CASE(_monitorenter): {
  // lockee 就是锁对象
  oop lockee = STACK_OBJECT(-1);
  // derefing's lockee ought to provoke implicit null check
  CHECK_NULL(lockee);
  // code 1：找到一个空闲的Lock Record
  BasicObjectLock* limit = istate->monitor_base();
  BasicObjectLock* most_recent = (BasicObjectLock*) istate->stack_base();
  BasicObjectLock* entry = NULL;
  while (most_recent != limit ) {
    if (most_recent->obj() == NULL) entry = most_recent;
    else if (most_recent->obj() == lockee) break;
    most_recent++;
  }
  //entry不为null，代表还有空闲的Lock Record
  if (entry != NULL) {
    // code 2：将Lock Record的obj指针指向锁对象
    entry->set_obj(lockee);
    int success = false;
    uintptr_t epoch_mask_in_place = (uintptr_t)markOopDesc::epoch_mask_in_place;
	// markoop即对象头的mark word
    markOop mark = lockee->mark();
    intptr_t hash = (intptr_t) markOopDesc::no_hash;
    // code 3：如果锁对象的mark word的状态是偏向模式
    if (mark->has_bias_pattern()) {
      uintptr_t thread_ident;
      uintptr_t anticipated_bias_locking_value;
      thread_ident = (uintptr_t)istate->thread();
     // code 4：这里有几步操作，下文分析
      anticipated_bias_locking_value =
        (((uintptr_t)lockee->klass()->prototype_header() | thread_ident) ^ (uintptr_t)mark) &
        ~((uintptr_t) markOopDesc::age_mask_in_place);
	 // code 5：如果偏向的线程是自己且epoch等于class的epoch
      if  (anticipated_bias_locking_value == 0) {
        // already biased towards this thread, nothing to do
        if (PrintBiasedLockingStatistics) {
          (* BiasedLocking::biased_lock_entry_count_addr())++;
        }
        success = true;
      }
       // code 6：如果偏向模式关闭，则尝试撤销偏向锁
      else if ((anticipated_bias_locking_value & markOopDesc::biased_lock_mask_in_place) != 0) {
        markOop header = lockee->klass()->prototype_header();
        if (hash != markOopDesc::no_hash) {
          header = header->copy_set_hash(hash);
        }
        // 利用CAS操作将mark word替换为class中的mark word
        if (Atomic::cmpxchg_ptr(header, lockee->mark_addr(), mark) == mark) {
          if (PrintBiasedLockingStatistics)
            (*BiasedLocking::revoked_lock_entry_count_addr())++;
        }
      }
         // code 7：如果epoch不等于class中的epoch，则尝试重偏向
      else if ((anticipated_bias_locking_value & epoch_mask_in_place) !=0) {
        // 构造一个偏向当前线程的mark word
        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);
        }
        // CAS替换对象头的mark word  
        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，代表存在多线程竞争，则调用monitorenter方法进行锁升级
          CALL_VM(InterpreterRuntime::monitorenter(THREAD, entry), handle_exception);
        }
        success = true;
      }
      else {
         // 走到这里说明当前要么偏向别的线程，要么是匿名偏向（即没有偏向任何线程）
       	// code 8：下面构建一个匿名偏向的mark word，尝试用CAS指令替换掉锁对象的mark word
        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));
        if (hash != markOopDesc::no_hash) {
          header = header->copy_set_hash(hash);
        }
        markOop new_header = (markOop) ((uintptr_t) header | thread_ident);
        // debugging hint
        DEBUG_ONLY(entry->lock()->set_displaced_header((markOop) (uintptr_t) 0xdeaddead);)
        if (Atomic::cmpxchg_ptr((void*)new_header, lockee->mark_addr(), header) == header) {
           // CAS修改成功
          if (PrintBiasedLockingStatistics)
            (* BiasedLocking::anonymously_biased_lock_entry_count_addr())++;
        }
        else {
          // 如果修改失败说明存在多线程竞争，所以进入monitorenter方法
          CALL_VM(InterpreterRuntime::monitorenter(THREAD, entry), handle_exception);
        }
        success = true;
      }
    }

