2025年畢業(yè)設計論文

參考文獻[1]百度百科．Java語言的特點和優(yōu)勢[EB/OL]．/view/edd3e74ffe4733687e21aac9.html[2]榮欽科技.Java2游戲設計[M]．北京．清華大學出版社．2004．[3]程成．軟件工程[M]．北京．機械工業(yè)出版社．2003．[4]袁海燕，王文濤．Java實用程序設計100例[M]．北京．人民郵電出版社．2005．[5]王伯欣．J2ME無線.Java[M]．北京．應用開發(fā)電子工業(yè)出版社．2004．[6]曹瑩，苗志剛，趙秀明．淺談Java優(yōu)勢[J]．中國科技信息，2006．[7]邵敏敏，陳德平．解析Java的多線程機制[J]．電腦知識與技術．2006．[8]朱平．Java程序設計教學[J]．湖北教育學院學報．2006．[9]猛偉君．李龍海．付少鋒．周利華．基于VC++和JAVA的多線程設計和實現(xiàn)[J]．2006．[10]秦琴．Java語言的異常處理設計[J]．中國科技信息．2006．[11]倪道義．手機游戲架構分析[J]．科技論壇，2011，(9)：90．[12]BennySkogberg．AndroidAppliactionDevelopment[D]．Sweden：MalmoUniversity，2010．[13]CayS．Horstmann，GaryCornell．IntroductiontoJava[M]．London:Macmillan．2006．[14]BruceEckel．ThinkinginJava[M]．London:PrenticeHallPTR．2003．

