jsp544洗衣店管理系統(tǒng)(ssh)論文

Withtheincreasinglyperfectandlaundryservicecomplex,simpletextrecordsbeforeartificialmanagementwaynotonlytheefficiencyislow,andpronetoerrors,leddirectlytothemanagementcostincreases,dropinthequalityofservice.Becauseofthisartificialmanagementmodecannotadaptthedevelopmentofdemand,aintelligentmanagementsystemsoarisesatthehistoricmoment.Thispaperaccordingtothesoftwareengineeringspecifications,accordingtotheactualdemanddescribesthewholeprocessofsystemanalysisanddesign,anintelligentcleaner'smanagementsystem.Itspurposeistorealizethechargeinformationmanagementandinandoutoftheclothesrecordquicklyandretrieval,theoperationissimpleandsafe,improveworkefficiencyandquality。Inthissystem,theusercanforthecorrespondingfunctionaloperation.Administratorscanallmembersofinformationaswellassaveclothing,clothingandotherinformationfrommodificationandinquiresthe,thememberinformationmanagement,collectionclothingsinformationmanagement,takeclothinginformationmanagement,accordingtothetimeperiodforturnoverstatisticsetc.Function。系統(tǒng)的響應(yīng)時間：用戶登錄時系統(tǒng)響應(yīng)時間在2秒鐘左?？煽啃院头€(wěn)定性要求：要具有實用性、界面美觀、操作簡單、條理清晰；應(yīng)用軟件與數(shù)據(jù)庫系統(tǒng)的設(shè)計要做到安全可靠，防止非法用戶的入侵。(1)會員信息實體E-R圖如圖1.1所示：(2)洗衣價格信息實體E-R圖如圖3.2所示：(3)收取衣物信息實體E-R圖如圖2.3所示(4)管理員信息E-R圖如圖3.7所示一個1:1聯(lián)系可以轉(zhuǎn)換為一個獨立的關(guān)系模式，也可以與任意一端對應(yīng)的關(guān)系模式合并；一個1:n聯(lián)系可以轉(zhuǎn)換為一個獨立的關(guān)系模式，也可以與n端對應(yīng)的關(guān)系模式合并；一個m:n聯(lián)系可以轉(zhuǎn)換為一個獨立的關(guān)系模式；一個多元聯(lián)系可以轉(zhuǎn)換為一個獨立的關(guān)系模式；具有相同碼的關(guān)系模式可以合并。J測試任何軟件產(chǎn)品都有兩種方法：如果已經(jīng)知道了產(chǎn)品應(yīng)該具有的功能，通過測試來檢驗是否每個功能都能正常使用；如果知道產(chǎn)品內(nèi)部工作過程，通過測試來檢驗產(chǎn)品內(nèi)部動作是否按規(guī)定正常進行。前一種方法稱為黑盒測試，后一種方法稱為白盒測試。對于軟件測試而言，黑盒測試法把程序看作一個黑盒子，完全不考慮程序的內(nèi)部結(jié)構(gòu)和處理過程。也就是說，黑盒測試是在程序接口進行的測試，它只檢查程序功能是否能正常使用，程序是否能適當?shù)慕邮蛰斎氲臄?shù)據(jù)并產(chǎn)生正確的輸出信息，程序運行過程中能否保持外部信息的完整性。黑盒測試又稱為功能測試。白盒測試法和黑盒測試法相反，它的前提是可以把程序看成裝在一個透明的白盒子里，測試者完全知道程序的結(jié)構(gòu)和處理算法。這種方法按照程序內(nèi)部的邏輯測試程序，檢測程序中的主要執(zhí)行通路是否都能按預定要求正確的工作。白盒測試又稱為結(jié)構(gòu)測試。通過單元測試，發(fā)現(xiàn)了系統(tǒng)中存在的一些錯誤和漏洞，將這些問題解決后，又對系統(tǒng)進行了集成測試。測試結(jié)果表明系統(tǒng)能達到所需要求，各部分測試都能達到理想結(jié)果。Aframeworkisareusable,semi-completeapplicationthatcanbespecializedtoproducecustomapplications[Johnson].Likepeople,softwareapplicationsaremorealikethantheyaredifferent.Theyrunonthesamecomputers,expectinputfromthesamedevices,outputtothesamedisplays,andsavedatatothesameharddisks.Developersworkingonconventionaldesktopapplicationsareaccustomedtotoolkitsanddevelopmentenvironmentsthatleveragethesamenessbetweenapplications.Applicationframeworksbuildonthiscommongroundtoprovidedeveloperswithareusablestructurethatcanserveasthefoundationfortheirownproducts.Aframeworkprovidesdeveloperswithasetofbackbonecomponentsthathavethefollowingcharacteristics:1.Theyareknowntoworkwellinotherapplications.2.Theyarereadytousewiththenextproject.3.Theycanalsobeusedbyotherteamsintheorganization.Frameworksaretheclassicbuild-versus-buyproposition.Ifyoubuildit,youwillunderstanditwhenyouaredone—buthowlongwillitbebeforeyoucanrollyourown?Ifyoubuyit,youwillhavetoclimbthelearningcurve—andhowlongisthatgoingtotake?Thereisnorightanswerhere,butmostobserverswouldagreethatframeworkssuchasStrutsprovideasignificantreturnoninvestmentcomparedtostartingfromscratch,especiallyforlargerprojects.Othertypesofframeworks:Theideaofaframeworkappliesnotonlytoapplicationsbuttoapplicationcomponentsaswell.Throughoutthisarticle,weintroduceothertypesofframeworksthatyoucanusewithStruts.TheseincludetheLucenesearchengine,theScaffoldtoolkit,theStrutsvalidator,andtheTilestaglibrary.Likeapplicationframeworks,thesetoolsprovidesemi-completeversionsofasubsystemthatcanbespecializedtoprovideacustomcomponent.Someframeworkshavebeenlinkedtoaproprietarydevelopmentenvironment.ThisisnotthecasewithStrutsoranyoftheotherframeworksshowninthisbook.YoucanuseanydevelopmentenvironmentwithStruts:VisualAgeforJava,JBuilder,Eclipse,Emacs,andTextpadareallpopularchoicesamongStrutsdevelopers.IfyoucanuseitwithJava,youcanuseitwithStruts.Enablingtechnologies:ApplicationsdevelopedwithStrutsarebasedonanumberofenablingtechnologies.ThesecomponentsarenotspecifictoStrutsandunderlieeveryJavawebapplication.AreasonthatdevelopersuseframeworkslikeStrutsistohidethenastydetailsbehindacronymslikeHTTP,CGI,andJSP.AsaStrutsdeveloper,youdon’tneedtobeanalphabetsoupguru,butaworkingknowledgeofthesebasetechnologiescanhelpyoudevisecreativesolutionstotrickyproblems.HypertextTransferProtocol(HTTP):Whenmediatingtalksbetweennations,diplomatsoftenfollowaformalprotocol.Diplomaticprotocolsaredesignedtoavoidmisunderstandingsandtokeepnegotiationsfrombreakingdown.Inasimilarvein,whencomputersneedtotalk,theyalsofollowaformalprotocol.Theprotocoldefineshowdataistransmittedandhowtodecodeitonceitarrives.WebapplicationsusetheHypertextTransferProtocol(HTTP)tomovedatabetweenthebrowserrunningonyourcomputerandtheapplicationrunningontheserver.ManyserverapplicationscommunicateusingprotocolsotherthanHTTP.Someofthesemaintainanongoingconnectionbetweenthecomputers.Theapplicationserverknowsexactlywhoisconnectedatalltimesandcantellwhenaconnectionisdropped.Becausetheyknowthestateofeachconnectionandtheidentityofeachpersonusingit,theseareknownasstatefulprotocols.Bycontrast,HTTPisknownasastatelessprotocol.AnHTTPserverwillacceptanyrequestfromanyclientandwillalwaysprovidesometypeofresponse,eveniftheresponseisjusttosayno.Withouttheoverheadofnegotiatingandretainingaconnection,statelessprotocolscanhandlealargevolumeofrequests.ThisisonereasonwhytheInternethasbeenabletoscaletomillionsofcomputers.AnotherreasonHTTPhasbecometheuniversalstandardisitssimplicity.AnHTTPrequestlookslikeanordinarytextdocument.ThishasmadeiteasyforapplicationstomakeHTTPrequests.YoucanevensendanHTTPrequestbyhandusingastandardutilitysuchasTelnet.WhentheHTTPresponsecomesback,itisalsoinplaintextthatdeveloperscanread.ThefirstlineintheHTTPrequestcontainsthemethod,followedbythelocationoftherequestedresourceandtheversionofHTTP.ZeroormoreHTTPrequestheadersfollowtheinitialline.TheHTTPheadersprovideadditionalinformationtotheserver.Thiscanincludethebrowsertypeandversion,acceptabledocumenttypes,andthebrowser’scookies,justtonameafew.Ofthesevenrequestmethods,GETandPOSTarebyfarthemostpopular.Oncetheserverhasreceivedandservicedtherequest,itwillissueanHTTPresponse.ThefirstlineintheresponseiscalledthestatuslineandcarriestheHTTPprotocolversion,anumericstatus,andabriefdescriptionofthestatus.Followingthestatusline,theserverwillreturnasetofHTTPresponseheadersthatworkinawaysimilartotherequestheaders.Aswementioned,HTTPdoesnotpreservestateinformationbetweenrequests.Theserverlogstherequest,sendstheresponse,andgoesblissfullyontothenextrequest.Whilesimpleandefficient,astatelessprotocolisproblematicfordynamicapplicationsthatneedtokeeptrackoftheirusers.