    // 如果偏向线程不是当前线程或没有开启偏向模式等原因都会导致success==false
    if (!success) {
      // 轻量级锁的逻辑
      //code 9: 构造一个无锁状态的Displaced Mark Word，并将Lock Record的lock指向它
      markOop displaced = lockee->mark()->set_unlocked();
      entry->lock()->set_displaced_header(displaced);
      //如果指定了-XX:+UseHeavyMonitors，则call_vm=true，代表禁用偏向锁和轻量级锁
      bool call_vm = UseHeavyMonitors;
      // 利用CAS将对象头的mark word替换为指向Lock Record的指针
      if (call_vm || Atomic::cmpxchg_ptr(entry, lockee->mark_addr(), displaced) != displaced) {
        // 判断是不是锁重入
        if (!call_vm && THREAD->is_lock_owned((address) displaced->clear_lock_bits())) {		//code 10: 如果是锁重入，则直接将Displaced Mark Word设置为null
          entry->lock()->set_displaced_header(NULL);
        } else {
          CALL_VM(InterpreterRuntime::monitorenter(THREAD, entry), handle_exception);
        }
      }
    }
    UPDATE_PC_AND_TOS_AND_CONTINUE(1, -1);
  } else {
    // lock record不够，重新执行
    istate->set_msg(more_monitors);
    UPDATE_PC_AND_RETURN(0); // Re-execute
  }
}

IRT_ENTRY_NO_ASYNC(void, InterpreterRuntime::monitorenter(JavaThread* thread, BasicObjectLock* elem))
  ...
  Handle h_obj(thread, elem->obj());
  assert(Universe::heap()->is_in_reserved_or_null(h_obj()),
         "must be NULL or an object");
  if (UseBiasedLocking) {
    // Retry fast entry if bias is revoked to avoid unnecessary inflation
    ObjectSynchronizer::fast_enter(h_obj, elem->lock(), true, CHECK);
  } else {
    ObjectSynchronizer::slow_enter(h_obj, elem->lock(), CHECK);
  }
  ...
IRT_END

void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock, bool attempt_rebias, TRAPS) {
 if (UseBiasedLocking) {
    if (!SafepointSynchronize::is_at_safepoint()) {
      BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
      if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {
        return;
      }
    } else {
      assert(!attempt_rebias, "can not rebias toward VM thread");
      BiasedLocking::revoke_at_safepoint(obj);
    }
    assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 }

 slow_enter (obj, lock, THREAD) ;
}

BiasedLocking::Condition BiasedLocking::revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS) {
  assert(!SafepointSynchronize::is_at_safepoint(), "must not be called while at safepoint");
    
