// 膨胀为重量级锁
// 只返回重量级锁(ObjectMonitor), 并不负责锁占有的逻辑
ObjectMonitor * ATTR ObjectSynchronizer::inflate (Thread * Self, oop object) {
  // Inflate mutates the heap ...
  // Relaxing assertion for bug 6320749.
  assert (Universe::verify_in_progress() ||
          // 必须在安全点才能膨胀
          !SafepointSynchronize::is_at_safepoint(), "invariant") ;

  // 先提前说下ObjectMonitor的获取和释放
  // 重量级锁的ObjectMonitor的存储使用了free list,
  // 获取时调用omAlloc,
  //    从omFreeList中获取, 如果omFreeList为空，则从JVM全局的gFreeList中分配一批ObjectMonitor到omFreeList中
  //    获取成功的ObjectMonitor放到omInUseList中,
  //    然后返回
  // 释放时调用omRelease, 从omInUseList换回到omFreeList中

  for (;;) {
    // 这是个for循环, 是为了处理多线程同时走膨胀逻辑的情况
    // 如果这样, CAS可能会失败, 需要获取最新状态并重新来
      const markOop mark = object->mark() ;
      assert (!mark->has_bias_pattern(), "invariant") ;

      // The mark can be in one of the following states:
      // MarkWord可以是以下几种状态
      // *  Inflated(膨胀完成, 已经是重量级锁)     - just return 直接返回
      // *  Stack-locked(轻量级锁) - coerce it to inflated 走膨胀逻辑
      // *  INFLATING(膨胀中)    - busy wait for conversion to complete 忙, 等待膨胀结束
      // *  Neutral(无锁)      - aggressively inflate the object. 走膨胀逻辑
      // *  BIASED(偏向锁)       - Illegal.  We should never see this 讲道理这里不该出现这种情况

      // 下面就是针对上面4中可能的情况走不同的处理逻辑

      // CASE: inflated
      if (mark->has_monitor()) {
          // 已经膨胀完成，已经是重量级锁的情况
          // 直接返回MarkWord中的ObjectMonitor
          ObjectMonitor * inf = mark->monitor() ;
          assert (inf->header()->is_neutral(), "invariant");
          assert (inf->object() == object, "invariant") ;
          assert (ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid");
          return inf ;
      }

      // CASE: inflation in progress - inflating over a stack-lock.
      // Some other thread is converting from stack-locked to inflated.
      // Only that thread can complete inflation -- other threads must wait.
      // The INFLATING value is transient.
      // Currently, we spin/yield/park and poll the markword, waiting for inflation to finish.
      // We could always eliminate polling by parking the thread on some auxiliary list.
      if (mark == markOopDesc::INFLATING()) {
         // 膨胀中的情况
         TEVENT (Inflate: spin while INFLATING) ;
         // 自旋等待，知道膨胀**状态解除, 这个方法中会有一个for循环用于等待,
         // 根据在for循环中的循环次数, 会有不同的等待策略, 可能会spin(自旋)/yield/park
         ReadStableMark(object) ;
         continue ;
      }

      // CASE: stack-locked
      // Could be stack-locked either by this thread or by some other thread.
      // 可以是当前线程的stack-lock, 也可以是其他线程的
      //
      // Note that we allocate the objectmonitor speculatively, _before_ attempting
      // to install INFLATING into the mark word.  We originally installed INFLATING,
      // allocated the objectmonitor, and then finally STed the address of the
      // objectmonitor into the mark.  This was correct, but artificially lengthened
      // the interval in which INFLATED appeared in the mark, thus increasing
      // the odds of inflation contention.
      // 翻译过来大致意思是:
      // 注意, 我们在将MarkWord状态更新为INLATING之前分配了ObjectMonitor.
      // 之前, 我们先将状态更新为INFLATING, 然后分配ObjectMonitor, 最后将ObjectMonitor的地址写到MarkWord中.
      // 这么做没错, 但是会人为拉长MarkWord为INFLATING状态的时间, 增加了膨胀的可能性.
      // 最后一句很难理解, 我觉得没增加膨胀的可能性, 拉长INFLATING状态的持续时间,
      // 会增加其他线程的等待时长, 导致其他线程被挂起, 从而总的等待时长增加.
      // 所以应该想办法减少INFLATING状态的持续时间
      //
      // We now use per-thread private objectmonitor free lists.
      // These list are reprovisioned from the global free list outside the
      // critical INFLATING...ST interval.  A thread can transfer
      // multiple objectmonitors en-mass from the global free list to its local free list.
      // This reduces coherency traffic and lock contention on the global free list.
      // Using such local free lists, it doesn't matter if the omAlloc() call appears
      // before or after the CAS(INFLATING) operation.
      // See the comments in omAlloc().
      // 说下ObjectMonitor的实现
      // OjbectMonitor的获取是基于free list实现的. free list有全局的和线程本地的两种.
      // 全局的free list是事先分配好的, 独立于膨胀流程的.
      // 每个线程一次会从全局的free list 获取多个块, 这种方式降低了为了一致性而加锁的成本,
      // 只在本次每次从全局获取时需要加锁, 在本地线程获取ObjectMonitor时并不需要加锁

      if (mark->has_locker()) {
          // 轻量级锁的情况
          ObjectMonitor * m = omAlloc (Self) ;
          // 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.
          // 第一件事是new了一个ObjectMonitor出来，会加到当前线程的omInUseList中
          // 对膨胀的成功率真的很乐观啊
          // 不过这样在其他线程抢先膨胀中的时候会节省时间
          m->Recycle();
          m->_Responsible  = NULL ;
          m->OwnerIsThread = 0 ;
          m->_recursions   = 0 ;
          // 设置自旋获取重量级锁的次数，默认5000
          m->_SpinDuration = ObjectMonitor::Knob_SpinLimit ;   // Consider: maintain by type/class

          // 设置MarkWord设置为INFLATING(0)状态
          markOop cmp = (markOop) Atomic::cmpxchg_ptr (markOopDesc::INFLATING(), object->mark_addr(), mark) ;
          if (cmp != mark) {
             // 设置失败，说明有其他线程抢先膨胀了
             // 将m从当前线程的omInUseList中移除, 其实是放弃自己初始化的ObjectMonitor
             omRelease (Self, m, true) ;
             // 再次尝试, 再次尝试时会重新判断状态
             continue ;       // Interference -- just retry
          }

          // We've successfully installed INFLATING (0) into the mark-word.
          // This is the only case where 0 will appear in a mark-work.
          // Only the singular thread that successfully swings the mark-word
          // to 0 can perform (or more precisely, complete) inflation.
          // 走到这里说明成功将MarkWord置为0
          // 这也是唯一能把锁标志位置0的方式
          // 只有成功设置的那一个线程才能执行锁膨胀.
          //
          // Why do we CAS a 0 into the mark-word instead of just CASing the
          // mark-word from the stack-locked value directly to the new inflated state?
          // Consider what happens when a thread unlocks a stack-locked object.
          // It attempts to use CAS to swing the displaced header value from the
          // on-stack basiclock back into the object header.  Recall also that the
          // header value (hashcode, etc) can reside in (a) the object header, or
          // (b) a displaced header associated with the stack-lock, or (c) a displaced
          // header in an objectMonitor.  The inflate() routine must copy the header
          // value from the basiclock on the owner's stack to the objectMonitor, all
          // the while preserving the hashCode stability invariants.  If the owner
          // decides to release the lock while the value is 0, the unlock will fail
          // and control will eventually pass from slow_exit() to inflate.  The owner
          // will then spin, waiting for the 0 value to disappear.   Put another way,
          // the 0 causes the owner to stall if the owner happens to try to
          // drop the lock (restoring the header from the basiclock to the object)
          // while inflation is in-progress.  This protocol avoids races that might
          // would otherwise permit hashCode values to change or "flicker" for an object.
          // Critically, while object->mark is 0 mark->displaced_mark_helper() is stable.
          // 0 serves as a "BUSY" inflate-in-progress indicator.
          // 为什么要先将MarkWord置为INFLATING(0),
          // 而不是将MarkWord直接从轻量级锁(stack-lock)直接变成重量级锁(inflated state)?
          // 为了防止竞争导致hashCode改变或者抖动!
          //
          // 解释下为什么会这样:
          // 回忆下对象头的值(hashCode等)可能在什么地方:
          //    1. 对象MarkWord,
          //    2. LockRecord(stack-lock)的displaced_header中
          //    3. ObjectMonitor的displaced_header中
          // 想象这样的场景:
          // 线程1:
          //    一个线程想要unlock轻量级锁(stack-lock), 此时MarkWord是指向LockRecord的
          //    这时需要将栈帧中的轻量级锁(LockRecord)的displaced_header写回到object的MarkWord中
          // 线程2:
          //    轻量级锁正在膨胀, 这个时候需要将LockRecord中的header设置到ObjectMonitor中,
          //    再将MarkWord指向ObjectMonitor
          //
          // 在这时, 如果需要计算hashCode, 因为的状态不稳定,
          // 所以计算出来的hashCode可能会放在LockRecord的displaced_header中,
          // 也可能在ObjectMonitor的displaced_header中
          // 读的时候也一样,
          // 导致hashCode被改变或者抖动, 这不是我们期望的情况.
          //
          // 所以加了一个中间状态INFLATING,
          // 表示膨胀中, 请勿打扰, 其他操作需要暂时等待, 包括对hashCode的操作
          // 这个时候如果要获取hashCode, 也得等待(暂时没空debug过去看, 猜的)


          // fetch the displaced mark from the owner's stack.
          // The owner can't die or unwind past the lock while our INFLATING
          // object is in the mark.  Furthermore the owner can't complete
          // an unlock on the object, either.
          // 从displaced_mark_helper中获取monitor_value,
          // 其实是obj原始的MarkWord, 不包括锁标志位
          // 这个过程中owner是不能挂的, 而且不能unlock
          // dmw应该是DisplacedMarkWord的意思
          markOop dmw = mark->displaced_mark_helper() ;
          assert (dmw->is_neutral(), "invariant") ;

          // Setup monitor fields to proper values -- prepare the monitor
          // 将dmw设置进ObjectMonitor
          m->set_header(dmw) ;

          // Optimization: if the mark->locker stack address is associated
          // with this thread we could simply set m->_owner = Self and
          // m->OwnerIsThread = 1. Note that a thread can inflate an object
          // that it has stack-locked -- as might happen in wait() -- directly
          // with CAS.  That is, we can avoid the xchg-NULL .... ST idiom.
          // 设置ObjectMonitor的owner为LockRecord，而不是当前线程(Self)
          // 因为触发锁膨胀的线程不一定是持有锁的线程, 可能是其他线程, 比如一个从wait中醒来的线程
          // 所以这里不会将owner设置为当前线程
          // 锁的占有可以在ObjectMonitor::enter中做
          m->set_owner(mark->locker());
          m->set_object(object);
          // TODO-FIXME: assert BasicLock->dhw != 0.

