Java 畢設(shè)論文 英文翻譯.doc

精品文檔JavaandtheInternetIfJavais,infact,yetanothercomputerprogramminglanguage,youmayquestionwhyitissoimportantandwhyitisbeingpromotedasarevolutionarystepincomputerprogramming.Theanswerisntimmediatelyobviousifyourecomingfromatraditionalprogrammingperspective.AlthoughJavaisveryusefulforsolvingtraditionalstandaloneprogrammingproblems,itisalsoimportantbecauseitwillsolveprogrammingproblemsontheWorldWideWeb.WhatistheWeb?TheWebcanseemabitofamysteryatfirst,withallthistalkof“surfing,”“presence,”and“homepages.”Itshelpfultostepbackandseewhatitreallyis,buttodothisyoumustunderstandclient/serversystems,anotheraspectofcomputingthatsfullofconfusingissues.Client/ServercomputingTheprimaryideaofaclient/serversystemisthatyouhaveacentralrepositoryofinformationsomekindofdata,ofteninadatabasethatyouwanttodistributeondemandtosomesetofpeopleormachines.Akeytotheclient/serverconceptisthattherepositoryofinformationiscentrallylocatedsothatitanbechangedandsothatthosechangeswillpropagateouttotheinformationconsumers.Takentogether,theinformationrepository,thesoftwarethatdistributestheinformation,andthemachine(s)wheretheinformationandsoftwareresideiscalledtheserver.Thesoftwarethatresidesontheremotemachine,communicateswiththeserver,fetchestheinformation,processesit,andthendisplaysitontheremotemachineiscalledtheclient.Thebasicconceptofclient/servercomputing,then,isnotsocomplicated.Theproblemsarisebecauseyouhaveasingleservertryingtoservemanyclientsatonce.Generally,adatabasemanagementsystemisinvolved,sothedesigner“balances”thelayoutofdataintotablesforoptimaluse.Inaddition,systemsoftenallowaclienttoinsertnewinformationintoaserver.Thismeansyoumustensurethatoneclientsnewdatadoesntwalkoveranotherclientsnewdata,orthatdataisntlostintheprocessofaddingittothedatabase(thisiscalledtransactionprocessing).Asclientsoftwarechanges,itmustbebuilt,debugged,andinstalledontheclientmachines,whichturnsouttobemorecomplicatedandexpensivethanyoumightthink.Itsespeciallyproblematictosupportmultipletypesofcomputersandoperatingsystems.Finally,therestheall-importantperformanceissue:Youmighthavehundredsofclientsmakingrequestsofyourserveratanyonetime,soanysmalldelayiscrucial.Tominimizelatency,programmersworkhardtooffloadprocessingtasks,oftentotheclientmachine,butsometimestoothermachinesattheserversite,usingso-calledmiddleware.