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jdk源码剖析二: 对象内存布局、synchronized终极原理

Posted on 2017-04-20 19:00  只会一点java  阅读(16311)  评论(33编辑  收藏  举报

 很多人一提到锁,自然第一个想到了synchronized,但一直不懂源码实现,现特地追踪到C++层来剥开synchronized的面纱。

网上的很多描述大都不全,让人看了不够爽,看完本章,你将彻底了解synchronized的核心原理。

一、启蒙知识预热

开启本文之前先介绍2个概念

1.1.cas操作

为了提高性能,JVM很多操作都依赖CAS实现,一种乐观锁的实现。本文锁优化中大量用到了CAS,故有必要先分析一下CAS的实现。

CAS:Compare and Swap。

JNI来完成CPU指令的操作:

unsafe.compareAndSwapInt(this, valueOffset, expect, update);

CAS有3个操作数,内存值V,旧的预期值A,要修改的新值B。如果A=V,那么把B赋值给V,返回V;如果A!=V,直接返回V。

打开源码:openjdk\hotspot\src\oscpu\windowsx86\vm\ atomicwindowsx86.inline.hpp,如下图:0

os::is_MP()  这个是runtime/os.hpp,实际就是返回是否多处理器,源码如下:

 

如上面源代码所示(看第一个int参数即可),LOCK_IF_MP:会根据当前处理器的类型来决定是否为cmpxchg指令添加lock前缀。如果程序是在多处理器上运行,就为cmpxchg指令加上lock前缀(lock cmpxchg)。反之,如果程序是在单处理器上运行,就省略lock前缀(单处理器自身会维护单处理器内的顺序一致性,不需要lock前缀提供的内存屏障效果)。

 

1.2.对象头

HotSpot虚拟机中,对象在内存中存储的布局可以分为三块区域:对象头(Header)、实例数据(Instance Data)和对齐填充(Padding)。

HotSpot虚拟机的对象头(Object Header)包括两部分信息:

第一部分"Mark Word":用于存储对象自身的运行时数据, 如哈希码(HashCode)、GC分代年龄、锁状态标志、线程持有的锁、偏向线程ID、偏向时间戳等等.

第二部分"Klass Pointer":对象指向它的类的元数据的指针,虚拟机通过这个指针来确定这个对象是哪个类的实例。(数组,对象头中还必须有一块用于记录数组长度的数据,因为虚拟机可以通过普通Java对象的元数据信息确定Java对象的大小,但是从数组的元数据中无法确定数组的大小。 )

32位的HotSpot虚拟机对象头存储结构:(下图摘自网络)

                                                              图1 32位的HotSpot虚拟机对象头Mark Word组成

为了证实上图的正确性,这里我们看openJDK--》hotspot源码markOop.hpp,虚拟机对象头存储结构:

                                             图2 HotSpot源码markOop.hpp中注释

单词解释:

hash: 保存对象的哈希码
age: 保存对象的分代年龄
biased_lock: 偏向锁标识位
lock: 锁状态标识位
JavaThread*: 保存持有偏向锁的线程ID
epoch: 保存偏向时间戳

上图中有源码中对锁标志位这样枚举

1 enum {   locked_value             = 0,//00 轻量级锁
2          unlocked_value           = 1,//01 无锁
3          monitor_value            = 2,//10 监视器锁,也叫膨胀锁,也叫重量级锁
4          marked_value             = 3,//11 GC标记
5          biased_lock_pattern      = 5 //101 偏向锁
6   };

下面是源码注释:

                                        图3 HotSpot源码markOop.hpp中锁标志位注释

看图3,不管是32/64位JVM,都是1bit偏向锁+2bit锁标志位。上面红框是偏向锁(第一行是指向线程的显示偏向锁,第二行是匿名偏向锁)对应枚举biased_lock_pattern,下面红框是轻量级锁、无锁、监视器锁、GC标记,分别对应上面的前4种枚举。我们甚至能看见锁标志11时,是GC的markSweep(标记清除算法)使用的。(这里就不再拓展了)

对象头中的Mark Word,synchronized源码实现就用了Mark Word来标识对象加锁状态。

二、JVM中synchronized锁实现原理(优化)

大家都知道java中锁synchronized性能较差,线程会阻塞。本节将以图文形式来描述JVM的synchronized锁优化。

在jdk1.6中对锁的实现引入了大量的优化来减少锁操作的开销:

锁粗化(Lock Coarsening):将多个连续的锁扩展成一个范围更大的锁,用以减少频繁互斥同步导致的性能损耗。

锁消除(Lock Elimination):JVM及时编译器在运行时,通过逃逸分析,如果判断一段代码中,堆上的所有数据不会逃逸出去从来被其他线程访问到,就可以去除这些锁。

轻量级锁(Lightweight Locking):JDK1.6引入。在没有多线程竞争的情况下避免重量级互斥锁,只需要依靠一条CAS原子指令就可以完成锁的获取及释放。

偏向锁(Biased Locking):JDK1.6引入。目的是消除数据再无竞争情况下的同步原语。使用CAS记录获取它的线程。下一次同一个线程进入则偏向该线程,无需任何同步操作。

适应性自旋(Adaptive Spinning):为了避免线程频繁挂起、恢复的状态切换消耗。产生了忙循环(循环时间固定),即自旋。JDK1.6引入了自适应自旋。自旋时间根据之前锁自旋时间和线程状态,动态变化,用以期望能减少阻塞的时间。

 锁升级:偏向锁--》轻量级锁--》重量级锁

2.1.偏向锁

  按照之前的HotSpot设计,每次加锁/解锁都会涉及到一些CAS操作(比如对等待队列的CAS操作),CAS操作会延迟本地调用,因此偏向锁的想法是一旦线程第一次获得了监视对象,之后让监视对象“偏向”这个线程,之后的多次调用则可以避免CAS操作。
  简单的讲,就是在锁对象的对象头(开篇讲的对象头数据存储结构)中有个ThreaddId字段,这个字段如果是空的,第一次获取锁的时候,就将自身的ThreadId写入到锁的ThreadId字段内,将锁头内的是否偏向锁的状态位置1.这样下次获取锁的时候,直接检查ThreadId是否和自身线程Id一致,如果一致,则认为当前线程已经获取了锁,因此不需再次获取锁,略过了轻量级锁和重量级锁的加锁阶段。提高了效率。
注意:当锁有竞争关系的时候,需要解除偏向锁,进入轻量级锁。