          // Must preserve store ordering. The monitor state must
          // be stable at the time of publishing the monitor address.
          guarantee (object->mark() == markOopDesc::INFLATING(), "invariant") ;
          // MarkWord设置为重量级锁
          object->release_set_mark(markOopDesc::encode(m));

          // Hopefully the performance counters are allocated on distinct cache lines
          // to avoid false sharing on MP systems ...
          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 ;
      }

      // CASE: neutral
      // TODO-FIXME: for entry we currently inflate and then try to CAS _owner.
      // If we know we're inflating for entry it's better to inflate by swinging a
      // pre-locked objectMonitor pointer into the object header.   A successful
      // CAS inflates the object *and* confers ownership to the inflating thread.
      // In the current implementation we use a 2-step mechanism where we CAS()
      // to inflate and then CAS() again to try to swing _owner from NULL to Self.
      // An inflateTry() method that we could call from fast_enter() and slow_enter()
      // would be useful.
      // 无锁的情况

      assert (mark->is_neutral(), "invariant");
      // 这里会先初始化一个ObjectMonitor对象，但没有设置owner,
      // 原因和上面一样, 请求线程不一定是轻量级锁持有的线程,
      // 在外层方法中将owner设置成当前线程
      ObjectMonitor * m = omAlloc (Self) ;
      // prepare m for installation - set monitor to initial state
      m->Recycle();
      m->set_header(mark);
      // owner为NULL
      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

      // 修改MarkWord为重量级锁状态
      if (Atomic::cmpxchg_ptr (markOopDesc::encode(m), object->mark_addr(), mark) != mark) {
          // 设置成功说明MarkWord被另外一个线程修改了, 释放ObjectMonitor
          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.
          // 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.
      }

      // Hopefully the performance counters are allocated on distinct
      // cache lines to avoid false sharing on MP systems ...
      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 ;
  }
}

          if (!success) {
            // 准备使用轻量级锁或者重量级锁
            // 用当前的MarkWord构造一个无锁的MarkWord作为DisplacedHeader
            markOop displaced = lockee->mark()->set_unlocked();
            // 保存到LockRecord中, 用于解锁时MarkWord的还原
            // 如果当前是轻量级锁, 那么这次是轻量级锁重入, 后面会将dispalced_header置为NULL
            // 如果当前是重量级锁状态, 这个dispalced_header不会被置为NULL, 区别于轻量级锁重入
            entry->lock()->set_displaced_header(displaced);
            bool call_vm = UseHeavyMonitors;
            // 如果参数强制指定使用重量级锁,
            // 或者CAS将MarkWord从无锁状态设置为轻量级锁失败
            if (call_vm || Atomic::cmpxchg_ptr(entry, lockee->mark_addr(), displaced) != displaced) {
              ...
              if (!call_vm && THREAD->is_lock_owned((address) displaced->clear_lock_bits())) {
                ...
              } else {
                // 膨胀
                CALL_VM(InterpreterRuntime::monitorenter(THREAD, entry), handle_exception);
              }
            }
          }

void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
  ...
  ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
}

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 ;

  // 没有被加锁(_owner==NULL), 设置_owner为当前线程
  cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
  if (cur == NULL) {
      // 设置成功
     // 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) {
     // 重入的情况
     // TODO-FIXME: check for integer overflow!  BUGID 6557169.
     _recursions ++ ;
     return ;
  }

  if (Self->is_lock_owned ((address)cur)) {
    // 当前线程持有该对象的轻量级锁，
    // 在膨胀的代码的轻量级锁膨胀逻辑中说了, 膨胀肯定不是轻量级锁对应线程触发的, 肯定是其他线程触发的
    // 这里应该是轻量级锁重入时发现自己的轻量级锁被膨胀了
    // 由轻量级锁膨胀为重量级锁，且第一次进入enter方法(升级是会设置_owner为LockRecord)，
    // 那cur是指向LockRecord的指针
    assert (_recursions == 0, "internal state error");
    // 重入计数置为1,
    // 为什么是1?
    // 之前说了这里对应的是轻量级锁被膨胀为重量级锁后第一次重入的情况, 重试次数设置成1
    // 确实是作为重量级锁的身份第一次重入, 但膨胀之前的轻量级锁可能已经重入过多次了, 轻量级锁的这些重入怎么算?
    // 这时是不是应该和当初偏向锁升级为轻量级锁一样, 遍历轻量级锁, 统计下从入次数, 后面每次解锁时减一?
    // 实际上没这么实现, ObjectMonitor又是不一样的思路,
    // 对于重量级锁的LockRecord, 其displaced_header是不为NULL的,
    // 对于轻量级锁的若干次重入, 解锁时判断displaced_header为NUll后直接跳过不操作ObjectMonitor,
    // 直到碰到第一次轻量级锁加锁时的displaced_header时的不为NULL的header时, 再对ObjectMonitor操作即可
    _recursions = 1 ;
    // Commute owner from a thread-specific on-stack BasicLockObject address to
    // a full-fledged "Thread *".
    // _owner设置为当前线程，在inflate方法中会设置为LockRecord(轻量级锁膨胀到重量级锁的情况), 或者是NULL(无锁膨胀到重量级锁的情况)
    _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.
  // 先尝试自旋获取锁(获取锁的过程就是CAS设置owner为自己)
  // 在进行昂贵的系统同步操作前再给自己一次获取锁的机会
  if (Knob_SpinEarly && TrySpin (Self) > 0) {
     // 如果获取到了, 直接返回
     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;

  // 设置线程状态为BLOCKED
  { // Change java thread status to indicate blocked on monitor enter.
    JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);

    Self->set_current_pending_monitor(this);

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

      // The current thread does not yet own the monitor and does not
      // yet appear on any queues that would get it made the successor.
      // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
      // handler cannot accidentally consume an unpark() meant for the
      // ParkEvent associated with this ObjectMonitor.
    }

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

    // 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()

      // 调用系统同步操作
      EnterI (THREAD) ;
... 后半部分在唤醒部分说

void ATTR ObjectMonitor::EnterI (TRAPS) {
    Thread * Self = THREAD ;
    assert (Self->is_Java_thread(), "invariant") ;
    assert (((JavaThread *) Self)->thread_state() == _thread_blocked   , "invariant") ;

    // Try the lock - TATAS
    // CAS设置owner为自己, 设置成功返回1,
    // 仍然不死心啊
    if (TryLock (Self) > 0) {
        assert (_succ != Self              , "invariant") ;
        assert (_owner == Self             , "invariant") ;
        assert (_Responsible != Self       , "invariant") ;
        return ;
    }

    DeferredInitialize () ;

    // We try one round of spinning *before* enqueueing Self.
    //
    // If the _owner is ready but OFFPROC we could use a YieldTo()
    // operation to donate the remainder of this thread's quantum
    // to the owner.  This has subtle but beneficial affinity
    // effects.

    // 来一轮自旋获取锁
    if (TrySpin (Self) > 0) {
        // 成功直接返回
        assert (_owner == Self        , "invariant") ;
        assert (_succ != Self         , "invariant") ;
        assert (_Responsible != Self  , "invariant") ;
        return ;
    }

    // The Spin failed -- Enqueue and park the thread ...
    // 终于放弃了, 乖乖入队等待了?
    assert (_succ  != Self            , "invariant") ;
    assert (_owner != Self            , "invariant") ;
    assert (_Responsible != Self      , "invariant") ;

    // Enqueue "Self" on ObjectMonitor's _cxq.
    //
    // Node acts as a proxy for Self.
    // As an aside(此外), if were to ever rewrite the synchronization code mostly
    // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
    // Java objects.  This would avoid awkward lifecycle and liveness issues,
    // as well as eliminate a subset of ABA issues.
    // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
    //

    // 将当前线程封装成ObjectWaiter
    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.
    // 将线程入队
    // 一旦进入cxq/EntryList, 线程在获得锁之前将一直排队.
    // 注意, 之前的自旋操作会降低线程出队的概率, 也就是说synchronized是非公平锁.
    ObjectWaiter * nxt ;
    // 不断尝试CAS入队
    for (;;) {
        // 头插法进入_cxq
        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.
        // CAS失败, 再次尝试获取锁, 这样可以降低进入队列等待的概率
        // 相当于逮到机会, 再尝试获取锁一下...
        // 这个代码有点可爱了
        if (TryLock (Self) > 0) {
            assert (_succ != Self         , "invariant") ;
            assert (_owner == Self        , "invariant") ;
            assert (_Responsible != Self  , "invariant") ;
            return ;
        }
    }

    // Check for cxq|EntryList edge transition to non-null.  This indicates
    // the onset of contention.  While contention persists exiting threads
    // will use a ST:MEMBAR:LD 1-1 exit protocol.  When contention abates exit
    // operations revert to the faster 1-0 mode.  This enter operation may interleave
    // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
    // arrange for one of the contending thread to use a timed park() operations
    // to detect and recover from the race.  (Stranding is form of progress failure
    // where the monitor is unlocked but all the contending threads remain parked).
    // That is, at least one of the contended threads will periodically poll _owner.
    // One of the contending threads will become the designated "Responsible" thread.
    // The Responsible thread uses a timed park instead of a normal indefinite park
    // operation -- it periodically wakes and checks for and recovers from potential
    // strandings admitted by 1-0 exit operations.   We need at most one Responsible
    // thread per-monitor at any given moment.  Only threads on cxq|EntryList may
    // be responsible for a monitor.
    //
    // Currently, one of the contended threads takes on the added role of "Responsible".
    // A viable alternative would be to use a dedicated "stranding checker" thread
    // that periodically iterated over all the threads (or active monitors) and unparked
    // successors where there was risk of stranding.  This would help eliminate the
    // timer scalability issues we see on some platforms as we'd only have one thread
    // -- the checker -- parked on a timer.

    // TODO-henry 不知道这要干嘛
    // SyncFlags默认是0, nxt==NULL&&_EntryList==NULL说明当前线程入队前_cxq和_EntryList都为空
    if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
        // Try to assume the role of responsible thread for the monitor.
        // CONSIDER:  ST vs CAS vs { if (Responsible==null) Responsible=Self }
        // 设置_Responsible为当前线程
        Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
    }

    // The lock have been released while this thread was occupied queueing
    // itself onto _cxq.  To close the race and avoid "stranding" and
    // progress-liveness failure we must resample-retry _owner before parking.
    // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
    // In this case the ST-MEMBAR is accomplished with CAS().
    //
    // TODO: Defer all thread state transitions until park-time.
    // Since state transitions are heavy and inefficient we'd like
    // to defer the state transitions until absolutely necessary,
    // and in doing so avoid some transitions ...

    TEVENT (Inflated enter - Contention) ;
    int nWakeups = 0 ;
    int RecheckInterval = 1 ;

    // 在这里会挂起-唤醒, 唯一能跳出循环的方式就是获取锁
    for (;;) {

        // 再次尝试获取锁...
        // 还不死心...
        if (TryLock (Self) > 0) break ;
        assert (_owner != Self, "invariant") ;

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

        // 终于乖乖挂起了(park)

        // park self
        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() ;
        }
... 后半部分是唤醒逻辑，后面会有解析

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 ;
   }
}

      CASE(_monitorexit): {
        // synchronized(obj)的obj
        oop lockee = STACK_OBJECT(-1);
        CHECK_NULL(lockee);
        // derefing's lockee ought to provoke implicit null check
        // find our monitor slot
        BasicObjectLock* limit = istate->monitor_base();
        // limit of expression stack
        BasicObjectLock* most_recent = (BasicObjectLock*) istate->stack_base();
        // 按enter时相同的顺序遍历LockRecord, 可以达到栈的效果
        while (most_recent != limit ) {
          if ((most_recent)->obj() == lockee) {
            // 找到当前对象持有的LockRecord
            BasicLock* lock = most_recent->lock();
            // 从LockRecord中取出刚开始的header
            markOop header = lock->displaced_header();
            // 解除关联, 偏向锁和重入的轻量级锁只需要这一步就够了
            most_recent->set_obj(NULL);
            if (!lockee->mark()->has_bias_pattern()) {
              // 当前不是偏向锁的情况, 可能是轻量级锁或者重量级锁
              bool call_vm = UseHeavyMonitors;
              // If it isn't recursive we either must swap old header or call the runtime
              if (header != NULL || call_vm) {
                // header != NULL说明不是轻量级锁的重入, call_vm==true说明是强制使用重量级锁
                // 对于不是重入的轻量级锁, 从DisplacedMarkWord还原回去就行,
                // 还原失败, 说明轻量级锁可能在某个地方膨胀后MarkWord被换成了重量级锁的
                // 需要走InterpreterRuntime::monitorexit
                // 对于call_vm==true, 也就是强制走重量级锁的情况, 直接InterpreterRuntime::monitorexit
                if (call_vm || Atomic::cmpxchg_ptr(header, lockee->mark_addr(), lock) != lock) {
                  // restore object for the slow case
                  // 先把obj设置回去, 这是为后面在InterpreterRuntime::monitorexit中为重量级锁解锁做准备
                  most_recent->set_obj(lockee);
                  CALL_VM(InterpreterRuntime::monitorexit(THREAD, most_recent), handle_exception);
                }
              }
            }
            // 只需要找到一个就退出, 对于重入的情况肯定是后获取的锁先释放
            UPDATE_PC_AND_TOS_AND_CONTINUE(1, -1);
          }
          // 下一个LockRecord
          most_recent++;
        }
        // Need to throw illegal monitor state exception
        CALL_VM(InterpreterRuntime::throw_illegal_monitor_state_exception(THREAD), handle_exception);
        ShouldNotReachHere();
      }

