计算机科学与技术专业毕业论文外文文献翻译Java垃圾收集器的工作方式.docx

计算机科学与技术专业毕业论文外文文献翻译Java垃圾收集器的工作方式.docx

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计算机科学与技术专业毕业论文外文文献翻译Java垃圾收集器的工作方式

计算机科学与技术专业毕业论文外文文献翻译：

Java垃圾收集器的工作方式

计算机科学与技术专业毕业论文外文文献翻译:

Java

垃圾收集器的工作方式

毕业设计论文

外文文献翻译

专业计算机科学与技术

学生姓名

班级

学号

指导教师

信息工程学院

英文原文

HowagarbagecollectorworksofJavaLanguage

Ifyoucomefromaprogramminglanguagewhereallocatingobjectsontheheapisexpensive,youmaynaturallyassumethatJava’sschemeofallocatingeverythingexceptprimitivesontheheapisalsoexpensive.However,itturnsoutthatthegarbagecollectorcanhaveasignificantimpactonincreasingthespeedofobjectcreation.Thismightsoundabitoddatfirst?

thatstoragereleaseaffectsstorageallocation?

butit’sthewaysomeJVMswork,anditmeansthatallocatingstorageforheapobjectsinJavacanbenearlyasfastascreatingstorageonthestack

inotherlanguages.

Forexample,youcanthinkoftheC++heapasayardwhereeachstakesoutitsownpieceofturfobject.Thisrealestatecanbecomeabandonedsometimelaterandmustbereused.InsomeJVMs,theJavaheapisquitedifferent;it’smorelikeaconveyorbeltthatmovesforwardeverytime

youallocateanewobject.Thismeansthatobjectstorageallocationisremarkablyrapid.The“heappointer”issimplymovedforwardintovirginterritory,soit’seffectivelythesameasC++’sstackallocation.Ofcourse,there’salittleextraoverheadforbookkeeping,butit’snothinglikesearchingforstorage.

Youmightobservethattheheapisn’tinfactaconveyorbelt,andifyoutreatitthatway,you’llstartpagingmemory?

movingitonandoffdisk,sothatyoucanappeartohavemorememorythanyouactuallydo.Pagingsignificantlyimpactsperformance.Eventually,afteryoucreateenoughobjects,you’llrunoutofmemory.Thetrickisthatthegarbagecollectorstepsin,andwhileitcollectsthegarbageitcompactsalltheobjectsintheheapsothatyou’veeffectivelymovedthe“heappointer”closertothebeginningoftheconveyorbeltandfartherawayfromapagefault.Thegarbagecollectorrearrangesthingsandmakesitpossibleforthehigh-speed,infinite-free-heapmodeltobeusedwhileallocatingstorageTounderstandgarbagecollectioninJava,it’shelpfullearnhowgarbage-collectionschemesworkinothersystems.A

simplebutslowgarbage-collectiontechniqueiscalledreferencecounting.Thismeansthateachobjectcontainsareferencecounter,andeverytimeareferenceisattachedtothatobject,thereferencecountisincreased.Everytimeareferencegoesoutofscopeorissettonull,thereferencecountisdecreased.Thus,managingreferencecountsisasmallbutconstantoverheadthathappensthroughoutthelifetimeofyourprogram.Thegarbagecollectormovesthroughtheentirelistofobjects,andwhenitfindsonewithareferencecountofzeroitreleasesthatstoragehowever,referencecountingschemesoftenreleaseanobjectassoonasthecountgoestozero.Theonedrawbackisthatifobjectscircularlyrefertoeachothertheycanhavenonzeroreferencecountswhilestillbeinggarbage.Locatingsuchself-referentialgroupsrequiressignificantextraworkforthegarbagecollector.Referencecountingiscommonlyusedtoexplainonekindofgarbagecollection,butitdoesn’tseemtobeusedinanyJVMimplementationsInfasterschemes,garbagecollectionisnotbasedonreferencecounting.Instead,itisbasedontheideathatanynon-deadobjectmustultimatelybetraceablebacktoareferencethatliveseitheronthestackorinstaticstorage.Thechainmightgothroughseverallayersofobjects.Thus,ifyoustartinthestackandinthestaticstorageareaandwalkthroughallthereferences,you’llfindalltheliveobjects.Foreachreferencethatyoufind,youmusttraceintotheobjectthatitpointstoandthenfollowallthereferencesin

thatobject,tracingintotheobjectstheypointto,etc.,untilyou’ve

movedthroughtheentireWebthatoriginatedwiththereferenceonthestackorinstaticstorage.Eachobjectthatyoumovethroughmuststillbealive.Notethatthereisnoproblemwithdetachedself-referentialgroups?

thesearesimplynotfound,andarethereforeautomaticallygarbageIntheapproachdescribedhere,theJVMusesanadaptivegarbage-collectionscheme,andwhatitdoeswiththeliveobjectsthatitlocatesdependsonthevariantcurrentlybeingused.Oneofthesevariantsisstop-and-copy.Thismeansthat?

forreasonsthatwillbecomeapparent?

theprogramisfirststoppedthisisnotabackgroundcollectionscheme.Then,eachliveobjectiscopiedfromoneheaptoanother,leavingbehindallthegarbage.Inaddition,astheobjectsarecopiedintothenewheap,theyarepackedend-to-end,thuscompactingthenewheapandallowingnewstoragetosimplybereeledofftheendaspreviouslydescribed.

