翻譯文獻(xiàn)-第二章 架構(gòu)

外文原文CHAPTER2ARCHITECTUREINTHEORYTHEREISNODIFFERENCEBETWEENTHEORYANDPRACTICEBUT,INPRACTICE,THEREISJANLAVANDESNEPSCHEUTARCHITECTUREITSTHATLEVELABOVEDESIGNWHEREWEGETOURFIRSTGLIMPSEOFWHATTHESYSTEMWILLULTIMATELYRESEMBLE,HOWITWILLBEBUILT,THEVISIONOFTHEAPPLICATIONANDSYSTEMASAWHOLEBYKEEPINGTHEADVICECONTAINEDINTHEITEMSTHATFOLLOWINMINDWHENLAYINGOUTTHEBASICFLOWANDDESIGNOFYOURENTERPRISEAPPLICATIONSANDSYSTEMS,YOULLMAKEPROGRESSTOWARDANOBLEGOALESTABLISHINGANARCHITECTURETHATENABLESAHIGHPERFORMANCE,HIGHSCALABILITYENTERPRISESYSTEMITEM1PREFERCOMPONENTSASTHEKEYELEMENTOFDEVELOPMENT,DEPLOYMENT,ANDREUSEONEOFTHEDIFFICULTIESMANYJAVADEVELOPERSFACEWHENATTEMPTINGTHEIRFIRSTJ2EEBASEDPROJECTISTHATJ2EEAPPLICATIONSAREBUILTDIFFERENTLYTHANTRADITIONALJAVAAPPLICATIONSRATHERTHANBUILDINGAPPLICATIONS,J2EEMANDATESTHECONSTRUCTIONOFCOMPONENTSTHATPLUGINTOANALREADYEXISTINGAPPLICATION,THATBEINGTHEJ2EECONTAINERITSELFTHISMAYNOTSEEMLIKEALARGEDIFFERENCEATFIRST,BUTITSIMPLICATIONSAREHUGE,INTWODIFFERENTDIRECTIONSFIRSTOFALL,THISMEANSTHATDEVELOPERSARENTREALLYINTERESTEDINCONSTRUCTINGOBJECTSPERSEBUTINCREATINGSTRONGLYENCAPSULATEDCOMPONENTSTHATINTHECASEOFJ2EEAREMADEUPOFTIGHTLYCOUPLEDCONSTITUENTOBJECTSSECOND,ITMEANSTHATTHEREAREASETOFSTRINGENTRULESTHATMUSTBEOBEYEDOVERWHICHWE,ASTHECOMPONENTBUILDER,HAVELITTLETONOINFLUENCEDEVELOPERSSTUDYINGTHEOBJECTORIENTEDPARADIGMHAVE,SINCETHEVERYEARLYDAYS,BEENREPEATEDLYTAUGHTTOPROMOTEENCAPSULATIONANDDATAHIDINGWEVEALLMEMORIZEDNUGGETSOFWISDOMHANDEDDOWNFROMONHIGH,SUCHAS“NEVERUSEPUBLICFIELDSINSTEADALWAYSCREATEACCESSORANDMUTATORMETHODSGETTERSANDSETTERS,INJAVABEANSLINGO,“FOREXAMPLE,OR“HIDEYOURIMPLEMENTATIONFROMCLIENTVIEW,“ANDSOONASARESULT,FOREVERYCLASSEVERWRITTEN,WEFAITHFULLYCREATEGET/SETMETHODPAIRSFOREVERYPRIVATEDATAFIELD,CREATEADEFAULTCONSTRUCTOR,ANDSOONUNFORTUNATELY,THISISHARDLYWHATTHEORIGINALPROPONENTSOFOBJECTORIENTATIONHADINMINDMERELYFORCINGCLIENTSTOGOTHROUGHAGETMETHODTOGETAREFERENCETOSOMEINTERNALLYHELDDATASTRUCTUREDOESNTCREATEENCAPSULATIONTHISHASLONGBEENDOCUMENTEDINSUCHBOOKSASEFFECTIVECMEYERS95AND,MORERECENTLY,EFFECTIVEJAVABLOCH,ITEM24MOREIMPORTANTLY,ITUNFORTUNATELYLEADSDEVELOPERSTOTHINKATASCALETOOSMALLFOREFFECTIVEREUSE,THATOFOBJECTSANDCLASSES,RATHERTHANTHINKINGATALARGERSCALE,SUCHASWHATWASORIGINALLYINTENDEDBYTHEJAVABEANSSPECIFICATIONNOTCONVINCEDCONSIDERATRADITIONALCOLLECTIONCLASS,SAY,ANARRAYLISTWECANUSETHEARRAYLISTASANINDEPENDENTOBJECT,BUTWHENITCOMESTIMETOWALKOVERTHECONTENTSOFTHECOLLECTIONITSELF,ANOTHEROBJECT,ANITERATORIMPLEMENTINGOBJECT,BECOMESDESIRABLE,IFNOTOUTRIGHTNECESSARYTHINKINGABOUTREUSEATTHECLASS/OBJECTLEVELMEANSWESERIOUSLYCONSIDERREUSINGTHEITERATORIMPLEMENTINGCLASSINDEPENDENTLYOFTHEARRAYLISTITSBOUNDTO,WHICHMAKESNOSENSEINSTEAD,BECAUSETHETWOAREINTENDEDTOBEUSEDASAPAIR,ITMEANSTHATWERELOOKINGATREUSEATAHIGHERLEVEL,WHATWEREFERTOASACOMPONENTINPRACTICALTERMS,THISUSUALLYMEANSASTANDALONEJARFILETHATCONTAINS,INTHISCASE,THEPAIROFCLASSESFORARRAYLISTANDITSINTERNALITERATORSUBTYPEINTHECOLLECTIONSCASE,ITWOULDALSOINCLUDERELEVANTINTERFACESLIKEITERATOR,COLLECTION,ANDLISTBECAUSETHEYALLHELPDEFINETHECONTRACTCLIENTSCANPLACEFAITHINTHISGETSUSINTOADISCUSSIONOFCOUPLINGTHEDEFINITIONOFTIGHTCOUPLINGSTATESTHATAGIVEN“THING“ISTIGHTLYCOUPLEDTOANOTHER“THING“IFONEHASTOCHANGEINRESPONSETOCHANGESINTHEOTHERINOTHERWORDS,CONSIDERARRAYLISTIFICHANGETHEDEFINITIONOFTHECOLLECTIONINTERFACE,WILLARRAYLISTNEEDTOCHANGEABSOLUTELY,SINCEITIMPLEMENTSCOLLECTIONTHROUGHLISTIFICHANGEARRAYLISTSINTERNALIMPLEMENTATION,WILLITSINTERNALITERATORIMPLEMENTATIONNEEDTOCHANGEABSOLUTELYHOWEVER,WILLCLIENTSNEEDTOCHANGENOTSOLONGASTHEYTREATTHEITERATORASAGENERALPURPOSEITERATORANDDONTDOWNCASTARRAYLISTISTIGHTLYCOUPLEDTOITSINTERNALITERATOR,ANDVICEVERSA,BUTCLIENTSCANREMAINLOOSELYCOUPLEDBYSTICKINGTOTHEINTERFACESLOOSECOUPLINGISEXPLAINEDINGREATERDETAILINITEM2,BUTINEEDTOFORWARDREFERENCEITHEREDONTSEETHERELEVANCERUNONOVERTOTHESERVLETBASEDWEBAPPLICATIONSIFWEOBEYTHETRADITIONALMODELVIEWCONTROLLERABSTRACTION,1THENWEREBUILDINGAMINIMUMOFTWOCLASSESTHATAREMOREORLESSTIGHTLYCOUPLEDTOONEANOTHERTHECONTROLLERSERVLETTHATDOESTHEINPUTPROCESSINGONANINCOMINGHTTPREQUEST,ANDTHEVIEWJSPTHATITFORWARDSTOTHECONTROLLERNEEDSTOKNOWWHATDATAELEMENTSTHEVIEWDEPENDSON,THEVIEWNEEDSTOKNOWWHATPROCESSINGHASALREADYBEENDONEBYTHECONTROLLERSOASNOTTODUPLICATETHATWORK,ANDTHETWOHAVETOAGREEONTHENAMESUNDERWHICHTHEDATAELEMENTSTHEVIEWDEPENDSONWILLBEBOUNDTHENAMEVALUEATTRIBUTESOFHTTPSESSIONPROVIDELATEBINDINGBUTNOTLOOSECOUPLINGAGAIN,SEEITEM2FORMOREINADDITION,BOTHCONTROLLERANDVIEWARETHEMSELVESTIGHTLYCOUPLEDAGAINSTTHEMODELCLASSES,SINCETHEYWILLNEEDTOKNOWWHATDATAELEMENTSAREPRESENTASPARTOFTHEMODELSUSEDASKYOURSELFTHESEQUESTIONSISITREALLYFEASIBLETOCONSIDERREUSINGTHECONTROLLERWITHOUTTHECORRESPONDINGVIEWAND/ORMODELCLASSESCOULDTHEVIEWEXECUTESUCCESSFULLYWITHOUTGOINGTHROUGHTHECONTROLLERSERVLETFIRST1THISISACTUALLYAMISNOMERTHEAPPROPRIATEPATTERNINQUESTIONISREALLYTHEPRESENTATIONABSTRACTIONCONTROLLERPOSA1,145,SINCETHEMODELVIEWCONTROLLERDEALSWITHMULTIPLEVIEWSANDTHEIRUPDATESIMULTANEOUSLY,SUCHASISCOMMONLYFOUNDINAGUIAPPLICATIONLIKEEXCELORWORDBECAUSETHEANSWERTOTHISQUESTIONISALMOSTUNIVER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)苛的規(guī)則，即使是作為構(gòu)件的開發(fā)者，我們也不能違反這一點。學(xué)習(xí)面向?qū)ο蠹夹g(shù)的開發(fā)者，從很早的時候起，就不斷地被告誡，要提倡封裝和數(shù)據(jù)隱藏。我們都還記得大師們教導(dǎo)的金玉良言。例如，“絕不使用公有域（PUBLICFIELD）；而是編寫讀取器（ACCESSOR）和修改器（MUTATOR）方法（用JAVABEAN術(shù)語表示，就是GETTER和SETTER方法）”，或是“對客戶的視圖隱藏你的實現(xiàn)”。因此，對曾經(jīng)寫過的每個類，我們都忠實地為每個私有字段加上GET/SET方法對，以及編寫缺省構(gòu)造器等等。遺憾的是，這樣就幾乎偏離了面向?qū)ο筇岢邆冊瓉淼南敕?。僅僅強迫客戶使用GET方法，以得到對某個內(nèi)部持有的數(shù)據(jù)結(jié)構(gòu)的引用，并不能稱為封裝。這個問題很久之前就已經(jīng)在EFFECTIVECMEYERS95一書里指出了，更近一點的，EFFECTIVEJAVABLOCH,第24項也同樣指出了這個問題。