IRT_ENTRY_NO_ASYNC(void, InterpreterRuntime::monitorexit(JavaThread* thread, BasicObjectLock* elem))
...
  ObjectSynchronizer::slow_exit(h_obj(), elem->lock(), thread);
... 后半部分代码后面分析
IRT_END

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

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();
  markOop mark ;
  // 不仅是轻量级锁钟乳, 当前是 重量级锁也可能会进到这里,
  // 如果膨胀为重量级锁前, 作为轻量级锁被重入多次, 重入的这些LockRecord会进到这里处理
  if (dhw == NULL) {
    // dhw为NULL，说明是轻量级重入的线程
    // 因为重入时displaced_header会被设置为NULL
    // 只做了些校验，没有实质性逻辑, 相当于啥也没做
     // 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() ;

  // If the object is stack-locked by the current thread, try to
  // swing the displaced header from the box back to the mark.
  // 只有轻量级锁会进
  if (mark == (markOop) lock) {
    // 因为对于轻量级锁来说, 锁标志位是00, 所以mark==(markOop)lock表示MarkWord指向LockRecord
    // 如果MarkWord就是LockRecord，将LockRecord的displaced_header写回MarkWord中
     assert (dhw->is_neutral(), "invariant") ;
     if ((markOop) Atomic::cmpxchg_ptr (dhw, object->mark_addr(), mark) == mark) {
       // 写成功就直接结束
       // 否则, 说明存在获取MarkWord后, MarkWord被改了, 存在竞争, 这时的轻量级锁应该升级为重量级锁了
       // 需要获取膨胀后的重量级锁, 然后释放
        TEVENT (fast_exit: release stacklock) ;
        return;
     }
  }

  // 上面没成功, 说明轻量级锁已经膨胀为重量级锁
  // 需要获取到重量级锁, 然后释放
  ObjectSynchronizer::inflate(THREAD, object)->exit (true, THREAD) ;
}

void ATTR ObjectMonitor::exit(bool not_suspended, TRAPS) {
   Thread * Self = THREAD ;
   if (THREAD != _owner) {
     // ObjectMonitor的owner不是当前线程
     if (THREAD->is_lock_owned((address) _owner)) {
       // 当前线程拥有轻量级锁,
       // 说明轻量级锁膨胀为重量级锁后, 还没调用enter方法, owner依然指向LockRecord
       // 通过is_lock_owned方法可以判断这个地址是否在LockRecord的栈上
       // 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设置为当前线程, 继续走后面的流程
       _owner = THREAD ;
       _recursions = 0 ;
       OwnerIsThread = 1 ;
     } else {
       // 抛错
       // 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;
     }
   }

   // 对于重入的情况, 重入-1
   if (_recursions != 0) {
     _recursions--;        // this is simple recursive enter
     TEVENT (Inflated exit - recursive) ;
     return ;
   }
... 此时当前线程完全释放锁，准备将owner交给其他线程了

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)

   // 设置假定继承人为wakee线程
   _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
   // 内存屏障#loadstore|#storestore;
   OrderAccess::release_store_ptr (&_owner, NULL) ;
   OrderAccess::fence() ;                               // ST _owner vs LD in unpark()

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

   // 唤醒选定的线程
   DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
   Trigger->unpark() ;

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

void ATTR ObjectMonitor::EnterI (TRAPS) {
... 省略入队代码
    
    for (;;) {

        ... 省略挂起代码

        // 被唤醒后继续获取锁, 如果成功直接退出
        if (TryLock(Self) > 0) break ;

        // The lock is still contested.
        // Keep a tally of the # of futile wakeups.
        // Note that the counter is not protected by a lock or updated by atomics.
        // That is by design - we trade "lossy" counters which are exposed to
        // races during updates for a lower probe effect.
        // 走到这里说明被唤醒后获取锁失败, 记录下
        TEVENT (Inflated enter - Futile wakeup) ;
        if (ObjectMonitor::_sync_FutileWakeups != NULL) {
           ObjectMonitor::_sync_FutileWakeups->inc() ;
        }
        ++ nWakeups ;

        // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
        // We can defer clearing _succ until after the spin completes
        // TrySpin() must tolerate being called with _succ == Self.
        // Try yet another round of adaptive spinning.
        // 尝试自旋获取锁
        if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;

        // We can find that we were unpark()ed and redesignated _succ while
        // we were spinning.  That's harmless.  If we iterate and call park(),
        // park() will consume the event and return immediately and we'll
        // just spin again.  This pattern can repeat, leaving _succ to simply
        // spin on a CPU.  Enable Knob_ResetEvent to clear pending unparks().
        // Alternately, we can sample fired() here, and if set, forgo spinning
        // in the next iteration.

        if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
           Self->_ParkEvent->reset() ;
           OrderAccess::fence() ;
        }
        // 如果还是失败, 放弃假定继承人
        if (_succ == Self) _succ = NULL ;

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

    // 走到这里应该获取到锁了

    // Egress :
    // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
    // Normally we'll find Self on the EntryList .
    // From the perspective of the lock owner (this thread), the
    // EntryList is stable and cxq is prepend-only.
    // The head of cxq is volatile but the interior is stable.
    // In addition, Self.TState is stable.
    // 入队(cxq/EntryList)后, 获取到了锁, 接下来从cxq/EntryList中出队
    // 正常情况下, 这个时候应该在EntryList中

    assert (_owner == Self      , "invariant") ;
    assert (object() != NULL    , "invariant") ;
    // I'd like to write:
    //   guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
    // but as we're at a safepoint that's not safe.

    // 出队(从_cxq或_EntryList中移除)
    UnlinkAfterAcquire (Self, &node) ;
    if (_succ == Self) _succ = NULL ;

    ... 省略内存屏障相关代码
    return ;
}

IRT_ENTRY_NO_ASYNC(void, InterpreterRuntime::monitorexit(JavaThread* thread, BasicObjectLock* elem))
...
  elem->set_obj(NULL);
...
IRT_END

static JNINativeMethod methods[] = {
    {"hashCode",    "()I",                    (void *)&JVM_IHashCode},
    {"wait",        "(J)V",                   (void *)&JVM_MonitorWait},
    {"notify",      "()V",                    (void *)&JVM_MonitorNotify},
    {"notifyAll",   "()V",                    (void *)&JVM_MonitorNotifyAll},
    {"clone",       "()Ljava/lang/Object;",   (void *)&JVM_Clone},
};

JVM_ENTRY(void, JVM_MonitorWait(JNIEnv* env, jobject handle, jlong ms))
  ...
  ObjectSynchronizer::wait(obj, ms, CHECK);
JVM_END

void ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
  if (UseBiasedLocking) {
    // 撤销偏向锁
    BiasedLocking::revoke_and_rebias(obj, false, THREAD);
    assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
  }
  ...
  // 膨胀为重量级锁
  ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
  DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis);
  // wait主逻辑
  monitor->wait(millis, true, THREAD);

  /* This dummy call is in place to get around dtrace bug 6254741.  Once
     that'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);
}

void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
   ... 省略检查逻辑和事件逻辑

   assert (Self->_Stalled == 0, "invariant") ;
   Self->_Stalled = intptr_t(this) ;
   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.
   ObjectWaiter node(Self);
   // 修改waiter状态为TS_WAIT
   node.TState = ObjectWaiter::TS_WAIT ;
   Self->_ParkEvent->reset() ;
   // 内存屏障
   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.

   // 对进入WaitSet的操作加锁
   Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
   // 进入WaitSet
   AddWaiter (&node) ;
   Thread::SpinRelease (&_WaitSetLock) ;

   if ((SyncFlags & 4) == 0) {
      _Responsible = NULL ;
   }
   intptr_t save = _recursions; // record the old recursion count
   _waiters++;                  // increment the number of waiters
   _recursions = 0;             // set the recursion level to be 1
   // 释放锁资源
   exit (true, Self) ;                    // exit the monitor
   guarantee (_owner != Self, "invariant") ;

   // 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
     // 设置操作系统线程状态
     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();

       // 挂起自己
       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) ;
         }
       }
       ...
}

JVM_ENTRY(void, JVM_MonitorNotify(JNIEnv* env, jobject handle))
  JVMWrapper("JVM_MonitorNotify");
  Handle obj(THREAD, JNIHandles::resolve_non_null(handle));
  ObjectSynchronizer::notify(obj, CHECK);
JVM_END

void ObjectSynchronizer::notify(Handle obj, TRAPS) {
 // 撤销偏向锁
 // 撤销时, 如果依然活跃会升级为轻量级锁
 if (UseBiasedLocking) {
    BiasedLocking::revoke_and_rebias(obj, false, THREAD);
    assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
  }

  markOop mark = obj->mark();
  if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
    // 能走到这里说明当前线程就是活跃的, 不用notify了
    return;
  }

  // 否则膨胀后notify
  ObjectSynchronizer::inflate(THREAD, obj())->notify(THREAD);
}

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") ;
  // 从WaitSet队列中获取一个waiter
  ObjectWaiter * iterator = DequeueWaiter() ;
  if (iterator != NULL) {
     TEVENT (Notify1 - Transfer) ;
     guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
     guarantee (iterator->_notified == 0, "invariant") ;
     if (Policy != 4) {
        iterator->TState = ObjectWaiter::TS_ENTER ;
     }
     iterator->_notified = 1 ;
     Thread * Self = THREAD;
     iterator->_notifier_tid = Self->osthread()->thread_id();

     ObjectWaiter * List = _EntryList ;
     if (List != NULL) {
        assert (List->_prev == NULL, "invariant") ;
        assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
        assert (List != iterator, "invariant") ;
     }

     if (Policy == 0) {       // prepend to EntryList
         // 头插到EntryList
         if (List == NULL) {
             iterator->_next = iterator->_prev = NULL ;
             _EntryList = iterator ;
         } else {
             List->_prev = iterator ;
             iterator->_next = List ;
             iterator->_prev = NULL ;
             _EntryList = iterator ;
        }
     } else
     if (Policy == 1) {      // append to EntryList
         // 尾插到EntryList
         if (List == NULL) {
             iterator->_next = iterator->_prev = NULL ;
             _EntryList = iterator ;
         } else {
            // CONSIDER:  finding the tail currently requires a linear-time walk of
            // the EntryList.  We can make tail access constant-time by converting to
            // a CDLL instead of using our current DLL.
            ObjectWaiter * Tail ;
            for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
            assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
            Tail->_next = iterator ;
            iterator->_prev = Tail ;
            iterator->_next = NULL ;
        }
     } else
     if (Policy == 2) {      // prepend to cxq
         // prepend to cxq
         // 头插到cxq, 默认
         if (List == NULL) {
             iterator->_next = iterator->_prev = NULL ;
             _EntryList = iterator ;
         } else {
            iterator->TState = ObjectWaiter::TS_CXQ ;
            for (;;) {
                ObjectWaiter * Front = _cxq ;
                iterator->_next = Front ;
                if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
                    break ;
                }
            }
         }
     } else
     if (Policy == 3) {      // append to cxq
         // 尾插到cxq
        iterator->TState = ObjectWaiter::TS_CXQ ;
        for (;;) {
            ObjectWaiter * Tail ;
            Tail = _cxq ;
            if (Tail == NULL) {
                iterator->_next = NULL ;
                if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
                   break ;
                }
            } else {
                while (Tail->_next != NULL) Tail = Tail->_next ;
                Tail->_next = iterator ;
                iterator->_prev = Tail ;
                iterator->_next = NULL ;
                break ;
            }
        }
     } else {
        // 取WaitSet第一个节点唤醒
        ParkEvent * ev = iterator->_event ;
        iterator->TState = ObjectWaiter::TS_RUN ;
        OrderAccess::fence() ;
        ev->unpark() ;
     }

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

     // _WaitSetLock protects the wait queue, not the EntryList.  We could
     // move the add-to-EntryList operation, above, outside the critical section
     // protected by _WaitSetLock.  In practice that's not useful.  With the
     // exception of  wait() timeouts and interrupts the monitor owner
     // is the only thread that grabs _WaitSetLock.  There's almost no contention
     // on _WaitSetLock so it's not profitable to reduce the length of the
     // critical section.
  }

  Thread::SpinRelease (&_WaitSetLock) ;

  if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) {
     ObjectMonitor::_sync_Notifications->inc() ;
  }
}

void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
   ... 省略进入WaitSet和释放资源的代码

   int ret = OS_OK ;
   int WasNotified = 0 ;
   { // State transition wrappers
     // 设置操作系统线程状态
     OSThread* osthread = Self->osthread();
     OSThreadWaitState osts(osthread, true);
     {
       ... 省略挂起代码

       // 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

     ...