每一个线程在准备获取共享资源时:

第一步,检查MarkWord里面是不是放的自己的ThreadId ,如果是,表示当前线程是处于 “偏向锁”.跳过轻量级锁直接执行同步体。

获得偏向锁如下图:

 

2.2.轻量级锁和重量级锁

如上图所示:

第二步,如果MarkWord不是自己的ThreadId,锁升级,这时候,用CAS来执行切换,新的线程根据MarkWord里面现有的ThreadId,通知之前线程暂停,之前线程将Markword的内容置为空。

第三步,两个线程都把对象的HashCode复制到自己新建的用于存储锁的记录空间,接着开始通过CAS操作,把共享对象的MarKword的内容修改为自己新建的记录空间的地址的方式竞争MarkWord.

第四步,第三步中成功执行CAS的获得资源,失败的则进入自旋.

第五步,自旋的线程在自旋过程中,成功获得资源(即之前获的资源的线程执行完成并释放了共享资源),则整个状态依然处于轻量级锁的状态,如果自旋失败 第六步,进入重量级锁的状态,这个时候,自旋的线程进行阻塞,等待之前线程执行完成并唤醒自己.

注意点:JVM加锁流程

偏向锁--》轻量级锁--》重量级锁

从左往右可以升级,从右往左不能降级

三、从C++源码看synchronized

前两节讲了synchronized锁实现原理,这一节我们从C++源码来剖析synchronized。

3.1 同步和互斥

同步:多个线程并发访问共享资源时,保证同一时刻只有一个(信号量可以多个)线程使用。

实现同步的方法有很多,常见四种如下:

1)临界区(CriticalSection,又叫关键段):通过对多线程的串行化来访问公共资源或一段代码,速度快,适合控制数据访问。进程内可用。

2)互斥量:互斥量用于线程的互斥。只能为0/1一个互斥量只能用于一个资源的互斥访问,可跨进程使用。

3)信号量:信号线用于线程的同步。可以为非负整数可实现多个同类资源的多线程互斥和同步。当信号量为单值信号量是,也可以完成一个资源的互斥访问。可跨进程使用。

4)事件:用来通知线程有一些事件已发生,从而启动后继任务的开始,可跨进程使用。

synchronized的底层实现就用到了临界区和互斥锁(重量级锁的情况下)这两个概念。

3.2 synchronized  C++源码

重点来了,之前在第一节中的图1,看过了对象头Mark Word。现在我们从C++源码来剖析具体的数据结构和获取释放锁的过程。

2.2.1 C++中的监视器锁数据结构

oopDesc--继承-->markOopDesc--方法monitor()-->ObjectMonitor-->enter、exit 获取、释放锁

1.oopDesc

openjdk\hotspot\src\share\vm\oops\oop.hpp下oopDesc类是JVM对象的顶级基类,故每个object都包含markOop。如下图所示:

 1 class oopDesc {
 2   friend class VMStructs;
 3  private:
 4   volatile markOop  _mark;//markOop:Mark Word标记字段
 5   union _metadata {
 6     Klass*      _klass;//对象类型元数据的指针
 7     narrowKlass _compressed_klass;
 8   } _metadata;
 9 
10   // Fast access to barrier set.  Must be initialized.
11   static BarrierSet* _bs;
12 
13  public:
14   markOop  mark() const         { return _mark; }
15   markOop* mark_addr() const    { return (markOop*) &_mark; }
16 
17   void set_mark(volatile markOop m)      { _mark = m;   }
18 
19   void    release_set_mark(markOop m);
20   markOop cas_set_mark(markOop new_mark, markOop old_mark);
21 
22   // Used only to re-initialize the mark word (e.g., of promoted
23   // objects during a GC) -- requires a valid klass pointer
24   void init_mark();
25 
26   Klass* klass() const;
27   Klass* klass_or_null() const volatile;
28   Klass** klass_addr();
29   narrowKlass* compressed_klass_addr();
....省略...
}

2.markOopDesc类

openjdk\hotspot\src\share\vm\oops\markOop.hpp下markOopDesc继承自oopDesc,并拓展了自己的方法monitor(),如下图

1 ObjectMonitor* monitor() const {
2     assert(has_monitor(), "check");
3     // Use xor instead of &~ to provide one extra tag-bit check.
4     return (ObjectMonitor*) (value() ^ monitor_value);
5   }

该方法返回一个ObjectMonitor*对象指针。

其中value()这样定义:

 1 uintptr_t value() const { return (uintptr_t) this; }

可知:将this转换成一个指针宽度的整数(uintptr_t),然后进行"异或"位操作。

monitor_value是常量
1 enum {   locked_value             = 0,//00轻量级锁
2          unlocked_value           = 1,//01无锁
3          monitor_value            = 2,//10监视器锁,又叫重量级锁
4          marked_value             = 3,//11GC标记
5          biased_lock_pattern      = 5 //101偏向锁
6   };
指针低2位00,异或10,结果还是10.(拿一个模板为00的数,异或一个二位数=数本身。因为异或:“相同为0,不同为1”.操作)

3.ObjectMonitor类

在HotSpot虚拟机中,最终采用ObjectMonitor类实现monitor。

openjdk\hotspot\src\share\vm\runtime\objectMonitor.hpp源码如下:

 1 ObjectMonitor() {
 2     _header       = NULL;//markOop对象头
 3     _count        = 0;
 4     _waiters      = 0,//等待线程数
 5     _recursions   = 0;//重入次数
 6     _object       = NULL;//监视器锁寄生的对象。锁不是平白出现的,而是寄托存储于对象中。
 7     _owner        = NULL;//指向获得ObjectMonitor对象的线程或基础锁
 8     _WaitSet      = NULL;//处于wait状态的线程,会被加入到wait set;
 9     _WaitSetLock  = 0 ;
10     _Responsible  = NULL ;
11     _succ         = NULL ;
12     _cxq          = NULL ;
13     FreeNext      = NULL ;
14     _EntryList    = NULL ;//处于等待锁block状态的线程,会被加入到entry set;
15     _SpinFreq     = 0 ;
16     _SpinClock    = 0 ;
17     OwnerIsThread = 0 ;// _owner is (Thread *) vs SP/BasicLock
18     _previous_owner_tid = 0;// 监视器前一个拥有者线程的ID
19   }

每个线程都有两个ObjectMonitor对象列表,分别为free和used列表,如果当前free列表为空,线程将向全局global list请求分配ObjectMonitor。

ObjectMonitor对象中有两个队列:_WaitSet 和 _EntryList,用来保存ObjectWaiter对象列表;

2.获取锁流程

 synchronized关键字修饰的代码段,在JVM被编译为monitorenter、monitorexit指令来获取和释放互斥锁.。

解释器执行monitorenter时会进入到InterpreterRuntime.cppInterpreterRuntime::monitorenter函数,具体实现如下:

 1 IRT_ENTRY_NO_ASYNC(void, InterpreterRuntime::monitorenter(JavaThread* thread, BasicObjectLock* elem))
 2 #ifdef ASSERT
 3   thread->last_frame().interpreter_frame_verify_monitor(elem);
 4 #endif
 5   if (PrintBiasedLockingStatistics) {
 6     Atomic::inc(BiasedLocking::slow_path_entry_count_addr());
 7   }
 8   Handle h_obj(thread, elem->obj());
 9   assert(Universe::heap()->is_in_reserved_or_null(h_obj()),
10          "must be NULL or an object");
11   if (UseBiasedLocking) {//标识虚拟机是否开启偏向锁功能,默认开启
12     // Retry fast entry if bias is revoked to avoid unnecessary inflation
13     ObjectSynchronizer::fast_enter(h_obj, elem->lock(), true, CHECK);
14   } else {
15     ObjectSynchronizer::slow_enter(h_obj, elem->lock(), CHECK);
16   }
17   assert(Universe::heap()->is_in_reserved_or_null(elem->obj()),
18          "must be NULL or an object");
19 #ifdef ASSERT
20   thread->last_frame().interpreter_frame_verify_monitor(elem);
21 #endif
22 IRT_END

 

先看一下入参:

1、JavaThread thread指向java中的当前线程;
2、BasicObjectLock基础对象锁:包含一个BasicLock和一个指向Object对象的指针oop。

openjdk\hotspot\src\share\vm\runtime\basicLock.hpp中BasicObjectLock类源码如下:
 1 class BasicObjectLock VALUE_OBJ_CLASS_SPEC {
 2   friend class VMStructs;
 3  private:
 4   BasicLock _lock;                                    // the lock, must be double word aligned
 5   oop       _obj;                                     // object holds the lock;
 6 
 7  public:
 8   // Manipulation
 9   oop      obj() const                                { return _obj;  }
10   void set_obj(oop obj)                               { _obj = obj; }
11   BasicLock* lock()                                   { return &_lock; }
12 
13   // Note: Use frame::interpreter_frame_monitor_size() for the size of BasicObjectLocks
14   //       in interpreter activation frames since it includes machine-specific padding.
15   static int size()                                   { return sizeof(BasicObjectLock)/wordSize; }
16 
17   // GC support
18   void oops_do(OopClosure* f) { f->do_oop(&_obj); }
19 
20   static int obj_offset_in_bytes()                    { return offset_of(BasicObjectLock, _obj);  }
21   static int lock_offset_in_bytes()                   { return offset_of(BasicObjectLock, _lock); }
22 };

3、BasicLock类型_lock对象主要用来保存:指向Object对象的对象头数据;

basicLock.hpp中BasicLock源码如下:
 1 class BasicLock VALUE_OBJ_CLASS_SPEC {
 2   friend class VMStructs;
 3  private:
 4   volatile markOop _displaced_header;//markOop是不是很熟悉?1.2节中讲解对象头时就是分析的markOop源码
 5  public:
 6   markOop      displaced_header() const               { return _displaced_header; }
 7   void         set_displaced_header(markOop header)   { _displaced_header = header; }
 8 
 9   void print_on(outputStream* st) const;
10 
11   // move a basic lock (used during deoptimization
12   void move_to(oop obj, BasicLock* dest);
13 
14   static int displaced_header_offset_in_bytes()       { return offset_of(BasicLock, _displaced_header); }
15 };

偏向锁的获取ObjectSynchronizer::fast_enter

在HotSpot中,偏向锁的入口位于openjdk\hotspot\src\share\vm\runtime\synchronizer.cpp文件的ObjectSynchronizer::fast_enter函数:

 1 void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock, bool attempt_rebias, TRAPS) {
 2  if (UseBiasedLocking) {
 3     if (!SafepointSynchronize::is_at_safepoint()) {
 4       BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
 5       if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {
 6         return;
 7       }
 8     } else {
 9       assert(!attempt_rebias, "can not rebias toward VM thread");
10       BiasedLocking::revoke_at_safepoint(obj);
11     }
12     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
13  }
14  //轻量级锁
15  slow_enter (obj, lock, THREAD) ;
16 }
偏向锁的获取由BiasedLocking::revoke_and_rebias方法实现,由于实现比较长,就不贴代码了,实现逻辑如下:
1、通过markOop mark = obj->mark()获取对象的markOop数据mark,即对象头的Mark Word;
2、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
3、判断mark中JavaThread的状态:如果为空,则进入步骤(4);如果指向当前线程,则执行同步代码块;如果指向其它线程,进入步骤(5);
4、通过CAS原子指令设置mark中JavaThread为当前线程ID,如果执行CAS成功,则执行同步代码块,否则进入步骤(5);
5、如果执行CAS失败,表示当前存在多个线程竞争锁,当达到全局安全点(safepoint),获得偏向锁的线程被挂起,撤销偏向锁,并升级为轻量级,升级完成后被阻塞在安全点的线程继续执行同步代码块;
偏向锁的撤销

