java面向特定群体的健康管理平台设计与实现毕业设计英文文献翻译.docx

java面向特定群体的健康管理平台设计与实现毕业设计英文文献翻译.docx

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java面向特定群体的健康管理平台设计与实现毕业设计英文文献翻译

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2014年6月

ExploreOnJAVA,JSPTechnologyAndThreeFrameworks

Duke,theJavamascotJamesGosling,MikeSheridan,andPatrickNaughtoninitiatedtheJavalanguageprojectinJune1991.Javawasoriginallydesignedforinteractivetelevision,butitwastooadvancedforthedigitalcabletelevisionindustryatthetime.ThelanguagewasinitiallycalledOakafteranoaktreethatstoodoutsideGosling'soffice;itwentbythenameGreenlater,andwaslaterrenamedJava,fromalistofrandomwords.GoslingaimedtoimplementavirtualmachineandalanguagethathadafamiliarC/C++styleofnotation.

SunMicrosystemsreleasedthefirstpublicimplementationasJava1.0in1995.Itpromised"WriteOnce,RunAnywhere",providingno-costrun-timesonpopularplatforms.Fairlysecureandfeaturingconfigurablesecurity,itallowednetwork-andfile-accessrestrictions.MajorwebbrowserssoonincorporatedtheabilitytorunJavaappletswithinwebpages,andJavaquicklybecamepopular.WiththeadventofJava2,newversionshadmultipleconfigurationsbuiltfordifferenttypesofplatforms.Forexample,J2EEtargetedenterpriseapplicationsandthegreatlystripped-downversionJ2MEformobileapplications,J2SEdesignatedtheStandardEdition.In2006,formarketingpurposes,SunrenamednewJ2versionsasJavaEE,JavaME,andJavaSE,respectively.

In1997,SunMicrosystemsapproachedtheISO/IECJTC1standardsbodyandlatertheEcmaInternationaltoformalizeJava,butitsoonwithdrewfromtheprocess.Javaremainsadefactostandard,controlledthroughtheJavaCommunityProcess.Atonetime,SunmademostofitsJavaimplementationsavailablewithoutcharge,despitetheirproprietarysoftwarestatus.SungeneratedrevenuefromJavathroughthesellingoflicensesforspecializedproductssuchastheJavaEnterpriseSystem.SundistinguishesbetweenitsSoftwareDevelopmentKit（SDK）andRuntimeEnvironment（JRE）（asubsetoftheSDK）;theprimarydistinctioninvolvestheJRE'slackofthecompiler,utilityprograms,andheaderfiles.OnNovember13,2006,SunreleasedmuchofJavaasopensourcesoftwareunderthetermsoftheGNUGeneralPublicLicense（GPL）.OnMay8,2007,Sunfinishedtheprocess,makingallofJava'scorecodeavailableunderfreesoftware/open-sourcedistributionterms,asidefromasmallportionofcodetowhichSundidnotholdthecopyright.Sun'svice-presidentRichGreenhassaidthatSun'sidealrolewithregardstoJavaisasan"evangelist."FollowingOracleCorporation'sacquisitionofSunMicrosystemsin2009–2010,Oraclehasdescribeditselfasthe"stewardofJavatechnologywitharelentlesscommitmenttofosteringacommunityofparticipationandtransparency".

PrinciplesTherewerefiveprimarygoalsinthecreationoftheJavalanguage:

1.Itshouldbe"simple,objectoriented,andfamiliar"2.Itshouldbe"robustandsecure".

3.Itshouldbe"architectureneutralandportable"4.Itshouldexecutewith"highperformance"5.Itshouldbe"interpreted,threaded,anddynamic".

JavaPlatformMainarticles:

Java（softwareplatform）andJavaVirtualMachineOnecharacteristicofJavaisportability,whichmeansthatcomputerprogramswrittenintheJavalanguagemustrunsimilarlyonanysupportedhardware/operating-systemplatform.ThisisachievedbycompilingtheJavalanguagecodetoanintermediaterepresentationcalledJavabytecode,insteadofdirectlytoplatform-specificmachinecode.Javabytecodeinstructionsareanalogoustomachinecode,butareintendedtobeinterpretedbyavirtualmachine（VM）writtenspecificallyforthehosthardware.End-userscommonlyuseaJavaRuntimeEnvironment（JRE）installedontheirownmachineforstandaloneJavaapplications,orinaWebbrowserforJavaapplets.Standardizedlibrariesprovideagenericwaytoaccesshost-specificfeaturessuchasgraphics,threading,andnetworking.

Amajorbenefitofusingbytecodeisporting.However,theoverheadofinterpretationmeansthatinterpretedprogramsalmostalwaysrunmoreslowlythanprogramscompiledtonativeexecutableswould.Just-in-Timecompilerswereintroducedfromanearlystagethatcompilebytecodestomachinecodeduringruntime.ImplementationsSunMicrosystemsofficiallylicensestheJavaStandardEditionplatformforLinux,MacOSX,andSolaris.AlthoughinthepastSunhaslicensedJavatoMicrosoft,thelicensehasexpiredandhasnotbeenrenewed.Throughanetworkofthird-partyvendorsandlicensees,alternativeJavaenvironmentsareavailablefortheseandotherplatforms.

