计算机科学与技术 外文翻译 外文文献 英文文献 Java垃圾收集器的工作方式Word格式文档下载.docx

计算机科学与技术 外文翻译 外文文献 英文文献 Java垃圾收集器的工作方式Word格式文档下载.docx

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allocation—butit’sthewaysomeJVMswork,anditmeansthatallocatingstoragefor

heapobjectsinJavacanbenearlyasfastascreatingstorageonthestackinotherlanguages.

Forexample,youcanthinkoftheC++heapasayardwhereeachstakesoutitsownpieceofturfobject.Thisrealestatecanbecomeabandonedsometimelaterandmustbereused.InsomeJVMs,theJavaheapisquitedifferent;

it’smorelikeaconveyorbeltthatmovesforwardeverytimeyouallocateanewobject.Thismeansthatobjectstorageallocationisremarkablyrapid.The“heappointer”issimplymovedforwardintovirginterritory,soit’seffectivelythesameasC++’sstackallocation.（Ofcourse,there’salittleextraoverheadforbookkeeping,butit’snothinglikesearchingforstorage.）

Youmightobservethattheheapisn’tinfactaconveyorbelt,andifyoutreatitthatway,you’llstartpagingmemory—movingitonandoffdisk,sothatyoucanappearto

havemorememorythanyouactuallydo.Pagingsignificantlyimpactsperformance.Eventually,afteryoucreateenoughobjects,you’llrunoutofmemory.Thetrickis

thatthegarbagecollectorstepsin,andwhileitcollectsthegarbageitcompactsalltheobjectsintheheapsothatyou’veeffectivelymovedthe“heappointer”closertothebeginningoftheconveyorbeltandfartherawayfromapagefault.Thegarbagecollectorrearrangesthingsandmakesitpossibleforthehigh-speed,infinite-free-heapmodeltobeusedwhileallocatingstorage.

TounderstandgarbagecollectioninJava,it’shelpfullearnhowgarbage-collection

schemesworkinothersystems.Asimplebutslowgarbage-collectiontechniqueis

calledreferencecounting.Thismeansthateachobjectcontainsareferencecounter,andeverytimeareferenceisattachedtothatobject,thereferencecountisincreased.Everytimeareferencegoesoutofscopeorissettonull,thereferencecountisdecreased.Thus,managingreferencecountsisasmallbutconstantoverheadthathappensthroughoutthelifetimeofyourprogram.Thegarbagecollectormovesthroughtheentirelistofobjects,andwhenitfindsonewithareferencecountofzeroitreleasesthatstorage（however,referencecountingschemesoftenreleaseanobjectassoonasthecountgoestozero）.Theonedrawbackisthatifobjectscircularlyrefertoeachothertheycanhavenonzeroreferencecountswhilestillbeinggarbage.Locatingsuchself-referentialgroupsrequiressignificantextraworkforthegarbagecollector.Referencecountingiscommonlyusedtoexplainonekindofgarbagecollection,butitdoesn’tseemtobeusedinanyJVMimplementations.

Infasterschemes,garbagecollectionisnotbasedonreferencecounting.Instead,itisbasedontheideathatanynon-deadobjectmustultimatelybetraceablebacktoareferencethatliveseitheronthestackorinstaticstorage.Thechainmightgothroughseverallayersofobjects.Thus,ifyoustartinthestackandinthestaticstorageareaandwalkthroughallthereferences,you’llfindalltheliveobjects.Foreachreferencethatyoufind,youmusttraceintotheobjectthatitpointstoandthenfollowallthereferencesinthatobject,tracingintotheobjectstheypointto,etc.,untilyou’vemovedthroughtheentireWebthatoriginatedwiththereferenceonthestackorinstaticstorage.Eachobjectthatyoumovethroughmuststillbealive.Notethatthereisnoproblemwithdetachedself-referentialgroups—thesearesimplynotfound,andare

thereforeautomaticallygarbage.

Intheapproachdescribedhere,theJVMusesanadaptivegarbage-collectionscheme,andwhatitdoeswiththeliveobjectsthatitlocatesdependsonthevariantcurrentlybeingused.Oneofthesevariantsisstop-and-copy.Thismeansthat—for

reasonsthatwillbecomeapparent—theprogramisfirststopped（thisisnota

backgroundcollectionscheme）.Then,eachliveobjectiscopiedfromoneheaptoanother,leavingbehindallthegarbage.Inaddition,astheobjectsarecopiedintothenewheap,theyarepackedend-to-end,thuscompactingthenewheap（andallowing

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newstoragetosimplybereeledofftheendaspreviouslydescribed）.Ofcourse,whenanobjectismovedfromoneplacetoanother,allreferencesthatpointattheobjectmustbechanged.Thereferencethatgoesfromtheheaporthestaticstorageareatotheobjectcanbechangedrightaway,buttherecanbeotherreferencespointingtothisobjectInitialization&

Cleanupthatwillbeencounteredlaterduringthe“walk.”Thesearefixedupastheyarefound（youcouldimagineatablethatmapsoldaddressestonewones）.