     // 从WaitSet中出队
     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'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() ;
     // 不允许notify后的自己称为假定的候选人, 
     // 因为后面需要走一遍enter的流程, 
     // 这段时间的空闲对应用程序来说是浪费, 所以要给别的线程机会去抢占锁资源
     if (_succ == Self) _succ = NULL ;
     WasNotified = node._notified ;
       
     // 重新获取锁

     // 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'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()) {
       // 难道是之前刚在notify中被唤醒就又被park了?
       JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);

       if (node._notified != 0 && _succ == Self) {
         // In this part of the monitor wait-notify-reenter protocol it
         // is possible (and normal) for another thread to do a fastpath
         // monitor enter-exit while this thread is still trying to get
         // to the reenter portion of the protocol.
         //
         // The ObjectMonitor was notified and the current thread is
         // the successor which also means that an unpark() has already
         // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
         // consume the unpark() that was done when the successor was
         // set because the same ParkEvent is shared between Java
         // monitors and JVM/TI RawMonitors (for now).
         //
         // We redo the unpark() to ensure forward progress, i.e., we
         // don't want all pending threads hanging (parked) with none
         // entering the unlocked monitor.
         node._event->unpark();
       }
     }

     if (event.should_commit()) {
       post_monitor_wait_event(&event, node._notifier_tid, millis, ret == OS_TIMEOUT);
     }

     OrderAccess::fence() ;

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

     assert (_owner != Self, "invariant") ;
     ObjectWaiter::TStates v = node.TState ;
     // 重新获取锁
     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);
     }
       
     // 获取锁成功

     // 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'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.
}

void SafepointSynchronize::begin() {
  ...
  // Call stuff that needs to be run when a safepoint is just about to be completed
  do_cleanup_tasks();
  ...
}

// Various cleaning tasks that should be done periodically at safepoints
void SafepointSynchronize::do_cleanup_tasks() {
  {
    TraceTime t1("deflating idle monitors", TraceSafepointCleanupTime);
    ObjectSynchronizer::deflate_idle_monitors();
  }
  ...
}

void ObjectSynchronizer::deflate_idle_monitors() {
  ...
  if (MonitorInUseLists) {
    int inUse = 0;
    // 遍历所有线程的omInUseList
    for (JavaThread* cur = Threads::first(); cur != NULL; cur = cur->next()) {
      nInCirculation+= cur->omInUseCount;
      int deflatedcount = walk_monitor_list(cur->omInUseList_addr(), &FreeHead, &FreeTail);
      cur->omInUseCount-= deflatedcount;
      // verifyInUse(cur);
      nScavenged += deflatedcount;
      nInuse += cur->omInUseCount;
     }

   // For moribund threads, scan gOmInUseList
   // 遍历gOmInUseList
   if (gOmInUseList) {
     nInCirculation += gOmInUseCount;
     int deflatedcount = walk_monitor_list((ObjectMonitor **)&gOmInUseList, &FreeHead, &FreeTail);
     gOmInUseCount-= deflatedcount;
     nScavenged += deflatedcount;
     nInuse += gOmInUseCount;
    }

  } ...

int ObjectSynchronizer::walk_monitor_list(ObjectMonitor** listheadp,
                                          ObjectMonitor** FreeHeadp, ObjectMonitor** FreeTailp) {
  ObjectMonitor* mid;
  ObjectMonitor* next;
  ObjectMonitor* curmidinuse = NULL; // 遍历过程中最新一个正在使用的节点
  int deflatedcount = 0;

  for (mid = *listheadp; mid != NULL; ) {
     oop obj = (oop) mid->object();
     bool deflated = false;
     if (obj != NULL) {
       deflated = deflate_monitor(mid, obj, FreeHeadp, FreeTailp);
     }
     if (deflated) {
       // extract from per-thread in-use-list
       // 如果降级了, 从xxInUseList中去除当前节点
       if (mid == *listheadp) {
         // 如果去除的是头部节点, 更改头部节点位置为降级节点的下一个节点
         *listheadp = mid->FreeNext;
       } else if (curmidinuse != NULL) {
         // 如果不是头部节点, 并且存在正在使用的节点, 将降级节点的下一节点接到它后面
         curmidinuse->FreeNext = mid->FreeNext; // maintain the current thread inuselist
       }
       next = mid->FreeNext;
       mid->FreeNext = NULL;  // This mid is current tail in the FreeHead list
       mid = next;
       deflatedcount++;
     } else {
       // 如果没降级, 则不去除当前节点, 继续遍历下一个
       curmidinuse = mid;
       mid = mid->FreeNext;
    }
  }
  return deflatedcount;
}

// Deflate a single monitor if not in use
// Return true if deflated, false if in use
bool ObjectSynchronizer::deflate_monitor(ObjectMonitor* mid, oop obj,
                                         ObjectMonitor** FreeHeadp, ObjectMonitor** FreeTailp) {
  bool deflated;
  // Normal case ... The monitor is associated with obj.
  guarantee (obj->mark() == markOopDesc::encode(mid), "invariant") ;
  guarantee (mid == obj->mark()->monitor(), "invariant");
  guarantee (mid->header()->is_neutral(), "invariant");

  // 如果owner, cxq, EntryList, WaitSet有一个不为空都认为busy
  if (mid->is_busy()) {
     if (ClearResponsibleAtSTW) mid->_Responsible = NULL ;
     deflated = false;
  } else {
     // Deflate the monitor if it is no longer being used
     // It's idle - scavenge and return to the global free list
     // plain old deflation ...
     TEVENT (deflate_idle_monitors - scavenge1) ;
     if (TraceMonitorInflation) {
       if (obj->is_instance()) {
         ResourceMark rm;
           tty->print_cr("Deflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
                (void *) obj, (intptr_t) obj->mark(), obj->klass()->external_name());
       }
     }

     // Restore the header back to obj
     // 还原MarkWord为无锁状态
     obj->release_set_mark(mid->header());
     // 清除owner和关联object
     mid->clear();

     assert (mid->object() == NULL, "invariant") ;

     // Move the object to the working free list defined by FreeHead,FreeTail.
     // 将当前节点移到xxFreeList中
     if (*FreeHeadp == NULL) *FreeHeadp = mid;
     if (*FreeTailp != NULL) {
       ObjectMonitor * prevtail = *FreeTailp;
       assert(prevtail->FreeNext == NULL, "cleaned up deflated?"); // TODO KK
       prevtail->FreeNext = mid;
      }
     *FreeTailp = mid;
     deflated = true;
  }
  return deflated;
}

...
          // implies UseBiasedLocking
          if (mark->has_bias_pattern()) {
            // 如果MarkWord是偏向锁(后三位101)
            uintptr_t thread_ident;
            uintptr_t anticipated_bias_locking_value;
            thread_ident = (uintptr_t)istate->thread();
            // 偏向锁状态的MarkWord结构: 线程ID+epoch+age+偏向锁标志+锁标志位
            // 锁标志位在无锁和偏向锁情况下都是01
            anticipated_bias_locking_value =
              // klass->prototype_header(), 包含epoch和偏向锁标志位, 拼上当前线程ID
              (((uintptr_t)lockee->klass()->prototype_header() | thread_ident)
              // 和对象的MarkWord异或, 不想等的位会是1
              ^ (uintptr_t)mark)
              // 比较结果中age的几位置为0
              & ~((uintptr_t) markOopDesc::age_mask_in_place);

            // 上面抹掉了age, 锁标志位在无锁和偏向锁情况下都是01, 所以相等
            // 如果不一样, 可能不同的地方有: 线程ID, epoch, 偏向锁标志位, 以下分情况处理

            if  (anticipated_bias_locking_value == 0) {
              // 说明完全一样, 偏向的是当前的线程, 并且epoch和clazz中的epoch相同
              // 什么都不做
              // already biased towards this thread, nothing to do
              if (PrintBiasedLockingStatistics) {
                (* BiasedLocking::biased_lock_entry_count_addr())++;
              }
              success = true;
            }
            else if ((anticipated_bias_locking_value & markOopDesc::biased_lock_mask_in_place) != 0) {
              ...
            }
            else if ((anticipated_bias_locking_value & epoch_mask_in_place) !=0) {
              // epoch不想等, 说明mark中的epoch过期(发生过批量重偏向),
              // epoch过期可以看作没有偏向任何线程, 可以重偏向
              // try rebias
              // 尝试重偏向
              // 构造一个偏向当前线程的MarkWord, 会设置最新的epoch
              markOop new_header = (markOop) ( (intptr_t) lockee->klass()->prototype_header() | thread_ident);
              if (hash != markOopDesc::no_hash) {
                new_header = new_header->copy_set_hash(hash);
              }
              // 用新的MarkWord重新偏向
              if (Atomic::cmpxchg_ptr((void*)new_header, lockee->mark_addr(), mark) == mark) {
                if (PrintBiasedLockingStatistics)
                  (* BiasedLocking::rebiased_lock_entry_count_addr())++;
              }
              else {
                // 重偏向失败, 说明已经被其他线程提前改了, 存在竞争, 进行膨胀
                CALL_VM(InterpreterRuntime::monitorenter(THREAD, entry), handle_exception);
              }
              success = true;
            }
            else {
              // try to bias towards thread in case object is anonymously biased
              // 如果这个时候最新的MarkWord是匿名偏向锁, 尝试指向自己, 这也是非公平锁的体现

              // 根据原MarkWord一个匿名偏向锁状态的MarkWord,
              // 保留了原mark的锁标志位+偏向锁标志+age+epoch
              markOop header = (markOop) ((uintptr_t) mark & ((uintptr_t)markOopDesc::biased_lock_mask_in_place |
                                                              (uintptr_t)markOopDesc::age_mask_in_place |
                                                              epoch_mask_in_place));
              // 设置hashcode
              if (hash != markOopDesc::no_hash) {
                header = header->copy_set_hash(hash);
              }
              // 偏向当前线程
              markOop new_header = (markOop) ((uintptr_t) header | thread_ident);
              // debugging hint
              DEBUG_ONLY(entry->lock()->set_displaced_header((markOop) (uintptr_t) 0xdeaddead);)
              // 设置MarkWord
              if (Atomic::cmpxchg_ptr((void*)new_header, lockee->mark_addr(), header) == header) {
                // 设置成功, 说明设置前的是匿名偏向锁, 通过CAS改成了偏向为自己的偏向锁
                if (PrintBiasedLockingStatistics)
                  (* BiasedLocking::anonymously_biased_lock_entry_count_addr())++;
              }
              else {
                // 重置失败, 说明当前已经偏向其他线程, 存在竞争, 进入膨胀流程
                CALL_VM(InterpreterRuntime::monitorenter(THREAD, entry), handle_exception);
              }
              success = true;
            }
          }
....