只有当其它线程尝试竞争偏向锁时,持有偏向锁的线程才会释放锁,偏向锁的撤销由BiasedLocking::revoke_at_safepoint方法实现:

 

 1 void BiasedLocking::revoke_at_safepoint(Handle h_obj) {
 2   assert(SafepointSynchronize::is_at_safepoint(), "must only be called while at safepoint");//校验全局安全点
 3   oop obj = h_obj();
 4   HeuristicsResult heuristics = update_heuristics(obj, false);
 5   if (heuristics == HR_SINGLE_REVOKE) {
 6     revoke_bias(obj, false, false, NULL);
 7   } else if ((heuristics == HR_BULK_REBIAS) ||
 8              (heuristics == HR_BULK_REVOKE)) {
 9     bulk_revoke_or_rebias_at_safepoint(obj, (heuristics == HR_BULK_REBIAS), false, NULL);
10   }
11   clean_up_cached_monitor_info();
12 }

1、偏向锁的撤销动作必须等待全局安全点;
2、暂停拥有偏向锁的线程,判断锁对象是否处于被锁定状态;
3、撤销偏向锁,恢复到无锁(标志位为 01)或轻量级锁(标志位为 00)的状态;

偏向锁在Java 1.6之后是默认启用的,但在应用程序启动几秒钟之后才激活,可以使用-XX:BiasedLockingStartupDelay=0参数关闭延迟,如果确定应用程序中所有锁通常情况下处于竞争状态,可以通过XX:-UseBiasedLocking=false参数关闭偏向锁。

轻量级锁的获取
当关闭偏向锁功能,或多个线程竞争偏向锁导致偏向锁升级为轻量级锁,会尝试获取轻量级锁,其入口位于ObjectSynchronizer::slow_enter
 1 void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
 2   markOop mark = obj->mark();
 3   assert(!mark->has_bias_pattern(), "should not see bias pattern here");
 4 
 5   if (mark->is_neutral()) {//是否为无锁状态001
 6     // Anticipate successful CAS -- the ST of the displaced mark must
 7     // be visible <= the ST performed by the CAS.
 8     lock->set_displaced_header(mark);
 9     if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {//CAS成功,释放栈锁
10       TEVENT (slow_enter: release stacklock) ;
11       return ;
12     }
13     // Fall through to inflate() ...
14   } else
15   if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
16     assert(lock != mark->locker(), "must not re-lock the same lock");
17     assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");
18     lock->set_displaced_header(NULL);
19     return;
20   }
21 
22 #if 0
23   // The following optimization isn't particularly useful.
24   if (mark->has_monitor() && mark->monitor()->is_entered(THREAD)) {
25     lock->set_displaced_header (NULL) ;
26     return ;
27   }
28 #endif
29 
30   // The object header will never be displaced to this lock,
31   // so it does not matter what the value is, except that it
32   // must be non-zero to avoid looking like a re-entrant lock,
33   // and must not look locked either.
34   lock->set_displaced_header(markOopDesc::unused_mark());
35   ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
36 }

1、markOop mark = obj->mark()方法获取对象的markOop数据mark;
2、mark->is_neutral()方法判断mark是否为无锁状态:mark的偏向锁标志位为 0,锁标志位为 01
3、如果mark处于无锁状态,则进入步骤(4),否则执行步骤(6);
4、把mark保存到BasicLock对象的_displaced_header字段;
5、通过CAS尝试将Mark Word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(6);
6、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;

假设线程A和B同时执行到临界区if (mark->is_neutral())
1、线程AB都把Mark Word复制到各自的_displaced_header字段,该数据保存在线程的栈帧上,是线程私有的;
2、Atomic::cmpxchg_ptr原子操作保证只有一个线程可以把指向栈帧的指针复制到Mark Word,假设此时线程A执行成功,并返回继续执行同步代码块;
3、线程B执行失败,退出临界区,通过ObjectSynchronizer::inflate方法开始膨胀锁;

轻量级锁的释放
轻量级锁的释放通过ObjectSynchronizer::slow_exit--->调用ObjectSynchronizer::fast_exit完成。
 1 void ObjectSynchronizer::fast_exit(oop object, BasicLock* lock, TRAPS) {
 2   assert(!object->mark()->has_bias_pattern(), "should not see bias pattern here");
 3   // if displaced header is null, the previous enter is recursive enter, no-op
 4   markOop dhw = lock->displaced_header();
 5   markOop mark ;
 6   if (dhw == NULL) {
 7      // Recursive stack-lock.
 8      // Diagnostics -- Could be: stack-locked, inflating, inflated.
 9      mark = object->mark() ;
10      assert (!mark->is_neutral(), "invariant") ;
11      if (mark->has_locker() && mark != markOopDesc::INFLATING()) {
12         assert(THREAD->is_lock_owned((address)mark->locker()), "invariant") ;
13      }
14      if (mark->has_monitor()) {
15         ObjectMonitor * m = mark->monitor() ;
16         assert(((oop)(m->object()))->mark() == mark, "invariant") ;
17         assert(m->is_entered(THREAD), "invariant") ;
18      }
19      return ;
20   }
21 
22   mark = object->mark() ;
23 
24   // If the object is stack-locked by the current thread, try to
25   // swing the displaced header from the box back to the mark.
26   if (mark == (markOop) lock) {
27      assert (dhw->is_neutral(), "invariant") ;
28      if ((markOop) Atomic::cmpxchg_ptr (dhw, object->mark_addr(), mark) == mark) {//成功的释放了锁
29         TEVENT (fast_exit: release stacklock) ;
30         return;
31      }
32   }
33 
34   ObjectSynchronizer::inflate(THREAD, object)->exit (true, THREAD) ;//锁膨胀升级
35 }
1、确保处于偏向锁状态时不会执行这段逻辑;
2、取出在获取轻量级锁时保存在BasicLock对象的mark数据dhw;
3、通过CAS尝试把dhw替换到当前的Mark Word,如果CAS成功,说明成功的释放了锁,否则执行步骤(4);
4、如果CAS失败,说明有其它线程在尝试获取该锁,这时需要将该锁升级为重量级锁,并释放;