更嚴(yán)重的是，這會誤導(dǎo)開發(fā)者在過于小的規(guī)模下（類和對象）考慮有效重用問題，而不是像JAVABEAN規(guī)范最初的意圖那樣，在較大的規(guī)模下去考慮重用。還不相信讓我們考慮一個常見的集合類，比如ARRAYLIST。我們可以把ARRAYLIST作為單獨對象使用。不過要是想遍歷集合的內(nèi)容，如果不是明顯地必需的話，至少也是希望能夠用另一個對象，也就是ITERATOR實現(xiàn)類的對象。在類/對象的層次考慮重用，意味著我們要著重于考慮對“獨立于ARRAYLIST綁定的ITERATOR實現(xiàn)類”進(jìn)行重用，這顯然沒有意義。由于這兩個類本來就應(yīng)該成對使用，所以這意味著我們應(yīng)該在更高層次去考慮重用，這也就是我們稱為構(gòu)件（COMPONENT）的層次。在實際環(huán)境下，它通常是一個單獨的JAR文件，在本例中它里面包含了兩個類ARRAYLIST及其內(nèi)部的ITERATOR子類型。在使用COLLECTION接口的情況下，還要包括像ITERATOR、COLLECTION和LIST這樣的相關(guān)接口，因為它們作為一個整體共同定義了客戶能夠信任的契約。這把我們帶入了對耦合的討論。緊耦合的定義是，如果一個給定的“事物”必須做出變化，以適應(yīng)另一“事物”的變化，我們就稱前者緊耦合于后者。換句話說，考慮ARRAYLIST，如果我改變了COLLECTION接口的定義，ARRAYLIST需要改變嗎當(dāng)然要，因為ARRAYLIST通過LIST也實現(xiàn)了COLLECTION接口。如果我改變ARRAYLIST的內(nèi)部實現(xiàn)，那么其內(nèi)部的ITERATOR實現(xiàn)需要改變嗎當(dāng)然要。不過，客戶端代碼需要改變嗎不久之前客戶端代碼還把ITERATOR當(dāng)作通用類型的ITERATOR（并且沒有向下轉(zhuǎn)型）。盡管ARRAYLIST與其內(nèi)部的ITERATOR緊密耦合（反之亦然），不過只要客戶端代碼嚴(yán)格遵循接口編程，就能與ARRAYLIST保持松耦合關(guān)系。（松耦合將在第2項中進(jìn)行詳細(xì)討論，不過這里我要提前引用一下。）還沒看到聯(lián)系嗎我們考慮一個基于SERVLET的WEB應(yīng)用程序。如果我們遵循傳統(tǒng)的模型視圖控制器模式（MVC）1，那么至少要編寫兩個類，它們或多或少是緊耦合關(guān)系控制器SERVLET用來對進(jìn)入的HTTP請求進(jìn)行處理，然后把請求轉(zhuǎn)發(fā)到視圖JSP?？刂破餍枰酪晥D。1這里其實有點用詞不當(dāng)符合題意的恰當(dāng)模式是表示抽象控制器模式的這些過程，除此之外還可以參考（PRESENTATIONABSTRACTIONCONTROLLER）POSA1,145，因為MVC用來處理多個視圖，并且視圖能同步更新，這種方式在EXCEL或WORD這樣的GUI應(yīng)用中很常見。所依賴的數(shù)據(jù)元素，視圖需要知道控制器已經(jīng)做了哪些處理，以免重復(fù)工作。二者必須對視圖所依賴的數(shù)據(jù)元素被綁定的名稱達(dá)成一致。（HTTPSESSION的“名稱值”屬性提供了延遲綁定，而不是松耦合；詳細(xì)討論請參閱第2項）此外，控制器和視圖都緊耦合于模型類，因為它們需要知道組成模型所使用的數(shù)據(jù)元素。問問你自己單單重用控制器而不管相應(yīng)的視圖類/模型類，真的可行嗎視圖能夠在不先進(jìn)入控制器SERVLET的情況下被成功執(zhí)行嗎對這個問題的回答一般總是“不行”。這表明，對于給定構(gòu)件里的類，本例中就是你的表示層構(gòu)件，相互之間暗含著緊耦合。這就產(chǎn)生了這樣一種現(xiàn)實狀況對于緊耦合已經(jīng)存在的地方，我們可以對傳統(tǒng)的“全面封裝”規(guī)則稍微放松一些，盡管我們曾經(jīng)如此盲目地遵守這個規(guī)則。當(dāng)然我并不是在建議，為了方便對字段進(jìn)行訪問立刻刪除所有GET/SET方法，我的建議是，你應(yīng)該對真正要保護(hù)的東西深思熟慮。