Ofcourse,whenanobjectismovedfromoneplacetoanother,allreferencesthatpointattheobjectmustbechanged.Thereferencethatgoesfromtheheaporthestaticstorageareatotheobjectcanbechangedrightaway,buttherecanbeotherreferencespointingtothisobjectInitialization&Cleanupthatwillbeencounteredlaterduringthe“walk.”Thesearefixedupastheyarefoundyoucouldimagineatablethatmapsoldaddressestonewones.

Therearetwoissuesthatmaketheseso-called“copycollectors”

inefficient.Thefirstistheideathatyouhavetwoheapsandyousloshallthememorybackandforthbetweenthesetwoseparateheaps,maintainingtwiceasmuchmemoryasyouactuallyneed.SomeJVMsdealwiththisbyallocatingtheheapinchunksasneededandsimplycopyingfromonechunktoanother.

Thesecondissueisthecopyingprocessitself.Onceyourprogrambecomesstable,itmightbegeneratinglittleornogarbage.Despitethat,acopycollectorwillstillcopyallthememoryfromoneplacetoanother,whichiswasteful.Topreventthis,someJVMsdetectthatnonewgarbageisbeinggeneratedandswitchtoadifferentschemethisisthe“adaptive”part.Thisotherschemeiscalledmark-and-sweep,andit’s

whatearlierversionsofSun’sJVMusedallthetime.Forgeneraluse,mark-and-sweepisfairlyslow,butwhenyouknowyou’regeneratinglittleornogarbage,it’sfast.Mark-and-sweepfollowsthesamelogicof

startingfromthestackandstaticstorage,andtracingthroughallthereferencestofindliveobjectsHowever,eachtimeitfindsaliveobject,thatobjectismarkedbysettingaflaginit,buttheobjectisn’tcollectedyet.Onlywhenthemarkingprocessisfinisheddoesthesweepoccur.Duringthesweep,thedeadobjectsarereleased.However,nocopyinghappens,soifthecollectorchoosestocompactafragmentedheap,itdoessobyshufflingobjectsaround.“Stop-and-copy”referstothe

ideathatthistypeofgarbagecollectionisnotdoneinthebackground;Instead,theprogramisstoppedwhilethegarbagecollectionoccurs.IntheSunliteratureyou’llfindmanyreferencestogarbagecollectionasalow-prioritybackgroundprocess,butitturnsoutthatthegarbagecollectionwasnotimplementedthatwayinearlierversionsoftheSunJVM.Instead,theSungarbagecollectorstoppedtheprogramwhenmemorygotlow.Mark-and-sweepalsorequiresthattheprogrambestopped.

Aspreviouslymentioned,intheJVMdescribedherememoryisallocatedinbigblocks.Ifyouallocatealargeobject,itgetsitsownblock.Strictstop-and-copyrequirescopyingeveryliveobjectfromthesourceheaptoanewheapbeforeyoucanfreetheoldone,whichtranslatestolotsofmemory.Withblocks,thegarbagecollectioncantypicallycopyobjectstodeadblocksasitcollects.Eachblockhasagenerationcounttokeeptrackofwhetherit’salive.Inthenormalcase,onlytheblockscreatedsincethelastgarbagecollectionarecompacted;allotherblocksgettheirgenerationcountbumpediftheyhavebeenreferencedfromsomewhere.Thishandlesthenormalcaseoflotsofshort-livedtemporaryobjects.Periodically,afullsweepismade?

largeobjectsarestillnotcopiedtheyjustgettheirgenerationcountbumped,andblockscontainingsmallobjectsarecopiedandcompacted.

TheJVMmonitorstheefficiencyofgarbagecollectionandifitbecomesawasteoftimebecauseallobjectsarelong-lived,thenit

switchestomark-andsweep.Similarly,theJVMkeepstrackofhowsuccessfulmark-and-sweepis,andiftheheapstartstobecomefragmented,itswitchesbacktostop-and-copy.Thisiswherethe“adaptive”part

comesin,soyouendupwithamouthful:

“Adaptivegenerationalstop-and-copymark-andsweep.”

ThereareanumberofadditionalspeedupspossibleinaJVM.Anespeciallyimportantoneinvolvestheoperationoftheloaderandwhatiscalledajust-in-timeJITcompiler.AJITcompilerpartiallyorfullyconvertsaprogramintonativemachinecodesothatitdoesn’tneedtobeinterpretedbytheJVMandthusrunsmuchfaster.Whenaclassmustbeloadedtypically,thefirsttimeyouwanttocreateanobjectofthatclass,the.classfileislocated,andthebytecodesforthatclassarebroughtintomemory.Atthispoint,oneapproachistosimplyJITcompileallthecode,butthishastwodrawbacks:

Ittakesalittlemoretime,which,compoundedthroughoutthelifeoftheprogram,canaddup;anditincreasesthesizeoftheexecutablebytecodesaresignificantlymorecompactthanexpandedJITcode,andthismightcausepaging,whichdefinitelyslowsdownaprogram.Analternativeapproachislazyevaluation,whichmeansthatthecodeisnotJITcompileduntilnecessary.Thus,codethatnevergetsexecutedmightneverbeJITcompiled.TheJavaHotspottechnologiesinrecentJDKstakeasimilarapproachbyincreasinglyoptimizingapieceofcodeeachtimeitisexecuted,sothe

morethecodeisexecuted,thefasteritgets.

中文译文Java垃圾收集器的工作方式

如果你学下过一种因为在堆里分配对象所以开销过大的编程语言,很自然你可能会假定