  markOop mark = obj->mark();
  if (mark->is_biased_anonymously() && !attempt_rebias) {
     //如果是匿名偏向且attempt_rebias==false会走到这里，如锁对象的hashcode方法被调用会出现这种情况，需要撤销偏向锁。
    markOop biased_value       = mark;
    markOop unbiased_prototype = markOopDesc::prototype()->set_age(mark->age());
    markOop res_mark = (markOop) Atomic::cmpxchg_ptr(unbiased_prototype, obj->mark_addr(), mark);
    if (res_mark == biased_value) {
      return BIAS_REVOKED;
    }
  } else if (mark->has_bias_pattern()) {
    // 锁对象开启了偏向模式会走到这里
    Klass* k = obj->klass();
    markOop prototype_header = k->prototype_header();
    //code 1： 如果对应class关闭了偏向模式
    if (!prototype_header->has_bias_pattern()) {
      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;
    //code2： 如果epoch过期
    } else if (prototype_header->bias_epoch() != mark->bias_epoch()) {
      if (attempt_rebias) {
        assert(THREAD->is_Java_thread(), "");
        markOop biased_value       = mark;
        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) {
          return BIAS_REVOKED_AND_REBIASED;
        }
      } else {
        markOop biased_value       = mark;
        markOop unbiased_prototype = markOopDesc::prototype()->set_age(mark->age());
        markOop res_mark = (markOop) Atomic::cmpxchg_ptr(unbiased_prototype, obj->mark_addr(), mark);
        if (res_mark == biased_value) {
          return BIAS_REVOKED;
        }
      }
    }
  }
  //code 3：批量重偏向与批量撤销的逻辑
  HeuristicsResult heuristics = update_heuristics(obj(), attempt_rebias);
  if (heuristics == HR_NOT_BIASED) {
    return NOT_BIASED;
  } else if (heuristics == HR_SINGLE_REVOKE) {
    //code 4：撤销单个线程
    Klass *k = obj->klass();
    markOop prototype_header = k->prototype_header();
    if (mark->biased_locker() == THREAD &&
        prototype_header->bias_epoch() == mark->bias_epoch()) {
      // 走到这里说明需要撤销的是偏向当前线程的锁，当调用Object#hashcode方法时会走到这一步
      // 因为只要遍历当前线程的栈就好了，所以不需要等到safepoint再撤销。
      ResourceMark rm;
      if (TraceBiasedLocking) {
        tty->print_cr("Revoking bias by walking my own stack:");
      }
      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 {
      // 下面代码最终会在VM线程中的safepoint调用revoke_bias方法
      VM_RevokeBias revoke(&obj, (JavaThread*) THREAD);
      VMThread::execute(&revoke);
      return revoke.status_code();
    }
  }
	
  assert((heuristics == HR_BULK_REVOKE) ||
         (heuristics == HR_BULK_REBIAS), "?");
   //code5：批量撤销、批量重偏向的逻辑
  VM_BulkRevokeBias bulk_revoke(&obj, (JavaThread*) THREAD,
                                (heuristics == HR_BULK_REBIAS),
                                attempt_rebias);
  VMThread::execute(&bulk_revoke);
  return bulk_revoke.status_code();
}

static BiasedLocking::Condition revoke_bias(oop obj, bool allow_rebias, bool is_bulk, JavaThread* requesting_thread) {
  markOop mark = obj->mark();
  // 如果没有开启偏向模式，则直接返回NOT_BIASED
  if (!mark->has_bias_pattern()) {
    ...
    return BiasedLocking::NOT_BIASED;
  }

  uint age = mark->age();
  // 构建两个mark word，一个是匿名偏向模式（101），一个是无锁模式（001）
  markOop   biased_prototype = markOopDesc::biased_locking_prototype()->set_age(age);
  markOop unbiased_prototype = markOopDesc::prototype()->set_age(age);

  ...

  JavaThread* biased_thread = mark->biased_locker();
  if (biased_thread == NULL) {
     // 匿名偏向。当调用锁对象的hashcode()方法可能会导致走到这个逻辑
     // 如果不允许重偏向，则将对象的mark word设置为无锁模式
    if (!allow_rebias) {
      obj->set_mark(unbiased_prototype);
    }
    ...
    return BiasedLocking::BIAS_REVOKED;
  }

  // code 1：判断偏向线程是否还存活
  bool thread_is_alive = false;
  // 如果当前线程就是偏向线程 
  if (requesting_thread == biased_thread) {
    thread_is_alive = true;
  } else {
     // 遍历当前jvm的所有线程，如果能找到，则说明偏向的线程还存活
    for (JavaThread* cur_thread = Threads::first(); cur_thread != NULL; cur_thread = cur_thread->next()) {
      if (cur_thread == biased_thread) {
        thread_is_alive = true;
        break;
      }
    }
  }
  // 如果偏向的线程已经不存活了
  if (!thread_is_alive) {
    // 允许重偏向则将对象mark word设置为匿名偏向状态，否则设置为无锁状态
    if (allow_rebias) {
      obj->set_mark(biased_prototype);
    } else {
      obj->set_mark(unbiased_prototype);
    }
    ...
    return BiasedLocking::BIAS_REVOKED;
  }

