并发基石-Markword与锁升级

迈不过友情╰ 2022-02-28 17:24 151阅读 0赞

### 并发基石-Markword与锁升级 ###

*   *  synchronized
     *  对象头Markword
     *  偏向锁、轻量级锁、重量级锁介绍
     *   *  偏向锁与无锁
         *  锁升级-轻量级锁
     *  锁升级-重量级锁
     *  代码验证

## synchronized ##

synchronized关键字是java提供的互斥锁关键字，我们常说的互斥锁一般都是非自旋锁，即竞争不到锁的线程会进入阻塞状态知道被唤醒  
今天我们来讲讲java中用来对synchronized进行优化的三种锁，同时会介绍markword对象头  
目前我在网上搜到的十几篇博客讲的都有问题,可能有写对的我没搜到.  
很多人不经过验证直接把markOop.hpp中的JavaThread\*当成ThreadId这是错误的，实际是java线程在C语言的指针  
并且未计算过hashCode和计算过hashCode的情况也是不一样的  
本篇博客最后会展示使用jol工具，读取展示对象头的结果进行验证  
附上openjdk的markOop.hpp[链接][Link 1]

## 对象头Markword ##

对象头是java将对象比较常用和重要的运行时数据，如hashCode、gc标识、存活年龄等等进行集中存储的一组数据  
占据8个字节，以下只讨论目前比较常见的64位的情况

## 偏向锁、轻量级锁、重量级锁介绍 ##

64bit下各锁状态的Markword格式

64 bits:
      --------
      unused:25 hash:31 -->| unused:1   age:4    biased_lock:1 lock:2 (normal object)
      JavaThread*:54 epoch:2 unused:1   age:4    biased_lock:1 lock:2 (biased object)
      PromotedObject*:61 --------------------->| promo_bits:3 ----->| (CMS promoted object)
      size:64 ----------------------------------------------------->| (CMS free block)
     
      unused:25 hash:31 -->| cms_free:1 age:4    biased_lock:1 lock:2 (COOPs && normal object)
      JavaThread*:54 epoch:2 cms_free:1 age:4    biased_lock:1 lock:2 (COOPs && biased object)
      narrowOop:32 unused:24 cms_free:1 unused:4 promo_bits:3 ----->| (COOPs && CMS promoted object)
      unused:21 size:35 -->| cms_free:1 unused:7 ------------------>| (COOPs && CMS free block)
     [ptr             | 00]  locked             ptr points to real header on stack
     [header      | 0 | 01]  unlocked           regular object header
     [ptr             | 10]  monitor            inflated lock (header is wapped out)
     [ptr             | 11]  marked             used by markSweep to mark an object

32bit下各锁状态的Markword格式

32 bits:
      --------
      hash:25 ------------>| age:4    biased_lock:1 lock:2 (normal object)
      JavaThread*:23 epoch:2 age:4    biased_lock:1 lock:2 (biased object)
      size:32 ------------------------------------------>| (CMS free block)
      PromotedObject*:29 ---------->| promo_bits:3 ----->| (CMS promoted object)

也就是对象头的最后两位，是作为锁的状态标志  
00—轻量锁  
01—偏向锁/无锁  
10—重量锁  
11—GC标记  
偏向锁状态和无锁状态通过区分对象头倒数第三位来确定，0代表无锁，1代表偏向锁  
注意，未计算hashCode的对象的初始状态为匿名偏向锁（线程指针为0，代表无线程获取）而非无锁

### 偏向锁与无锁 ###

java中对于锁的实际获取依赖于UnSafe调用native方法去实现不同操作系统上的cas原语操作  
如果一个对象，在每次作业的运行始终处于单一线程，那每次对于锁的检测、获取和释放都会对性能造成不小的消耗  
于是java引入了偏向锁  
当一个处于匿名偏向锁状态的对象，第一次被一个线程竞争时,其对象头会被标记为偏向锁,同时存储其线程指针  
接下来每次该线程对该锁的获取都不需要经过不必要的cas判断锁资源从而优化了性能，这也是偏向的由来

匿名偏向锁状态的对象如果计算了hashCode，则会变为无锁状态。hashCode存在markword，并且接下来不会再进去偏向锁

匿名偏向锁状态的对象被获取时，进入非匿名偏向锁状态，markword存储持有者的java线程在操作系统的C语言指针

无锁状态下的对象被获取时，会直接跳到轻量级锁（因为偏向锁下markword没有记录hashCode，没办法存储hashCode,而轻量级锁的下面讲）

非匿名偏向锁状态的对象计算了hashCode以后,会直接进入重量级锁，此时的重量级锁会（重量级锁的下面讲）

开启偏向锁的支持需要添加两个虚拟机参数

-XX:+UseBiasedLocking
    -XX:BiasedLockingStartupDelay=0

第一个参数是开启偏向锁  
第二个参数是指定立即开启，因为默认偏向锁的开启时在虚拟机运行后延时5秒  
如果没有线程竞争，非匿名偏向锁偏向锁释放后会变回匿名偏向锁状态