重量级锁

重量级锁通过对象内部的监视器(monitor)实现,其中monitor的本质是依赖于底层操作系统的Mutex Lock实现,操作系统实现线程之间的切换需要从用户态到内核态的切换,切换成本非常高。

锁膨胀过程

锁的膨胀过程通过ObjectSynchronizer::inflate函数实现

  1 ObjectMonitor * ATTR ObjectSynchronizer::inflate (Thread * Self, oop object) {
  2   // Inflate mutates the heap ...
  3   // Relaxing assertion for bug 6320749.
  4   assert (Universe::verify_in_progress() ||
  5           !SafepointSynchronize::is_at_safepoint(), "invariant") ;
  6 
  7   for (;;) {//自旋
  8       const markOop mark = object->mark() ;
  9       assert (!mark->has_bias_pattern(), "invariant") ;
 10 
 11       // The mark can be in one of the following states:
 12       // *  Inflated     - just return
 13       // *  Stack-locked - coerce it to inflated
 14       // *  INFLATING    - busy wait for conversion to complete
 15       // *  Neutral      - aggressively inflate the object.
 16       // *  BIASED       - Illegal.  We should never see this
 17 
 18       // CASE: inflated已膨胀,即重量级锁
 19       if (mark->has_monitor()) {//判断当前是否为重量级锁
 20           ObjectMonitor * inf = mark->monitor() ;//获取指向ObjectMonitor的指针
 21           assert (inf->header()->is_neutral(), "invariant");
 22           assert (inf->object() == object, "invariant") ;
 23           assert (ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid");
 24           return inf ;
 25       }
 26 
 27       // CASE: inflation in progress - inflating over a stack-lock.膨胀等待(其他线程正在从轻量级锁转为膨胀锁)
 28       // Some other thread is converting from stack-locked to inflated.
 29       // Only that thread can complete inflation -- other threads must wait.
 30       // The INFLATING value is transient.
 31       // Currently, we spin/yield/park and poll the markword, waiting for inflation to finish.
 32       // We could always eliminate polling by parking the thread on some auxiliary list.
 33       if (mark == markOopDesc::INFLATING()) {
 34          TEVENT (Inflate: spin while INFLATING) ;
 35          ReadStableMark(object) ;
 36          continue ;
 37       }
 38 
 39       // CASE: stack-locked栈锁(轻量级锁) 
 40       // Could be stack-locked either by this thread or by some other thread.
 41       //
 42       // Note that we allocate the objectmonitor speculatively, _before_ attempting
 43       // to install INFLATING into the mark word.  We originally installed INFLATING,
 44       // allocated the objectmonitor, and then finally STed the address of the
 45       // objectmonitor into the mark.  This was correct, but artificially lengthened
 46       // the interval in which INFLATED appeared in the mark, thus increasing
 47       // the odds of inflation contention.
 48       //
 49       // We now use per-thread private objectmonitor free lists.
 50       // These list are reprovisioned from the global free list outside the
 51       // critical INFLATING...ST interval.  A thread can transfer
 52       // multiple objectmonitors en-mass from the global free list to its local free list.
 53       // This reduces coherency traffic and lock contention on the global free list.
 54       // Using such local free lists, it doesn't matter if the omAlloc() call appears
 55       // before or after the CAS(INFLATING) operation.
 56       // See the comments in omAlloc().
 57 
 58       if (mark->has_locker()) {
 59           ObjectMonitor * m = omAlloc (Self) ;//获取一个可用的ObjectMonitor 
 60           // Optimistically prepare the objectmonitor - anticipate successful CAS
 61           // We do this before the CAS in order to minimize the length of time
 62           // in which INFLATING appears in the mark.
 63           m->Recycle();
 64           m->_Responsible  = NULL ;
 65           m->OwnerIsThread = 0 ;
 66           m->_recursions   = 0 ;
 67           m->_SpinDuration = ObjectMonitor::Knob_SpinLimit ;   // Consider: maintain by type/class
 68 
 69           markOop cmp = (markOop) Atomic::cmpxchg_ptr (markOopDesc::INFLATING(), object->mark_addr(), mark) ;
 70           if (cmp != mark) {//CAS失败//CAS失败,说明冲突了,自旋等待//CAS失败,说明冲突了,自旋等待//CAS失败,说明冲突了,自旋等待
 71              omRelease (Self, m, true) ;//释放监视器锁
 72              continue ;       // Interference -- just retry
 73           }
 74 
 75           // We've successfully installed INFLATING (0) into the mark-word.
 76           // This is the only case where 0 will appear in a mark-work.
 77           // Only the singular thread that successfully swings the mark-word
 78           // to 0 can perform (or more precisely, complete) inflation.
 79           //
 80           // Why do we CAS a 0 into the mark-word instead of just CASing the
 81           // mark-word from the stack-locked value directly to the new inflated state?
 82           // Consider what happens when a thread unlocks a stack-locked object.
 83           // It attempts to use CAS to swing the displaced header value from the
 84           // on-stack basiclock back into the object header.  Recall also that the
 85           // header value (hashcode, etc) can reside in (a) the object header, or
 86           // (b) a displaced header associated with the stack-lock, or (c) a displaced
 87           // header in an objectMonitor.  The inflate() routine must copy the header
 88           // value from the basiclock on the owner's stack to the objectMonitor, all
 89           // the while preserving the hashCode stability invariants.  If the owner
 90           // decides to release the lock while the value is 0, the unlock will fail
 91           // and control will eventually pass from slow_exit() to inflate.  The owner
 92           // will then spin, waiting for the 0 value to disappear.   Put another way,
 93           // the 0 causes the owner to stall if the owner happens to try to
 94           // drop the lock (restoring the header from the basiclock to the object)
 95           // while inflation is in-progress.  This protocol avoids races that might
 96           // would otherwise permit hashCode values to change or "flicker" for an object.
 97           // Critically, while object->mark is 0 mark->displaced_mark_helper() is stable.
 98           // 0 serves as a "BUSY" inflate-in-progress indicator.
 99 
100 
101           // fetch the displaced mark from the owner's stack.
