外文文獻：EvolvingJavaWithoutChangingtheLanguageIn"TheFeelofJava"JamesGoslingstatedthat:Javaisabluecollarlanguage.It'snotPhDthesismaterialbutalanguageforajob.JavafeelsveryfamiliartomanydifferentprogrammersbecauseIhadaverystrongtendencytopreferthingsthathadbeenusedalotoverthingsthatjustsoundedlikeagoodidea.TheextraordinarysuccessofJavaoffersweighttothenotionthatthiswasasensibleapproach,andifitremainsanimportantgoalforJavatoday,thenitmakessensethatthelanguageshouldcontinuetoevolverelativelyslowly.Inadditiontothis,thefactthatJavaisamature,widelyusedlanguagecausesitsevolutiontobefraughtwithdifficulty.Foronething,eachfeatureaddedtothelanguagecanchangethewayitfeelsinsubtleandoftenunpredictableways,riskingalienatingdeveloperswhohavealreadyadopteditastheirlanguageofchoice.Foranother,afeaturethatmakesperfectsenseonitsownmayinteractwithotherfeaturesofthelanguageinawkwardorunexpectedways.Worse,oncealanguagefeaturehasbeenaddeditisallbutimpossibletoremoveevenifitturnsouttobedetrimentaltothelanguageasawhole.Tojustifyaddinganewfeature,alanguagedesignermustbehighlyconfidentthatitwillbeoflongtermbenefittothelanguageratherthanashorttermorfashionablesolutiontoaproblemthatrapidlybecomesredundant.Tomitigatetheriskalanguagedesignerwilltypicallyexperimentbycreatingaseparatelanguageorbranch,suchasthePizzalanguageusedtoexperimentwithJava'sgenerics,priortotheirimplementation.Theproblemwiththisapproachisthattheaudienceforsuchexperimentsisbothsmallandself-selecting;obviouslytheywillallbeinterestedinlanguagefeatures,andmanymaybeacademicsorresearchers.Anideawhichplayswelltosuchanaudiencemaystillplaybadlywhenitisincorporatedintothemainlanguageandgeneralprogrammersstarttoworkwithit.Togetasenseofthis,considertheclosuresdebatethatbecamesoheatedforJava7.Implementationsforthemainproposals(andsomeothers)havebeenavailableforsometimebutnoconsensushasemerged.InconsequenceSundecidedthatJDK7willnotgetfullclosuressupport.ThecoreargumentcamedowntowhetherJavahadbecomeascomplexasitcouldaffordtobewhengenerics(andinparticularthewildcardsyntax)wereaddedtoJava5;andwhethertheadditionoffullsupportforclosureswasjustifiedwhenJavaalreadyhasamorelimitedformthroughanonymousinnerclasses.Twoimportantusecasesforaddingfullclosuressupportweretosimplifyworkingwiththefork/joinAPIthatisbeingaddedtoJDK7toimprovemulti-coreprogramming,andtohelpwithresourceclean-up.JoshBloch'sARMblockproposal,whichisnowexpectedtobeinJDK7viaProjectCoin,offersanalternativesolutiontothelatterproblem.Dr.CliffClick'sresearchonascalable,non-blockingprogrammingstyleforJavaoffersanalternativeapproachtofork/jointhatmaybemoreappropriateasthenumberofprocessorcoresincreases.Ifthisweretohappen,thentheusesforclosuresinJavamayarguablybetoolimitedtojustifytheirinclusion.Itremainsimportantthoughthataprogramminglanguagecontinuestodevelopatsomelevel.ThisarticlethereforeexaminesthreealternativetechniquesforaddingnewlanguagefeaturestoJavathatdon'trequirechangestothelanguageitself-usingacustomDomainSpecificLanguage,