### 锁升级-轻量级锁 ###

当一个对象处于非匿名偏向锁状态下，如果有别的线程过来竞争，另一个线程尝试竞争锁，竞争失败并给予jvm一个竞争的信号以后进入自旋（不断尝试获取锁）  
接下来在持有该锁的线程执行来到安全点时，会触发stop the world并将膨胀为轻量级锁  
轻量级锁会创建一份锁记录(Lock Record)在当前持有他的线程的线程栈里  
LockRecord中包含一个owner属性指向锁对象，而锁对象的markword中也会保存一个执行该LockRecord的指针

这时的竞争就是轻量级锁的竞争了，轻量级锁的竞争时，竞争锁的线程会在一个周期时间内不断的自旋获取锁，如果获取失败就会进入阻塞并将markword的锁标记标记为10（重量级锁）

因为LockRecord复制了markword，所以在执行同步块时并不去关注markword，只有到了释放时

1.锁对象的markword升级为了重量级锁，将锁对象升级为重量级锁，锁对象的markword存储一个指向一个由操作系统实现的mutex互斥变量，唤醒阻塞的竞争线程

2.markword没变化，释放锁，对象锁恢复到无锁状态（如果LockRecord记录的是偏向锁，则恢复到匿名偏向锁，否则恢复到无锁状态）

## 锁升级-重量级锁 ##

当对象来到重量级锁以后，新被从竞争队列挑选出来一部分竞争锁的线程队列会一起竞争锁  
最终竞争到锁的一个线程会继续运行，竞争失败的线程进入阻塞队列  
处于执行状态的线程执行完同步代码块后，会释放锁并唤醒阻塞队列中的线程  
将他们加入新的挑选出来的竞争锁的线程队列，并重新竞争锁，重复以上操作  
因为需要阻塞和唤醒线程，所以需要从用户态到系统态切换，所以重量级锁下的系统开销很大

## 代码验证 ##

Thread currentThread = Thread.currentThread();
    		System.out.println("threadId : "+Long.toBinaryString(currentThread.getId()));
    		Object o = new Object();
    		System.out.println("-------------------------------------------------------------------------------------------------------");
    		System.out.println("init object info");
    		System.out.println(ClassLayout.parseInstance(o).toPrintable());
    		System.out.println("-------------------------------------------------------------------------------------------------------");
    		synchronized (o) { 
    			System.out.println("synchronized lock object info");
    			System.out.println(ClassLayout.parseInstance(o).toPrintable());
    			System.out.println("synchronized finished");
    			System.out.println("-------------------------------------------------------------------------------------------------------");
    		}
    		Thread.sleep(2000);
    		System.out.println("after synchronized object info");
    		System.out.println(ClassLayout.parseInstance(o).toPrintable());
    		System.out.println("binary hashCode : "+Integer.toBinaryString(o.hashCode()));
    		System.out.println("-------------------------------------------------------------------------------------------------------");
    		
    		System.out.println("after calculate hashcode object info");
    		System.out.println(ClassLayout.parseInstance(o).toPrintable());
    		System.out.println("-------------------------------------------------------------------------------------------------------");
    		
    		synchronized (o) { 
    			System.out.println("synchronized lock object info");
    			System.out.println(ClassLayout.parseInstance(o).toPrintable());
    			System.out.println("synchronized finished");
    			System.out.println("-------------------------------------------------------------------------------------------------------");
    		}
    		Object o2 = new Object();
    		System.out.println("o2 hashCode : "+Long.toBinaryString(o2.hashCode()));
    		System.out.println("init lock object2 info");
    		System.out.println(ClassLayout.parseInstance(o2).toPrintable());
    		System.out.println("-------------------------------------------------------------------------------------------------------");
    		synchronized (o2) { 
    			System.out.println("synchronized lock object2 info");
    			System.out.println(ClassLayout.parseInstance(o2).toPrintable());
    			System.out.println("synchronized finished");
    			System.out.println("-------------------------------------------------------------------------------------------------------");
    		}
    		System.out.println("after lock object2 info");
    		System.out.println(ClassLayout.parseInstance(o2).toPrintable());
    		System.out.println("-------------------------------------------------------------------------------------------------------");
    		//计算过hashcode的会直接进入轻量锁