102           // The owner can't die or unwind past the lock while our INFLATING
103           // object is in the mark.  Furthermore the owner can't complete
104           // an unlock on the object, either.
105           markOop dmw = mark->displaced_mark_helper() ;
106           assert (dmw->is_neutral(), "invariant") ;
107           //CAS成功,设置ObjectMonitor的_header、_owner和_object等
108           // Setup monitor fields to proper values -- prepare the monitor
109           m->set_header(dmw) ;
110 
111           // Optimization: if the mark->locker stack address is associated
112           // with this thread we could simply set m->_owner = Self and
113           // m->OwnerIsThread = 1. Note that a thread can inflate an object
114           // that it has stack-locked -- as might happen in wait() -- directly
115           // with CAS.  That is, we can avoid the xchg-NULL .... ST idiom.
116           m->set_owner(mark->locker());
117           m->set_object(object);
118           // TODO-FIXME: assert BasicLock->dhw != 0.
119 
120           // Must preserve store ordering. The monitor state must
121           // be stable at the time of publishing the monitor address.
122           guarantee (object->mark() == markOopDesc::INFLATING(), "invariant") ;
123           object->release_set_mark(markOopDesc::encode(m));
124 
125           // Hopefully the performance counters are allocated on distinct cache lines
126           // to avoid false sharing on MP systems ...
127           if (ObjectMonitor::_sync_Inflations != NULL) ObjectMonitor::_sync_Inflations->inc() ;
128           TEVENT(Inflate: overwrite stacklock) ;
129           if (TraceMonitorInflation) {
130             if (object->is_instance()) {
131               ResourceMark rm;
132               tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
133                 (void *) object, (intptr_t) object->mark(),
134                 object->klass()->external_name());
135             }
136           }
137           return m ;
138       }
139 
140       // CASE: neutral 无锁
141       // TODO-FIXME: for entry we currently inflate and then try to CAS _owner.
142       // If we know we're inflating for entry it's better to inflate by swinging a
143       // pre-locked objectMonitor pointer into the object header.   A successful
144       // CAS inflates the object *and* confers ownership to the inflating thread.
145       // In the current implementation we use a 2-step mechanism where we CAS()
146       // to inflate and then CAS() again to try to swing _owner from NULL to Self.
147       // An inflateTry() method that we could call from fast_enter() and slow_enter()
148       // would be useful.
149 
150       assert (mark->is_neutral(), "invariant");
151       ObjectMonitor * m = omAlloc (Self) ;
152       // prepare m for installation - set monitor to initial state
153       m->Recycle();
154       m->set_header(mark);
155       m->set_owner(NULL);
156       m->set_object(object);
157       m->OwnerIsThread = 1 ;
158       m->_recursions   = 0 ;
159       m->_Responsible  = NULL ;
160       m->_SpinDuration = ObjectMonitor::Knob_SpinLimit ;       // consider: keep metastats by type/class
161 
162       if (Atomic::cmpxchg_ptr (markOopDesc::encode(m), object->mark_addr(), mark) != mark) {
163           m->set_object (NULL) ;
164           m->set_owner  (NULL) ;
165           m->OwnerIsThread = 0 ;
166           m->Recycle() ;
167           omRelease (Self, m, true) ;
168           m = NULL ;
169           continue ;
170           // interference - the markword changed - just retry.
171           // The state-transitions are one-way, so there's no chance of
172           // live-lock -- "Inflated" is an absorbing state.
173       }
174 
175       // Hopefully the performance counters are allocated on distinct
176       // cache lines to avoid false sharing on MP systems ...
177       if (ObjectMonitor::_sync_Inflations != NULL) ObjectMonitor::_sync_Inflations->inc() ;
178       TEVENT(Inflate: overwrite neutral) ;
179       if (TraceMonitorInflation) {
180         if (object->is_instance()) {
181           ResourceMark rm;
182           tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
183             (void *) object, (intptr_t) object->mark(),
184             object->klass()->external_name());
185         }
186       }
187       return m ;
188   }
189 }
膨胀过程的实现比较复杂,大概实现过程如下:
1、整个膨胀过程在自旋下完成;
2、mark->has_monitor()方法判断当前是否为重量级锁(上图18-25行),即Mark Word的锁标识位为 10,如果当前状态为重量级锁,执行步骤(3),否则执行步骤(4);
3、mark->monitor()方法获取指向ObjectMonitor的指针,并返回,说明膨胀过程已经完成;
4、如果当前锁处于膨胀中(上图33-37行),说明该锁正在被其它线程执行膨胀操作,则当前线程就进行自旋等待锁膨胀完成,这里需要注意一点,虽然是自旋操作,但不会一直占用cpu资源,每隔一段时间会通过os::NakedYield方法放弃cpu资源,或通过park方法挂起;如果其他线程完成锁的膨胀操作,则退出自旋并返回;
5、如果当前是轻量级锁状态(上图58-138行),即锁标识位为 00,膨胀过程如下:
  1. 通过omAlloc方法,获取一个可用的ObjectMonitor monitor,并重置monitor数据;
  2. 通过CAS尝试将Mark Word设置为markOopDesc:INFLATING,标识当前锁正在膨胀中,如果CAS失败,说明同一时刻其它线程已经将Mark Word设置为markOopDesc:INFLATING,当前线程进行自旋等待膨胀完成;
  3. 如果CAS成功,设置monitor的各个字段:_header、_owner和_object等,并返回;
6、如果是无锁(中立,上图150-186行),重置监视器值;
monitor竞争
当锁膨胀完成并返回对应的monitor时,并不表示该线程竞争到了锁,真正的锁竞争发生在ObjectMonitor::enter方法中。
  1 void ATTR ObjectMonitor::enter(TRAPS) {
  2   // The following code is ordered to check the most common cases first
  3   // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
  4   Thread * const Self = THREAD ;
  5   void * cur ;
  6 
  7   cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
  8   if (cur == NULL) {//CAS成功
  9      // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
 10      assert (_recursions == 0   , "invariant") ;
 11      assert (_owner      == Self, "invariant") ;
 12      // CONSIDER: set or assert OwnerIsThread == 1
 13      return ;
 14   }
 15 
 16   if (cur == Self) {//重入锁
 17      // TODO-FIXME: check for integer overflow!  BUGID 6557169.
 18      _recursions ++ ;
 19      return ;
 20   }
 21 
 22   if (Self->is_lock_owned ((address)cur)) {
 23     assert (_recursions == 0, "internal state error");
 24     _recursions = 1 ;
 25     // Commute owner from a thread-specific on-stack BasicLockObject address to
 26     // a full-fledged "Thread *".
 27     _owner = Self ;
 28     OwnerIsThread = 1 ;
 29     return ;
 30   }
 31 
 32   // We've encountered genuine contention.