exploitingtheJava6annotationprocessortoaddoptionallanguagefeaturesviaalibrary,andmovingthesyntacticsugarfromthelanguagetotheIDE.Eachoffersthepotentialtoallowawideaudienceofmainstreamdeveloperstoexperimentwiththenewfeaturesoverthemediumterminanon-invasivemanner,andthebestideascanthenfilterdownforinclusioninthecorelanguage.CustomDSLsThemostwidelydiscussedofthethreeistheDomain-SpecificLanguageorDSL.Thereissomedisagreementonexactlywhatthetermmeans,butforthepurposesofthisdiscussionwe'llrefertoitsimplyasalanguagethathasbeencreatedwithanarrowfocustosolveaparticularproblem,ratherthanasageneralpurposelanguagedesignedtosolveeverycomputingproblem.AssuchwewouldexpectaDSLtobenon-Turingcompleteandforthemostpartthisisthecase.Thereareedgecasesofcourse.Postscript,forexample,isaTuringcompletelanguagebutalsoqualifiesasaDSLusingourdefinition.Astheaboveexamplealsoillustrates,theideaofaDSLisnotnew.OtherfamiliarDSLsincludeRegularExpressions,XSLT,Ant,andJSP,allofwhichrequiresomesortofcustomparsertoprocessthem.MartinFowleralsosuggeststhatfluentinterfaces/APIscanbeconsideredasecondtypeofDSL,whichhereferstoasaninternalDSL.HisdefinitionisthataninternalDSLisdevelopeddirectlywithinthehostlanguage.ThiswasacommonpracticeamongstbothLispandSmalltalkprogrammers,andmorerecentlytheRubycommunityhasbeenpopularisingthetechnique.Whilstmanywell-knownDSLsarecommerciallydevelopedandmaintained,someenterprisedevelopmentteamshaveusedthetechniquetocreatealanguagethatallowsthemtorapidlyexploreaspectsoftheirproblemdomain.Itisn'thoweverascommonasitmightbe,perhapsbecauseDSLshaveafairlyintimidatingbarriertoentry.Theteamhastodesignthelanguage,buildtheparserandpossiblyothertoolstosupporttheprogrammingteam,andtraineachnewdeveloperthatjoinstheteamonhowtheDSLworks.HeretheemergenceoftoolstospecificallysupportDSLdevelopmentcouldsignificantlychangethelandscape.IntentionalSoftware'sIntentionalDomainWorkbench,whichhasbeenindevelopmentlongerthanJavahasbeenaround,isthefirstsignificantimplementationofsuchatool.TheprojectstartedlifeatMicrosoftResearch,andDr.CharlesSimonyi's1995paper"TheDeathofComputerLanguages,theBirthofIntentionalProgramming"describeshisvision.In2002SimonyifoundedIntentionalSoftwaretocontinueworkingonhisideasandahugelyimpressivevideodemoofthesystemisavailable.Theproductitselfisat1.0status,butaccessisrestrictedtoverylimitedpartners.Othersoftwarehousesarealsoexploringtheconcepts,amongstthemJetBrains,wellrespectedfortheirIntelliJIDEAJavaIDE,whohaverecentlyreleasedthe1.0versionoftheirMetaProgrammingSystem(MPS).MPSdoesn'tuseaparser,insteadworkingwiththeAbstractSyntaxTree(AST)directly.Itprovidesatext-likeprojectionaleditorwhichallowstheprogrammertomanipulatetheAST,andisusedtowritelanguagesandprograms.Foreachnodeinthetreeatextualprojectioniscreated-astheprogrammerworkswiththeprojection,thechangeisreflectedinthenode.Thisapproachallowsyoutoextendandembedlanguagesinanycombination(oftenreferredtoaslanguagecomposing)promotinglanguagere-use.JetBrainsareusingtheproductinternallyandhaverecentlyreleasedYouTrack,abugtrackingproductdevelopedusingthesystem.TheJava6AnnotationProcessorWhilstDSLsarelesscommoninmoremainstreamlanguagessuchasJavathantheyareinRuby,SmalltalkandLisp,recentdevelopmentsintheJavalanguage,inparticulartheannotationprocessorwhichwasaddedinJava6,offernewpossibilitiesfordeveloperslookingtousetheminJava.TheJPA2.0criteriaAPIthatwillshipaspartofJavaEE6,itselfaDSL,offersanexample.Heretheannotationprocessorbuildsupametamodeltypeforeachpersistentclassintheapplication.WhilstitwouldbeperfectlypossibleforthedevelopertohandcraftthemetamodelinJava,itwouldbebothtediousanderrorprone.Theuseoftheannotationprocessoreliminatesthatpainand,sincetheannotationprocessorisbuiltintoJava6,theapproachrequiresnospecificIDEsupport-anIDEdelegatestotheannotationprocessortriggeredbythecompiler,andthemetadatamodelisgeneratedonthefly.Usingtheannotationprocessoritisalsopossibleforalibrarytoaddanewlanguagefeature.BruceChapman'sprototype"noclosures"proposal,forexample,usesthetechniquetoprovideamechanismforcastingamethodtoaSingleAbstractMethod(SAM)typewhichcompilesontopofJava6.DuringourconversationChapmanpointedoutthattheSAMtypealsosupportsfreevariables,akeyaspectofaclosure:ThemethodbodycandeclareadditionalparametersbeyondthoserequiredfortheSingleAbstractMethodusingthe@As.Additionalannotation.TheseparameterscanhavevaluesboundtothematthepointwhereyouobtainaninstanceoftheSAMtype,andarethenpassedtothemethodeachtimeitisinvoked.ChapmanalsosetuptheRaptprojecttoexploreotherusesofthetechnique,andhasaddedimplementationsfortwolanguagechanges-MultilineStringsandXMLliterals-thatwereconsideredforJDK7butwon'tnowmakeitintothefinalrelease.Javacouldevengetaformofclosuressupportusingthisapproach.Whenaskedaboutthis,Chapmansaid:WearejustfinishingaSwingprojectwhichweuseditfor.Wehavefoundacoupleofminorbugsaroundgenerictypes,onerecentlydiscoveredremainstobefixedbutotherthanthatitseemsquitenicetouse,andnobodyhasbeenwantingtorushbacktouseconventionalanonymousinnerclasses.ProjectLombok,anotherprojectexploringthetheannotationprocessor,pushesthetechniquestillfurther.IneffectLombokusesannotationprocessingasahooktorunaJavaagentthatre-writesvariousjavacinternalsbasedontheannotations.Sinceitismanipulatinginternalclassesitisprobablynotsuitedtoproductionuse(internalclassescanchangeevenbetweenminorreleasesoftheJVM)buttheprojectisaneye-openingexampleofjustwhatcanbedoneusingtheannotationprocessor,including:Supportforpropertiesusingapairof@Getterand/or@Setterannotationswithvaryingaccesslevels,e.g.