致謝花有重開日，人無再少年。隨著畢業(yè)論文的提交，大學生活畫上了完美的句號。但這同時也象征著新的開始，自己將獨自走向新的臺階，邁向人生征程新的開始。回想大學四年的學習生活，許多事情浮現(xiàn)腦海，仿佛前不久才發(fā)生。剛進入大學校園時與父母歡樂和興奮的場景還歷歷在目。一切都是那么新鮮，那么富有吸引力。有快樂也有艱辛，有收獲也有失落。感謝軟件學院給予幫助過我的所有老師。18班的友情也同樣終身難忘難忘，是你們陪我共同度過了人生重要的一段路程，無論將來走到哪里，你們都是我人身中珍貴的回憶。我要感謝指導教師和杰普老師。這個論文是在她們的悉心指導和熱情幫助下完成的，老師們認真負責的工作態(tài)度，精益求精的工作作風以及平易近人的人格魅力都使我受益匪淺。對于在畢業(yè)設計完成過程中遇到的問題，她們無論在理論上還是在實踐中，都對我進行了認真的指導，使我的專業(yè)實踐能力得到很大提高，這對于我以后的學習和工作都有幫助。在此，特別向指導老師表示衷心的感謝，請兩位老師接受我最誠摯的敬意！謝謝你們認真輔導，使我能夠順利完成畢業(yè)設計的開發(fā)工作。同時也要感謝幫助過我的同學們，在略微乏味的學海中，讓我體會到友情的無價。同時，我也想感謝我的家人，他們不僅幫助我茁壯成長、給予我關懷和照顧，而且，是他們培養(yǎng)了我對科學文化知識的興趣。沒有他們，就沒有我今天的成績。在今后的日子里，我想說，我會一如既往的認真下去，將自己在學生階段所學知識運用到社會實踐當中，我會不忘初心，用自己的努力報答你們！由于本人水平有限以及時間緊湊，本文可能會出現(xiàn)一些不足之處，這這里，我懇請各位老師給予指正和幫助。附錄200334thAnnualIEEEComputerSoftwareandApplicationsConferenceWorkshopsCS1andCS2:writecomputergamesinJavaShinNAKAJIMAKeijiHOKAMURANaoyasuUBAYASHINationalInstituteofInformaticsKyushuUniversityKyushuUniversityTokyo,JapanFukuoka,JapanFukuoka,JapanEmail:nkjm@nii.ac.jpEmail:polikeiji@Email:ubayashi@Abstract—Javahasbecomeahugeandcomplextechnologyplatform,fordevelopers,inordertobettermastertheJavatechnology,in-depthunderstandingoftheunderlyingtechnicalprocessingdetailsisessential.Themasteryofthecoreconceptsandideascanhelpuslearnbyanalogy,helpustoenhanceourunderstandingoftheentireJavaplatform.HerearetheJavatechnologyplatformdescribedinseveralcoreconcepts,whichcontainstheideaswillhelpusamoreprofoundunderstandingofJavatechnology.Keywords:Java;VirtualMachine;Classfile;API;I.INTRODUCTIONJavaisanobject-orientedprogramminglanguage,notonlyabsorbthevariousadvantagesoftheC++language,butalsotoabandontheC++difficulttounderstandthemulti-inheritance,pointerandotherconcepts,sotheJavalanguagehasapowerfulandeasytousetwofeatures.Javalanguageasastaticobject-orientedprogramminglanguagerepresentatives,anexcellentrealizationoftheobject-orientedtheory,allowingprogrammerstoelegantwayofthinkingcomplexprogramming[1].Javahasthecharacteristicsofsimplicity,object-oriented,distributed,robustness,security,platformindependenceandportability,multi-thread,anddynamic[2].Javacanwritedesktopapplications,Webapplications,distributedsystemsandembeddedsystemapplications[3].II.JAVAVIRTUALMACHINEThemaintaskoftheJavavirtualmachineistoinstalltheclassfileandexecutethebytecode.TheJavavirtualmachinecontainsaclassloaderthatcanloadclassfilesfromprogramsandAPIs.OnlythoseclassesthatarerequiredbytheprogramtobeexecutedintheJavaAPIwillbeloaded.Bytecodeisexecutedbytheexecutionengine.IndifferentJavavirtualmachines,theexecutionenginemaybeverydifferent.Inthevirtualmachineimplementedbythesoftware,thesimplestexecutionengineisaone-timeinterpretationofthebytecode.Anotherexecutionengineisfaster,butalsoconsumesmorememory,called"just-in-timecompiler".Inthiscase,thefirstbytecodetobeexecutedwillbecompiledintothelocalmachinecode.Thecompiledlocalmachinecodeiscachedandcanbereusedwhenthemethodiscalledlater.Thethirdexecutionengineistheadaptiveoptimizer.Inthisway,thevirtualmachinebeginsbyinterpretingthebytecode,butitmonitorstheactivityoftherunningprogramandrecordsthemostfrequentlyusedcodesegments.Whentheprogramisrunning,thevirtualmachineonlycompilesthemostfrequentlycompiledcodeintolocalcode,andtheothercodeiskeptveryfrequentlyandiskeptasbytecode-thevirtualmachinecontinuestoexplainthem.AnadaptiveoptimizerallowstheJavavirtualmachinetoperformoptimizedlocalcodein80%to90%ofthetime,butonly10%to20%ofthecodethataffectsperformance.WhentheJavavirtualmachineisimplementedbythesoftwareonthehostoperatingsystem,theJavaprograminteractswiththehostbycallingthenativemethod.TherearetwomethodsinJava:Javamethodsandlocalmethods.JavamethodiswrittenbytheJavalanguage,compiledintobytecodefiles,storedintheclassfile.Localmethodsarewritteninotherlanguages(suchasc,c++orassemblylanguage),compiledintoprocessor-relatedmachinecode.Localmethodsarestoredinthedynamiclinklibrary,theformatisproprietarytoeachplatform.WhenaJavaprogramcallsalocalmethod,thevirtualmachineloadsthedynamiclibrarythatcontainsthelocalmethodandcallsthemethod.ThelocalmethodistoconnecttheJavaprogramandtheunderlyinghostoperatingsystemconnectionmethod.Accesstheunderlyingoperatingsystemresources.AlocalNativeinterface(JNI)-allowslocalmethodstorunonanyJavaplatformimplementationofaparticularhostsystem.Ifyouwanttouseresourcesonspecifichost,theycannotaccessfromtheJavaAPI,youcanwriteaplatform-relatedJavaprogramtocallthelocalmethod.Ifyouwanttoensurethattheprogramplatformindependence,thenonlythroughtheJavaAPItoaccesstheunderlyingsystemresources.Figure1JavaVirtualMachineWorkingprincipleIII. CLASSLOADERARCHITECTUREJavaapplicationcanusetwoclassloaders:the"bootstrap"classloaderandtheuser-definedclassloader.Startingtheclassloader(whichisuniqueinthesystem)ispartoftheJavaVirtualMachineimplementation.Thebootclassloadertypicallyusesadefaultwaytoloadclassesfromalocaldisk,includingtheJavaAPIclasses(thebootloaderisalsoknownastheoriginalclassloader,thesystemclassloader,orthedefaultclassloader).Javaapplicationscaninstalluser-definedclassloadersatruntime,andthisclassloadercanuseacustomwaytoloadclasses.Forexample,downloadtheclassfilefromthenetwork.Althoughthebootloaderisanessentialpartofthevirtualmachineimplementationandtheuser-definedclassloaderisnot,theuser-definedclassloadercanbewritteninJavaandcanbecompiledintoaclassfilethatcanbeloadedbyavirtualmachine.Asinstantiatedasotherobjects.Becausethereisauser-definedclassloader,sodonothavetocompilethetimetoknowtherunningJavaapplicationswilleventuallyjoinalltheclass.User-definedclassloadersmakeitpossibletorunextendedJavaapplications.Whenitisrunning,theapplicationcansolvewhichadditionalclassesitneedstobeabletodecidewhethertouseoneormoreuser-definedclassloaderstoload.SinceclassloadersarewritteninJava,classloadingisdonewithanystylethatcanbeexpressedinJavacode.Theseclassescanbedownloadedoverthenetwork,canbeobtainedfromsomedatabases,andcanevenbedynamicallygenerated.Wheneachclassisloaded,theJavavirtualmachinemonitorstheclassandseeswhetheritisloadedbytheclassloaderorbytheuser-definedclassloader.Whentheloadedclassreferencesanotherclass,thevirtualmachineloadsthereferencedclassusingtheclassloaderthatloadsthefirstclass.Forexample,ifavirtualmachineloadsaVolcanoclassusingaspecificclassloader,itwillusetheclassloadertoloadallclassesusedbytheVolcanoclassSincetheJavavirtualmachinetakestheloadoftheclassinthisway,theloadedclasscanonlyseeotherclassesthatareloadedbythesameclassloaderbydefault.Inthisway,theJavaarchitectureallowsyoutocreatemultiplenamespacesinaJavaapplication.EachclassloaderintheruntimeJavaprogramhasitsownnamespace.Javaapplicationscancreatehowmany(orhowmany)classesthatareloadedbydifferentclassloadersarestoredindifferentnamespaces,andtheycannotaccesseachotherunlesstheapplicationexplicitlyallowsittodoso.WhenwritingaJavaapplication,classesthatareloadedfromdifferentsourcefilescanbeseparatedindifferentnamespaces.Inthisway,youcanusetheJavaclassloaderarchitecturetocontroltheinteractionbetweenthecodeloadedinanyofthedifferentsourcefiles,inparticular,topreventmaliciouscodefromgainingaccesstoordamagetogoodwillcode.TheWebbrowserisanexampleofadynamicextensionthatusesauser-definedclassloadertodownloadclassfilesforJavaappletsfromthenetwork.TheWebbrowserusesaJavaapplicationtoinstalltheuser-definedclassloader.Thisuser-definedclassloaderisoftenreferredtoastheJavaAppletclassloader,whichknowshowtorequestaclassfilefromanHTTPserver.JavaAppletcanbeusedasadynamicextensionoftheexample,becausetheJavaapplicationdoesnotknowwhenitwillstartfromthenetworktodownloadthebrowserrequestclassfile.OnlywhenthebrowserencounteredaJavaappletpage,itwasdecidedwhethertheneedtodownloadclassfiles.Webbrowser-initiatedJavaapplicationstypicallycreatedifferentuser-definedclassloadersforeachnetworkaddressthatprovidesclassfiles,sodifferentuser-definedclassloadersloaddifferentsourceclassfiles.WhichcanbeplacedseparatelyunderthedifferentnamespaceoftheJavahostapplication.BecausedifferentJavaappletfilesareplacedindifferentnamespaces,maliciousJavaappletcodedoesnotdirectlyaccessclassfilesdownloadedfromotherplaces.Thiscanlimitorpreventmutualaccessbetweencodefromdifferentsources.Figure2 classloaderarchitectureVI.JAVACLASSFILEJavaclassfilesmakeJavamoresuitableforthenetworkintermsofplatformindependenceandnetworkmobility.ItstaskinplatformindependenceistoprovideJavaprogramswithbinaryformsofservicesthatareindependentoftheunderlyinghostplatform.ThisapproachbreaksthetraditionofCorC++andotherlanguages,andprogramswritteninthesetraditionallanguagesareusuallycompiledfirstandthenconnectedintoseparate,binaryfilesthatspecificallysupportspecifichardwareplatformsandoperatingsystems.Often,binaryexecutablesonaplatformcannotworkonotherplatforms.JavaclassfilescanrunonanyhardwareplatformthatsupportstheJavavirtualmachineandthebinaryfilesontheoperatingsystem.Figure3javaclassfileWhencompilingandconnectingaC++program,theresultingexecutablebinariescanonlyberunonthespecifiedhardwareplatformandoperatingsystembecausethebinariescontainthemachinelanguageforthetargetprocessor.TheJavacompilertoJavasourcefileinstructionstranslatedintobytecode,thisbytecodeistheJavavirtualmachine"machinelanguage."Classfilesaredesignedsothattheycanbequicklytransmittedoverthenetwork.Second,becausetheJavaprogramisdynamicallylinkedanddynamicallyexpanded,classfilescanbedownloadedwhenneeded.ThisfeatureallowsJavaapplicationstoschedulethetimetodownloadclassfilesfromthenetwork,therebyminimizingend-userlatency.Figure4javaclassV.APIAPI(ApplicationProgrammingInterface)isapre-definedfunction,thepurposeistoprovideapplicationsanddevelopersbasedonasoftwareorhardwaretoaccessasetofroutines,withouthavingtoaccessthesourcecode,orunderstandtheinternalworkMechanismdetails.ThestandardscurrentlyappliedtotheAPIincludetheANSIstandardSQLAPI.Therearealsosomeothertypesofstandardsarestillbeingdeveloped.APIcanbeappliedtoallcomputerplatformsandoperatingsystems.TheseAPIsconnectdataindifferentformats.Eachdataformatrequiresdifferentdatacommandsandparameterstoachievethecorrectdatacommunication,butatthesametimewillproducedifferenttypesoferrors.Therefore,inadditiontotheknowledgerequiredtoperformdatasharingtasks,thesetypesofAPIsmustalsoaddressmanynetworkparameterproblemsandpossibleerrorconditions,thatis,eachapplicationmustknowwhetherithasstrongperformancesupportforinter-programcommunication.Onthecontrary,sincethisAPIonlyhandlesaninformationformat,theinformationdeliveryAPIinthiscaseonlyprovidessmallercommands,networkparameters,andsubsetoferrorconditions.Becauseofthis,deliveryAPIapproachgreatlyreducesthecomplexityofthesystem,sowhentheapplicationneedstoachievedatasharingthroughmultipleplatforms,theuseofinformationdeliveryAPItypeistheidealchoice.Informationdelivery:referstotheloosecouplingortightcouplingbetweentheapplicationofsmallformatinformation,throughthedirectcommunicationbetweenprogramstoachievedatasharing.ThestandardscurrentlyappliedtotheAPIincludetheANSIstandardSQLAPI.Therearealsosomeothertypesofstandardsarestillbeingdeveloped.APIcanbeappliedtoallcomputerplatformsandoperatingsystems.TheseAPIsconnectdataindifferentformats.Eachdataformatrequiresdifferentdatacommandsandparameterstoachievethecorrectdatacommunication,butatthesametimewillproducedifferenttypesoferrors.Therefore,inadditiontotheknowledgerequiredtoperformdatasharingtasks,thesetypesofAPIsmustalsoaddressmanynetworkparameterproblemsandpossibleerrorconditions,thatis,eachapplicationmustknowwhetherithasstrongperformancesupportforinter-programcommunication.Onthecontrary,sincethisAPIonlyhandlesaninformationformat,theinformationdeliveryAPIinthiscaseonlyprovidessmallercommands,networkparameters,andsubsetoferrorconditions.Becauseofthis,deliveryAPIapproachgreatlyreducesthecomplexityofthesystem,sowhentheapplicationneedstoachievedatasharingthroughmultipleplatforms,theuseofinformationdeliveryAPItypeistheidealchoice.Informationdelivery:referstotheloosecouplingortightcouplingbetweentheapplicationofsmallformatinformation,throughthedirectcommunicationbetweenprogramstoachievedatasharingRemoteProcedureCall(RPC):implementscommunicationbetweenprogramsbyactingonashareddatabufferStandardQueryLanguage(SQL):isthestandardaccesstothedataquerylanguage,throughthecommondatabasetoachievedatasharingbetweenapplications.FileTransfer:FileTransferDatasharingbetweenapplicationsisachievedbysendingformattedfiles.Informationdelivery:referstotheloosecouplingortightcouplingbetweentheapplicationofsmallformatinformation,throughthedirectcommunicationbetweenprogramstoachievedatasharing.Figure5javarelationshipdiagramofthecollectionclassBeforeaplatformcanevensupportJavaprograms,theAPImustbeexplicitlyimplementedonthisparticularplatform.Toaccessthelocalresourcesonthehost,theJavaAPIcallsthelocalmethod.SinceJavaAPIclassfilescalllocalmethods,Javaprogramsdonotneedtocallthemagain.Inthisway,theJavaAPIclassfileprovidesJavaprogramswithplatform-independent,standardinterfacesfortheunderlyinghost.ForJavaprograms,theJavaAPIhasthesameperformanceandpredictablebehavior,regardlessoftheplatform.ItisbecausetheJavavirtualmachineandtheJavaAPIareexplicitlyimplementedoneachparticularhostplatform,sotheJavaprogramitselfcanbecomeaplatform-independentprogramTheJavaAPIalsocontributestotheJavasecuritymodel.WhentheJavaAPImethodperformsanypotentiallydangerousoperation(suchasmakingalocaldiskwrite),itchecksthroughtheaccesscontrollertoverifythatitisauthorized.Theaccesscontrollerisaclassthatisusedtoperformastackcheckandhasdecidedwhetherornottoallowanoperation.VI.JAVACHARACTERISTICSJavahasthefollowingcharacteristics:Simple:OneoftheprinciplesofJavaistobuildasimpleandeasy-to-usesystemthatallowsuserstostartdesigningprogramswithouthavingtobetrained,soJava'ssyntaxisasmuchaspossiblethecurrentprogramissimilartomanydesignershaveusedCandC++languages.Also,JavaremovedC,andC++manyrarelyused,difficulttounderstandoroftenconfusedfunction,alotofinheritance,pointerandsoon.Object-OrientedObject-orientedObject-orientedistheconceptandtechnologywidelyusedintheinformationindustryinrecentyears.Ithasmanygoodfeatures.Objectencapsulationcanmaketheobjectinterfacedefinitionclear;inheritancecanincreasethereusabilityofthesoftware,contributetoclassificationandtemplatedesign.Implementingtheobject-orientedconceptanditsvariousgoodfeaturesisoneofthedesignconceptsofJava.Thedevelopmentofdistributedcomputernetworksmakestheapplicationofinformationtowardsadistributedenvironment,Sothemoderninformationdevelopmentlanguageandtheenvironmentshouldbewiththedistributedcharacteristicsandfunctions.Javahasanetwork-enabledlibrarythatincludestheabilitytointegratewithTCP/IPnetworkcommunicationprotocolssuchasHTTPandFTP.4.RobustTheprogramwrittenbyJavaiscapableofrunninginavarietyofsituationsandmusthavehighstability.Javainthedevelopmentofthetimetoaddtopreventthememoryisoverwrittenanddatacorruptionrelatedprocessingmechanism.Security(Java)isdesignedforuseinthenetworkanddistributedenvironment,sosecurityisaveryimportantconsideration.Javahasanumberofsimpletocomplexsecuritymeasures,caneffectivelypreventtheinvasionanddestructionofthevirusoccurred.ArchitectureNeutralTherearemanydifferenttypesofcomputersonthenetwork,andthereareverydifferentdifferencesfromthecentralprocessortotheoperatingsystem.Soitisquitedifficulttomaketheapplicationrunoneverymachine.Forthispurpose,theJavacompilercanproduceastructure-neutralobjectcodefileformat-ByteCode.Thisbytecodecanrunonmanydifferentcomputers.7.MultithreadingMultithreadingisoneofthetoolsnecessarytodevelopcomplexandpowerfulprograms,andJavaalsosupportsthisimportantfeature.Sourcefile:Useatexteditor,suchasEditorNotepadtowritesourcefiles.unavailableWordeditorbecauseitcontainsinvisiblecharacters.Willbecompiledsourcefilessaved,thesourcefileextensionmustbeJava.Compiler:Thesourcefileiscompiledbythecompiler(Javac.exe)togenerateabytecodefilewitha.Classextension.Bytecodefilesarecomposedofplatform-independentbinarycode,whichisinterpretedbytheinterpreterasalocalmachinecode.RunningJavaprograms:Javaprogramsaredividedintotwocategories-Javaapplications(Application)andJavaapplet(applet).JavaapplicationsmustpasstheJavainterpreter(java.exe)Toexplaintheimplementationofitsbytecodefile;JavaappletcanbesupportedbytheJavastandardbrowsertosolveReleaseexecution.VII.CONCLUSIONTheJavalanguageisarapiddevelopmentofthecomputerlanguageprogram,whichprofoundlyshowstheessenceofthepreparationoftheprogram,coupledwithitssimpleandrigorousstructureandsimplegrammarforitsfuturedevelopmentandmaintenanceprovidesaguarantee.Asaresultofthenetworkapplicationsupportandmultimediaaccess,willpromotetheInternetandenterprisenetworkWebapplications[6].Inaddition,inordertokeepJavagrowingandadvancingtheJavacommunity,SunannouncedtheopeningoftheJavacoresourcecodeattheJavaOneDeveloperConferencetoencouragemorepeopletoparticipateinJavacommunityactivities.FromtheJavacommunityandIBMandotherglobaltechnologypartnersintwoareasofsupport,Javatechnologyininnovationandsocialprogresscontinuetoplayastrongandimportantrole[4],andwiththedifficultyofwritingitsprogramsothatmoreprofessionalswillfocusonPlacedinthepreparationoftheJavalanguageandthedesignoftheframework[3].ACKNOWLEDGMENTThereportedworkherewasconductedincollaborationwithRossJACKSON(ENSIMAG,France)andKellyTOMWELL(UniversitedePierreetCurie,France)whentheyvisitedNIIduringtheirinternshipinthesummerof2003.REFERENCESR.J(Bud)Bates.GPRS:GeneralPacketRadioService.McGraw-Hill,2003,.Cameron,J.R.JSP&JSDtheJacksonapproachtosoftwaredevelopment.SilverSpringMD:IEEEComputerSocietyPress,1983,.M.J.King,J.P.Pardoe.ProgramdesignusingJSPapracticalintroduction.BasingstokeHampshire:Macmillan,1992,.JamesGoodwill.PureJavaServerPages.IndianapolisInd:Sams,2000,.GwenaelLeBodic.MobilemessagingtechnologiesandservicesSMSEMSandMMS.NewYork:J.Wiley,2005,.[6]HenryLabord(?)re,VincentJonack.SMSandMMSinterworkinginmobilenetworks.Boston:ArtechHouse,2004,CS1和CS2:用Java編寫電腦游戲摘要Java已經(jīng)成為一個龐大而復雜的技術平臺，對于開發(fā)人員而言，要想更好的掌握Java技術，深入理解底層的技術處理細節(jié)必不可少。對核心概念和思想的掌握可以幫助我們舉一反三、觸類旁通，有助于提升我們對整個Java平臺的理解力。這里所介紹的是Java技術平臺的幾個核心概念，其中所蘊含的思想有助于我們更深刻的理解Java技術。關鍵字：Java；虛擬機；Class文件；API;1引言Java是一門面向?qū)ο缶幊陶Z言，不僅吸收了C++語言的各種優(yōu)點，還摒棄了C++里難以理解的多繼承、指針等概念，因此Java語言具有功能強大和簡單易用兩個特征。Java語言作為靜態(tài)面向?qū)ο缶幊陶Z言的代表，極好地實現(xiàn)了面向?qū)ο罄碚?，允許程序員以優(yōu)雅的思維方式進行復雜的編程[1]。Java具有簡單性、面向?qū)ο?、分布式、健壯性、安全性、平臺獨立與可移植性、多線程、動態(tài)性等特點[2]。Java可以編寫桌面應用程序、Web應用程序、分布式系統(tǒng)和嵌入式系統(tǒng)應用程序等[3]。2Java虛擬機Java虛擬機的主要任務是裝在class文件并且執(zhí)行其中的字節(jié)碼。Java虛擬機包含一個類裝載器，它可以從程序和API中裝載class文件。JavaAPI中只有程序執(zhí)行時需要的那些類才會被裝載。字節(jié)碼由執(zhí)行引擎來執(zhí)行。

