文章简介

前面我有文章介绍了synchronized的基本原理，这篇文章我会从jvm源码分析synchronized的实现逻辑，希望让大家有一个更加深度的认识

内容导航

1. 从synchronized的字节码说起
2. 什么是monitor
3. 分析synchronized的源码

从synchronized的字节码说起

由于synchronized的实现是在jvm层面，所以我们如果要看它的源码，需要从字节码入手。这段代码演示了synchronized作为实例锁的两种用法，我们观察一下这段代码生成的字节码

public class App 
{
    public synchronized void test1(){
    }
    public void test2(){
        synchronized (this){

        }
    }
    public static void main( String[] args ){
        System.out.println( "Hello World!" );
    }
}

进入classpath目录下找到App.class文件， 在cmd中输入 javap -v App.class查看字节码

public synchronized void test1();
    descriptor: ()V
    flags: ACC_PUBLIC, ACC_SYNCHRONIZED
    Code:
      stack=0, locals=1, args_size=1
         0: return
      LineNumberTable:
        line 10: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       1     0  this   Lcom/gupaoedu/openclass/App;

  public void test2();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=3, args_size=1
         0: aload_0
         1: dup
         2: astore_1
         3: monitorenter  //监视器进入，获取锁
         4: aload_1
         5: monitorexit  //监视器退出，释放锁
         6: goto          14
         9: astore_2
        10: aload_1
        11: monitorexit
        12: aload_2
        13: athrow
        14: return

通过字节码我们可以发现，修饰在方法层面的同步关键字，会多一个 ACC_SYNCHRONIZED的flag；修饰在代码块层面的同步块会多一个 monitorenter和 monitorexit关键字。无论采用哪一种方式，本质上都是对一个对象的监视器(monitor)进行获取，而这个获取的过程是排他的，也就是同一个时刻只能有一个线程获得同步块对象的监视器。
在 synchronized的原理分析这篇文章中，有提到对象监视器。

synchronized关键字经过编译之后，会在同步块的前后分别形成monitorenter和monitorexit这两个字节码指令。当我们的JVM把字节码加载到内存的时候，会对这两个指令进行解析。这两个字节码都需要一个Object类型的参数来指明要锁定和解锁的对象。如果Java程序中的synchronized明确指定了对象参数，那么这个对象就是加锁和解锁的对象；如果没有明确指定，那就根据synchronized修饰的是实例方法还是类方法，获取对应的对象实例或Class对象来作为锁对象

什么是monitor

在分析源代码之前需要了解oop, oopDesc, markOop等相关概念，在Synchronized的原理分析这篇文章中，我们讲到了synchronized的同步锁实际上是存储在对象头中，这个对象头是一个Java对象在内存中的布局的一部分。Java中的每一个Object在JVM内部都会有一个native的C++对象oop/oopDesc与之对应。在hotspot源码 oop.hpp中oopDesc的定义如下

class oopDesc {
  friend class VMStructs;
 private:
  volatile markOop  _mark;
  union _metadata {
    Klass*      _klass;
    narrowKlass _compressed_klass;
  } _metadata;

其中 markOop就是我们所说的Mark Word，用于存储锁的标识。
hotspot源码 markOop.hpp文件代码片段

class markOopDesc: public oopDesc {
 private:
  // Conversion
  uintptr_t value() const { return (uintptr_t) this; }

 public:
  // Constants
  enum { age_bits                 = 4,
         lock_bits                = 2,
         biased_lock_bits         = 1,
         max_hash_bits            = BitsPerWord - age_bits - lock_bits - biased_lock_bits,
         hash_bits                = max_hash_bits > 31 ? 31 : max_hash_bits,
         cms_bits                 = LP64_ONLY(1) NOT_LP64(0),
         epoch_bits               = 2
  };
  ...
}

markOopDesc继承自oopDesc，并且扩展了自己的monitor方法，这个方法返回一个ObjectMonitor指针对象，在hotspot虚拟机中，采用ObjectMonitor类来实现monitor

bool has_monitor() const {
    return ((value() & monitor_value) != 0);
  }
  ObjectMonitor* monitor() const {
    assert(has_monitor(), "check");
    // Use xor instead of &~ to provide one extra tag-bit check.
    return (ObjectMonitor*) (value() ^ monitor_value);
  }