...
      CASE(_monitorexit): {
        // synchronized(obj)的obj
        oop lockee = STACK_OBJECT(-1);
        CHECK_NULL(lockee);
        // derefing's lockee ought to provoke implicit null check
        // find our monitor slot
        BasicObjectLock* limit = istate->monitor_base();
        // limit of expression stack
        BasicObjectLock* most_recent = (BasicObjectLock*) istate->stack_base();
        // 按enter时相同的顺序遍历LockRecord, 可以达到栈的效果
        while (most_recent != limit ) {
          if ((most_recent)->obj() == lockee) {
            // 找到当前对象持有的LockRecord
            BasicLock* lock = most_recent->lock();
            // 从LockRecord中取出刚开始的header
            markOop header = lock->displaced_header();
            // 解除关联, 偏向锁和重入的轻量级锁只需要这一步就够了
            most_recent->set_obj(NULL);
            ...
            // 只需要找到一个就退出, 对于重入的情况肯定是后获取的锁先释放
            UPDATE_PC_AND_TOS_AND_CONTINUE(1, -1);
          }
          // 下一个LockRecord
          most_recent++;
        }
...

...
        if (entry != NULL) {
          ...
          // implies UseBiasedLocking
          if (mark->has_bias_pattern()) {
            ...

            if  (anticipated_bias_locking_value == 0) {
              ...
            }
            else if ((anticipated_bias_locking_value & markOopDesc::biased_lock_mask_in_place) != 0) {
              // 偏向锁标志位不同, 而刚才的mark通过了偏向锁检查, 所以是klass中的偏向锁模式关闭
              // 没法继续使用偏向锁了, 需要撤销当前的偏向锁, 然后升级为轻量级锁或者重量级锁
              // try revoke bias
              // 尝试撤销偏向锁
              // 此时的lockee->klass()->prototype_header()应该是无锁状态的MarkWord
              markOop header = lockee->klass()->prototype_header();
              // 重置MarkWord, 如果有hash, 带上hash
              if (hash != markOopDesc::no_hash) {
                header = header->copy_set_hash(hash);
              }
              if (Atomic::cmpxchg_ptr(header, lockee->mark_addr(), mark) == mark) {
                // 修改成功, 下面可能会直接升级为轻量级锁
                if (PrintBiasedLockingStatistics)
                  (*BiasedLocking::revoked_lock_entry_count_addr())++;
              }
              // 后面的代码会继续走轻量级锁或者重量级锁的获取逻辑
            }
            else if ((anticipated_bias_locking_value & epoch_mask_in_place) !=0) {
              ...
              // 用新的MarkWord重新偏向
              if (Atomic::cmpxchg_ptr((void*)new_header, lockee->mark_addr(), mark) == mark) {
                ...
              }
              else {
                // 重偏向失败, 说明已经被其他线程提前改了, 存在竞争, 进行膨胀
                CALL_VM(InterpreterRuntime::monitorenter(THREAD, entry), handle_exception);
              }
              success = true;
            }
            else {
              ...
              if (Atomic::cmpxchg_ptr((void*)new_header, lockee->mark_addr(), header) == header) {
                ...
              }
              else {
                // 重置失败, 说明当前已经偏向其他线程, 存在竞争, 进入膨胀流程
                CALL_VM(InterpreterRuntime::monitorenter(THREAD, entry), handle_exception);
              }
              success = true;
            }
          }

          // traditional lightweight locking
          // 处理非偏向锁的情况
          if (!success) {
            // 准备使用轻量级锁或者重量级锁
            // 用当前的MarkWord构造一个无锁的MarkWord作为DisplacedHeader
            markOop displaced = lockee->mark()->set_unlocked();
            // 保存到LockRecord中, 用于解锁时MarkWord的还原
            entry->lock()->set_displaced_header(displaced);
            bool call_vm = UseHeavyMonitors;
            // 如果参数强制指定使用重量级锁,
            // 或者CAS将MarkWord从无锁状态设置为轻量级锁失败
            if (call_vm || Atomic::cmpxchg_ptr(entry, lockee->mark_addr(), displaced) != displaced) {
              // Is it simple recursive case?
              // 判断是否轻量级锁重入
              if (!call_vm && THREAD->is_lock_owned((address) displaced->clear_lock_bits())) {
                // 轻量级锁重入时, 重入的锁的DisplacedMarkWord为NULL
                entry->lock()->set_displaced_header(NULL);
              } else {
                // 膨胀
                CALL_VM(InterpreterRuntime::monitorenter(THREAD, entry), handle_exception);
              }
            }
          }

...

 ...
            else if ((anticipated_bias_locking_value & markOopDesc::biased_lock_mask_in_place) != 0) {
              // 偏向锁标志位不同, 而刚才的mark通过了偏向锁检查, 所以是klass中的偏向锁模式关闭
              // 没法继续使用偏向锁了, 需要撤销当前的偏向锁, 然后升级为轻量级锁或者重量级锁
              // try revoke bias
              // 尝试撤销偏向锁
              // 此时的lockee->klass()->prototype_header()应该是无锁状态的MarkWord
              markOop header = lockee->klass()->prototype_header();
              // 重置MarkWord, 如果有hash, 带上hash
              if (hash != markOopDesc::no_hash) {
                header = header->copy_set_hash(hash);
              }
              if (Atomic::cmpxchg_ptr(header, lockee->mark_addr(), mark) == mark) {
                // 修改成功, 下面可能会直接升级为轻量级锁
                if (PrintBiasedLockingStatistics)
                  (*BiasedLocking::revoked_lock_entry_count_addr())++;
              }
              // 后面的代码会继续走轻量级锁或者重量级锁的获取逻辑
            }
...

...
          if (!success) {
            // 准备使用轻量级锁或者重量级锁
            // 用当前的MarkWord构造一个无锁的MarkWord作为DisplacedHeader
            markOop displaced = lockee->mark()->set_unlocked();
            // 保存到LockRecord中, 用于解锁时MarkWord的还原
            entry->lock()->set_displaced_header(displaced);
            ...
            // 如果参数强制指定使用重量级锁,
            // 或者CAS将MarkWord从无锁状态设置为轻量级锁失败
            if (call_vm || Atomic::cmpxchg_ptr(entry, lockee->mark_addr(), displaced) != displaced) {
...

...
IRT_ENTRY_NO_ASYNC(void, InterpreterRuntime::monitorenter(JavaThread* thread, BasicObjectLock* elem))
  ...
  // 如果允许偏向锁
  if (UseBiasedLocking) {
    // 加偏向锁
    // Retry fast entry if bias is revoked to avoid unnecessary inflation
    ObjectSynchronizer::fast_enter(h_obj, elem->lock(), true, CHECK);
  } else {
    // 加轻量级锁
    ObjectSynchronizer::slow_enter(h_obj, elem->lock(), CHECK);
  }
  ...
IRT_END
...

void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock, bool attempt_rebias, TRAPS) {
  // 走到这里肯定是之前获取偏向锁失败了,
  // 如果没有出现批量重偏向, 后面肯定得撤销偏向锁, 从轻量级锁开始加起了
 if (UseBiasedLocking) {
    // 是否在安全点
    if (!SafepointSynchronize::is_at_safepoint()) {
      // 撤销然后重偏向
      // 重偏向只有一种情况下能成功, 发生了批量重偏向, 其他情况都是撤销或者升级为轻量级锁
      // 如果当前对象偏向锁没有偏向, 会偏向当前线程, 然后退出
      // 否则,
      //  如果当前对象偏向的线程已经死亡,
      //    会变成匿名偏向锁(attempt_rebias为true)/无锁状态(attempt_rebias为false)
      //  如果当前对象偏向的线程依然存活, 会将偏向锁升级为轻量级锁, 但是不改变偏向
      //      因为升级为轻量级锁后需要改变偏向说明存在竞争, 存在竞争就得膨为重量级锁
      // 这个方法中的部分逻辑是可以不在安全点执行的, 因为这部分逻辑完全是在单线程范围内的, 不存在并发问题
      // 对于存在并发问题的部分, 还是会等待到安全点再执行
      BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
      if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {
        // 如果重偏向成功, 流程结束
        return;
      }
    } else {
      assert(!attempt_rebias, "can not rebias toward VM thread");
      // 这段代码是在安全点执行的, 逻辑和revoke_and_rebias中需要在安全点执行的代码一样
      // 这个当中是不可能重偏向的只会撤销或者升级为轻量级锁
      BiasedLocking::revoke_at_safepoint(obj);
    }

    // 这行代码很明显, 到这里偏向锁肯定被撤销了
    assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 }

 // 很显然到这里, 偏向锁肯定被撤销了
 // 加轻量级锁
 slow_enter (obj, lock, THREAD) ;
}

BiasedLocking::Condition BiasedLocking::revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS) {
  // 不需要在安全点被调用
  assert(!SafepointSynchronize::is_at_safepoint(), "must not be called while at safepoint");

  // step1 高效的撤消

  // We can revoke the biases of anonymously-biased objects
  // efficiently enough that we should not cause these revocations to
  // update the heuristics because doing so may cause unwanted bulk
  // revocations (which are expensive) to occur.
  // 获取MarkWord
  markOop mark = obj->mark();
  if (mark->is_biased_anonymously() && !attempt_rebias) {
    // step1.1 匿名偏向锁, 并且不需要重偏向, 那直接撤销就好
    // 这里的匿名重偏向是在那里设置的?
    // 匿名偏向在两个地方会设置:
    //   1. 这个对象刚new出来, 如果其类型是允许偏向的, 会设置成匿名偏向状态
    //   2. 经历了批量重偏向, 批量重偏向中会先置为匿名偏向, 然后再偏向当前线程, 是在safepoint做的
    // 按理说这两种情况都不会走到这里, 所以我也不知道什么情况会走到这里了, 可能好似其他地方会用到吧
    // We are probably trying to revoke the bias of this object due to
    // an identity hash code computation. Try to revoke the bias
    // without a safepoint. This is possible if we can successfully
    // compare-and-exchange an unbiased header into the mark word of
    // the object, meaning that no other thread has raced to acquire
    // the bias of the object.
    // 有匿名偏向锁(线程ID为0的偏向锁)并且attempt_rebias==false
    //
    // 计算hashcode会走到这里,
    // 如果当前是匿名偏向锁, 说明线程ID和epoch没用, 可以用来存放hashCode
    // 所以要将偏向锁标志位先置为0
    //
    // 撤销偏向锁, 变为无锁状态
    markOop biased_value       = mark;
    // new一个无锁状态的MarkWord
    markOop unbiased_prototype = markOopDesc::prototype()->set_age(mark->age());
    // CAS替换
    markOop res_mark = (markOop) Atomic::cmpxchg_ptr(unbiased_prototype, obj->mark_addr(), mark);
    if (res_mark == biased_value) {
      // 撤销成功
      return BIAS_REVOKED;
    }
  } else if (mark->has_bias_pattern()) {
    // step1.2 偏向锁的情况
    Klass* k = obj->klass();
    markOop prototype_header = k->prototype_header();
    if (!prototype_header->has_bias_pattern()) {
      // step1.2.1 obj的klass不支持偏向锁的情况, 说明经历了批量重撤销,
      // 而且批量撤销时当前对象没有被改掉, 当前对象的偏向状态已经没用了, 那直接撤销就好
      // This object has a stale bias from before the bulk revocation
      // for this data type occurred. It's pointless to update the
      // heuristics at this point so simply update the header with a
      // CAS. If we fail this race, the object's bias has been revoked
      // by another thread so we simply return and let the caller deal
      // with it.
      // clazz关闭了偏向锁的情况, 和外层情况类似, 外层也做过一次CAS, 应该是失败了进这里的, 因为成功了mark就不是偏向锁了
      // 为什么mark中是偏向锁而class不是偏向锁了呢? 因为发生了批量撤销, 修改了class中偏向锁标志, 修改时这个对象应该没被使用, 所以没被改
      // 所以这里直接CAS设置下header, 但不关心CAS的结果,
      // 因为如果CAS失败, 肯定是其他线程在操作, 在其他线程中应该会被撤销或者升级
      // 这里是不用考虑重偏向的, 因为都不支持偏向锁了, 还重偏向什么, 直接撤销, 后面会走slow_enter逻辑加锁
      markOop biased_value       = mark;
      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;
    } else if (prototype_header->bias_epoch() != mark->bias_epoch()) {
      // step1.2.2 epoch过期的情况, 说明经历了批量重偏向, 而这个对象当时没被使用, epoch没更新
      // 这说明之前发生过批量重偏向, 更新了epoch, 此时需要重偏向直接CAS就解决, 很高效
      // The epoch of this biasing has expired indicating that the
      // object is effectively unbiased. Depending on whether we need
      // to rebias or revoke the bias of this object we can do it
      // efficiently enough with a CAS that we shouldn't update the
      // heuristics. This is normally done in the assembly code but we
      // can reach this point due to various points in the runtime
      // needing to revoke biases.
      // 既然epoch过期, 说明MarkWord无效, 说明当前可以随便改
      if (attempt_rebias) {
        // step1.2.2.1 如果需要重偏向, 就重偏向当前线程
        assert(THREAD->is_Java_thread(), "");
        markOop biased_value       = mark;
        // new一个重偏向当前线程的MarkWord
        markOop rebiased_prototype = markOopDesc::encode((JavaThread*) THREAD, mark->age(), prototype_header->bias_epoch());
        markOop res_mark = (markOop) Atomic::cmpxchg_ptr(rebiased_prototype, obj->mark_addr(), mark);
        if (res_mark == biased_value) {
          // CAS设置成功, 返回偏向锁撤销并重偏向
          return BIAS_REVOKED_AND_REBIASED;
        }
      } else {
        // step1.2.2.2 否则直接撤销, 变为无锁状态
        markOop biased_value       = mark;
        // new一个偏向锁标志位为0, 锁标志位为01, hashCode为空, 年龄为mark->age()的MarkWord
        markOop unbiased_prototype = markOopDesc::prototype()->set_age(mark->age());
        // CAS替换
        markOop res_mark = (markOop) Atomic::cmpxchg_ptr(unbiased_prototype, obj->mark_addr(), mark);
        if (res_mark == biased_value) {
          // CAS设置成功, 返回撤销成功
          return BIAS_REVOKED;
        }
      }
    }
  }