  // 线程还存活则遍历线程栈中所有的Lock Record
  GrowableArray<MonitorInfo*>* cached_monitor_info = get_or_compute_monitor_info(biased_thread);
  BasicLock* highest_lock = NULL;
  for (int i = 0; i < cached_monitor_info->length(); i++) {
    MonitorInfo* mon_info = cached_monitor_info->at(i);
    // 如果能找到对应的Lock Record说明偏向的线程还在执行同步代码块中的代码
    if (mon_info->owner() == obj) {
      ...
      // 需要升级为轻量级锁，直接修改偏向线程栈中的Lock Record。为了处理锁重入的case，在这里将Lock Record的Displaced Mark Word设置为null，第一个Lock Record会在下面的代码中再处理
      markOop mark = markOopDesc::encode((BasicLock*) NULL);
      highest_lock = mon_info->lock();
      highest_lock->set_displaced_header(mark);
    } else {
      ...
    }
  }
  if (highest_lock != NULL) {
    // 修改第一个Lock Record为无锁状态，然后将obj的mark word设置为指向该Lock Record的指针
    highest_lock->set_displaced_header(unbiased_prototype);
    obj->release_set_mark(markOopDesc::encode(highest_lock));
    ...
  } else {
    // 走到这里说明偏向线程已经不在同步块中了
    ...
    if (allow_rebias) {
       //设置为匿名偏向状态
      obj->set_mark(biased_prototype);
    } else {
      // 将mark word设置为无锁状态
      obj->set_mark(unbiased_prototype);
    }
  }

  return BiasedLocking::BIAS_REVOKED;
}

CASE(_monitorexit): {
  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();
  BasicObjectLock* most_recent = (BasicObjectLock*) istate->stack_base();
  // 从低往高遍历栈的Lock Record
  while (most_recent != limit ) {
    // 如果Lock Record关联的是该锁对象
    if ((most_recent)->obj() == lockee) {
      BasicLock* lock = most_recent->lock();
      markOop header = lock->displaced_header();
      // 释放Lock Record
      most_recent->set_obj(NULL);
      // 如果是偏向模式，仅仅释放Lock Record就好了。否则要走轻量级锁or重量级锁的释放流程
      if (!lockee->mark()->has_bias_pattern()) {
        bool call_vm = UseHeavyMonitors;
        // header!=NULL说明不是重入，则需要将Displaced Mark Word CAS到对象头的Mark Word
        if (header != NULL || call_vm) {
          if (call_vm || Atomic::cmpxchg_ptr(header, lockee->mark_addr(), lock) != lock) {
            // CAS失败或者是重量级锁则会走到这里，先将obj还原，然后调用monitorexit方法
            most_recent->set_obj(lockee);
            CALL_VM(InterpreterRuntime::monitorexit(THREAD, most_recent), handle_exception);
          }
        }
      }
      //执行下一条命令
      UPDATE_PC_AND_TOS_AND_CONTINUE(1, -1);
    }
    //处理下一条Lock Record
    most_recent++;
  }
  // Need to throw illegal monitor state exception
  CALL_VM(InterpreterRuntime::throw_illegal_monitor_state_exception(THREAD), handle_exception);
  ShouldNotReachHere();
}

BiasedLocking::Condition BiasedLocking::revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS) {
  ...
  //code 1：重偏向的逻辑
  HeuristicsResult heuristics = update_heuristics(obj(), attempt_rebias);
  // 非重偏向的逻辑
  ...
      
  assert((heuristics == HR_BULK_REVOKE) ||
         (heuristics == HR_BULK_REBIAS), "?");	
   //code 2：批量撤销、批量重偏向的逻辑
  VM_BulkRevokeBias bulk_revoke(&obj, (JavaThread*) THREAD,
                                (heuristics == HR_BULK_REBIAS),
                                attempt_rebias);
  VMThread::execute(&bulk_revoke);
  return bulk_revoke.status_code();
}

static HeuristicsResult update_heuristics(oop o, bool allow_rebias) {
  markOop mark = o->mark();
  //如果不是偏向模式直接返回
  if (!mark->has_bias_pattern()) {
    return HR_NOT_BIASED;
  }
 