不同的Java虛擬機中，執(zhí)行引擎可能實現(xiàn)得非常不同。在由軟件實現(xiàn)的虛擬機中，最簡單的執(zhí)行引擎就是一次性解釋字節(jié)碼。另一種執(zhí)行引擎更快，但是也更消耗內(nèi)存，叫做"即時編譯器(just-in-timecompiler)"。在這種情況下，第一次被執(zhí)行的字節(jié)碼會被編譯成本地機器代碼。編譯出的本地機器代碼會被緩存，當方法以后被調(diào)用的時候可以重用。第三種執(zhí)行引擎是自適應優(yōu)化器。在這種方法里，虛擬機開始的時候解釋字節(jié)碼，但是會監(jiān)視運行中程序的活動，并且記錄下使用最頻繁的代碼段。程序運行的時候，虛擬機只把那些活動最頻繁的代碼編譯成本地代碼，其他的代碼由于使用得不是很頻繁，繼續(xù)保留為字節(jié)碼-由虛擬機繼續(xù)解釋它們。一個自適應的優(yōu)化器可以使得Java虛擬機在80%~90%的時間里執(zhí)行被優(yōu)化過的本地代碼，而只需要編譯10%~20%的對性能有影響的代碼。

當Java虛擬機是由主機操作系統(tǒng)上的軟件實現(xiàn)的時候，Java程序通過調(diào)用本地方法(nativemethod)和主機交互。Java中有兩種方法:Java方法和本地方法。Java方法是由Java語言編寫，編譯成字節(jié)碼文件，存儲在class文件中的。本地方法是由其他語言(比如c,c++或匯編語言)編寫的，編譯成何處理器相關的機器代碼。本地方法保存在動態(tài)鏈接庫中,格式是各個平臺專有的。運行中Java程序調(diào)用本地方法時，虛擬機裝載包含這個本地方法的動態(tài)庫，并調(diào)用這個方法。本地方法是聯(lián)系Java程序和底層主機操作系統(tǒng)的連接方法。圖1Java虛擬機工作原理

