鎖的原理一首先我要去研究java對(duì)象頭呢

這這jv的對(duì)象頭在對(duì)象的不同狀態(tài)下會(huì)有不同的表現(xiàn)形式，主要有三種狀態(tài)，無(wú)鎖狀態(tài)、加鎖狀態(tài)、gc標(biāo)記狀態(tài)。那么我可以理解jva碼塊。但是v當(dāng)中的鎖有分為很多種，從上圖可以看出大體分為偏向鎖、輕量析，我們只有合理的設(shè)計(jì)代碼，才能合理的利用鎖、那么這三種鎖的原理是什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]對(duì)應(yīng)：[Oop(OrdinaryObjectPointer),boolean,byte,char,short,int,float,long,double]大小整個(gè)對(duì)象一共B，其中對(duì)象頭（bjecthrBB字節(jié)（因?yàn)樵?位虛擬機(jī)上對(duì)象的大小必須是的倍數(shù)）B？?jī)蓚€(gè)問(wèn)題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整個(gè)對(duì)象的大小還是沒(méi)有改變一共16B，其中對(duì)象頭（Object關(guān)于java首先openjdk文檔當(dāng)中對(duì)對(duì)象頭的解objectCommonstructureatthebeginningofeveryGC-managedheapobject.(Everyooppointstoanobjectheader.)Includesfundamentalinformationabouttheheapobject'slayout,type,GCstate,synchronizationstate,andidentityhashcode.Consistsoftwowords.Inarraysitisimmediayfollowedbyalengthfield.NotethatbothJavaobjectsandVM-internalobjectshaveacommonobjectheaderformat上述中提到一個(gè)java對(duì)象頭包含了2個(gè)word，并且好包含了堆對(duì)象的布markThefirstwordofeveryobjectheader.Usuallyasetofbitfieldsincludingsynchronizationstateandidentityhashcode.Mayalsobeapointer(withcharacteristiclowbitencoding)tosynchronizationrelatedinformation.DuringGC,maycontainGCstatebits. klassThesecondwordofeveryobjectheader.Pointstoanotherobject(ametaobject)whichdescribesthelayoutandbehavioroftheoriginalobject.ForJavaobjects,the"klass"containsaC++style"vtable". 的源碼注釋中得知到一個(gè)markword一個(gè)是64bit，那么klass的長(zhǎng)度是多少呢？根據(jù)上述利用JOL打印的對(duì)象頭信息可以知道一個(gè)對(duì)象頭是12B，其中8B是markword4B就是klassword了，和鎖相關(guān)的就是markword了，那么接下來(lái)重點(diǎn)分析markword里面信息publicstaticvoidmain(String[]args)throwsExceptionAa=newout.println("befor//JVM計(jì)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分析結(jié)果 印完hashcode之后 儲(chǔ)。其中2行是我們通過(guò)hashcde方法打印的結(jié)果，行是我根據(jù)-B的信息計(jì)算出來(lái)的hashcde，所以以確定對(duì)象頭當(dāng)中的markwok里面的后七個(gè)字節(jié)的是hashcd信息，那么第一個(gè)字節(jié)當(dāng)中的八位分別存的就是分帶、偏向鎖信息，和對(duì)象狀態(tài)，這個(gè)t分別表示的信息如下圖（其實(shí)上圖也有信息），關(guān)于對(duì)象狀態(tài)一共分為五種狀態(tài)，分別是無(wú)鎖、偏向鎖、輕量鎖、重量鎖、C標(biāo)記，那么t，如何能表示五種狀態(tài)（bt4中狀態(tài)分別是：1，m做的比較好的是把偏向鎖和無(wú)鎖狀態(tài)表示為同一個(gè)狀態(tài)，根據(jù)偏向標(biāo)識(shí)標(biāo)識(shí)鎖還向鎖。什思呢個(gè)代碼分析一下，在寫(xiě)代碼之前我們先記得無(wú)鎖狀態(tài)下的信息 ，然寫(xiě)一個(gè)偏向鎖的例子看看結(jié)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)分析結(jié)果上面這個(gè)程序只有一個(gè)線程去調(diào)用sync 和無(wú)鎖的時(shí)候一模一 們可以直接通過(guò)JVM的參數(shù)來(lái)禁用延遲-XX:+UseBiasedLocking-packagepackageimportpackagepublicclassAintpublicsynchronizedvoid}7importimportstatic//‐XX:BiasedLockingStartupDelay=20000publicclassJOLExample3publicstaticvoidmain(String[]args)throwsExceptionAa=newlongstart=9for(inti=0;i<}Llongend=System.out.println(String.format("%sms",end‐ 19那么問(wèn)題來(lái)了，什么是輕量級(jí)鎖呢？工作原理是什么呢？為什么比偏向鎖慢？輕量級(jí)鎖嘗試在應(yīng)用層面解決線程同步問(wèn)題，而不觸發(fā)操作系統(tǒng)的互斥操作，輕量級(jí)鎖減少多線程進(jìn)入互斥的幾率，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來(lái)計(jì) 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(