@Setter(AccessLevel.PROTECTED)privateStringname;The@EqualsAndHashCodeannotation,whichgenerateshashCode()andequals()implementationsfromthefieldsofyourobjectThe@ToStringannotation,whichgeneratesanimplementationofthetoString()methodThe@datamethod,whichisequivalenttocombining@ToString,@EqualsAndHashCode,@Getteronallfields,and@Setteronallnon-finalfieldsalongwithaconstructortoinitializeyourfinalfieldsOtherlanguageexperimentation,suchasremovingcheckedexceptionsfromJava,canalsobedoneusingthisapproach.Whilsttheannotationprocessortechniqueopensupawelcomenewroutetolanguageexperimentation,careneedstobetakenthatthegeneratedcodecanbeeasilyreadbydevelopers,notjustbythemachine.Chapmanmadeanumberofsuggestionsduringourconversation:Generatesourcecodenotbytecode,andpayattentiontoformatting(indentingespecially)inthegeneratedcode.Thecompilerwon'tcarewhetheritisallononelineornot,butyouruserswill.Ievensometimesaddcommentsandjavadocinthesourcecodegeneratedbymyannotationprocessorswhereappropriate.HopefullyifthetechniquebecomesmoreprevalentIDEswillalsomakeiteasiertoviewthecodethatistobegeneratedatcompiletime.SyntacticSugarintheIDEBruceChapmanalsotouchesonourthirdtechnique-movingthesyntacticsugarfromthelanguagetotheIDE-inhisblogandheelaboratedonhisideasduringourconversation.ItisalreadyroutineforJavaIDEstocreateportionsofboilerplatecodeforyousuchasthegettersandsettersofaclass,butIDEdevelopersarebeginningtopushtheconceptfurther.JetBrains'IntelliJ9offersatersecodeblocksyntaxforinnerclassessimilartoaclosure,whichadevelopercanalsotype.Actinglikecodefolds,thesecanthenbeexpandedintothefullanonymousinnerclasseswhichthecompilerworkswith-thisallowsdeveloperswhoprefertostickwiththestandardanonymousinnerclasssyntaxtodoso.Asimilarplug-inforEclipsealsoexists.Thekeypointhereisthatthe"alternate"syntaxisjustaviewoftheactualcodewhichthecompilerandanysourcemanagementtoolscontinuetoworkwith.Thusthedevelopershouldbeabletoswitchviewsbetweeneitherform(likeexpandingorcollapsingacodefold),andanyonewithoutaccesstothedefinitionofthesugarjustseesthenormalJavacode.Chapmanwrites:Therearemanydetailstoworkoutinordertomakethiseasilyaccessible,butlongtermIseedevelopersrelativelyeasilydefiningatwowaysugaring/desugaringtransformation(jackpotisagoodstartforhowthismightbedone),tryingthemout,evolvingthemandsharingthegoodoneswithcolleaguesandthecommunity.Theadvantagesofthisarealmostthesameasforalanguagechange,withoutthedisadvantages.Theverybestcouldbecomeubiquitousandthenformthebasisofanactuallanguagechangeifnecessarytogetridofanyremaining"noise"notpossiblewiththisapproach.Sincesyntacticsugarhastomaptoanother(moreverbose)languagefeatureitcannotoffercompleteclosuresupport;therearesomefeaturesofBGGAclosuresforexamplethatcannotbemappedtoanonymousinnerclasses,andsotheycouldn'tbeimplementedthroughthisapproach.Neverthelesstheideaopensupthepossibilityofhavingvariousnewsyntaxesforrepresentinganonymousinnerclasses,similartoBGGAsyntaxorFCMsyntax,andallowingdeveloperstopickthesyntaxtheywanttoworkwith.Otherlanguagefeatures,suchasthenull-safeElvisoperator,couldcertainlybedonethisway.ToexperimentfurtherwiththeideathisNetBeansmodulealsodevelopedbyChapman,iswhathedescribesasa"barelyfunctional"prototypeforPropertiesusingthisapproach.ConclusionInlanguagedevelopmentthereisalwaysatrade-offbetweenstabilityandprogress.Theadvantagethatallofthesetechniquesbringisthattheydon'taffecttheplatformorthelanguage.Inconsequencetheyaremoretoleranttomistakesandarethereforemoreconducivetorapidandradicalexperimentation.Withdevelopersfreelyabletoexperimentweshouldbegintoseemorepeopleseparatelytacklingthepoorsignaltonoiseratioofsomecommonboilerplatesuchastheanonymousinnerclasssyntax,mixingandevolvingtheseideastosomeoptimumformthataddsthemostvalueinthemostcases.ItwillbefascinatingtoseehowdevelopersusethesedifferentapproachestopushtheJavaplatforminnewdirections.