通過本地方法，Java程序可以直接訪問底層操作系統(tǒng)的資源。一個本地方法接口(JavaNativeInterface,JNI)-使得本地方法可以在特定主機系統(tǒng)的任何一個Java平臺實現(xiàn)上運行。

如果希望使用特定主機上的資源，它們又無法從JavaAPI訪問，那么可以寫一個平臺相關的Java程序來調(diào)用本地方法。如果希望保證程序的平臺無關性，那么只能通過JavaAPI來訪問底層系統(tǒng)資源。3類裝載器的體系結(jié)構

一個Java應用程序可以使用兩種類裝載器："啟動(bootstrap)"類裝載器和用戶定義的類裝載器。啟動類裝載器(這是系統(tǒng)中唯一的)是Java虛擬機實現(xiàn)的一部分。啟動類裝載器通常使用某種默認方式從本地磁盤中裝載類，包括JavaAPI類(啟動類裝載器也被稱為原始類器、系統(tǒng)類裝載器或者默認類裝載器)。

Java應用程序能夠在運行時安裝用戶定義的類裝載器，這種類裝載器能夠使用自定義的方式來裝載類。例如，從網(wǎng)絡下載class文件。盡管啟動類裝載器是虛擬機實現(xiàn)的本質(zhì)部分，而用戶定義的類裝載器不是，但用戶定義的類裝載器能夠用Java來編寫,能夠被編譯成class文件，能夠被虛擬機裝載，還能夠像其它對象一樣實例化。

由于有用戶定義類裝載器，所以不必再編譯的時候就知道運行中的Java應用程序中最終會加入的所有的類。用戶定義的類裝載器使得在運行擴展Java應用程序成為可能。當它運行時，應用程序能夠解決它需要哪些額外的類，能夠決定是使用一個或是更多的用戶定義的類裝載器來裝載。由于類裝載器是用Java編寫的，所以用任何在Java代碼中可以表述的風格來進行類裝載。這些類可以通過網(wǎng)絡下載，可以從某些數(shù)據(jù)庫中獲取，甚至可以動態(tài)生成。