(Middlewareisalsousedtoimprovemaintainability.)Thesimpleideaofdistributinginformationhassomanylayersofcomplexitythatthewholeproblemcanseemhopelesslyenigmatic.Andyetitscrucial:Client/servercomputingaccountsforroughlyhalfofallprogrammingactivities.Itsresponsibleforeverythingfromtakingordersandcredit-cardtransactionstothedistributionofanykindofdatastockmarket,scientific,government,younameit.Whatwevecomeupwithinthepastisindividualsolutionstoindividualproblems,inventinganewsolutioneachtime.Thesewerehardtocreateandhardtouse,andtheuserhadtolearnanewinterfaceforeachone.Theentireclient/serverproblemneedstobesolvedinabigway.The Web asagiant serverTheWebisactuallyonegiantclient/serversystem.Itsabitworsethanthat,sinceyouhavealltheserversandclientscoexistingonasinglenetworkatonce.Youdontneedtoknowthat,becauseallyoucareaboutisconnectingtoandinteractingwithoneserveratatime(eventhoughyoumightbehoppingaroundtheworldinyoursearchforthecorrectserver).Initiallyitwasasimpleone-wayprocess.Youmadearequestofaserverandithandedyouafile,whichyourmachinesbrowsersoftware(i.e.,theclient)wouldinterpretbyformattingontoyourlocalmachine.Butinshortorderpeoplebeganwantingtodomorethanjustdeliverpagesfromaserver.Theywantedfullclient/servercapabilitysothattheclientcouldfeedinformationbacktotheserver,forexample,tododatabaselook upsontheserver,toaddnewinformationtotheserver,ortoplaceanorder(whichrequiredmoresecuritythantheoriginalsystemsoffered).ThesearethechangeswevebeenseeinginthedevelopmentoftheWeb.TheWebbrowserwasabigstepforward:theconceptthatonepieceof informationcouldbedisplayedonanytypeofcomputerwithoutchange.However,browserswerestillratherprimitiveandrapidlyboggeddownbythedemandsplacedonthem.Theywerentparticularlyinteractive,andtendedtoclogupboththeserverandtheInternetbecauseanytimeyouneededtodosomethingthatrequiredprogrammingyouhadtosendinformationbacktotheservertobeprocessed.Itcouldtakemanysecondsorminutestofindoutyouhadmisspelledsomethinginyourrequest.Sincethebrowserwasjustavieweritcouldntperformeventhesimplestcomputingtasks.(Ontheother hand,itwassafe,becauseitcouldntexecuteanyprogramsonyourlocalmachinethatmightcontainbugsorviruses.)Tosolvethisproblem,differentapproacheshavebeentaken.Tobeginwith,graphicsstandardshavebeenenhancedtoallowbetteranimationandvideowithinbrowsers.Theremainderoftheproblemcanbesolvedonlybyincorporatingtheabilitytorunprogramsontheclientend,underthebrowser.Thisiscalledclient-sideprogramming.Client-sideprogrammingTheWebsinitialserver-browserdesignprovidedforinteractivecontent,buttheinteractivitywascompletelyprovidedbytheserver.Theserverproducedstaticpagesfortheclientbrowser,whichwouldsimplyinterpretanddisplaythem.Basic