 33   assert (Self->_Stalled == 0, "invariant") ;
 34   Self->_Stalled = intptr_t(this) ;
 35 
 36   // Try one round of spinning *before* enqueueing Self
 37   // and before going through the awkward and expensive state
 38   // transitions.  The following spin is strictly optional ...
 39   // Note that if we acquire the monitor from an initial spin
 40   // we forgo posting JVMTI events and firing DTRACE probes.
 41   if (Knob_SpinEarly && TrySpin (Self) > 0) {
 42      assert (_owner == Self      , "invariant") ;
 43      assert (_recursions == 0    , "invariant") ;
 44      assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
 45      Self->_Stalled = 0 ;
 46      return ;
 47   }
 48 
 49   assert (_owner != Self          , "invariant") ;
 50   assert (_succ  != Self          , "invariant") ;
 51   assert (Self->is_Java_thread()  , "invariant") ;
 52   JavaThread * jt = (JavaThread *) Self ;
 53   assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
 54   assert (jt->thread_state() != _thread_blocked   , "invariant") ;
 55   assert (this->object() != NULL  , "invariant") ;
 56   assert (_count >= 0, "invariant") ;
 57 
 58   // Prevent deflation at STW-time.  See deflate_idle_monitors() and is_busy().
 59   // Ensure the object-monitor relationship remains stable while there's contention.
 60   Atomic::inc_ptr(&_count);
 61 
 62   EventJavaMonitorEnter event;
 63 
 64   { // Change java thread status to indicate blocked on monitor enter.
 65     JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
 66 
 67     DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
 68     if (JvmtiExport::should_post_monitor_contended_enter()) {
 69       JvmtiExport::post_monitor_contended_enter(jt, this);
 70     }
 71 
 72     OSThreadContendState osts(Self->osthread());
 73     ThreadBlockInVM tbivm(jt);
 74 
 75     Self->set_current_pending_monitor(this);
 76 
 77     // TODO-FIXME: change the following for(;;) loop to straight-line code.
 78     for (;;) {
 79       jt->set_suspend_equivalent();
 80       // cleared by handle_special_suspend_equivalent_condition()
 81       // or java_suspend_self()
 82 
 83       EnterI (THREAD) ;
 84 
...省略...139 }
1、通过CAS尝试把monitor的_owner字段设置为当前线程;
2、如果设置之前的_owner指向当前线程,说明当前线程再次进入monitor,即重入锁,执行_recursions ++ ,记录重入的次数;
3、如果之前的_owner指向的地址在当前线程中,这种描述有点拗口,换一种说法:之前_owner指向的BasicLock在当前线程栈上,说明当前线程是第一次进入该monitor,设置_recursions为1,_owner为当前线程,该线程成功获得锁并返回;
4、如果获取锁失败,则等待锁的释放;
monitor等待
monitor竞争失败的线程,通过自旋执行ObjectMonitor::EnterI方法等待锁的释放,EnterI方法的部分逻辑实现如下:
 1 ObjectWaiter node(Self) ;
 2     Self->_ParkEvent->reset() ;
 3     node._prev   = (ObjectWaiter *) 0xBAD ;
 4     node.TState  = ObjectWaiter::TS_CXQ ;
 5 
 6     // Push "Self" onto the front of the _cxq.
 7     // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
 8     // Note that spinning tends to reduce the rate at which threads
 9     // enqueue and dequeue on EntryList|cxq.
10     ObjectWaiter * nxt ;
11     for (;;) {
12         node._next = nxt = _cxq ;
13         if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;
14 
15         // Interference - the CAS failed because _cxq changed.  Just retry.
16         // As an optional optimization we retry the lock.
17         if (TryLock (Self) > 0) {
18             assert (_succ != Self         , "invariant") ;
19             assert (_owner == Self        , "invariant") ;
20             assert (_Responsible != Self  , "invariant") ;
21             return ;
22         }
23     }
1、当前线程被封装成ObjectWaiter对象node,状态设置成ObjectWaiter::TS_CXQ;
2、在for循环中,通过CAS把node节点push到_cxq列表中,同一时刻可能有多个线程把自己的node节点push到_cxq列表中;
3、node节点push到_cxq列表之后,通过自旋尝试获取锁,如果还是没有获取到锁,则通过park将当前线程挂起,等待被唤醒,实现如下:
 1 for (;;) {
 2 
 3         if (TryLock (Self) > 0) break ;
 4         assert (_owner != Self, "invariant") ;
 5 
 6         if ((SyncFlags & 2) && _Responsible == NULL) {
 7            Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
 8         }
 9 
10         // park self
11         if (_Responsible == Self || (SyncFlags & 1)) {
12             TEVENT (Inflated enter - park TIMED) ;
13             Self->_ParkEvent->park ((jlong) RecheckInterval) ;
14             // Increase the RecheckInterval, but clamp the value.
15             RecheckInterval *= 8 ;
16             if (RecheckInterval > 1000) RecheckInterval = 1000 ;
17         } else {
18             TEVENT (Inflated enter - park UNTIMED) ;
19             Self->_ParkEvent->park() ;//当前线程挂起
20         }
21 
22         if (TryLock(Self) > 0) break ;
23 
24         // The lock is still contested.
25         // Keep a tally of the # of futile wakeups.
26         // Note that the counter is not protected by a lock or updated by atomics.
27         // That is by design - we trade "lossy" counters which are exposed to
28         // races during updates for a lower probe effect.
29         TEVENT (Inflated enter - Futile wakeup) ;
30         if (ObjectMonitor::_sync_FutileWakeups != NULL) {
31            ObjectMonitor::_sync_FutileWakeups->inc() ;
32         }
33         ++ nWakeups ;
34 
35         // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
36         // We can defer clearing _succ until after the spin completes
37         // TrySpin() must tolerate being called with _succ == Self.
38         // Try yet another round of adaptive spinning.
39         if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;
40 
41         // We can find that we were unpark()ed and redesignated _succ while
42         // we were spinning.  That's harmless.  If we iterate and call park(),
43         // park() will consume the event and return immediately and we'll
44         // just spin again.  This pattern can repeat, leaving _succ to simply
45         // spin on a CPU.  Enable Knob_ResetEvent to clear pending unparks().
46         // Alternately, we can sample fired() here, and if set, forgo spinning
47         // in the next iteration.
48 
49         if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
50            Self->_ParkEvent->reset() ;
51            OrderAccess::fence() ;
52         }
53         if (_succ == Self) _succ = NULL ;
54 
55         // Invariant: after clearing _succ a thread *must* retry _owner before parking.
56         OrderAccess::fence() ;
57     }