例如，如果你的模型對象并不在表示層（注意這個術(shù)語；請參閱第3項）之外使用，尤其是當(dāng)這些模型對象只是對數(shù)據(jù)的簡單包裝時，比如“數(shù)據(jù)傳輸對象”（DATATRANSFEROBJECT）FOWLER,401，還要為每個字段加上GET/SET方法，真的有意義嗎請記住，封裝是用來在具體實現(xiàn)發(fā)生變化的時候，為了保護(hù)客戶端代碼而設(shè)計的，其目的并不是在構(gòu)件自身發(fā)生改變時，來保護(hù)構(gòu)件。（開發(fā)者幾乎不能從對單個的類進(jìn)行封裝中得到益處，因此只要把構(gòu)件想象成更大、粒度更粗的類，你就離目標(biāo)不遠(yuǎn)了。）假定我們有幾個類，它們相互之間緊密協(xié)作，以完成一些有用的工作。這些類要么被一起部署，要么就都不被部署。我們在尋找某種“大于對象”的東西，作為部署的核心單元。換句話說，我們要尋求的正是構(gòu)件。SERVLET規(guī)范已經(jīng)拋棄了把單個SERVLET部署到SERVLET容器中的作法，而是采用WEB應(yīng)用的思想。WEB應(yīng)用是一系列資源，諸如SERVLET、JSP、模型類、工具類、以及靜態(tài)資源（HTML、圖像、聲音文件等），它們共同地相互協(xié)作以提供所需功能。WEB應(yīng)用采用單一的WAR文件部署，所以不可能出現(xiàn)（A）只部署了部分應(yīng)用；或（B）應(yīng)用的各個部件之間的版本不匹配。實際上，WAR文件和JAVA的JAR文件目的相同，都是為了使應(yīng)用的部署更加容易；如果你曾經(jīng)使用過JAVA10，那么你一定還記得“把CLASS文件解壓縮到CLASSPATH指定的路徑下”這種部署風(fēng)格，同時你也會認(rèn)同，那時候的JAVA應(yīng)用的部署方式不那么優(yōu)雅。WAR文件提供的原子性部署意味著，只有那些被認(rèn)為是WEB應(yīng)用的組成部分的資源才會真正出現(xiàn)在部署之后的應(yīng)用里。也就是說，只有你故意強行拷貝單獨的文件到部署后的應(yīng)用目錄中，才會出現(xiàn)局部的基于“替換文件”的部署。這么做并不好，原因很多。一個重要原因是可能會引入版本不匹配的問題?！爸豢截惏l(fā)生變化的文件到部署目錄”看起來很有誘惑力，但是人們太容易忘記什么發(fā)生了改變，更重要的是，SERVLET容器并不能以我們的觀點去看待應(yīng)用發(fā)生了什么樣的變化?？紤]一下CLASSLOADER。還記得嗎它由SERVLET容器創(chuàng)建，用來把你的WEB應(yīng)用從硬盤加載到JAVA虛擬機。每當(dāng)WEB應(yīng)用發(fā)生“改變”，通常是“硬盤部署目錄的內(nèi)容發(fā)生變化”的時候，SERVLET容器需要創(chuàng)建一個新的CLASSLOADER。但你要是仔細(xì)閱讀SERVLET規(guī)范就會發(fā)現(xiàn)，當(dāng)新的CLASSLOADER被啟動時，它加載的是整個WEB應(yīng)用，而不是某個單獨的SERVLET。因為容器把整個WEB應(yīng)用當(dāng)作單個構(gòu)件，所以組成WEB應(yīng)用的所有CLASS都要由同一個CLASSLOADER加載。因此我們完全可以想象每當(dāng)有一個文件被拷貝到部署目錄時，就會產(chǎn)生一個新的CLASSLOADER實例，從而要執(zhí)行加載整個WEB程序的所有額外工作，這并不會帶來任何實際效益。順便提一下，要確保理解類加載策略所產(chǎn)生的切實效果，請閱讀第70項。這些問題的關(guān)鍵是，SERVLET（或J2EE中的其它）規(guī)范希望你使用構(gòu)件來構(gòu)造WEB應(yīng)用，而不是使用單獨的類。JSP規(guī)范則更進(jìn)一步通過鼓勵“可重用標(biāo)記庫”的概念，促進(jìn)了較小規(guī)模構(gòu)件在WEB應(yīng)用內(nèi)部以及跨越WEB應(yīng)用中的開發(fā)和使用。規(guī)范并不關(guān)心你