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1、本科生毕业设计（论文）第9页HowagarbagecollectorworksofJavaLanguageIfyoucomefromaprogramminglanguagewhereallocatingobjectsontheheapisexpensive,youmaynaturallyassumethatJava’sschemeofallocatingeverything(exceptprimitives)ontheheapisalsoexpensive.However,itturnsoutthattheg
2、arbagecollectorcanhaveasignificantimpactonincreasingthespeedofobjectcreation.Thismightsoundabitoddatfirst—thatstoragereleaseaffectsstorageallocation—butit’sthewaysomeJVMswork,anditmeansthatallocatingstorageforheapobjectsinJavacanbenearlyasfastascreatingstor
3、ageonthestackinotherlanguages.Forexample,youcanthinkoftheC++heapasayardwhereeachstakesoutitsownpieceofturfobject.Thisrealestatecanbecomeabandonedsometimelaterandmustbereused.InsomeJVMs,theJavaheapisquitedifferent;it’smorelikeaconveyorbeltthatmovesforwardeve
4、rytimeyouallocateanewobject.Thismeansthatobjectstorageallocationisremarkablyrapid.The“heappointer”issimplymovedforwardintovirginterritory,soit’seffectivelythesameasC++’sstackallocation.(Ofcourse,there’salittleextraoverheadforbookkeeping,butit’snothinglikese
5、archingforstorage.)Youmightobservethattheheapisn’tinfactaconveyorbelt,andifyoutreatitthatway,you’llstartpagingmemory—movingitonandoffdisk,sothatyoucanappeartohavemorememorythanyouactuallydo.Pagingsignificantlyimpactsperformance.Eventually,afteryoucreateenou
6、ghobjects,you’llrunoutofmemory.Thetrickisthatthegarbagecollectorstepsin,andwhileitcollectsthegarbageitcompactsalltheobjectsintheheapsothatyou’veeffectivelymovedthe“heappointer”closertothebeginningoftheconveyorbeltandfartherawayfromapagefault.Thegarbagecolle
7、ctorrearrangesthingsandmakesitpossibleforthehigh-speed,infinite-free-heapmodeltobeusedwhileallocatingstorage.TounderstandgarbagecollectioninJava,it’shelpfullearnhowgarbage-collectionschemesworkinothersystems.Asimplebutslowgarbage-collectiontechniqueiscalled
8、referencecounting.Thismeansthateachobjectcontainsareferencecounter,本科生毕业设计（论文）第9页andeverytimeareferenceisattachedtothatobject,thereferencecountisincreased.Everytimeareferencegoesoutofscopeorissettonull