HyperTextMarkupLanguage(HTML)containssimplemechanismsfordatagathering:text-entryboxes,checkboxes,radioboxes,listsanddrop-downlists,aswellasabuttonthatcanonlybeprogrammedtoresetthedata ontheformor“submit”thedataontheformbacktotheserver.ThissubmissionpassesthroughtheCommonGatewayInterface(CGI)providedonallWebservers.ThetextwithinthesubmissiontellsCGIwhattodowithit.Themostcommonactionistorunaprogramlocatedontheserverinadirectorythatstypicallycalled“cgi-bin.”(IfyouwatchtheaddresswindowatthetopofyourbrowserwhenyoupushabuttononaWebpage,youcansometimessee“cgi-bin”withinallthegobbledygookthere.)Theseprogramscanbewritteninmostlanguages.Perlhasbeenacommonchoicebecauseitisdesignedfortextmanipulationandisinterpreted,soitcanbeinstalledonanyserverregardlessofprocessororoperatingsystem.However,Python(myfavoriteseewww.P)hasbeenmakinginroadsbecauseofitsgreaterpowerandsimplicity.ManypowerfulWebsitestodayarebuiltstrictlyonCGI,andyoucaninfactdonearlyanythingwithCGI.However,WebsitesbuiltonCGIprogramscanrapidlybecomeoverlycomplicatedtomaintain,andthereisalsotheproblemofresponsetime.TheresponseofaCGIprogramdependsonhowmuchdatamustbesent,aswellastheloadonboththeserverandtheInternet.(Ontopofthis,startingaCGIprogramtendstobeslow.)TheinitialdesignersoftheWebdidnotforeseehowrapidlythisbandwidthwouldbeexhaustedforthekindsofapplicationspeopledeveloped.Forexample,anysortofdynamicgraphingisnearlyimpossibletoperformwithconsistencybecauseaGraphicsInterchangeFormat(GIF)filemustbecreatedandmovedfromtheservertotheclientforeachversionofthegraph.Andyouvenodoubthaddirectexperiencewithsomethingassimpleasvalidatingthedataonaninputform.Youpressthesubmitbuttononapage;thedataisshippedbacktotheserver;theserverstartsaCGIprogramthatdiscoversanerror,formatsanHTML pageinformingyouoftheerror,andthensendsthepagebacktoyou;youmustthenbackupapageandtryagain.Notonlyisthisslow,itsinelegant.Thesolutionisclient-sideprogramming.MostmachinesthatrunWebbrowsersarepowerfulenginescapableofdoingvastwork,andwiththeoriginalstaticHTMLapproachtheyaresittingthere,justidlywaitingfortheservertodishupthenextpage.Client-sideprogrammingmeansthattheWebbrowserisharnessedtodowhateverworkitcan,andtheresultfortheuserisamuchspeedierandmoreinteractiveexperienceatyourWebsite.Theproblemwithdiscussionsofclient-sideprogrammingisthattheyarentverydifferentfromdiscussionsofprogrammingingeneral.Theparametersarealmostthesame,buttheplatformisdifferent;aWebbrowserislikealimitedoperatingsystem.Intheend,youmuststillprogram,andthisaccountsforthedizzyingarrayofproblemsandsolutionsproducedbyclient-sideprogramming.Therestofthissectionprovidesanoverviewoftheissuesandapproachesinclient-sideprogramming.Plug-insOneofthemostsignificantstepsforwardinclient-sideprogrammingisthedevelopmentoftheplug-in.Thisisawayforaprogrammertoaddnewfunctionalitytothebrowserbydownloadingapieceofcodethatplugsitselfintotheappropriatespotinthebrowser.Ittellsthebrowser“fromnowonyoucanperformthisnewactivity.”(Youneedtodownloadtheplug-inonlyonce.)Somefastandpowerfulbehaviorisaddedtobrowsersviaplug-ins,butwritingaplug-inisnotatrivialtask,andisntsomethingyoudwanttodoaspartoftheprocessofbuildingaparticularsite.Thevalueoftheplug-inforclient-sideprogrammingisthatitallowsanexpertprogrammertodevelopanewlanguageandaddthatlanguagetoabrowserwithoutthepermissionofthebrowsermanufacturer.Thus,plug-insprovidea“backdoor”thatallowsthecreationofnewclient-sideprogramminglanguages(althoughnotalllanguagesareimplementedasplug-ins).ScriptinglanguagesPlug-insresultedinanexplosionofscriptinglanguages.Withascriptinglanguage,youembedthesourcecodeforyourclient-sideprogramdirectlyintotheHTMLpage,andtheplug-inthatinterpretsthatlanguageisautomaticallyactivatedwhiletheHTMLpageisbeingdisplayed.Scriptinglanguagestendtobereasonablyeasytounderstandand,becausetheyaresimplytextthatispartofanHTMLpage,theyloadveryquicklyaspartofthesingleserverhitrequiredtoprocurethatpage.Thetrade-offisthatyourcode isexposedforeveryonetosee(andsteal).Generally,however,youarentdoingamazinglysophisticatedthingswithscriptinglanguages,sothisisnottoomuchofahardship.ThispointsoutthatthescriptinglanguagesusedinsideWebbrowsersarereallyintendedtosolvespecifictypesofproblems,primarilythecreationofricherandmoreinteractivegraphicaluserinterfaces(GUIs).However,ascriptinglanguagemightsolve80percentoftheproblemsencounteredinclient-sideprogramming.Yourproblemsmightverywellfitcompletelywithinthat80percent,andsincescriptinglanguagescanalloweasierandfasterdevelopment,youshouldprobablyconsiderascriptinglanguagebeforelookingatamoreinvolvedsolutionsuchasJavaorActiveXprogramming.ThemostcommonlydiscussedbrowserscriptinglanguagesareJavaScript(whichhasnothingtodowithJava;itsnamedthatwayjusttograbsomeof