现，而monitor的本质是依赖操作系统底层的MutexLock实现的。我们先来看锁的膨胀过程，从前面的分析中已经知道了所膨胀的过程是通过 ObjectSynchronizer::inflate方法实现的，代码如下：

ObjectMonitor * ATTR ObjectSynchronizer::inflate (Thread * Self, oop object) {
  // Inflate mutates the heap ...
  // Relaxing assertion for bug 6320749.
  assert (Universe::verify_in_progress() ||
          !SafepointSynchronize::is_at_safepoint(), "invariant") ;

  for (;;) { //通过无意义的循环实现自旋操作
      const markOop mark = object->mark() ;
      assert (!mark->has_bias_pattern(), "invariant") ;

      if (mark->has_monitor()) {//has_monitor是markOop.hpp中的方法，如果为true表示当前锁已经是重量级锁了
          ObjectMonitor * inf = mark->monitor() ;//获得重量级锁的对象监视器直接返回
          assert (inf->header()->is_neutral(), "invariant");
          assert (inf->object() == object, "invariant") ;
          assert (ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid");
          return inf ;
      }

      if (mark == markOopDesc::INFLATING()) {//膨胀等待，表示存在线程正在膨胀，通过continue进行下一轮的膨胀
         TEVENT (Inflate: spin while INFLATING) ;
         ReadStableMark(object) ;
         continue ;
      }

      if (mark->has_locker()) {//表示当前锁为轻量级锁，以下是轻量级锁的膨胀逻辑
          ObjectMonitor * m = omAlloc (Self) ;//获取一个可用的ObjectMonitor
          // Optimistically prepare the objectmonitor - anticipate successful CAS
          // We do this before the CAS in order to minimize the length of time
          // in which INFLATING appears in the mark.
          m->Recycle();
          m->_Responsible  = NULL ;
          m->OwnerIsThread = 0 ;
          m->_recursions   = 0 ;
          m->_SpinDuration = ObjectMonitor::Knob_SpinLimit ;   // Consider: maintain by type/class
          /**将object->mark_addr()和mark比较，如果这两个值相等，则将object->mark_addr()
          改成markOopDesc::INFLATING()，相等返回是mark，不相等返回的是object->mark_addr()**/
                     markOop cmp = (markOop) Atomic::cmpxchg_ptr (markOopDesc::INFLATING(), object->mark_addr(), mark) ;
          if (cmp != mark) {//CAS失败
             omRelease (Self, m, true) ;//释放监视器
             continue ;       // 重试
          }

          markOop dmw = mark->displaced_mark_helper() ;
          assert (dmw->is_neutral(), "invariant") ;

          //CAS成功以后，设置ObjectMonitor相关属性
          m->set_header(dmw) ;


          m->set_owner(mark->locker());
          m->set_object(object);
          // TODO-FIXME: assert BasicLock->dhw != 0.


          guarantee (object->mark() == markOopDesc::INFLATING(), "invariant") ;
          object->release_set_mark(markOopDesc::encode(m));


          if (ObjectMonitor::_sync_Inflations != NULL) ObjectMonitor::_sync_Inflations->inc() ;
          TEVENT(Inflate: overwrite stacklock) ;
          if (TraceMonitorInflation) {
            if (object->is_instance()) {
              ResourceMark rm;
              tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
                (void *) object, (intptr_t) object->mark(),
                object->klass()->external_name());
            }
          }
          return m ; //返回ObjectMonitor
      }
      //如果是无锁状态
      assert (mark->is_neutral(), "invariant");
      ObjectMonitor * m = omAlloc (Self) ; ////获取一个可用的ObjectMonitor
      //设置ObjectMonitor相关属性
      m->Recycle();
      m->set_header(mark);
      m->set_owner(NULL);
      m->set_object(object);
      m->OwnerIsThread = 1 ;
      m->_recursions   = 0 ;
      m->_Responsible  = NULL ;
      m->_SpinDuration = ObjectMonitor::Knob_SpinLimit ;       // consider: keep metastats by type/class
      /**将object->mark_addr()和mark比较，如果这两个值相等，则将object->mark_addr()
          改成markOopDesc::encode(m)，相等返回是mark，不相等返回的是object->mark_addr()**/
      if (Atomic::cmpxchg_ptr (markOopDesc::encode(m), object->mark_addr(), mark) != mark) {
          //CAS失败，说明出现了锁竞争，则释放监视器重行竞争锁
          m->set_object (NULL) ;
          m->set_owner  (NULL) ;
          m->OwnerIsThread = 0 ;
          m->Recycle() ;
          omRelease (Self, m, true) ;
          m = NULL ;
          continue ;
          // interference - the markword changed - just retry.
          // The state-transitions are one-way, so there's no chance of
          // live-lock -- "Inflated" is an absorbing state.
      }

      if (ObjectMonitor::_sync_Inflations != NULL) ObjectMonitor::_sync_Inflations->inc() ;
      TEVENT(Inflate: overwrite neutral) ;
      if (TraceMonitorInflation) {
        if (object->is_instance()) {
          ResourceMark rm;
          tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
            (void *) object, (intptr_t) object->mark(),
            object->klass()->external_name());
        }
      }
      return m ; //返回ObjectMonitor对象
  }
}