输出结果

threadId : 1
    -------------------------------------------------------------------------------------------------------
    init object info
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           05 00 00 00 (00000101 00000000 00000000 00000000) (5)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           e5 01 00 20 (11100101 00000001 00000000 00100000) (536871397)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    -------------------------------------------------------------------------------------------------------
    synchronized lock object info
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           05 e8 09 01 (00000101 11101000 00001001 00000001) (17426437)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           e5 01 00 20 (11100101 00000001 00000000 00100000) (536871397)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    synchronized finished
    -------------------------------------------------------------------------------------------------------
    after synchronized object info
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           05 e8 09 01 (00000101 11101000 00001001 00000001) (17426437)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           e5 01 00 20 (11100101 00000001 00000000 00100000) (536871397)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    binary hashCode : 100000111110100010001111000001
    -------------------------------------------------------------------------------------------------------
    after calculate hashcode object info
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           01 c1 23 fa (00000001 11000001 00100011 11111010) (-98320127)
          4     4        (object header)                           20 00 00 00 (00100000 00000000 00000000 00000000) (32)
          8     4        (object header)                           e5 01 00 20 (11100101 00000001 00000000 00100000) (536871397)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    -------------------------------------------------------------------------------------------------------
    synchronized lock object info
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           50 f7 c4 02 (01010000 11110111 11000100 00000010) (46462800)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           e5 01 00 20 (11100101 00000001 00000000 00100000) (536871397)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    synchronized finished
    -------------------------------------------------------------------------------------------------------
    o2 hashCode : 110101100000011100010111110011
    init lock object2 info
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           01 f3 c5 81 (00000001 11110011 11000101 10000001) (-2117733631)
          4     4        (object header)                           35 00 00 00 (00110101 00000000 00000000 00000000) (53)
          8     4        (object header)                           e5 01 00 20 (11100101 00000001 00000000 00100000) (536871397)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    -------------------------------------------------------------------------------------------------------
    synchronized lock object2 info
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           50 f7 c4 02 (01010000 11110111 11000100 00000010) (46462800)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           e5 01 00 20 (11100101 00000001 00000000 00100000) (536871397)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    synchronized finished
    -------------------------------------------------------------------------------------------------------
    after lock object2 info
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           01 f3 c5 81 (00000001 11110011 11000101 10000001) (-2117733631)
          4     4        (object header)                           35 00 00 00 (00110101 00000000 00000000 00000000) (53)
          8     4        (object header)                           e5 01 00 20 (11100101 00000001 00000000 00100000) (536871397)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    -------------------------------------------------------------------------------------------------------

多线程竞争

Thread currentThread = Thread.currentThread();
    		System.out.println("threadId : "+Long.toBinaryString(currentThread.getId()));
    		Object o = new Object();
    		System.out.println("-------------------------------------------------------------------------------------------------------");
    		System.out.println("init object info");
    		System.out.println(ClassLayout.parseInstance(o).toPrintable());
    		System.out.println("-------------------------------------------------------------------------------------------------------");
    		
    		new Thread() { 
    			public void run() { 
    				synchronized (o) { 
    					System.out.println("-------------------------------------------------------------------------------------------------------");
    					System.out.println("-------------------------------------------------------------------------------------------------------");
    					System.out.println("another thread id "+Long.toBinaryString(this.getId()));
    					System.out.println("synchronized lock object info");
    					System.out.println(ClassLayout.parseInstance(o).toPrintable());
    					System.out.println("synchronized finished");
    					System.out.println("-------------------------------------------------------------------------------------------------------");
    					System.out.println("------------------------------------------------------------------------------------------------------");
    				}
    			}
    		}.start();
    		
    		synchronized (o) { 
    			System.out.println("synchronized lock object info");
    			System.out.println(ClassLayout.parseInstance(o).toPrintable());
    			
    			//有锁未计算hashCode状态下计算hashCode
    			
    			o.hashCode();
    			System.out.println("when synchronized calculate hashcode ");
    			System.out.println(ClassLayout.parseInstance(o).toPrintable());
    			
    			System.out.println("synchronized finished");
    			System.out.println("-------------------------------------------------------------------------------------------------------");
    		}
    		Thread.sleep(2000);
    		System.out.println("after synchronized object info");
    		System.out.println(ClassLayout.parseInstance(o).toPrintable());
    		System.out.println("binary hashCode : "+Integer.toBinaryString(o.hashCode()));
    		System.out.println("-------------------------------------------------------------------------------------------------------");
    		