Javasmarketingmomentum),VBScript(whichlookslikeVisualBASIC),andTcl/Tk,whichcomesfromthepopularcross-platformGUI-buildinglanguage.Thereareothersoutthere,andnodoubtmoreindevelopment.JavaScriptisprobablythemostcommonlysupported.ItcomesbuiltintobothNetscapeNavigatorandtheMicrosoftInternetExplorer(IE).Unfortunately,theflavorofJavaScriptonthetwobrowserscanvarywidely(theMozillabrowser,freelydownloadablefromwww.M,supportstheECMAScriptstandard,whichmayonedaybecomeuniversallysupported).Inaddition,thereareprobablymoreJavaScriptbooksavailablethantherearefortheotherbrowserlanguages,andsometoolsautomaticallycreatepagesusingJavaScript.However,ifyourealreadyfluentinVisualBASICorTcl/Tk,youllbemoreproductiveusingthosescriptinglanguagesratherthanlearning anewone.(YoullhaveyourhandsfulldealingwiththeWebissuesalready.)java和因特網(wǎng)既然Java不過另一種類型的程序設(shè)計(jì)語言，大家可能會奇怪它為什么值得如此重視，為什么還有這么多的人認(rèn)為它是計(jì)算機(jī)程序設(shè)計(jì)的一個(gè)里程碑呢？如果您來自一個(gè)傳統(tǒng)的程序設(shè)計(jì)背景，那么答案在剛開始的時(shí)候并不是很明顯。java除了可解決傳統(tǒng)的程序設(shè)計(jì)問題以外，還能解決WorldWideWeb（萬維網(wǎng)）上的編程問題。1.11.1什么是web？web這個(gè)詞剛開始顯得有些泛泛，似乎“沖浪”、“網(wǎng)上存在”以及“主頁”等等都和它拉上了一些關(guān)系。甚至還有一種“Internet綜合癥”的說法，對許多人狂熱的上網(wǎng)行為提出了質(zhì)疑。我們在這里有必要作一些深入的探討，但在這之前，必須理解客戶機(jī)服務(wù)器系統(tǒng)的概念，這是充斥著許多令人迷惑的問題的又一個(gè)計(jì)算領(lǐng)域。1.客戶機(jī)服務(wù)器計(jì)算客戶機(jī)服務(wù)器系統(tǒng)的基本思想是我們能在一個(gè)統(tǒng)一的地方集中存放信息資源。一般將數(shù)據(jù)集中保存在某個(gè)數(shù)據(jù)庫中，根據(jù)其他人或者機(jī)器的請求將信息投遞給對方?？蛻魴C(jī)服務(wù)器概述的一個(gè)關(guān)鍵在于信息是“集中存放”的。所以我們能方便地更改信息，然后將修改過的信息發(fā)放給信息的消費(fèi)者。將各種元素集中到一起，信息倉庫、用于投遞信息的軟件以及信息及軟件所在的那臺機(jī)器，它們聯(lián)合起來便叫作“服務(wù)器”（server）。而對那些駐留在程機(jī)器上的軟件，它們需要與服務(wù)器通信，取回信息，進(jìn)行適當(dāng)?shù)奶幚恚缓笤谶h(yuǎn)程機(jī)器上顯示出來，這些就叫作“客戶”（client）。這樣看來，客戶機(jī)服務(wù)器的基本概念并不復(fù)雜。這里要注意的一個(gè)主要問題是單個(gè)服務(wù)器需要同時(shí)向多個(gè)客戶提供服務(wù)。在這一機(jī)制中，通常少不了一套數(shù)據(jù)庫管理系統(tǒng)，使設(shè)計(jì)人員能將數(shù)據(jù)布局封裝到表格中，以獲得最優(yōu)的使用。除此以外，系統(tǒng)經(jīng)常允許客戶將新信息插入一個(gè)服務(wù)器。這意味著必須確?？蛻舻男聰?shù)據(jù)不會與其他客戶的新數(shù)據(jù)沖突，或者說需要保證那些數(shù)據(jù)在加入數(shù)據(jù)庫的時(shí)候不會丟失（用數(shù)據(jù)庫的術(shù)語來說，這叫作“事務(wù)處理”）?？蛻糗浖l(fā)生了改變之后，它們必須在客戶機(jī)器上構(gòu)建、調(diào)試以及安裝。所有這些會使問題變得比我們一般想象的復(fù)雜得多。另外，對多種類型的計(jì)算機(jī)和操作系統(tǒng)的支持也是一個(gè)大問題。最后，性能的問題顯得尤為重要：可能會有數(shù)百個(gè)客戶同時(shí)向服務(wù)器發(fā)出請求。所以任何微小的延誤都是不能忽視的。為盡可能緩解潛伏的問題，程序員需要謹(jǐn)慎地分散任務(wù)的處理負(fù)擔(dān)。一般可以考慮讓客戶機(jī)負(fù)擔(dān)部分處理任務(wù)，但有時(shí)亦可分派給服務(wù)器所在地的其他機(jī)器，那些機(jī)器亦叫作“中間件”（中間件也用于改進(jìn)對系統(tǒng)的維護(hù)）。所以在具體實(shí)現(xiàn)的時(shí)候，其他人發(fā)布信息這樣一個(gè)簡單的概念可能變得異常復(fù)雜。有時(shí)甚至?xí)谷水a(chǎn)生完全無從著手的感覺?？蛻魴C(jī)服務(wù)器的概念在這時(shí)就可以大顯身手了。事實(shí)上，大約有一半的程序設(shè)計(jì)活動(dòng)都可以采用客戶機(jī)服務(wù)器的結(jié)構(gòu)。這種系統(tǒng)可負(fù)責(zé)從處理訂單及信用卡交易，一直到發(fā)布各類數(shù)據(jù)的方方面面的任務(wù)股票市場、科學(xué)研究、政府運(yùn)作等等。在過去，我們一般為單獨(dú)的問題采取單獨(dú)的解決方案；每次都要設(shè)計(jì)一套新方案。這些方案無論創(chuàng)建還是使用都比較困難，用戶每次都要學(xué)習(xí)和適應(yīng)新界面?？蛻魴C(jī)服務(wù)器問題需要從根本上加以變革！2、web是一個(gè)巨大的服務(wù)器web實(shí)際就是一套規(guī)模巨大的客戶機(jī)服務(wù)器系統(tǒng)。但它的情況要復(fù)雜一些，因?yàn)樗蟹?wù)器和客戶都同時(shí)存在于單個(gè)網(wǎng)絡(luò)上面。但我們沒必要了解更進(jìn)一步的細(xì)節(jié)，因?yàn)槲ㄒ灰P(guān)心的就是一次建立同一個(gè)服務(wù)器的連接，并同它打交道（即使可能要在全世界的范圍內(nèi)搜索正確的服務(wù)器）。最開始的時(shí)候，這是一個(gè)簡單的單向操作過程。我們向一個(gè)服務(wù)器發(fā)出請求它向我們回傳一個(gè)文件，由于本機(jī)的瀏覽器軟件（亦即“客戶”或“客戶程序”）負(fù)責(zé)解釋和格式化，并在我們面前的屏幕上正確地顯示出來。但人們不久就不滿足于只從一個(gè)服務(wù)器傳遞網(wǎng)頁。他們希望獲得完全的客戶機(jī)服務(wù)器能力，使客戶（程序）也能反饋一些信息到服務(wù)器。比如希望對服務(wù)器上的數(shù)據(jù)庫進(jìn)行檢索，向服務(wù)器添加新信息，或者下一份訂單等等（這也提供了比以前的系統(tǒng)更高的安全要求）。在web的發(fā)展過程中，我們可以很清晰地看出這些令人心喜的變化。 web瀏覽器的發(fā)展終于邁出了重要的一步：某個(gè)信息可在任何類型的計(jì)算機(jī)上顯示出來，毋需任何改動(dòng)。然而，瀏覽器仍然顯得很原始，在用戶迅速增多的要求面前顯得有些力不從心。它們的交互能力不夠強(qiáng)，而且對服務(wù)器和因特網(wǎng)都造成了一定程度的干擾。這是由于每次采取一些要求編程的操作時(shí)，必須將信息反饋回服務(wù)器，在服務(wù)器那一端進(jìn)行處理。所以完全可能需要等待數(shù)秒乃至數(shù)分鐘的時(shí)間才會發(fā)現(xiàn)自己剛才拼錯(cuò)了一個(gè)單詞。由于瀏覽器只是一個(gè)純粹的查看程序，所以連最簡單的計(jì)算任務(wù)都不能進(jìn)行（當(dāng)然在另一方面，它也顯得非常安全，因?yàn)椴荒茉诒緳C(jī)上面執(zhí)行任何程序，避開了程序錯(cuò)誤或者病毒的騷擾）。為解決這個(gè)問題，人們采取了許多不同的方法。最開始的時(shí)候，人們對圖形標(biāo)準(zhǔn)進(jìn)行了改進(jìn)，使瀏覽器能顯示更好的動(dòng)畫和視頻。為解決剩下的問題，唯一的辦法就是在客戶端（瀏覽器）內(nèi)運(yùn)行程序。這就叫作“客戶端編程”，它是對傳統(tǒng)的“服務(wù)器端編程”的一個(gè)非常重要的拓展。1.11.2客戶端編程（注釋）web最初采用的“服務(wù)器瀏覽器”方案可提供交互式內(nèi)容，但這種交互能力完全由服務(wù)器提供，為服務(wù)器和因特網(wǎng)帶來了不小的負(fù)擔(dān)。服務(wù)器一般為客戶瀏覽器產(chǎn)生靜態(tài)網(wǎng)頁，由后者簡單地解釋并顯示出來?；綡TML語言提供了簡單的數(shù)據(jù)收集機(jī)制：文字輸入框、復(fù)選框、單選鈕、列表以及下拉列表等，另外還有一個(gè)按鈕，只能由程序規(guī)定重新設(shè)置表單中的數(shù)據(jù)，以便回傳給服務(wù)器。用戶提交的信息通過所有web服務(wù)器均能支持的“通用網(wǎng)關(guān)接口”（CGI）回傳到服務(wù)器。包含在提交數(shù)據(jù)中的文字指示該如何操作。最常見的行動(dòng)是運(yùn)行位于服務(wù)器的一個(gè)程序。那個(gè)程序一般保存在一個(gè)名為“cgi-bin”的目錄中（按下web頁內(nèi)的一個(gè)按鈕時(shí)，請注意一下瀏覽器頂部的地址窗，經(jīng)常都能發(fā)現(xiàn)“cgi-bin”的字樣）。大多數(shù)語言都可用來編制這些程序，但其中最常見的是Perl。這是由于Perl是專為文字的處理及解釋而設(shè)計(jì)的，所以能在任何服務(wù)器上安裝和使用，無論采用的處理器或操作系統(tǒng)是什么。：本節(jié)內(nèi)容改編自某位作者的一篇文章。那篇文章最早出現(xiàn)在位于的Mainspring上。本節(jié)的采用已征得了對方的同意。今天的許多Web站點(diǎn)都嚴(yán)格地建立在CGI的基礎(chǔ)上，事實(shí)上幾乎所有事情都可用CGI做到。唯一的問題就是響應(yīng)時(shí)間。CGI程序的響應(yīng)取決于需要傳送多少數(shù)據(jù)，以及服務(wù)器和因特網(wǎng)兩方面的負(fù)擔(dān)有多重（而且CGI程序的啟動(dòng)比較慢）。Web的早期設(shè)計(jì)者并未預(yù)料到當(dāng)初綽綽有余的帶寬很快就變得不夠用，這正是大量應(yīng)用充斥網(wǎng)上造成的結(jié)果。例如此時(shí)任何形式的動(dòng)態(tài)圖形顯示都幾乎不能連貫地顯示，因?yàn)榇藭r(shí)必須創(chuàng)建一個(gè)GIF文件，再將圖形的每種變化從服務(wù)器傳遞給客戶。而且大家應(yīng)該對輸入表單上的數(shù)據(jù)校驗(yàn)有著深刻的體會。原來的方法是我們按下網(wǎng)頁上的提交按鈕(Submit)；數(shù)據(jù)回傳給服務(wù)器；服務(wù)器啟動(dòng)一個(gè)CGI程序，檢查用戶輸入是否有錯(cuò)；格式化一個(gè)HTML頁，通知可能遇到的錯(cuò)誤，并將這