Therearetwoissuesthatmaketheseso-called“copycollectors”inefficient.The

firstistheideathatyouhavetwoheapsandyousloshallthememorybackandforthbetweenthesetwoseparateheaps,maintainingtwiceasmuchmemoryasyouactuallyneed.SomeJVMsdealwiththisbyallocatingtheheapinchunksasneededandsimplycopyingfromonechunktoanother.

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

whatearlierversionsofSun’sJVMusedallthetime.Forgeneraluse,mark-and-sweepisfairlyslow,butwhenyouknowyou’regeneratinglittleornogarbage,it’sfast.Mark-and-sweepfollowsthesamelogicofstartingfromthestackandstaticstorage,andtracingthroughallthereferencestofindliveobjects.However,eachtimeitfindsaliveobject,thatobjectismarkedbysettingaflaginit,buttheobjectisn’tcollectedyet.Onlywhenthemarkingprocessisfinisheddoesthe

sweepoccur.Duringthesweep,thedeadobjectsarereleased.However,nocopyinghappens,soifthecollectorchoosestocompactafragmentedheap,itdoessobyshufflingobjectsaround.“Stop-and-copy”referstotheideathatthistypeofgarbage

collectionisnotdoneinthebackground;

Instead,theprogramisstoppedwhilethegarbagecollectionoccurs.IntheSunliteratureyou’llfindmanyreferencestogarbagecollectionasalow-prioritybackgroundprocess,butitturnsoutthatthegarbagecollectionwasnotimplementedthatwayinearlierversionsoftheSunJVM.Instead,

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theSungarbagecollectorstoppedtheprogramwhenmemorygotlow.

Mark-and-sweepalsorequiresthattheprogrambestopped.

Aspreviouslymentioned,intheJVMdescribedherememoryisallocatedinbigblocks.Ifyouallocatealargeobject,itgetsitsownblock.Strictstop-and-copyrequirescopyingeveryliveobjectfromthesourceheaptoanewheapbeforeyoucanfreetheoldone,whichtranslatestolotsofmemory.Withblocks,thegarbagecollectioncantypicallycopyobjectstodeadblocksasitcollects.Eachblockhasagenerationcounttokeeptrackofwhetherit’salive.Inthenormalcase,onlytheblockscreatedsincethelastgarbagecollectionarecompacted;

allotherblocksgettheirgenerationcountbumpediftheyhavebeenreferencedfromsomewhere.Thishandlesthenormalcaseoflotsofshort-livedtemporaryobjects.Periodically,afullsweepismade—largeobjectsarestillnotcopied（theyjustgettheirgenerationcountbumped）,andblockscontainingsmallobjectsarecopiedandcompacted.TheJVMmonitorstheefficiencyofgarbagecollectionandifitbecomesawasteoftimebecauseallobjectsarelong-lived,thenitswitchestomark-andsweep.Similarly,theJVMkeepstrackofhowsuccessfulmark-and-sweepis,andiftheheapstartstobecomefragmented,itswitchesbacktostop-and-copy.Thisiswherethe“adaptive”

partcomesin,soyouendupwithamouthful:

“Adaptivegenerationalstop-and-copy

mark-andsweep.”

ThereareanumberofadditionalspeedupspossibleinaJVM.Anespeciallyimportantoneinvolvestheoperationoftheloaderandwhatiscalledajust-in-time（JIT）compiler.AJITcompilerpartiallyorfullyconvertsaprogramintonativemachinecodesothatitdoesn’tneedtobeinterpretedbytheJVMandthusrunsmuchfaster.Whenaclassmustbeloaded（typically,thefirsttimeyouwanttocreateanobjectofthatclass）,the.classfileislocated,andthebytecodesforthatclassarebroughtintomemory.Atthispoint,oneapproachistosimplyJITcompileallthecode,butthishastwodrawbacks:

Ittakesalittlemoretime,which,compoundedthroughoutthelifeoftheprogram,canaddup;

anditincreasesthesizeoftheexecutable（bytecodesaresignificantlymorecompactthanexpandedJITcode）,andthismightcausepaging,whichdefinitelyslowsdownaprogram.Analternative

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approachislazyevaluation,whichmeansthatthecodeisnotJITcompileduntilnecessary.Thus,codethatnevergetsexecutedmightneverbeJITcompiled.TheJavaHotspottechnologiesinrecentJDKstakeasimilarapproachbyincreasinglyoptimizingapieceofcodeeachtimeitisexecuted,sothemorethecodeisexecuted,thefasteritgets.

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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Java在堆里为每一样东西（除了primitives）分配内存资源的机