    		System.out.println("after calculate hashcode object info");
    		System.out.println(ClassLayout.parseInstance(o).toPrintable());
    		System.out.println("-------------------------------------------------------------------------------------------------------");
    		
    		synchronized (o) { 
    			System.out.println("synchronized lock object info");
    			System.out.println(ClassLayout.parseInstance(o).toPrintable());
    			System.out.println("synchronized finished");
    			System.out.println("-------------------------------------------------------------------------------------------------------");
    		}
    		Object o2 = new Object();
    		System.out.println("o2 hashCode : "+Long.toBinaryString(o2.hashCode()));
    		System.out.println("init lock object2 info");
    		System.out.println(ClassLayout.parseInstance(o2).toPrintable());
    		System.out.println("-------------------------------------------------------------------------------------------------------");
    		synchronized (o2) { 
    			System.out.println("synchronized lock object2 info");
    			System.out.println(ClassLayout.parseInstance(o2).toPrintable());
    			System.out.println("synchronized finished");
    			System.out.println("-------------------------------------------------------------------------------------------------------");
    		}
    		System.out.println("after lock object2 info");
    		System.out.println(ClassLayout.parseInstance(o2).toPrintable());
    		System.out.println("-------------------------------------------------------------------------------------------------------");
    		//计算过hashcode的会直接进入轻量锁

输出结果

threadId : 1
    -------------------------------------------------------------------------------------------------------
    init object info
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           05 00 00 00 (00000101 00000000 00000000 00000000) (5)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           e5 01 00 20 (11100101 00000001 00000000 00100000) (536871397)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    -------------------------------------------------------------------------------------------------------
    synchronized lock object info
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           ba c4 c4 02 (10111010 11000100 11000100 00000010) (46449850)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           e5 01 00 20 (11100101 00000001 00000000 00100000) (536871397)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    when synchronized calculate hashcode 
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           ba c4 c4 02 (10111010 11000100 11000100 00000010) (46449850)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           e5 01 00 20 (11100101 00000001 00000000 00100000) (536871397)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    synchronized finished
    -------------------------------------------------------------------------------------------------------
    -------------------------------------------------------------------------------------------------------
    -------------------------------------------------------------------------------------------------------
    another thread id 1010
    synchronized lock object info
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           ba c4 c4 02 (10111010 11000100 11000100 00000010) (46449850)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           e5 01 00 20 (11100101 00000001 00000000 00100000) (536871397)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    synchronized finished
    -------------------------------------------------------------------------------------------------------
    ------------------------------------------------------------------------------------------------------
    after synchronized object info
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           01 c1 23 fa (00000001 11000001 00100011 11111010) (-98320127)
          4     4        (object header)                           20 00 00 00 (00100000 00000000 00000000 00000000) (32)
          8     4        (object header)                           e5 01 00 20 (11100101 00000001 00000000 00100000) (536871397)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    binary hashCode : 100000111110100010001111000001
    -------------------------------------------------------------------------------------------------------
    after calculate hashcode object info
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           01 c1 23 fa (00000001 11000001 00100011 11111010) (-98320127)
          4     4        (object header)                           20 00 00 00 (00100000 00000000 00000000 00000000) (32)
          8     4        (object header)                           e5 01 00 20 (11100101 00000001 00000000 00100000) (536871397)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    -------------------------------------------------------------------------------------------------------
    synchronized lock object info
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           90 f5 ad 02 (10010000 11110101 10101101 00000010) (44955024)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           e5 01 00 20 (11100101 00000001 00000000 00100000) (536871397)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    synchronized finished
    -------------------------------------------------------------------------------------------------------
    o2 hashCode : 110101100000011100010111110011
    init lock object2 info
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           01 f3 c5 81 (00000001 11110011 11000101 10000001) (-2117733631)
          4     4        (object header)                           35 00 00 00 (00110101 00000000 00000000 00000000) (53)
          8     4        (object header)                           e5 01 00 20 (11100101 00000001 00000000 00100000) (536871397)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    -------------------------------------------------------------------------------------------------------
    synchronized lock object2 info
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           90 f5 ad 02 (10010000 11110101 10101101 00000010) (44955024)
          4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4        (object header)                           e5 01 00 20 (11100101 00000001 00000000 00100000) (536871397)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    synchronized finished
    -------------------------------------------------------------------------------------------------------
    after lock object2 info
    java.lang.Object object internals:
     OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
          0     4        (object header)                           01 f3 c5 81 (00000001 11110011 11000101 10000001) (-2117733631)
          4     4        (object header)                           35 00 00 00 (00110101 00000000 00000000 00000000) (53)
          8     4        (object header)                           e5 01 00 20 (11100101 00000001 00000000 00100000) (536871397)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    -------------------------------------------------------------------------------------------------------

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[Link 1]: http://hg.openjdk.java.net/jdk8/jdk8/hotspot/file/87ee5ee27509/src/share/vm/oops/markOop.hpp
[20200521183346518.jpg]: /images/20220227/4c67e76c072f4198bdc1cd2218f0e8ee.png