  // 可以发现,
  // 上面的情况都是明确不支持偏向锁,
  // 或者可以修改偏向的状态(匿名偏向锁, epoch过期)
  // 没有并发问题, 只需要一个CAS, 很高效
    
  // 如果走到这里说明上面CAS可能失败了,
  ...
  
  ...
  } else if (heuristics == HR_SINGLE_REVOKE) {
    // 单个撤销
    Klass *k = obj->klass();
    markOop prototype_header = k->prototype_header();
    if (mark->biased_locker() == THREAD &&
        prototype_header->bias_epoch() == mark->bias_epoch()) {
      // 只有当前线程在用这个偏向锁
      // A thread is trying to revoke the bias of an object biased
      // toward it, again likely due to an identity hash code
      // computation. We can again avoid a safepoint in this case
      // since we are only going to walk our own stack. There are no
      // races with revocations occurring in other threads because we
      // reach no safepoints in the revocation path.
      // Also check the epoch because even if threads match, another thread
      // can come in with a CAS to steal the bias of an object that has a
      // stale epoch.
      // 撤销当前线程的偏向锁, 判断时除了判断线程外, 还判断了epoch是否过期,
      // 因为epoch过期, 说明发生过批量重偏向, MarkWord中的线程ID是个无效值, 等于没有
      // 如果都是true就说明当前锁是一个在用的锁, 并且被当前线程持有
      //
      // 计算hashcode的时候会走到这里,
      // 而且这个时候发现没法将hashCode存在MarkWord中了(只有在匿名偏向的情况下可以存到MarkWord中, 上面处理了这种情况)
      // 需要先膨胀, 再找地方存hashCode
      //
      // 这里的撤销不用等到safepoint的, 因为只需要遍历当前线程的栈即可, 不需要等待所有线程都到达safepoint
      ResourceMark rm;
      if (TraceBiasedLocking) {
        tty->print_cr("Revoking bias by walking my own stack:");
      }
      // revoke_bias中, 可能会升级为轻量级锁(obj依然在使用), 也可能撤消(无锁)
      BiasedLocking::Condition cond = revoke_bias(obj(), false, false, (JavaThread*) THREAD);
      ((JavaThread*) THREAD)->set_cached_monitor_info(NULL);
      assert(cond == BIAS_REVOKED, "why not?");
      return cond;
    } ...
...
}

// 撤销偏向锁
// 如果allow_rebias==false, 最后要么膨胀为轻量级锁, 要么变为无锁状态
// 如果allow_rebias==true, 貌似只有在批量重偏向中对当前对象重偏向可能是true, 最后要么膨胀为轻量级锁, 要么是匿名偏向状态(为后面重偏向做准备)
static BiasedLocking::Condition revoke_bias(oop obj, bool allow_rebias, bool is_bulk, JavaThread* requesting_thread) {
  markOop mark = obj->mark();
  if (!mark->has_bias_pattern()) {
    // 不是偏向锁状态, 不用处理
    if (TraceBiasedLocking) {
      ResourceMark rm;
      tty->print_cr("  (Skipping revocation of object of type %s because it's no longer biased)",
                    obj->klass()->external_name());
    }
    return BiasedLocking::NOT_BIASED;
  }

  uint age = mark->age();
  // 匿名偏向锁(101, 且线程ID为0)
  markOop   biased_prototype = markOopDesc::biased_locking_prototype()->set_age(age);
  // 无锁(001)
  markOop unbiased_prototype = markOopDesc::prototype()->set_age(age);

  if (TraceBiasedLocking && (Verbose || !is_bulk)) {
    ResourceMark rm;
    tty->print_cr("Revoking bias of object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s , prototype header " INTPTR_FORMAT " , allow rebias %d , requesting thread " INTPTR_FORMAT,
                  p2i((void *)obj), (intptr_t) mark, obj->klass()->external_name(), (intptr_t) obj->klass()->prototype_header(), (allow_rebias ? 1 : 0), (intptr_t) requesting_thread);
  }

  // 偏向的线程
  JavaThread* biased_thread = mark->biased_locker();
  if (biased_thread == NULL) {
    // 匿名偏向锁的情况,
    // Object is anonymously biased. We can get here if, for
    // example, we revoke the bias due to an identity hash code
    // being computed for an object.
    // 比如计算hashcode会走到这里
    if (!allow_rebias) {
      // 如果不允许重偏向, MarkWord变为无锁(001)
      // 批量重偏向时, allow_rebias==true
      obj->set_mark(unbiased_prototype);
    }
    if (TraceBiasedLocking && (Verbose || !is_bulk)) {
      tty->print_cr("  Revoked bias of anonymously-biased object");
    }
    // 返回偏向锁撤销
    return BiasedLocking::BIAS_REVOKED;
  }

  // Handle case where the thread toward which the object was biased has exited
  // 处理偏向的线程已经退出的情况
  // 判断线程是否存活, 以下两种情况认为存活:
  //     1. 偏向线程就是当前请求的线程
  //     2. 偏向线程存在于JVM活跃线程列表中, 需要遍历所有jvm线程
  bool thread_is_alive = false;
  if (requesting_thread == biased_thread) {
    thread_is_alive = true;
  } else {
    for (JavaThread* cur_thread = Threads::first(); cur_thread != NULL; cur_thread = cur_thread->next()) {
      if (cur_thread == biased_thread) {
        thread_is_alive = true;
        break;
      }
    }
  }
  if (!thread_is_alive) {
    // 偏向的线程死亡的情况
    if (allow_rebias) {
      // 如果需要重偏向, 设置为匿名偏向状态, 外面可以自己设置为偏向xx线程的
      obj->set_mark(biased_prototype);
    } else {
      // 否则, 设置为无锁状态
      obj->set_mark(unbiased_prototype);
    }
    if (TraceBiasedLocking && (Verbose || !is_bulk)) {
      tty->print_cr("  Revoked bias of object biased toward dead thread");
    }
    return BiasedLocking::BIAS_REVOKED;
  }

  // Thread owning bias is alive.
  // Check to see whether it currently owns the lock and, if so,
  // write down the needed displaced headers to the thread's stack.
  // Otherwise, restore the object's header either to the unlocked
  // or unbiased state.
  // 偏向的线程依然存活的情况
  // 获取偏向线程所有LockRecord(不仅仅是当前对象的, 是所有), 按时间倒序
  GrowableArray<MonitorInfo*>* cached_monitor_info = get_or_compute_monitor_info(biased_thread);
  BasicLock* highest_lock = NULL;
  // 遍历所有LockRecord
  for (int i = 0; i < cached_monitor_info->length(); i++) {
    MonitorInfo* mon_info = cached_monitor_info->at(i);
    if (mon_info->owner() == obj) {
      // LockRecord的持有者是当前对象
      // 说明能找到, 代码块肯定没退出
      // 此时只撤销是不行的了, 因为这个锁还要用, 并且原先的MarkWord还需要保留,
      // 需要升级为轻量级锁了
      if (TraceBiasedLocking && Verbose) {
        tty->print_cr("   mon_info->owner (" PTR_FORMAT ") == obj (" PTR_FORMAT ")",
                      p2i((void *) mon_info->owner()),
                      p2i((void *) obj));
      }
      // Assume recursive case and fix up highest lock later
      // 升级为轻量级锁, 且displaced_header为NULL, 表示重入的轻量级锁
      // 为什么是重入的轻量级锁?
      // 因为所有轻量级锁中, 只有第一个进来的displaced_header不为NULL,
      // 因为是按时间倒序的, 最先进来的LockRecord最后处理,
      // 等到循环退出时, highest_lock就是最先的LockRecord, 到时候再将它的NULL改掉就可以了(这里剧透了)
      markOop mark = markOopDesc::encode((BasicLock*) NULL);
      highest_lock = mon_info->lock();
      highest_lock->set_displaced_header(mark);
      // 从这里也可以看出来, 只有还在使用的LockRecord才有可能膨胀为轻量级锁
    } else {
      if (TraceBiasedLocking && Verbose) {
        tty->print_cr("   mon_info->owner (" PTR_FORMAT ") != obj (" PTR_FORMAT ")",
                      p2i((void *) mon_info->owner()),
                      p2i((void *) obj));
      }
    }
  }
  // 处理最先的LockRecord
  if (highest_lock != NULL) {
    // highest_lock存在, 说明代码块没有退出
    // Fix up highest lock to contain displaced header and point
    // object at it
    // 设置第一个LockRecord的DisplacedMarkWord为无锁状态(01)
    highest_lock->set_displaced_header(unbiased_prototype);
    // Reset object header to point to displaced mark.
    // Must release storing the lock address for platforms without TSO
    // ordering (e.g. ppc).
    // 让obj的mark指向这个LockRecord
    obj->release_set_mark(markOopDesc::encode(highest_lock));
    assert(!obj->mark()->has_bias_pattern(), "illegal mark state: stack lock used bias bit");
    if (TraceBiasedLocking && (Verbose || !is_bulk)) {
      tty->print_cr("  Revoked bias of currently-locked object");
    }
    // 到这里所有偏向锁都变成了轻量级锁
    // 也就是说, 如果当前锁还在使用, 并且有另一个线程想要加锁, 这个锁就会升级为轻量级锁
  } else {
    // 走到这里说明highest_lock没找到, 也就是synchronized(obj)偏向的线程的代码块退出了
    if (TraceBiasedLocking && (Verbose || !is_bulk)) {
      tty->print_cr("  Revoked bias of currently-unlocked object");
    }
    if (allow_rebias) {
        // 允许重偏向, MarkWord设置为匿名偏向状态
      obj->set_mark(biased_prototype);
    } else {
      // Store the unlocked value into the object's header.
      // 否则MarkWord设置为无锁状态
      obj->set_mark(unbiased_prototype);
    }
  }

  return BiasedLocking::BIAS_REVOKED;
}

BiasedLocking::Condition BiasedLocking::revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS) {
  // 不需要在安全点被调用
  assert(!SafepointSynchronize::is_at_safepoint(), "must not be called while at safepoint");

  ...