中文譯文：不改變語言的前提下推進Java演進JamesGosling在“TheFeelofJava”中說過：Java是一種藍領語言，它并不是博士的論文材料而是可以完成工作上的語言。很多不同的程序員都非常熟悉Java，因為我有一種趨勢去選擇普及的東西，這是一個不錯的選擇。Java的這種成功證明了它的這種方法是正確的，如果Java今天仍然以這個為目標，那么它的語言演進將變得很緩慢。Java作為一個成熟，使用廣泛的語言也將導致其演進過程充滿了困難。一方面，每個特性添加到語言中都有可能造成不可預知到后果，這么做會疏遠那些熟悉）@丫@的人，另一方面，本身很完美的特性可能在與其他特性進行交互時產生未知的影響。更糟的是，一旦增加了某個語言特性，幾乎就不可能再將它移除，即使這個特性會對整個語言產生危害也沒辦法。語言設計者為了證明某個特性是正確的，應該從長遠的角度來考慮，不應該在短期內解決這個問題，而之后就變成了多余的。為了降低風險，語言設計者一般會創建單獨的一種語言或者分支來進行試驗，比如Pizza語言就是在完成前用來測試Java泛型的。這種方式的問題在于試驗的參與者是比較少的人群，并且是自愿的。他們對語言特性很感興趣，很多人是學者或者研究員。但是，在普通的程序員使用這些特性時，那些學者或是研究員認為很好的特性可能變得很糟。為了感受一下這種情況，請考慮Java7閉包特性的激烈爭論。很長時間以來有人在提案中給出了閉包的實現，但最終卻還是沒有達成共識。然后，$口口公司不在JDK7中添加完整的閉包支持。爭論的焦點轉向為添加泛型會不會導致Java變得越來越復雜，在Java中已經通過匿名內部類部分實現該功能的情況下，完整閉包還是正確的嗎。需要完整閉包支持的是簡化fork/joinAPI（添加到了JDK7中以改進多核編程）的使用以及清理資源。JoshBloch的ARMblock提案對第二個問題給出了另一種解決方案。CliffClick博士在面向Java的可擴展，非阻塞變成風格的研究中給出了關于fork/join的另一種方案，這種方案隨著核心處理器的不斷增加看起來也很合理，如果這種成為可能的話，那么Java中使用閉包的地方將會很少了，語言就不需要提供這個特性了。雖然這樣，但是對編程語言來說。不斷的平穩發展還是非常重要的。因此下面講了3種技術，向Java中增加新的語言特性而又不改變語言本身，他們是客戶化領域特定語言（DSL）、Java6的注解處理器（用庫來添加可選擇的語言特性）以及將語法糖從語言般向IDE。每個技術都能讓主流開發者用非侵入方式體驗這些新特性，好的想法可以融合到語言核心當中。客戶化DSL在這3個技術中，討論最多的還是特定領域語言或者說是DSL。關于DSL的定義有很多爭議，但是為了方便我們討論，我們把它當作一個簡單的語言而不是一個解決計算機問題的語言。那樣DSL就不是圖靈完備的了。當然也有邊緣情況，例如，Postscript是圖靈完備的語言但是也可以作為DSL來定義。正如上面的例子，一個DSL也不是新的想法。其他熟悉的DSL包括正則表達式，XSLT，Ant和JSP，所有這樣都需要一個定制的分析器排序，來處理他們。MartinFowler也指出interfaces/API可以被認為是一個第二種類型的內部DSL。他認為一個內部DSL應該被開發語言直接開發，這是Lisp和Smalltalk程序員普遍的做法，以及最近Ruby社區也一直致力推廣這一技術。雖然很多知名的DSL是為了商業上的開發和維護，但是一些企業已經使用這項技術來創造一種語言，但還是小部分人，可能是DSL門檻較高吧。這個小組必須設計語言，建立解析器和其他可能的工具來支持這個程序，以及培養一個新加入者如何用DSL。這是出現來可以支持DSL開發的工具，這大大的改變了局面。IntentionalSoftware所開發的IntentionalDomainWorkbench比Java還要久，它首先實現了該工具的功能。該項目開始于微軟研究院，CharlesSimonyi博士在1995年所發表的論文“TheDeathofComputerLanguages，theBirthofIntentionalProgramming”中描繪了其愿景。2002年，Simonyi創建了IntentionalSoftware以繼續實現他的想法，一個巨大的系統中獲得的令人印象深刻的視頻。該產品現在是1.0版本，但訪問權僅限于有限合伙人。其他的一些公司也開始研究這項技術，其中包括一IntrelliJIEDAJavaIDE而出名的JetBrains，它最近發布了MetaProgrammingSystem（MPS）1.0版。MPS并沒有使用分析器，而是直接使用AbstactSyntaxTree（AST）。他提供了一個可以允許程序員來操作的AST以及可以編寫語言和程序的文本編輯器。當程序員使用投影時就會為樹上的每個借點創建一個文本，這樣變化就會反映到節點當中。開發者能通過這樣的方法開任意組合。JetBrains正在內部使用這個產品，最近發布了bug追蹤產品YouTrack就是使用該系統開發的。Java6注解處理器在很多主流語言像Java中，DSL遠不如RubySmalltalk和Lisp普遍，最近在Java語言中出現的變化特別是Java6中新增的注解處理器為開發者提供了新的機遇去使用Java。JavaEE6中的JPA2.0，某些API本身就是DSL，這就是一個例子。這種處理器會在應用中建立一個持久的元模型。雖然開發者可以很好的用手工去處理Java元模型，但是它會很枯燥無味而且容易出現錯誤。注解處理器的使用解決了這種痛苦，因為這種處理器是建立在Java內部，而且不需要特殊的IDE支持一IDE會代理處理器自己建立元模型。使用這種注解處理器也可以為程序庫增加新的語言特性。舉個例子來說，BruceChapman的原型“noclosures”提案就是憑借這種技術將方法轉換為SingleAbstractMethod(SAM)類型，然后在Java6上編譯。在我們的會話中Chapman指