每一個類被裝載的時候，Java虛擬機都監(jiān)視這個類，看到它到底是被啟動類裝載器還是被用戶定義類裝載器裝載。當被裝載的類引用了另外一個類時，虛擬機就會使用裝載第一個類的類裝載器裝載引用的類。例如，如果虛擬機使用一個特定的類裝載器裝載Volcano這個類，它就會使用這個類裝載器裝載Volcano類使用的所有類。

由于Java虛擬機采取這種方式進行類的裝載，所以被裝載的類默認情況下只能看到被同一個類裝載器裝載的別的類。通過這種方法，Java的體系結(jié)構允許在一個Java應用程序中建立多個命名空間。運行時的Java程序中的每一個類裝載器都有自己的命名空間。

Java應用程序可以創(chuàng)建多少個(或多少種)被不同的類裝載器裝載的類存放在不同的命名空間中，它們不能相互訪問，除非應用程序顯示地允許這么做。當編寫一個Java應用程序的時候，從不同源文件裝載的類可以分隔在不同的命名空間中。通過這種方法，就能夠使用Java類裝載器的體系結(jié)構來控制任何不同源文件中裝載的代碼之間的相互影響，特別是能夠阻止惡意代碼獲取訪問或破壞善意代碼的權限。

Web瀏覽器是一個動態(tài)擴展的例子，Web瀏覽器使用用戶定義的類裝載器從網(wǎng)絡下載用于Javaapplet的class文件。Web瀏覽器使用一個用來安裝用戶定義類裝載器的Java應用程序。這個用戶定義的類裝載器通常被稱為JavaApplet類裝載器，它知道如何向HTTP服務器請求class文件。JavaApplet可以作為動態(tài)擴展的例子，因為Java應用程序并不知道它什么時候會開始從網(wǎng)絡下載瀏覽器請求的class文件。只有當瀏覽器遇到有Javaapplet的頁面時，才決定是否需要下載class文件。

Web瀏覽器啟動的Java應用程序通常為每個提供class文件的網(wǎng)絡地址分別創(chuàng)建不同的用戶定義類裝載器，因此，不同的用戶定義類裝載器裝載不同來源的class文件。這就可以把它們分別放置在Java主機應用程序的不同命名空間之下。由于不同來源的Javaapplet文件放置在不同的命名空間中，惡意的Javaapplet代碼就不會直接訪問從別的地方下載的class文件。這就能夠限制或阻止不同來源的代碼之間的相互訪問。圖2類裝載器的體系結(jié)構4Javaclass文件

Javaclass文件主要在平臺無關性和網(wǎng)絡移動性方面使Java更適合網(wǎng)絡。它在平臺無關性方面的任務是：為Java程序提供獨立于底層主機平臺的二進制形式的服務。這種途徑途徑打破了C或者C++等語言所遵循的傳統(tǒng)，使用這些傳統(tǒng)語言寫的程序通常首先被編譯，然后被連接成單獨的、專門支持特定硬件平臺和操作系統(tǒng)的二進制文件。通常情況下，一個平臺上的二進制可執(zhí)行文件不能在其他平臺上工作。而Javaclass文件時可以運行在任何支持Java虛擬機的硬件平臺和操作系統(tǒng)上的二進制文件。圖3Java類文件

當編譯和連接一個C++程序時，所獲得的可執(zhí)行二進制文件只能在指定的硬件平臺和操作系統(tǒng)上運行，因為這個二進制文件包含了對目標處理器的機器語言。而Java編譯器把Java源文件的指令翻譯成字節(jié)碼，這種字節(jié)碼就是Java虛擬機的"機器語言"。

class文件設計得緊湊，因此它們可以快速地在網(wǎng)絡上傳送。其次，由于Java程序是動態(tài)連接和動態(tài)擴展的，class文件可以在需要的時候才下載。這個特點使得Java應用程序能夠安排從網(wǎng)絡上下載class文件的時間，從而可以最大限度地減少終端用戶的等待時間。圖4JavaAPI5JavaAPI

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