4、当该线程被唤醒时,会从挂起的点继续执行,通过ObjectMonitor::TryLock尝试获取锁,TryLock方法实现如下:

 1 int ObjectMonitor::TryLock (Thread * Self) {
 2    for (;;) {
 3       void * own = _owner ;
 4       if (own != NULL) return 0 ;
 5       if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {//CAS成功,获取锁
 6          // Either guarantee _recursions == 0 or set _recursions = 0.
 7          assert (_recursions == 0, "invariant") ;
 8          assert (_owner == Self, "invariant") ;
 9          // CONSIDER: set or assert that OwnerIsThread == 1
10          return 1 ;
11       }
12       // The lock had been free momentarily, but we lost the race to the lock.
13       // Interference -- the CAS failed.
14       // We can either return -1 or retry.
15       // Retry doesn't make as much sense because the lock was just acquired.
16       if (true) return -1 ;
17    }
18 }

其本质就是通过CAS设置monitor的_owner字段为当前线程,如果CAS成功,则表示该线程获取了锁,跳出自旋操作,执行同步代码,否则继续被挂起;

monitor释放

当某个持有锁的线程执行完同步代码块时,会进行锁的释放,给其它线程机会执行同步代码,在HotSpot中,通过退出monitor的方式实现锁的释放,并通知被阻塞的线程,具体实现位于ObjectMonitor::exit方法中。

 1 void ATTR ObjectMonitor::exit(bool not_suspended, TRAPS) {
 2    Thread * Self = THREAD ;
 3    if (THREAD != _owner) {
 4      if (THREAD->is_lock_owned((address) _owner)) {
 5        // Transmute _owner from a BasicLock pointer to a Thread address.
 6        // We don't need to hold _mutex for this transition.
 7        // Non-null to Non-null is safe as long as all readers can
 8        // tolerate either flavor.
 9        assert (_recursions == 0, "invariant") ;
10        _owner = THREAD ;
11        _recursions = 0 ;
12        OwnerIsThread = 1 ;
13      } else {
14        // NOTE: we need to handle unbalanced monitor enter/exit
15        // in native code by throwing an exception.
16        // TODO: Throw an IllegalMonitorStateException ?
17        TEVENT (Exit - Throw IMSX) ;
18        assert(false, "Non-balanced monitor enter/exit!");
19        if (false) {
20           THROW(vmSymbols::java_lang_IllegalMonitorStateException());
21        }
22        return;
23      }
24    }
25 
26    if (_recursions != 0) {
27      _recursions--;        // this is simple recursive enter
28      TEVENT (Inflated exit - recursive) ;
29      return ;
30    }
...省略...

1、如果是重量级锁的释放,monitor中的_owner指向当前线程,即THREAD == _owner;
2、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,实现如下:
 1 void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
 2    assert (_owner == Self, "invariant") ;
 3 
 4    // Exit protocol:
 5    // 1. ST _succ = wakee
 6    // 2. membar #loadstore|#storestore;
 7    // 2. ST _owner = NULL
 8    // 3. unpark(wakee)
 9 
10    _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;
11    ParkEvent * Trigger = Wakee->_event ;
12 
13    // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
14    // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
15    // out-of-scope (non-extant).
16    Wakee  = NULL ;
17 
18    // Drop the lock
19    OrderAccess::release_store_ptr (&_owner, NULL) ;
20    OrderAccess::fence() ;                               // ST _owner vs LD in unpark()
21 
22    if (SafepointSynchronize::do_call_back()) {
23       TEVENT (unpark before SAFEPOINT) ;
24    }
25 
26    DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
27    Trigger->unpark() ;
28 
29    // Maintain stats and report events to JVMTI
30    if (ObjectMonitor::_sync_Parks != NULL) {
31       ObjectMonitor::_sync_Parks->inc() ;
32    }
33 }

 

3、被唤醒的线程,继续执行monitor的竞争;

四.总结

本文重点介绍了Synchronized原理以及JVM对Synchronized的优化。简单来说解决三种场景:

1)只有一个线程进入临界区,偏向锁

2)多个线程交替进入临界区,轻量级锁

3)多线程同时进入临界区,重量级锁

如果还是模糊,看一张图,截自网络

 

========================================================

 

参考:

《深入理解 Java 虚拟机》

JVM源码分析之synchronized实现

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