锁膨胀的过程稍微有点复杂，整个锁膨胀的过程是通过自旋来完成的，具体的实现逻辑简答总结以下几点：

• mark->has_monitor() 判断如果当前锁对象为重量级锁，也就是lock:10，则执行第二步骤,否则执行第三步骤。
• 通过 mark->monitor获得重量级锁的对象监视器ObjectMonitor并返回，锁膨胀过程结束。
• 如果当前锁处于 INFLATING,说明有其他线程在执行锁膨胀，那么当前线程通过自旋等待其他线程锁膨胀完成。
• 如果当前是轻量级锁状态 mark->has_locker(),则进行锁膨胀。首先，通过omAlloc方法获得一个可用的ObjectMonitor，并设置初始数据；然后通过CAS将对象头设置为`markOopDesc:INFLATING，表示当前锁正在膨胀，如果CAS失败，继续自旋。
• 如果是无锁状态，逻辑类似第四步骤。

锁膨胀的过程实际上是获得一个ObjectMonitor对象监视器，而真正抢占锁的逻辑，在 ObjectMonitor::enter方法里面。

5.8、重量级锁的竞争逻辑

重量级锁的竞争，在 ObjectMonitor::enter方法中，代码文件在 objectMonitor.cpp重量级锁的代码就不一一分析了，简单说一下下面这段代码主要做的几件事：

• 通过CAS将monitor的 _owner字段设置为当前线程，如果设置成功，则直接返回。
• 如果之前的 _owner指向的是当前的线程，说明是重入，执行 _recursions++增加重入次数。
• 如果当前线程获取监视器锁成功，将 _recursions设置为1， _owner设置为当前线程。
• 如果获取锁失败，则等待锁释放。

void ATTR ObjectMonitor::enter(TRAPS) {
  // The following code is ordered to check the most common cases first
  // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
  Thread * const Self = THREAD ;
  void * cur ;

  cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
  if (cur == NULL) {//CAS成功
     // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
     assert (_recursions == 0   , "invariant") ;
     assert (_owner      == Self, "invariant") ;
     // CONSIDER: set or assert OwnerIsThread == 1
     return ;
  }

  if (cur == Self) {
     // TODO-FIXME: check for integer overflow!  BUGID 6557169.
     _recursions ++ ;
     return ;
  }

  if (Self->is_lock_owned ((address)cur)) {
    assert (_recursions == 0, "internal state error");
    _recursions = 1 ;
    // Commute owner from a thread-specific on-stack BasicLockObject address to
    // a full-fledged "Thread *".
    _owner = Self ;
    OwnerIsThread = 1 ;
    return ;
  }

  // We've encountered genuine contention.
  assert (Self->_Stalled == 0, "invariant") ;
  Self->_Stalled = intptr_t(this) ;

  // Try one round of spinning *before* enqueueing Self
  // and before going through the awkward and expensive state
  // transitions.  The following spin is strictly optional ...
  // Note that if we acquire the monitor from an initial spin
  // we forgo posting JVMTI events and firing DTRACE probes.
  if (Knob_SpinEarly && TrySpin (Self) > 0) {
     assert (_owner == Self      , "invariant") ;
     assert (_recursions == 0    , "invariant") ;
     assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
     Self->_Stalled = 0 ;
     return ;
  }

  assert (_owner != Self          , "invariant") ;
  assert (_succ  != Self          , "invariant") ;
  assert (Self->is_Java_thread()  , "invariant") ;
  JavaThread * jt = (JavaThread *) Self ;
  assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
  assert (jt->thread_state() != _thread_blocked   , "invariant") ;
  assert (this->object() != NULL  , "invariant") ;
  assert (_count >= 0, "invariant") ;

  // Prevent deflation at STW-time.  See deflate_idle_monitors() and is_busy().
  // Ensure the object-monitor relationship remains stable while there's contention.
  Atomic::inc_ptr(&_count);

  EventJavaMonitorEnter event;

  { // Change java thread status to indicate blocked on monitor enter.
    JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);

    DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
    if (JvmtiExport::should_post_monitor_contended_enter()) {
      JvmtiExport::post_monitor_contended_enter(jt, this);
    }

