锁的原理一首先我要去研究java对象头呢

这这jv的对象头在对象的不同状态下会有不同的表现形式，主要有三种状态，无锁状态、加锁状态、gc标记状态。那么我可以理解jva码块。但是v当中的锁有分为很多种，从上图可以看出大体分为偏向锁、轻量析，我们只有合理的设计代码，才能合理的利用锁、那么这三种锁的原理是什publicpublicclassA3packagepackageimportimportimportstatic5publicclassJOLExample1publicstaticvoidmain(String[]args)throwsException}11))))#Running64‐bitHotSpot#Usingcompressedoopwith0‐bit#Usingcompressedklasswith3‐bit#Objectsare8bytes#Fieldsizesbytype:4,1,1,2,2,4,4,8,8#Arrayelementsizes:4,1,1,2,2,4,4,8,87com.luban.layout.AobjectOFFSETSIZETYPEDESCRIPTION04(objectheader)0100000044(objectheader)00000000(84(objectheader)82220120)124(lossduetothenextobjectInstancesize:16Spacelosses:0bytesinternal+4bytesexternal=4bytes)1Field1Fieldsizesbytype411224488[bytes]对应：[Oop(OrdinaryObjectPointer),boolean,byte,char,short,int,float,long,double]大小整个对象一共B，其中对象头（bjecthrBB字节（因为在4位虚拟机上对象的大小必须是的倍数）B？两个问题publicpublicclassAbooleanflag4#Running#Running64‐bitHotSpot#Usingcompressedoopwith0‐bit#Usingcompressedklasswith3‐bit#Objectsare8bytes#Fieldsizesbytype:4,1,1,2,2,4,4,8,8#Arrayelementsizes:4,1,1,2,2,4,4,8,878com.luban.layout.Aobject9OFFSETSIZETYPEDESCRIPTION04(objectheader)01000000))44(object00000000)84(object82220120)()121boolean133133(lossduetothenextobjectInstancesize:16Spacelosses:0bytesinternal+3bytesexternal=3bytes整个对象的大小还是没有改变一共16B，其中对象头（Object关于java首先openjdk文档当中对对象头的解objectCommonstructureatthebeginningofeveryGC-managedheapobject.(Everyooppointstoanobjectheader.)Includesfundamentalinformationabouttheheapobject'slayout,type,GCstate,synchronizationstate,andidentityhashcode.Consistsoftwowords.Inarraysitisimmediayfollowedbyalengthfield.NotethatbothJavaobjectsandVM-internalobjectshaveacommonobjectheaderformat上述中提到一个java对象头包含了2个word，并且好包含了堆对象的布markThefirstwordofeveryobjectheader.Usuallyasetofbitfieldsincludingsynchronizationstateandidentityhashcode.Mayalsobeapointer(withcharacteristiclowbitencoding)tosynchronizationrelatedinformation.DuringGC,maycontainGCstatebits. klassThesecondwordofeveryobjectheader.Pointstoanotherobject(ametaobject)whichdescribesthelayoutandbehavioroftheoriginalobject.ForJavaobjects,the"klass"containsaC++style"vtable". 的源码注释中得知到一个markword一个是64bit，那么klass的长度是多少呢？根据上述利用JOL打印的对象头信息可以知道一个对象头是12B，其中8B是markword4B就是klassword了，和锁相关的就是markword了，那么接下来重点分析markword里面信息publicstaticvoidmain(String[]args)throwsExceptionAa=newout.println("befor//JVM计算的out.println("after13beforcom.luban.layout.AobjectOFFSETSIZETYPEDESCRIPTION04(objectheader)01000000 44(objectheader)00000000 6678984(objectheader)43c10020)121booleanA.flag133(lossduetothenextobjectInstancesize:16Spacelosses:0bytesinternal+3bytesexternal=3bytes)afterhashcom.luban.layout.Aobjectinternals:OFFSETSIZETYPEDESCRIPTIONVALUE04(objectheader)019409e4)44(objectheader)6a000000))(84(objectheader)43c10020)121booleanA.flag133(lossduetothenextobjectalignment)Instancesize:16bytesSpacelosses:0bytesinternal+3bytesexternal=3bytes)26Processfinishedwithexitcode Stringcode= 20分析结果 印完hashcode之后 储。其中2行是我们通过hashcde方法打印的结果，行是我根据-B的信息计算出来的hashcde，所以以确定对象头当中的markwok里面的后七个字节的是hashcd信息，那么第一个字节当中的八位分别存的就是分带、偏向锁信息，和对象状态，这个t分别表示的信息如下图（其实上图也有信息），关于对象状态一共分为五种状态，分别是无锁、偏向锁、轻量锁、重量锁、C标记，那么t，如何能表示五种状态（bt4中状态分别是：1，m做的比较好的是把偏向锁和无锁状态表示为同一个状态，根据偏向标识标识锁还向锁。什思呢个代码分析一下，在写代码之前我们先记得无锁状态下的信息 ，然写一个偏向锁的例子看看结果packagepackageimportimportstatic45publicclassJOLExample2staticstaticApublicstaticvoidmain(String[]args)throwsExceptiona=newout.println("befreout.println("after}publicstaticvoidsync()throwsInterruptedExceptionsynchronized} 23404 544 684 ))7121boolean133(lossduetothenextobjectInstancesize:16Spacelosses:0bytesinternal+3bytesexternal=3bytesaftercom.luban.layout.AobjectOFFSETSIZETYPEDESCRIPTION04(objectheader)01000000)44(objectheader)00000000)(84(objectheader)43c10020)121booleanA.flag133(lossduetothenextobject)分析结果上面这个程序只有一个线程去调用sync 和无锁的时候一模一 们可以直接通过JVM的参数来禁用延迟-XX:+UseBiasedLocking-packagepackageimportpackagepublicclassAintpublicsynchronizedvoid}7importimportstatic//‐XX:BiasedLockingStartupDelay=20000publicclassJOLExample3publicstaticvoidmain(String[]args)throwsExceptionAa=newlongstart=9for(inti=0;i<}Llongend=System.out.println(String.format("%sms",end‐ 19那么问题来了，什么是轻量级锁呢？工作原理是什么呢？为什么比偏向锁慢？轻量级锁尝试在应用层面解决线程同步问题，而不触发操作系统的互斥操作，轻量级锁减少多线程进入互斥的几率，不能代替互斥packagepackageimportimportstatic4publicclassJOLExample5staticApublicstaticvoidmain(String[]args)throwsExceptiona=newout.println("befreout.println("after} publicstaticvoidsync()throwsInterruptedExceptionsynchronizedsynchronizedout.println("lock}}221package1package23import4568publicclassJOLExample4staticCountDownLatchcountDownLatch=newpublicstaticvoidmain(String[]args)throwsException{finalAa=newA();79 longstart= L来计 for(intnewpublicvoidrun()while(countDownLatch.getCount()>0)}}}longend=System.out.println(String.format("%sms",end‐ 30packageimportimportstatic4publicclassJOLExample6staticApublicstaticvoidmain(String[]args)throwsExceptiona=newout.println("befreThreadt1=newpublicvoidrun()synchronizedtrySystem.out.println("t1}catch(InterruptedExceptione)}}}out.println("t1lockout.println("afterout.println("after}publicstaticvoidsync()throwsInterruptedExceptionsynchronizedSystem.out.println("t1main}}441package1package23import45importstatic6publicclassJOLExample7staticApublicstaticvoidmain(String[]args)throwsExceptiona=newout.println("befreThreadt1=newpublicvoidrun()synchronizedtrysynchronized(a)System.out.println("beforeSystem.out.println("after}}catch(