  // 如果走到这里说明上面CAS可能失败了,
  //
  // 或者偏向锁正偏向其他线程, 并且偏向的还不是当前线程,
  // 但偏向的线程不一定存活, 也就是说这个锁不一定被使用
  // 由此可见偏向锁的条件有多苛刻, 它只适用于同一个线程反复进入临界区的场景,
  // 就算这个线程退出了, 偏向的线程ID都不会改, 其他线程都不能直接用
  //
  // 这两种情况都会认为偏向锁不再适用, 需要撤销或者升级
  // 先思考下撤销和升级时需要做什么
  // 首先, 得找到对象持有的所有LockRecord, 然后修改,
  // 如果加锁的不是当前线程, 说明这是并发场景, 还得先等待加锁线程到安全点再操作
  // 这两个都是有成本的, 特别是等待安全点
  //
  // 如果重偏向或者撤销的次数多了, 开销很大
  // 于是就有了批量重偏向和批量撤销:
  // 当在一段时间内的撤销次数达到一定数量
  // 如果达到批量撤销的阈值(默认40),
  //    干脆直接不让用偏向锁了, 直接把这个对象类型的对象的锁全撤销为无锁, 如果还在用就升级为轻量级锁
  // 如果达到批量重偏向的阈值(默认20),
  //    会遍历所有线程找到所有在用的LockRecord, 修改其epoch, 然后尝试重偏向当前线程,
  //    其实批量重偏向过程中, 真正可能立即偏向的只有请求线程, 还只是可能, 并不是字面意思那样将大量锁偏向到了一个线程
  //    大部分工作只是更新了存活对象的epoch, 变相地将没标记的置为失效了, 给重偏向提高可能
  //    这样下次进入时, epoch失效的就可以当作偏向锁使用了.
  // 对经常撤销的偏向锁来说, 批量撤销和批量重偏向可以减少等待safepoint的次数, 提高效率

  // 批量重偏向和撤销逻辑

  // 先判断下怎么撤销或者重定向, 主要是批量重偏向和批量撤销的判断
  // update_heuristics会对撤销次数进行统计, 并给出后续的处理意见
  // 为什么前面的没有统计?
  // 因为前面偏向锁没用到(匿名偏向)或者失效(epoch过期),
  // 如果没有竞争, 替换成本就一个CAS, 可以忽略不计
  HeuristicsResult heuristics = update_heuristics(obj(), attempt_rebias);
  if (heuristics == HR_NOT_BIASED) {
    // 不是偏向锁, 不用撤销
    return NOT_BIASED;
  } else if (heuristics == HR_SINGLE_REVOKE) {
    // 单个撤销
    Klass *k = obj->klass();
    markOop prototype_header = k->prototype_header();
    if (mark->biased_locker() == THREAD &&
        prototype_header->bias_epoch() == mark->bias_epoch()) {
      // 只有当前线程在用这个偏向锁
      // A thread is trying to revoke the bias of an object biased
      // toward it, again likely due to an identity hash code
      // computation. We can again avoid a safepoint in this case
      // since we are only going to walk our own stack. There are no
      // races with revocations occurring in other threads because we
      // reach no safepoints in the revocation path.
      // Also check the epoch because even if threads match, another thread
      // can come in with a CAS to steal the bias of an object that has a
      // stale epoch.
      // 撤销当前线程的偏向锁, 判断时除了判断线程外, 还判断了epoch是否过期,
      // 因为epoch过期, 说明发生过批量重偏向, MarkWord中的线程ID是个无效值, 等于没有
      // 如果都是true就说明当前锁是一个在用的锁, 并且被当前线程持有
      //
      // 计算hashcode的时候会走到这里,
      // 而且这个时候发现没法将hashCode存在MarkWord中了(只有在匿名偏向的情况下可以存到MarkWord中, 上面处理了这种情况)
      // 需要先膨胀, 再找地方存hashCode
      //
      // 这里的撤销不用等到safepoint的, 因为只需要遍历当前线程的栈即可, 不需要等待所有线程都到达safepoint
      ResourceMark rm;
      if (TraceBiasedLocking) {
        tty->print_cr("Revoking bias by walking my own stack:");
      }
      // revoke_bias中, 可能会升级为轻量级锁(obj依然在使用), 也可能撤消(无锁)
      BiasedLocking::Condition cond = revoke_bias(obj(), false, false, (JavaThread*) THREAD);
      ((JavaThread*) THREAD)->set_cached_monitor_info(NULL);
      assert(cond == BIAS_REVOKED, "why not?");
      return cond;
    } else {
      // 走到这里说明当前偏向锁偏向的不是当前线程,
      // 或者当前偏向锁的MarkWord过期了,
      // 在存在并发时会这样,
      //
      // 所以, 这两种情况在当前线程中都不能直接对偏向锁做修改,
      // 需要等到一个稳定的状态再处理才保险, 也就是安全点
      //
      // 以下是通过VM线程去执行的
      // 等待到安全点, 然后调用revoke_bias
      VM_RevokeBias revoke(&obj, (JavaThread*) THREAD);
      VMThread::execute(&revoke);
      return revoke.status_code();
    }
  }

  assert((heuristics == HR_BULK_REVOKE) ||
         (heuristics == HR_BULK_REBIAS), "?");
  // 以下是通过VM线程执行的, 会等到安全点再执行
  // 批量撤销和批量重偏向的逻辑: bulk_revoke_or_rebias_at_safepoint
  VM_BulkRevokeBias bulk_revoke(&obj, (JavaThread*) THREAD,
                                (heuristics == HR_BULK_REBIAS),
                                attempt_rebias);
  VMThread::execute(&bulk_revoke);
  return bulk_revoke.status_code();
}

// 更新撤销计数, 判断是否需要批量重定向或批量撤消
static HeuristicsResult update_heuristics(oop o, bool allow_rebias) {
  markOop mark = o->mark();
  if (!mark->has_bias_pattern()) {
    // 如果不是偏向锁, 直接返回
    return HR_NOT_BIASED;
  }

  // Heuristics to attempt to throttle the number of revocations.
  // Stages:
  // 1. Revoke the biases of all objects in the heap of this type,
  //    but allow rebiasing of those objects if unlocked.
  // 2. Revoke the biases of all objects in the heap of this type
  //    and don't allow rebiasing of these objects. Disable
  //    allocation of objects of that type with the bias bit set.
  // 锁对象的类, 批量撤销是以类为单位的
  Klass* k = o->klass();
  // 当前时间
  jlong cur_time = os::javaTimeMillis();
  // clazz上次批量撤销的时间
  jlong last_bulk_revocation_time = k->last_biased_lock_bulk_revocation_time();
  // clazz偏向锁撤销次数
  int revocation_count = k->biased_lock_revocation_count();
  // BiasedLockingBulkRebiasThreshold是批量重偏向阈值
  // BiasedLockingBulkRevokeThreshold是批量撤销阈值
  // BiasedLockingDecayTime 偏向锁统计的衰退时间
  if ((revocation_count >= BiasedLockingBulkRebiasThreshold) &&
      (revocation_count <  BiasedLockingBulkRevokeThreshold) &&
      (last_bulk_revocation_time != 0) &&
      (cur_time - last_bulk_revocation_time >= BiasedLockingDecayTime)) {
    // 上面两个值的统计不是一直累加, 而是以BiasedLockingDecayTime为周期的
    // 如果发现撤销数量达到批量重定向阈值, 还没到批量撤销阈值, 但是已经超过BiasedLockingDecayTime了, 会将撤销次数清零
    // This is the first revocation we've seen in a while of an
    // object of this type since the last time we performed a bulk
    // rebiasing operation. The application is allocating objects in
    // bulk which are biased toward a thread and then handing them
    // off to another thread. We can cope with this allocation
    // pattern via the bulk rebiasing mechanism so we reset the
    // klass's revocation count rather than allow it to increase
    // monotonically. If we see the need to perform another bulk
    // rebias operation later, we will, and if subsequently we see
    // many more revocation operations in a short period of time we
    // will completely disable biasing for this type.
    k->set_biased_lock_revocation_count(0);
    revocation_count = 0;
  }

  // Make revocation count saturate just beyond BiasedLockingBulkRevokeThreshold
  // 撤销放次数+1
  if (revocation_count <= BiasedLockingBulkRevokeThreshold) {
    revocation_count = k->atomic_incr_biased_lock_revocation_count();
  }

  if (revocation_count == BiasedLockingBulkRevokeThreshold) {
    // 达到批量撤销阈值(40), 结果为批量撤销
    return HR_BULK_REVOKE;
  }

  if (revocation_count == BiasedLockingBulkRebiasThreshold) {
    // 达到批量重定向阈值(20), 结果批量重偏向
    return HR_BULK_REBIAS;
  }

  // 否则, 单个撤销就可以了
  return HR_SINGLE_REVOKE;
}

class VM_BulkRevokeBias : public VM_RevokeBias {
private:
  bool _bulk_rebias;
  bool _attempt_rebias_of_object;

public:
  VM_BulkRevokeBias(Handle* obj, JavaThread* requesting_thread,
                    bool bulk_rebias,
                    bool attempt_rebias_of_object)
    : VM_RevokeBias(obj, requesting_thread)
    , _bulk_rebias(bulk_rebias)
    , _attempt_rebias_of_object(attempt_rebias_of_object) {}

  virtual VMOp_Type type() const { return VMOp_BulkRevokeBias; }
  virtual bool doit_prologue()   { return true; }

  virtual void doit() {
    _status_code = bulk_revoke_or_rebias_at_safepoint((*_obj)(), _bulk_rebias, _attempt_rebias_of_object, _requesting_thread);
    clean_up_cached_monitor_info();
  }
};

// 批量撤销或者重偏向逻辑
static BiasedLocking::Condition bulk_revoke_or_rebias_at_safepoint(oop o,
                                                                   bool bulk_rebias,
                                                                   bool attempt_rebias_of_object,
                                                                   JavaThread* requesting_thread) {
  // 必须在safepoint执行
  assert(SafepointSynchronize::is_at_safepoint(), "must be done at safepoint");

  if (TraceBiasedLocking) {
    tty->print_cr("* Beginning bulk revocation (kind == %s) because of object "
                  INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
                  (bulk_rebias ? "rebias" : "revoke"),
                  p2i((void *) o), (intptr_t) o->mark(), o->klass()->external_name());
  }

  // 设置偏向锁批量撤销时间为当前时间
  jlong cur_time = os::javaTimeMillis();
  o->klass()->set_last_biased_lock_bulk_revocation_time(cur_time);


  Klass* k_o = o->klass();
  Klass* klass = k_o;

  if (bulk_rebias) {
    // 批量重偏向
    // Use the epoch in the klass of the object to implicitly revoke
    // all biases of objects of this data type and force them to be
    // reacquired. However, we also need to walk the stacks of all
    // threads and update the headers of lightweight locked objects
    // with biases to have the current epoch.
    // 使用obj的klass中的epoch来隐式地撤销所有这个类型的锁, 然后强制他们重新获取.
    // 这一过程中也会遍历所有线程的所有栈, 更新轻量级锁的LockRecord中的obj为当前epoch

    // If the prototype header doesn't have the bias pattern, don't
    // try to update the epoch -- assume another VM operation came in
    // and reset the header to the unbiased state, which will
    // implicitly cause all existing biases to be revoked
    // 如果klass不支持偏向锁, 不要更新epoch.
    // 因为这说明在另一个VM线程中将klass中的偏向锁标志置为0了, 置为0的同时已经批量撤销过了
    if (klass->prototype_header()->has_bias_pattern()) {
      int prev_epoch = klass->prototype_header()->bias_epoch();

      // klass中的epoch自增, epoch只有2位哦, 总共就4个值...
      klass->set_prototype_header(klass->prototype_header()->incr_bias_epoch());
      int cur_epoch = klass->prototype_header()->bias_epoch();

