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History

Duke,theJavamascotJamesGosling,MikeSheridan,andPatrickNaughtoninitiatedtheJavalanguageprojectinJune1991.Javawasoriginallydesignedforinteractivetelevision,butitwastooadvancedforthedigitalcabletelevisionindustryatthetime.ThelanguagewasinitiallycalledOakafteranoaktreethatstoodoutsideGosling'

soffice;

itwentbythenameGreenlater,andwaslaterrenamedJava,fromalistofrandomwords.GoslingaimedtoimplementavirtualmachineandalanguagethathadafamiliarC/C++styleofnotation.

SunMicrosystemsreleasedthefirstpublicimplementationasJava1.0in1995.Itpromised"

WriteOnce,RunAnywhere"

（WORA）,providing

no-costrun-timesonpopularplatforms.Fairlysecureandfeaturingconfigurablesecurity,itallowednetwork-andfile-accessrestrictions.MajorwebbrowserssoonincorporatedtheabilitytorunJavaappletswithinwebpages,andJavaquicklybecamepopular.WiththeadventofJava2（releasedinitiallyasJ2SE1.2inDecember1998–1999）,newversionshadmultipleconfigurationsbuiltfordifferenttypesofplatforms.Forexample,J2EEtargetedenterpriseapplicationsandthegreatlystripped-downversionJ2MEformobileapplications（MobileJava）.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）.On

May8,2007,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）andJavaVirtualMachine

OnecharacteristicofJavaisportability,whichmeansthatcomputerprogramswrittenintheJavalanguagemustrunsimilarlyonanysupportedhardware/operating-systemplatform.ThisisachievedbycompilingtheJavalanguagecodetoanintermediaterepresentationcalledJavabytecode,insteadofdirectlytoplatform-specificmachinecode.Java

bytecodeinstructionsareanalogoustomachinecode,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,as

wellasacourtorderenforcingthetermsofthelicensefromSun.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.）,andoptimizationsintheJava

VirtualMachineitself,suchasHotSpotbecomingthedefaultforSun'

sJVMin2000.Currently,JavacodehasapproximatelyhalftheperformanceofCcode.

SomeplatformsofferdirecthardwaresupportforJava;

therearemicrocontrollersthatcanrunjavainhardwareinsteadofasoftwareJVM,andARMbasedprocessorscanhavehardwaresupportforexecutingJavabytecodethroughitsJazelleoption.

AutomaticmemorymanagementJavausesanautomaticgarbagecollectortomanagememoryintheobjectlifecycle.Theprogrammerdetermineswhenobjectsarecreated,andtheJavaruntimeisresponsibleforrecoveringthememoryonceobjectsarenolongerinuse.Oncenoreferencestoanobjectremain,theunreachablememorybecomeseligibletobefreedautomaticallybythegarbagecollector.Somethingsimilartoamemoryleakmaystilloccurifaprogrammer'

scodeholdsareferencetoanobjectthatisnolongerneeded,typicallywhenobjectsthatarenolongerneededarestoredincontainersthatarestillinuse.Ifmethodsforanonexistentobjectarecalled,a"

nullpointerexception"

isthrown.

OneoftheideasbehindJava'

sautomaticmemorymanagementmodelisthatprogrammerscanbesparedtheburdenofhavingtoperformmanualmemorymanagement.Insomelanguages,memoryforthecreationofobjectsisimplicitlyallocatedonthestack,orexplicitlyallocatedanddeallocatedfromtheheap.Inthelattercasethe

responsibilityofmanagingmemoryresideswiththeprogrammer.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,autoboxingenablesprogrammerstoproceedasifprimitivetypeswereinstancesoftheirwrappercl