    OSThreadContendState osts(Self->osthread());
    ThreadBlockInVM tbivm(jt);

    Self->set_current_pending_monitor(this);

    // TODO-FIXME: change the following for(;;) loop to straight-line code.
    for (;;) {
      jt->set_suspend_equivalent();
      // cleared by handle_special_suspend_equivalent_condition()
      // or java_suspend_self()

      EnterI (THREAD) ;

      if (!ExitSuspendEquivalent(jt)) break ;

      //
      // We have acquired the contended monitor, but while we were
      // waiting another thread suspended us. We don't want to enter
      // the monitor while suspended because that would surprise the
      // thread that suspended us.
      //
          _recursions = 0 ;
      _succ = NULL ;
      exit (false, Self) ;

      jt->java_suspend_self();
    }
    Self->set_current_pending_monitor(NULL);
  }
...//此处省略无数行代码

如果获取锁失败，则需要通过自旋的方式等待锁释放，自旋执行的方法是 ObjectMonitor::EnterI，部分原理以及代码如下：

• 将当前线程封装成ObjectWaiter对象node，状态设置成TS_CXQ。
• 通过自旋操作将node节点push到_cxq队列。
• node节点添加到_cxq队列之后，继续通过自旋尝试获取锁，如果在指定的阈值范围内没有获得锁，则通过park将当前线程挂起，等待被唤醒。

void ATTR ObjectMonitor::EnterI (TRAPS) {
    Thread * Self = THREAD ;
    ...//省略很多代码
    ObjectWaiter node(Self) ;
    Self->_ParkEvent->reset() ;
    node._prev   = (ObjectWaiter *) 0xBAD ;
    node.TState  = ObjectWaiter::TS_CXQ ;

    // Push "Self" onto the front of the _cxq.
    // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
    // Note that spinning tends to reduce the rate at which threads
    // enqueue and dequeue on EntryList|cxq.
    ObjectWaiter * nxt ;
    for (;;) { //自旋，讲node添加到_cxq队列
        node._next = nxt = _cxq ;
        if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;

        // Interference - the CAS failed because _cxq changed.  Just retry.
        // As an optional optimization we retry the lock.
        if (TryLock (Self) > 0) {
            assert (_succ != Self         , "invariant") ;
            assert (_owner == Self        , "invariant") ;
            assert (_Responsible != Self  , "invariant") ;
            return ;
        }
    }
    ...//省略很多代码
    //node节点添加到_cxq队列之后，继续通过自旋尝试获取锁，如果在指定的阈值范围内没有获得锁，则通过park将当前线程挂起，等待被唤醒
    for (;;) {
        if (TryLock (Self) > 0) break ;
        assert (_owner != Self, "invariant") ;

        if ((SyncFlags & 2) && _Responsible == NULL) {
           Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
        }

        // park self //通过park挂起当前线程
        if (_Responsible == Self || (SyncFlags & 1)) {
            TEVENT (Inflated enter - park TIMED) ;
            Self->_ParkEvent->park ((jlong) RecheckInterval) ;
            // Increase the RecheckInterval, but clamp the value.
            RecheckInterval *= 8 ;
            if (RecheckInterval > 1000) RecheckInterval = 1000 ;
        } else {
            TEVENT (Inflated enter - park UNTIMED) ;
            Self->_ParkEvent->park() ;//当前线程挂起
        }

        if (TryLock(Self) > 0) break ; //当线程被唤醒时，会从这里继续执行


        TEVENT (Inflated enter - Futile wakeup) ;
        if (ObjectMonitor::_sync_FutileWakeups != NULL) {
           ObjectMonitor::_sync_FutileWakeups->inc() ;
        }
        ++ nWakeups ;

        if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;

        if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
           Self->_ParkEvent->reset() ;
           OrderAccess::fence() ;
        }
        if (_succ == Self) _succ = NULL ;

        // Invariant: after clearing _succ a thread *must* retry _owner before parking.
        OrderAccess::fence() ;
    }
    ...//省略很多代码
}

TryLock(self)的代码是在 ObjectMonitor::TryLock定义的，代码的实现如下：

代码的实现原理很简单，通过自旋，CAS设置monitor的_owner字段为当前线程，如果成功，表示获取到了锁，如果失败，则继续被挂起。

int ObjectMonitor::TryLock (Thread * Self) {
   for (;;) {
      void * own = _owner ;
      if (own != NULL) return 0 ;
      if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
         // Either guarantee _recursions == 0 or set _recursions = 0.
         assert (_recursions == 0, "invariant") ;
         assert (_owner == Self, "invariant") ;
         // CONSIDER: set or assert that OwnerIsThread == 1
         return 1 ;
      }
      // The lock had been free momentarily, but we lost the race to the lock.
      // Interference -- the CAS failed.
      // We can either return -1 or retry.
      // Retry doesn't make as much sense because the lock was just acquired.
      if (true) return -1 ;
   }
}