      // Now walk all threads' stacks and adjust epochs of any biased
      // and locked objects of this data type we encounter
      // 遍历所有线程的栈, 更新偏向锁的LockRecord的epoch
      for (JavaThread* thr = Threads::first(); thr != NULL; thr = thr->next()) {
        GrowableArray<MonitorInfo*>* cached_monitor_info = get_or_compute_monitor_info(thr);
        for (int i = 0; i < cached_monitor_info->length(); i++) {
          MonitorInfo* mon_info = cached_monitor_info->at(i);
          oop owner = mon_info->owner();
          markOop mark = owner->mark();
          if ((owner->klass() == k_o) && mark->has_bias_pattern()) {
            // 找到当前对象类型在栈中有偏向锁的对象
            // 更新他们的epoch,
            // 但不会重偏向, 因为这些锁正在使用中, 不能直接重偏向了
            // 从这里也能看出, 只会处理正在使用的LockRecord,
            // 如果不在使用, 所以每次monitorenter入口进来后都需要和klass的epoch比较, 看是否过期
            // We might have encountered this object already in the case of recursive locking
            assert(mark->bias_epoch() == prev_epoch || mark->bias_epoch() == cur_epoch, "error in bias epoch adjustment");
            owner->set_mark(mark->set_bias_epoch(cur_epoch));
          }
        }
      }
    }

    // At this point we're done. All we have to do is potentially
    // adjust the header of the given object to revoke its bias.
    // 当前obj的重偏向操作, 变为可重偏向的匿名重偏向状态
    revoke_bias(o, attempt_rebias_of_object && klass->prototype_header()->has_bias_pattern(), true, requesting_thread);
  } else {
    // 批量撤销逻辑
    if (TraceBiasedLocking) {
      ResourceMark rm;
      tty->print_cr("* Disabling biased locking for type %s", klass->external_name());
    }

    // Disable biased locking for this data type. Not only will this
    // cause future instances to not be biased, but existing biased
    // instances will notice that this implicitly caused their biases
    // to be revoked.
    // 设置klass的MarkWord为无锁的状态, 后面实例化的这个类型的对象的MarkWord都是无锁的
    // 如果要加锁, 得从轻量级锁开始加
    klass->set_prototype_header(markOopDesc::prototype());

    // Now walk all threads' stacks and forcibly revoke the biases of
    // any locked and biased objects of this data type we encounter.
    for (JavaThread* thr = Threads::first(); thr != NULL; thr = thr->next()) {
      GrowableArray<MonitorInfo*>* cached_monitor_info = get_or_compute_monitor_info(thr);
      for (int i = 0; i < cached_monitor_info->length(); i++) {
        MonitorInfo* mon_info = cached_monitor_info->at(i);
        oop owner = mon_info->owner();
        markOop mark = owner->mark();
        if ((owner->klass() == k_o) && mark->has_bias_pattern()) {
          // 找到该类型的偏向锁状态的对象
          // 撤销偏向锁, 参数中第二个参数表示不允许重偏向
          revoke_bias(owner, false, true, requesting_thread);
        }
      }
    }

    // Must force the bias of the passed object to be forcibly revoked
    // as well to ensure guarantees to callers
    // 撤销当前对象的偏向锁
    revoke_bias(o, false, true, requesting_thread);
  }

  if (TraceBiasedLocking) {
    tty->print_cr("* Ending bulk revocation");
  }

  BiasedLocking::Condition status_code = BiasedLocking::BIAS_REVOKED;

  // 如果需要重偏向, 再设置下obj的MarkWord, 偏向请求线程
  // bulk_rebias==false, attempt_rebias_of_object==true
  if (attempt_rebias_of_object &&
      o->mark()->has_bias_pattern() &&
      klass->prototype_header()->has_bias_pattern()) {
    // 如果希望尝试重偏向, 并且当前支持偏向锁
    // 构造一个偏向当前线程的偏向锁
    markOop new_mark = markOopDesc::encode(requesting_thread, o->mark()->age(),
                                           klass->prototype_header()->bias_epoch());
    o->set_mark(new_mark);
    status_code = BiasedLocking::BIAS_REVOKED_AND_REBIASED;
    if (TraceBiasedLocking) {
      tty->print_cr("  Rebiased object toward thread " INTPTR_FORMAT, (intptr_t) requesting_thread);
    }
  }

  assert(!o->mark()->has_bias_pattern() ||
         (attempt_rebias_of_object && (o->mark()->biased_locker() == requesting_thread)),
         "bug in bulk bias revocation");

  return status_code;
}

    ...
      CASE(_monitorenter): {
        // 获取对象
        oop lockee = STACK_OBJECT(-1);
        // derefing's lockee ought to provoke implicit null check
        CHECK_NULL(lockee);
        // find a free monitor or one already allocated for this object
        // if we find a matching object then we need a new monitor
        // since this is recursive enter
        // 找到一个可用的LockRecord,
        // 遍历栈中的LockRecord
        // base of monitors on the native stack
        BasicObjectLock* limit = istate->monitor_base();
        // base of monitors on the native stack
        BasicObjectLock* most_recent = (BasicObjectLock*) istate->stack_base();
        BasicObjectLock* entry = NULL;
        // 从栈底向栈顶遍历
        while (most_recent != limit ) {
          // obj为NULL代表可用,
          if (most_recent->obj() == NULL) entry = most_recent;
          else if (most_recent->obj() == lockee) break;  // 说明锁重入了, 取刚才最近的一个为空的LockRecord
          most_recent++;
        }
        // entry不为空, 说明存在可用的LockRecord
        if (entry != NULL) {
          // LockRecord指向当前对象
          entry->set_obj(lockee);
          int success = false;
          uintptr_t epoch_mask_in_place = (uintptr_t)markOopDesc::epoch_mask_in_place;
          // 获取MarkWord
          markOop mark = lockee->mark();
          intptr_t hash = (intptr_t) markOopDesc::no_hash;
...
          UPDATE_PC_AND_TOS_AND_CONTINUE(1, -1);
        } else {
          // 说明LockRecord没获取到,
          // 什么情况下获取不到, 跟本次无关, 先不看
          istate->set_msg(more_monitors);
          // 重新执行
          UPDATE_PC_AND_RETURN(0); // Re-execute
        }
      }
...

...
      CASE(_monitorenter): {
      ...
            else if ((anticipated_bias_locking_value & markOopDesc::biased_lock_mask_in_place) != 0) {
              // 偏向锁标志位不同, 而刚才的mark通过了偏向锁检查, 所以是klass中的偏向锁模式关闭
              // 没法继续使用偏向锁了, 需要撤销当前的偏向锁, 然后升级为轻量级锁或者重量级锁
              // try revoke bias
              // 尝试撤销偏向锁
              // 此时的lockee->klass()->prototype_header()应该是无锁状态的MarkWord
              markOop header = lockee->klass()->prototype_header();
              // 重置MarkWord, 如果有hash, 带上hash
              if (hash != markOopDesc::no_hash) {
                header = header->copy_set_hash(hash);
              }
              if (Atomic::cmpxchg_ptr(header, lockee->mark_addr(), mark) == mark) {
                // 修改成功
                // 这里修改为无锁的MarkWord可能是想提前修改, 后面少几次判断, 算是个优化吧
                if (PrintBiasedLockingStatistics)
                  (*BiasedLocking::revoked_lock_entry_count_addr())++;
              }
              // 后面的代码会继续走轻量级锁或者重量级锁的获取逻辑
    ...
          // traditional lightweight locking
          // 处理非偏向锁的情况
          if (!success) {
            // 准备使用轻量级锁或者重量级锁
            // 用当前的MarkWord构造一个无锁的MarkWord作为DisplacedHeader
            markOop displaced = lockee->mark()->set_unlocked();
            // 保存到LockRecord中, 用于解锁时MarkWord的还原
            entry->lock()->set_displaced_header(displaced);
            bool call_vm = UseHeavyMonitors;
            // 如果参数强制指定使用重量级锁,
            // 或者CAS将MarkWord从无锁状态设置为轻量级锁失败
            if (call_vm || Atomic::cmpxchg_ptr(entry, lockee->mark_addr(), displaced) != displaced) {
              // Is it simple recursive case?
              // 判断是否轻量级锁重入
              if (!call_vm && THREAD->is_lock_owned((address) displaced->clear_lock_bits())) {
                // 轻量级锁重入时, 重入的锁的DisplacedMarkWord为NULL
                entry->lock()->set_displaced_header(NULL);
              } else {
                // 膨胀
                CALL_VM(InterpreterRuntime::monitorenter(THREAD, entry), handle_exception);
              }
            }
          }
...

IRT_ENTRY_NO_ASYNC(void, InterpreterRuntime::monitorenter(JavaThread* thread, BasicObjectLock* elem))
#ifdef ASSERT
  thread->last_frame().interpreter_frame_verify_monitor(elem);
#endif
  ...
  // 如果允许偏向锁
  if (UseBiasedLocking) {
    ...
  } else {
    // 加轻量级锁
    ObjectSynchronizer::slow_enter(h_obj, elem->lock(), CHECK);
  }
  assert(Universe::heap()->is_in_reserved_or_null(elem->obj()),
         "must be NULL or an object");
#ifdef ASSERT
  thread->last_frame().interpreter_frame_verify_monitor(elem);
#endif
IRT_END

void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock, bool attempt_rebias, TRAPS) {
  ...
 if (UseBiasedLocking) {
    ...
 }

 // 很显然到这里, 偏向锁肯定被撤销了
 // 加轻量级锁
 slow_enter (obj, lock, THREAD) ;
}

// 加轻量级锁
void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
  // 获取Mark Word
  markOop mark = obj->mark();
  assert(!mark->has_bias_pattern(), "should not see bias pattern here");

  if (mark->is_neutral()) {
    // 无锁(001)的情况
    // Anticipate successful CAS -- the ST of the displaced mark must
    // be visible <= the ST performed by the CAS.
    // 尝试升级为轻量级锁
    lock->set_displaced_header(mark);
    if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {
      // 升级成功, 结束
      TEVENT (slow_enter: release stacklock) ;
      return ;
    }
    // 如果失败, 说明存在竞争, 膨胀为重量级锁
    // Fall through to inflate() ...
  } else
  if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
    // 轻量级锁重入的情况
    assert(lock != mark->locker(), "must not re-lock the same lock");
    assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");
    // 重入的轻量级锁，LockRecord的displaced_header会被置为NULL
    lock->set_displaced_header(NULL);
    return;
  }

  // The object header will never be displaced to this lock,
  // so it does not matter what the value is, except that it
  // must be non-zero to avoid looking like a re-entrant lock,
  // and must not look locked either.
  // 以上都失败, 轻量级锁膨胀为重量级锁
  //
  // 标记MarkWord为unused_mark(0b011),
  // 其实这里标记为11只是为了设置一个有别于轻量级锁的特殊值
  // 如果没改, 后面没法判断是轻量级锁重入还是轻量级锁膨胀
  lock->set_displaced_header(markOopDesc::unused_mark());
  // 调用重量级锁方法
  ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
}

      CASE(_monitorexit): {
        ...
        // 按enter时相同的顺序遍历LockRecord, 可以达到栈的效果
        while (most_recent != limit ) {
          if ((most_recent)->obj() == lockee) {
            // 找到当前对象持有的LockRecord
            BasicLock* lock = most_recent->lock();
            // 从LockRecord中取出刚开始的header
            markOop header = lock->displaced_header();
            // 解除关联, 偏向锁和重入的轻量级锁只需要这一步就够了
            most_recent->set_obj(NULL);
            if (!lockee->mark()->has_bias_pattern()) {
              // 当前不是偏向锁的情况, 可能之前膨胀了
              bool call_vm = UseHeavyMonitors;
              // If it isn't recursive we either must swap old header or call the runtime
              if (header != NULL || call_vm) {
                // header != NULL说明不是轻量级锁的重入, call_vm==true说明是强制使用重量级锁
                // 对于不是重入的轻量级锁, 从DisplacedMarkWord还原回去就行,
                //  如果失败, 说明存在竞争, 可能在某个地方膨胀了,
                //  需要走InterpreterRuntime::monitorexit
                // 对于call_vm==true, 也就是强制走重量级锁的情况, 直接InterpreterRuntime::monitorexit
                if (call_vm || Atomic::cmpxchg_ptr(header, lockee->mark_addr(), lock) != lock) {
                  ...
                }
              }
            }
            // 只需要找到一个就退出, 对于重入的情况肯定是后获取的锁先释放
            UPDATE_PC_AND_TOS_AND_CONTINUE(1, -1);
          }
          // 下一个LockRecord
          most_recent++;
        }