在 ObjectMonitor.hpp中，可以看到ObjectMonitor的定义

class ObjectMonitor {
...
  ObjectMonitor() {
    _header       = NULL; //markOop对象头
    _count        = 0;    
    _waiters      = 0,   //等待线程数
    _recursions   = 0;   //重入次数
    _object       = NULL;  
    _owner        = NULL;  //获得ObjectMonitor对象的线程
    _WaitSet      = NULL;  //处于wait状态的线程，会被加入到waitSet
    _WaitSetLock  = 0 ; 
    _Responsible  = NULL ;
    _succ         = NULL ;
    _cxq          = NULL ;
    FreeNext      = NULL ;
    _EntryList    = NULL ; //处于等待锁BLOCKED状态的线程
    _SpinFreq     = 0 ;   
    _SpinClock    = 0 ;
    OwnerIsThread = 0 ; 
    _previous_owner_tid = 0; //监视器前一个拥有线程的ID
  }
...

简单总结一下，同步块的实现使用 monitorenter和 monitorexit指令，而同步方法是依靠方法修饰符上的flag ACC_SYNCHRONIZED来完成。其本质是对一个对象监视器(monitor)进行获取，这个获取过程是排他的，也就是同一个时刻只能有一个线程获得由synchronized所保护对象的监视器。所谓的监视器，实际上可以理解为一个同步工具，它是由Java对象进行描述的。在Hotspot中，是通过ObjectMonitor来实现，每个对象中都会内置一个ObjectMonitor对象

简单分析synchronized的源码

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

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

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

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

UseBiasedLocking是在JVM启动的时候，是否启动偏向锁的标识

1. 如果支持偏向锁,则执行 ObjectSynchronizer::fast_enter的逻辑
2. 如果不支持偏向锁,则执行 ObjectSynchronizer::slow_enter逻辑，绕过偏向锁，直接进入轻量级锁

ObjectSynchronizer::fast_enter的实现在 synchronizer.cpp文件中，代码如下

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

 slow_enter (obj, lock, THREAD) ;
}

fast_enter方法的主要流程做一个简单的解释

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

偏向锁的获取逻辑

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

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

偏向锁的撤销

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

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

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

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

轻量级锁的获取逻辑

轻量级锁的获取，是调用 ::slow_enter方法，该方法同样位于 synchronizer.cpp文件中

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

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

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

轻量级锁的获取逻辑简单再整理一下

1. mark->is_neutral()方法, is_neutral这个方法是在 markOop.hpp中定义，如果 biased_lock:0且lock:01表示无锁状态
2. 如果mark处于无锁状态，则进入步骤(3)，否则执行步骤(5)
3. 把mark保存到BasicLock对象的displacedheader字段
4. 通过CAS尝试将markword更新为指向BasicLock对象的指针，如果更新成功，表示竞争到锁，则执行同步代码，否则执行步骤(5)
5. 如果当前mark处于加锁状态，且mark中的ptr指针指向当前线程的栈帧，则执行同步代码，否则说明有多个线程竞争轻量级锁，轻量级锁需要膨胀升级为重量级锁

轻量级锁的释放逻辑

轻量级锁的释放是通过 monitorexit调用

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

这段代码中主要是通过 ObjectSynchronizer::slow_exit来执行

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

ObjectSynchronizer::fast_exit的代码如下

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

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

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

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

锁膨胀的过程分析

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

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

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

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

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

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

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

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


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


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


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

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

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

1. mark->has_monitor() 判断如果当前锁对象为重量级锁，也就是lock:10，则执行(2),否则执行(3)
2. 通过 mark->monitor获得重量级锁的对象监视器ObjectMonitor并返回，锁膨胀过程结束
3. 如果当前锁处于 INFLATING,说明有其他线程在执行锁膨胀，那么当前线程通过自旋等待其他线程锁膨胀完成
4. 如果当前是轻量级锁状态 mark->has_locker(),则进行锁膨胀。首先，通过omAlloc方法获得一个可用的ObjectMonitor，并设置初始数据；然后通过CAS将对象头设置为`markOopDesc:INFLATING，表示当前锁正在膨胀，如果CAS失败，继续自旋
5. 如果是无锁状态，逻辑类似第4步骤

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

重量级锁的竞争逻辑

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

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

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

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

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

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

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

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

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

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

  EventJavaMonitorEnter event;

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

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

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

    Self->set_current_pending_monitor(this);

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

      EnterI (THREAD) ;

      if (!ExitSuspendEquivalent(jt)) break ;

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

重量级锁的释放

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

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

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

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

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

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

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

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

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

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

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

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

分析源码，需要很大的耐心，希望大家能有耐心看下去，有疑问欢迎微信留言