5.9、重量级锁的释放

重量级锁的释放是通过 ObjectMonitor::exit来实现的，释放以后会通知被阻塞的线程去竞争锁：

• 判断当前锁对象中的owner没有指向当前线程，如果owner指向的BasicLock在当前线程栈上,那么将_owner指向当前线程。
• 如果当前锁对象中的_owner指向当前线程，则判断当前线程重入锁的次数，如果不为0，继续执行ObjectMonitor::exit()，直到重入锁次数为0为止。
• 释放当前锁，并根据QMode的模式判断，是否将_cxq中挂起的线程唤醒。还是其他操作。

void ATTR ObjectMonitor::exit(bool not_suspended, TRAPS) {
   Thread * Self = THREAD ;
   if (THREAD != _owner) {//如果当前锁对象中的_owner没有指向当前线程
     //如果_owner指向的BasicLock在当前线程栈上,那么将_owner指向当前线程
     if (THREAD->is_lock_owned((address) _owner)) {
       // Transmute _owner from a BasicLock pointer to a Thread address.
       // We don't need to hold _mutex for this transition.
       // Non-null to Non-null is safe as long as all readers can
       // tolerate either flavor.
       assert (_recursions == 0, "invariant") ;
       _owner = THREAD ;
       _recursions = 0 ;
       OwnerIsThread = 1 ;
     } else {
       // NOTE: we need to handle unbalanced monitor enter/exit
       // in native code by throwing an exception.
       // TODO: Throw an IllegalMonitorStateException ?
       TEVENT (Exit - Throw IMSX) ;
       assert(false, "Non-balanced monitor enter/exit!");
       if (false) {
          THROW(vmSymbols::java_lang_IllegalMonitorStateException());
       }
       return;
     }
   }
   //如果当前，线程重入锁的次数，不为0，那么就重新走ObjectMonitor::exit，直到重入锁次数为0为止
   if (_recursions != 0) {
     _recursions--;        // this is simple recursive enter
     TEVENT (Inflated exit - recursive) ;
     return ;
   }
  ...//此处省略很多代码
  for (;;) {
    if (Knob_ExitPolicy == 0) {
      OrderAccess::release_store(&_owner, (void*)NULL);   //释放锁
      OrderAccess::storeload();                        // See if we need to wake a successor
      if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
        TEVENT(Inflated exit - simple egress);
        return;
      }
      TEVENT(Inflated exit - complex egress);
      //省略部分代码...
    }
    //省略部分代码...
    ObjectWaiter * w = NULL;
    int QMode = Knob_QMode;
    //根据QMode的模式判断，
    //如果QMode == 2则直接从_cxq挂起的线程中唤醒    
    if (QMode == 2 && _cxq != NULL) {
      w = _cxq;
      ExitEpilog(Self, w);
      return;
    }
     //省略部分代码... 省略的代码为根据QMode的不同，不同的唤醒机制
  }
}

根据不同的策略(由QMode指定)，从cxq或EntryList中获取头节点，通过ObjectMonitor::ExitEpilog方法唤醒该节点封装的线程，唤醒操作最终由unpark完成。

void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
{
   assert (_owner == Self, "invariant") ;

   // Exit protocol:
   // 1. ST _succ = wakee
   // 2. membar #loadstore|#storestore;
   // 2. ST _owner = NULL
   // 3. unpark(wakee)

   _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;
   ParkEvent * Trigger = Wakee->_event ;

   // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
   // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
   // out-of-scope (non-extant).
   Wakee  = NULL ;

   // Drop the lock
   OrderAccess::release_store_ptr (&_owner, NULL) ;
   OrderAccess::fence() ;                               // ST _owner vs LD in unpark()

   if (SafepointSynchronize::do_call_back()) {
      TEVENT (unpark before SAFEPOINT) ;
   }

   DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
   Trigger->unpark() ; //unpark唤醒线程

   // Maintain stats and report events to JVMTI
   if (ObjectMonitor::_sync_Parks != NULL) {
      ObjectMonitor::_sync_Parks->inc() ;
   }
}

再次提醒：关于源码分析如果不是功底特别深厚的小伙伴可能需要用心的去细心咀嚼，千万不要抱着看一边就能懂的心态学习，不然最终也没有任何作用。

六、参考资料

• 《深入理解JVM虚拟机》
• 《Java并发编程之美》
• 《Java高并发程序设计》
• 《亿级流量网站架构核心技术》
• 《Java并发编程实战》