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英文原文Java2StandardEditionandtheWorldofJava1IntroductionThecomputerrevolutionofthe1970sincreasedthedemandforsophisticatedcomputersoftwaretotakeadvantageoftheeverincreasingcapacityofcomputerstoprocessdata.TheCprogramminglanguagebecamethelinchpinthatenabledprogrammerstobuildsoftwarethatwasjustasrobustasthecomputeritranon.Asthe1980sapproached,programmerswerewitnessinganotherspurtintheevolutionofprogramminglanguage.ComputertechnologyadvancedbeyondthecapabilitiesoftheCprogramminglanguage.Theproblemwasntnew.Itoccurredpreviouslyandcausedthedemiseofgenerationsofprogramminglanguages.Theproblemwasthatprogramswerebecomingtoocomplicatedtodesign,write,andmanagetokeepupwiththecapabilitiesofcomputers.ItwasaroundthistimethatadesignconceptbasedonSimula67andSmalltalkfromthelate1960smovedprogrammingtothenextevolutionarystep.ThiswastheperiodwhenobjectorientedprogrammingOOP,andwithitanewprogramminglanguagecalledC,tookprogrammersbystorm.In1979,BjarneStroustrupofBellLaboratoriesinNewJerseyenhancedtheCprogramminglanguagetoincludeobjectorientedfeatures.HecalledthelanguageC.TheistheincrementaloperatorintheCprogramminglanguage.CistrulyanenhancementoftheCprogramminglanguage,anditbeganasapreprocessorlanguagethatwastranslatedintoCsyntaxbeforetheprogramwasprocessedbythecompiler.StroustrupbuiltontheconceptofaclasstakenfromSimula67andSmalltalk,fromwhichinstancesofobjectsarecreated.Aclasscontainsdatamembersandmemberfunctionsthatdefineanobjectsdataandfunctionality.Healsointroducedtheconceptofinheritance,whichenabledaclasstoinheritsomeoralldatamembersandmemberfunctionsfromoneormoreotherclassesallofwhichcomplementstheconceptsofobjectorientedprogramming.2EnterJavaJustasCwasbecomingthelanguageofchoiceforbuildingindustrialstrengthapplications,anothergrowthspurtintheevolutionofprogramminglanguagewasbudding,fertilizedbythelatestdisruptivetechnologytheWorldWideWeb.TheInternethadbeenawellkeptsecretfordecadesbeforetheNationalScienceFoundationwhooversawtheInternetremovedbarriersthatpreventedcommercialization.Until1991whenitwasopenedtocommerce,theInternetwasthealmostexclusivedomainofgovernmentagenciesandtheacademiccommunity.Oncethebarriertocommercializationwaslifted,theWorldWideWeboneofseveralservicesofferedontheInternetbecameavirtualcommunitycenterwherevisitorscouldgetfreeinformationaboutpracticallyanythingandbrowsethroughthousandsofvirtualstores.BrowserspowertheWorldWideWeb.AbrowsertranslatesASCIItextfileswritteninHTMLintoaninteractivedisplaythatcanbeinterpretedonanymachine.AslongasthebrowseriscompatiblewiththecorrectversionofHTMLandHTTPimplementation,anycomputerrunningthebrowsercanusethesameHTMLdocumentwithouthavingtomodifyitforaparticulartypeofcomputer,whichwassomethingunheardofatthetime.ProgramswritteninCorCaremachinedependentandcannotrunonadifferentmachineunlesstheprogramisrecompiled.ThesuccessoftheInternetgaverenewedfocustodevelopingamachineindependentprogramminglanguage.AndthesameyeartheInternetwascommercialized,fivetechnologistsatSunMicrosystemssetouttodojustthat.JamesGosling,PatrickNaughton,ChrisWarth,EdFrank,andMikeSheridanspent18monthsdevelopingtheprogramminglanguagetheycalledOak,whichwasrenamedJavawhenthisnewlanguagemadeitsdebutin1995.Javawentthroughnumerousiterationsbetween1991and1995,duringwhichtimemanyothertechnologistsatSunmadesubstantialcontributionstothelanguage.TheseincludedBillJoy,ArthurvanHoff,JonathanPayne,FrankYelin,andTimLindholm.AlthoughJavaiscloselyassociatedwiththeInternet,itwasdevelopedasalanguageforprogrammingsoftwarethatcouldbeembeddedintoelectronicdevicesregardlessofthetypeofCPUusedbythedevice.ThisisknownastheEmbeddedJavaplatformandisincontinuoususetodayforclosedsystems.TheJavateamfromSunsucceededincreatingaportableprogramminglanguage,somethingthathadeludedprogrammerssincecomputerswerefirstprogrammed.Theirsuccess,however,wasfarbeyondtheirwildestdreams.ThesameconceptusedtomakeJavaprogramsportabletoelectronicdevicesalsocouldbeusedtomakeJavaprogramsrunoncomputersrunningMicrosoftWindows,UNIX,andMacintosh.Timingwasperfect.TheInternet/intranethadwhettedcorporateAmericasappetiteforcosteffective,portableprogramsthatcouldreplacemissioncriticalapplicationswithinthecorporation.AndJavahadprovenitselfasaprogramminglanguageusedtosuccessfullydevelopmachineindependentapplications.3JavaVirtualMachineJavavirtualmachineJVMismethodforthespecificationofthecomputerequipment.Theavailabilityofdifferentwayssoftwareorhardwarecanbeimplemented.Compilingthevirtualmachineinstructionset,thecompilerofthemicroprocessorinstructionsetisverysimilar.TheJavavirtualmachineincludesasetofbytecodeinstructionset,asetofregisters,astack,agarbagecollectedheapandastoragedomain.JavaVirtualMachineJVMisanimaginarycomputerthatcanrunJavacode.AccordingtotheJVMspecificationdescribestheinterpreterportedtoaparticularcomputer,wecanguaranteethecompiledJavacodetorunonthesystem.TheJavaVirtualMachineisanimaginarymachineontheactualcomputersoftwaresimulation.Javavirtualmachinehardwareofhisownimagining,suchasprocessor,stack,registers,etc.,butalsohasacorrespondinginstruction.4BacktotheFutureJ2EEJ2EEisacompletelydifferentfromthetraditionalapplicationdevelopmenttechnologyarchitecture,containsanumberofcomponents,mainlytosimplifyandstandardizetheapplicationsystemdevelopmentanddeployment,andtoimproveportability,securityandreusevalue.J2EECoreisasetoftechnicalspecificationsandguidelines,whichincludevarioustypesofcomponents,services,structureandtechnicallevelhavecommonstandardsandspecifications,thevariousfollowtheJ2EEarchitecturebetweendifferentplatforms,thereisagoodcompatibility,solveenterprisebackendinformationproductsnotcompatiblewitheachother,leadingtothedifficultdilemmabetweentheinternalorexternal.IntheJ2EEarchitecture,developerscanfollowthenormativebasis,ledtothedevelopmentofenterpriseapplicationsdifferentJ2EEsuppliers,withsupportdifferentversionsofJ2EEstandardselaborated,inordertoensurecompatibilitybetweenthedifferentJ2EEplatformsandproducts.Inotherwords,theapplicationofplantbasedJ2EEarchitecture,canbasicallybedeployedindifferentapplicationservers,nooronlyneedasmallamountofcodechanges,whichcansignificantlyimprovetheportabilityoftheapplicationsystemPortability.TheJ2EEmaincollaborativeindustrycommontechnicalspecificationsdrawnupbytheSunSUNwithIBMandothermanufacturers,orientedtobusinesstobusinessoperationsbetweentheJavadevelopmentenvironment.J2EEarchitecturedefinesthedifferenttypesofcomponents,suchasWebComponent,EJBComponentandallkindsofcomponentscanbereusedreuse,sothathasbeendevelopedcomponents,orcomponentsderivedthroughthepurchaseonthemarket,canfurtherassembledintothetypeofsystem.Fordevelopersonlyneedtofocusonthebusinesslogicofthesystemarchitecturedesignforavarietyofapplications,Asforthewritingoftheunderlyingcumbersomeprocedures,canbeusedwithdifferentdevelopmentplatformtoenablethedevelopmentanddeploymentofapplicationsystemthatincreasestheefficiency.CorespecificationforJ2EEEnterpriseisJavaBeansEJBs.EJBaccordingtodifferentcharacteristics,dividedintothreetypes,namelySessionBean,EntityBean,MessageDrivenBean.SessionBeanandEntitybeansregardedastheancestoroftheEJBandbothEJBspecificationsalreadyexistintheEJB1.xversionandMessageDrivenBeaninEJB2.0specifications.TheindustryprogrammersorwebdesignersmakemoreuseoftheconvenienceofJSP/Servlet,whichdevelopedabovetheJ2EEserverapplication,ortheintegrationofvariousresourceswithinthecompany.5TheadvantagesofJ2EEJ2EEbuildscalability,flexibility,andeaseofmaintenanceofbusinesssystemsprovidesagoodmechanism1.PreservetheexistingITassetsCompaniesmustadapttothenewbusinessneeds,theuseofexistingenterpriseinformationsystemsinvestments,ratherthanreenactacomprehensiveprogrambecomesveryimportant.Thus,anincrementalratherthanaradicalnegatedcreateserversideplatformontopoftheexistingsystemmechanismiswantedcompany.J2EEarchitecturecantakefulladvantageoftheusersoriginalinvestment,suchasusedbysomecompaniesBEATuxedo,IBMCICS,IBMEncina,InpriseVisiBrokerandNetscapeApplicationServer.ThiswasmadepossiblebecausetheJ2EEhasbroadindustrysupportandsomeimportantareasofenterprisecomputingtheparticipationofsuppliers.EachsupplierexistingcustomersupgradepathwithoutabandoningexistinginvestmentstoenterthefieldofportableJ2EE.BasedonJ2EEplatformproductsalmostrunonanyoperatingsystemandhardwareconfigurationcanalsobereservedforusebytheoperatingsystemandhardware.2.EfficientdevelopmentJ2EEallowssomegeneric,verycumbersometaskservertotheintermediatesuppliertocomplete.Sothatdeveloperscanfocusonhowtocreatebusinesslogic,toshortenthedevelopmenttime.Advancedmiddlewarevendorsprovidethesecomplexmiddlewareservicesstatemanagementserviceenablesdeveloperstowritelesscode,donotcareabouthowtomanagethestate,thiscanbedonefasterprogramdevelopment.persistentmanagementservicesothatdevelopersdonotneedtoencodethedataaccesslogicwillbeabletowriteapplications,cangeneratemorecompact,withadatabaseindependentapplications,suchapplicationseasiertodevelopandmaintain.distributedshareddataobjectcacheserviceallowsdeveloperstothepreparationofhighperformancesystem,whichgreatlyimprovethescalabilityoftheoveralldeployment.3.SupportheterogeneousenvironmentsJ2EEdeploymentcanbeabletoinheterogeneousenvironmentstodevelopportableprograms.J2EEbasedapplicationdoesnotrelyonanyparticularoperatingsystem,middlewareandhardware.Therefore,therationaldesignofJ2EEbasedprogramtodeveloponcedeployedtoavarietyofplatforms.Thisinatypicalheterogeneousenterprisecomputingenvironmentiscrucial.TheJ2EEstandardalsoallowscustomerstoorderJ2EEcompliantthirdpartyofftheshelfcomponents,deploythemtoaheterogeneousenvironment,savingthecostoftheentireprogramdevelopedbytheirown.4.ScalabilityThecompanieswillhavetochooseaserversideplatformthatshouldbeabletoprovideexcellentscalabilitytomeetintheirsystemonalargenumberofnewcustomersinthecommercialoperation.J2EEplatformbasedapplicationscanbedeployedonavarietyofoperatingsystems.Forexample,canbedeployedtohighendUNIXandmainframesystems,thissystemsupports64256processors,whichisbeyondthereachofNTservertheJ2EEfieldofsupplierstoprovideawiderrangeofloadbalancingstrategy.Toeliminatebottlenecksinthesystem,allowingmultipleserversintegrateddeployment.Suchadeploymentofuptothousandsofprocessors,toachieveahighlyscalablesystemtomeetthefutureneedsofbusinessapplications.5.StableAvailabilityAserversideplatformmustbeabletooperatearoundtheclocktomeettheneedsofourcustomers,partners.BecausetheInternetisglobal,ubiquitous,evenatnightasplanneddowntimemayalsocauseseriousdamage.Ifunplanneddowntime,itwillhavedisastrousconsequences.J2EEdeploymenttoareliableoperatingenvironment,theysupportthelongtermavailability.SomeJ2EEdeploymentintheWindowsenvironment,thecustomercanalsochooserobustnessbetteroperatingsystemssuchasSunSolaris,IBMOS/390.Therobustnessofthebestoperatingsystemupto99.999availability,oronlyfiveminutesdowntime.Thisistheidealchoiceforstrongrealtimebusinesssystems.
编号:201311171517576767    大小:109.50KB    格式:DOC    上传时间:2013-11-17
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