  // 锁对象的类
  Klass* k = o->klass();
  // 当前时间
  jlong cur_time = os::javaTimeMillis();
  // 该类上一次批量撤销的时间
  jlong last_bulk_revocation_time = k->last_biased_lock_bulk_revocation_time();
  // 该类偏向锁撤销的次数
  int revocation_count = k->biased_lock_revocation_count();
  // BiasedLockingBulkRebiasThreshold是重偏向阈值（默认20），BiasedLockingBulkRevokeThreshold是批量撤销阈值（默认40），BiasedLockingDecayTime是开启一次新的批量重偏向距离上次批量重偏向的后的延迟时间，默认25000。也就是开启批量重偏向后，经过了一段较长的时间（>=BiasedLockingDecayTime），撤销计数器才超过阈值，那我们会重置计数器。
  if ((revocation_count >= BiasedLockingBulkRebiasThreshold) &&
      (revocation_count <  BiasedLockingBulkRevokeThreshold) &&
      (last_bulk_revocation_time != 0) &&
      (cur_time - last_bulk_revocation_time >= BiasedLockingDecayTime)) {
    // 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;
  }

  // 自增撤销计数器
  if (revocation_count <= BiasedLockingBulkRevokeThreshold) {
    revocation_count = k->atomic_incr_biased_lock_revocation_count();
  }
  // 如果达到批量撤销阈值则返回HR_BULK_REVOKE
  if (revocation_count == BiasedLockingBulkRevokeThreshold) {
    return HR_BULK_REVOKE;
  }
  // 如果达到批量重偏向阈值则返回HR_BULK_REBIAS
  if (revocation_count == BiasedLockingBulkRebiasThreshold) {
    return HR_BULK_REBIAS;
  }
  // 没有达到阈值则撤销单个对象的锁
  return HR_SINGLE_REVOKE;
}

static BiasedLocking::Condition bulk_revoke_or_rebias_at_safepoint(oop o,
                                                                   bool bulk_rebias,
                                                                   bool attempt_rebias_of_object,
                                                                   JavaThread* requesting_thread) {
  ...
  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) {
    // 批量重偏向的逻辑
    if (klass->prototype_header()->has_bias_pattern()) {
      // 自增前类中的的epoch
      int prev_epoch = klass->prototype_header()->bias_epoch();
      // code 1：类中的epoch自增
      klass->set_prototype_header(klass->prototype_header()->incr_bias_epoch());
      int cur_epoch = klass->prototype_header()->bias_epoch();

      // code 2：遍历所有线程的栈，更新类型为该klass的所有锁实例的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()) {
            // 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));
          }
        }
      }
    }

    // 接下来对当前锁对象进行重偏向
    revoke_bias(o, attempt_rebias_of_object && klass->prototype_header()->has_bias_pattern(), true, requesting_thread);
  } else {
    ...

    // code 3：批量撤销的逻辑，将类中的偏向标记关闭，markOopDesc::prototype()返回的是一个关闭偏向模式的prototype
    klass->set_prototype_header(markOopDesc::prototype());

    // code 4：遍历所有线程的栈，撤销该类所有锁的偏向
    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()) {
          revoke_bias(owner, false, true, requesting_thread);
        }
      }
    }

    // 撤销当前锁对象的偏向模式
    revoke_bias(o, false, true, requesting_thread);
  }

  ...
  
  BiasedLocking::Condition status_code = BiasedLocking::BIAS_REVOKED;

  if (attempt_rebias_of_object &&
      o->mark()->has_bias_pattern() &&
      klass->prototype_header()->has_bias_pattern()) {
    // 构造一个偏向请求线程的mark word
    markOop new_mark = markOopDesc::encode(requesting_thread, o->mark()->age(),
                                           klass->prototype_header()->bias_epoch());
    // 更新当前锁对象的mark word
    o->set_mark(new_mark);
    status_code = BiasedLocking::BIAS_REVOKED_AND_REBIASED;
    ...
  }

  ...

  return status_code;
}