Sun'strademarklicenseforusageoftheJavabrandinsiststhatallimplementationsbe"compatible".ThisresultedinalegaldisputewithMicrosoftafterSunclaimedthattheMicrosoftimplementationdidnotsupportRMIorJNIandhadaddedplatform-specificfeaturesoftheirown.Sunsuedin1997,andin2001wonasettlementofUS$20million,aswellasacourtorderenforcingthetermsofthelicensefromSun.Asaresult,MicrosoftnolongershipsJavawithWindows,andinrecentversionsofWindows,InternetExplorercannotsupportJavaappletswithoutathird-partyplugin.Sun,andothers,havemadeavailablefreeJavarun-timesystemsforthoseandotherversionsofWindows.Platform-independentJavaisessentialtotheJavaEEstrategy,andanevenmorerigorousvalidationisrequiredtocertifyanimplementation.Thisenvironmentenablesportableserver-sideapplications,suchasWebservices,JavaServlets,andEnterpriseJavaBeans,aswellaswithembeddedsystemsbasedonOSGi,usingEmbeddedJavaenvironments.ThroughthenewGlassFishproject,Sunisworkingtocreateafullyfunctional,unifiedopensourceimplementationoftheJavaEEtechnologies.SunalsodistributesasupersetoftheJREcalledtheJavaDevelopmentKit（commonlyknownastheJDK）,whichincludesdevelopmenttoolssuchastheJavacompiler,Javadoc,Jar,anddebugger.

Javaperformanceandgarbagecollectors

ProgramswritteninJavahaveareputationforbeingslowerandrequiringmorememorythanthosewritteninC.However,Javaprograms'executionspeedimprovedsignificantlywiththeintroductionofJust-in-timecompilationin1997/1998forJava1.1,theadditionoflanguagefeaturessupportingbettercodeanalysis（suchasinnerclasses,StringBufferclass,optionalassertions,etc.）,andoptimizationsintheJavaVirtualMachineitself,suchasHotSpotbecomingthedefaultforSun'sJVMin2000.Currently,JavacodehasapproximatelyhalftheperformanceofCcode.

SomeplatformsofferdirecthardwaresupportforJava;therearemicrocontrollersthatcanrunjavainhardwareinsteadofasoftwareJVM,andARMbasedprocessorscanhavehardwaresupportforexecutingJavabytecodethroughitsJazelleoption.

Automaticmemorymanagementjavausesanautomaticgarbagecollectortomanagememoryintheobjectlifecycle.Theprogrammerdetermineswhenobjectsarecreated,andtheJavaruntimeisresponsibleforrecoveringthememoryonceobjectsarenolongerinuse.Oncenoreferencestoanobjectremain,theunreachablememorybecomeseligibletobefreedautomaticallybythegarbagecollector.Somethingsimilartoamemoryleakmaystilloccurifaprogrammer'scodeholdsareferencetoanobjectthatisnolongerneeded,typicallywhenobjectsthatarenolongerneededarestoredincontainersthatarestillinuse.Ifmethodsforanonexistentobjectarecalled,a"nullpointerexception"isthrown.

OneoftheideasbehindJava'sautomaticmemorymanagementmodelisthatprogrammerscanbesparedtheburdenofhavingtoperformmanualmemorymanagement.Insomelanguages,memoryforthecreationofobjectsisimplicitlyallocatedonthestack,orexplicitlyallocatedanddeallocatedfromtheheap.Inthelattercasetheresponsibilityofmanagingmemoryresideswiththeprogrammer.Iftheprogramdoesnotdeallocateanobject,amemoryleakoccurs.Iftheprogramattemptstoaccessordeallocatememorythathasalreadybeendeallocated,theresultisundefinedanddifficulttopredict,andtheprogramislikelytobecomeunstableand/orcrash.Thiscanbepartiallyremediedbytheuseofsmartpointers,buttheseaddoverheadandcomplexity.Notethatgarbagecollectiondoesnotprevent"logical"memoryleaks,i.e.thosewherethememoryisstillreferencedbutneverused.

Garbagecollectionmayhappenatanytime.Ideally,itwilloccurwhenaprogramisidle.Itisguaranteedtobetriggeredifthereisinsufficientfreememoryontheheaptoallocateanewobject;thiscancauseaprogramtostallmomentarily.ExplicitmemorymanagementisnotpossibleinJava.

JavadoesnotsupportC/C++stylepointerarithmetic,whereobjectaddressesandunsignedintegers（usuallylongintegers）canbeusedinterchangeably.Thisallowsthegarbagecollectortorelocatereferencedobjectsandensurestypesafetyandsecurity.AsinC++andsomeotherobject-orientedlanguages,variablesofJava'sprimitivedatatypesarenotobjects.Valuesofprimitivetypesareeitherstoreddirectlyinfields（forobjects）oronthestack（formethods）ratherthanontheheap,ascommonlytrueforobjects（butseeEscapeanalysis）.ThiswasaconsciousdecisionbyJava'sdesignersforperformancereasons.Becauseofthis,Javawasnotconsideredtobeapureobject-orientedprogramminglanguage.However,asofJava5.0,autoboxingenablesprogrammerstoproceedasifprimitivetypeswereinstancesoftheirwrapperclass.Javacontainsmultipletypesofgarbagecollectors.Bydefault,HotSpotusestheConcurrentMarkSweepcollector,alsoknownastheCMSGarbageCollector.However,therearealsoseveralothergarbagecollectorsthatcanbeusedtomanagetheHeap.For90%ofapplicationsinJa