(Ignoranceisnotalwaysbliss.CookiesandURLrewritingaretwocommonwaystokeeptrackofusersbetweenrequests.Acookieisaspecialpacketofinformationontheuser’scomputer.URLrewritingstoresaspecialreferenceinthepageaddressthataJavaservercanusetotrackusers.Neitherapproachisseamless,andusingeithermeansextraworkwhendevelopingawebapplication.Onitsown,astandardHTTPwebserverdoesnottrafficindynamiccontent.Itmainlyusestherequesttolocateafileandthenreturnsthatfileintheresponse.ThefileistypicallyformattedusingHypertextMarkupLanguage(HTML)[W3C,HTML]thatthewebbrowsercanformatanddisplay.TheHTMLpageoftenincludeshypertextlinkstootherwebpagesandmaydisplayanynumberofothergoodies,suchasimagesandvideos.Theuserclicksalinktomakeanotherrequest,andtheprocessbeginsanew.Standardwebservershandlestaticcontentandimagesquitewellbutneedahelpinghandtoprovideuserswithacustomized,dynamicresponse.DEFINITION:StaticcontentontheWebcomesdirectlyfromtextordatafiles,likeHTMLorJPEGfiles.Thesefilesmightbechangedfromtimetotime,buttheyarenotalteredautomaticallywhenrequestedbyawebbrowser.Dynamiccontent,ontheotherhand,isgeneratedonthefly,typicallyinresponsetoanindividualizedrequestfromabrowser.CommonGatewayInterface(CGI):ThefirstwidelyusedstandardforproducingdynamiccontentwastheCommonGatewayInterface(CGI).CGIusesstandardoperatingsystemfeatures,suchasenvironmentvariablesandstandardinputandoutput,tocreateabridge,orgateway,betweenthewebserverandotherapplicationsonthehostmachine.Theotherapplicationscanlookattherequestsenttothembythewebserverandcreateacustomizedresponse.Whenawebserverreceivesarequestthat’sintendedforaCGIprogram,itrunsthatprogramandprovidestheprogramwithinformationfromtheincomingrequest.TheCGIprogramrunsandsendsitsoutputbacktotheserver.Thewebserverthenrelaystheresponsetothebrowser.CGIdefinesasetofconventionsregardingwhatinformationitwillpassasenvironmentvariablesandhowitexpectsstandardinputandoutputtobeused.LikeHTTP,CGIisflexibleandeasytoimplement,andagreatnumberofCGI-awareprogramshavebeenwritten.ThemaindrawbacktoCGIisthatitmustrunanewcopyoftheCGI-awareprogramforeachrequest.Thisisarelativelyexpensiveprocessthatcanbogdownhigh-volumesiteswherethousandsofrequestsareservicedperminute.AnotherdrawbackisthatCGIprogramstendtobeplatformdependent.ACGIprogramwrittenforoneoperatingsystemmaynotrunonanother.Javaservlets:Sun’sJavaServletplatformdirectlyaddressesthetwomaindrawbacksofCGIprograms.First,servletsofferbetterperformanceandutilizationofresourcesthanconventionalCGIprograms.Second,thewrite-once,run-anywherenatureofJavameansthatservletsareportablebetweenoperatingsystemsthathaveaJavaVirtualMachine(JVM).Aservletlooksandfeelslikeaminiaturewebserver.Itreceivesarequestandrendersaresponse.But,unlikeconventionalwebservers,theservletapplicationprogramminginterface(API)isspecificallydesignedtohelpJavadeveloperscreatedynamicapplications.TheservletitselfissimplyaJavaclassthathasbeencompiledintobytecode,likeanyotherJavaobject.TheservlethasaccesstoarichAPIofHTTP-specificservices,butitisstilljustanotherJavaobjectrunninginanapplicationandcanleverageallyourotherJavaassets.Togiveconventionalwebserversaccesstoservlets,theservletsarepluggedintocontainers.Theservletcontainerisattachedtothewebserver.EachservletcandeclarewhatURLpatternsitwouldliketohandle.Whenarequestmatchingaregisteredpatternarrives,thewebserverpassestherequesttothecontainer,andthecontainerinvokestheservlet.ButunlikeCGIprograms,anewservletisnotcreatedforeachrequest.Oncethecontainerinstantiatestheservlet,itwilljustcreateanewthreadforeachrequest.JavathreadsaremuchlessexpensivethantheserverprocessesusedbyCGIprograms.Oncetheservlethasbeencreated,usingitforadditionalrequestsincursverylittleoverhead.Servletdeveloperscanusetheinit()methodtoholdreferencestoexpensiveresources,suchasdatabaseconnectionsorEJBHomeInterfaces,sothattheycanbesharedbetweenrequests.Acquiringresourceslikethesecantakeseveralseconds—whichislongerthanmanysurfersarewillingtowait.Theotheredgeoftheswordisthat,sinceservletsaremultithreaded,servletdevelopersmusttakespecialcaretobesuretheirservletsarethread-safe.Tolearnmoreaboutservletprogramming,werecommendJavaServletsbyExample,byAlanR.Williamson[Williamson].ThedefinitivesourceforServletinformationistheJavaServletSpecification[Sun,JST].JavaServerPages:WhileJavaservletsareabigstepupfromCGIprograms,theyarenotapanacea.Togeneratetheresponse,developersarestillstuckwithusingprintlnstatementstorendertheHTML.Codethatlookslike:out.println("<P>OnelineofHTML.</P>");out.println("<P>AnotherlineofHTML.</P>");isalltoocommoninservletsthatgeneratetheHTTPresponse.TherearelibrariesthatcanhelpyougenerateHTML,butasapplicationsgrowmorecomplex,JavadevelopersendupbeingcastintotheroleofHTMLpagedesigners.Meanwhile,giventhechoice,mostprojectmanagersprefertodividedevelopmentteamsintospecializedgroups.TheylikeHTMLdesignerstobeworkingonthepresentationwhileJavaengineerssweatthebusinesslogic.Usingservletsaloneencouragesmixingmarkupwithbusinesslogic,makingitdifficultforteammemberstospecialize.Tosolvethisproblem,Sunturnedtotheideaofusingserverpagestocombinescriptingandtemplatingtechnologiesintoasinglecomponent.TobuildJavaServerPages,developersstartbycreatingHTMLpagesinthesameoldway,usingthesameoldHTMLsyntax.Tobringdynamiccontentintothepage,thedevelopercanalsoplaceJSPscriptingelementsonthepage.ScriptingelementsaretagsthatencapsulatelogicthatisrecognizedbytheJSP.YoucaneasilypickoutscriptingelementsonJSPpagesbylookingforcodethatbeginswith<%andendswith%>.TobeseenasaJSPpage,thefilejustneedstobesavedwithanextensionof.jsp.WhenaclientrequeststheJSPpage,thecontainertranslatesthepageintoasourcecodefileforaJavaservletandcompilesthesourceintoaJavaclassfile—justasyouwoulddoifyouwerewritingaservletfromscratch.Atruntime,thecontainercanalsocheckthelastmodifieddateoftheJSPfileagainsttheclassfile.IftheJSPfilehaschangedsinceitwaslastcompiled,thecontainerwillretranslateandrebuildthepagealloveragain.ProjectmanagerscannowassignthepresentationlayertoHTMLdevelopers,whothenpassontheirworktoJavadeveloperstocompletethebusiness-logicportion.TheimportantthingtorememberisthataJSPpageisreallyjustaservlet.Anythingyoucandowithaservlet,youcandowithaJSP.JavaBeans:JavaBeansareJavaclasseswhichconformtoasetofdesignpatternsthatmakethemeasiertousewithdevelopmenttoolsandothercomponents.DEFINITIONAJavaBeanisareusablesoftwarecomponentwritteninJava.ToqualifyasaJavaBean,theclassmustbeconcreteandpublic,andhaveanoargumentconstructor.JavaBeansexposeinternalfieldsaspropertiesbyprovidingpublicmethodsthatfollowaconsistentdesignpattern.Knowingthatthepropertynamesfollowthispattern,otherJavaclassesareabletouseintrospectiontodiscoverandmanipulateJavaBeanproperties.TheJavaBeandesignpatternsprovideaccesstothebean’sinternalstatethroughtwoflavorsofmethods:accessorsareusedtoreadaJavaBean’sstate;mutatorsareusedtochangeaJavaBean’sstate.Mutatorsarealwaysprefixedwithlowercasetokensetfollowedbythepropertyname.Thefirstcharacterinthepropertynamemustbeuppercase.Thereturnvalueisalwaysvoid—mutatorsonlychangepropertyvalues;theydonotretrievethem.Themutatorforasimplepropertytakesonlyoneparameterinitssignature,whichcanbeofanytype.Mutatorsareoftennicknamedsettersaftertheirprefix.ThemutatormethodsignatureforaweightpropertyofthetypeDoublewouldbe:publicvoidsetWeight(Doubleweight)Asimilardesignpatternisusedtocreatetheaccessormethodsignature.Accessormethodsarealwaysprefixedwiththelowercasetokenget,followedbythepropertyname.Thefirstcharacterinthepropertynamemustbeuppercase.Thereturnvaluewillmatchthemethodparameterinthecorrespondingmutator.Accessorsforsimplepropertiescannotacceptparametersintheirmethodsignature.Notsurprisingly,accessorsareoftencalledgetters.Theaccessormethodsignatureforourweightpropertyis：publicDoublegetWeight()Iftheaccessorreturnsalogicalvalue,thereisavariantpattern.Insteadofusingthelowercasetokenget,alogicalpropertycanusetheprefixis,followedbythepropertyname.Thefirstcharacterinthepropertynamemustbeuppercase.Thereturnvaluewillalwaysbealogicalvalue—eitherbooleanorBoolean.Logicalaccessorscannotacceptparametersintheirmethodsignature.ThebooleanaccessormethodsignatureforanonpropertywouldbepublicbooleanisOn()ThecanonicalmethodsignaturesplayanimportantrolewhenworkingwithJava-Beans.OthercomponentsareabletousetheJavaReflectionAPItodiscoveraJavaBean’spropertiesbylookingformethodsprefixedbyset,is,orget.IfacomponentfindssuchasignatureonaJavaBean,itknowsthatthemethodcanbeusedtoaccessorchangethebean’sproperties.SunintroducedJavaBeanstoworkwithGUIcomponents,buttheyarenowusedwitheveryaspectofJavadevelopment,includingwebapplications.WhenSunengineersdevelopedtheJSPtagextensionclasses,theydesignedthemtoworkwithJavaBeans.ThedynamicdataforapagecanbepassedasaJavaBean,andtheJSPtagcanthenusethebean’spropertiestocustomizetheoutput.FormoreonJavaBeans,wehighlyrecommendTheAwesomePowerofJavaBeans,byLawrenceH.Rodrigues[Rodrigues].ThedefinitivesourceforJavaBeaninformationistheJavaBeanSpecification[Sun,JBS].Model2:The0.92releaseoftheServlet/JSPSpecificationdescribedModel2asanarchitecturethatusesservletsandJSPpagestogetherinthesameapplication.ThetermModel2disappearedfromlaterreleases,butitremainsinpopularuseamongJavawebdevelopers.UnderModel2,servletshandlethedataaccessandnavigationalflow,whileJSPpageshandlethepresentation.Model2letsJavaengineersandHTMLdeveloperseachworkontheirownpartoftheapplication.AchangeinonepartofaModel2applicationdoesnotmandateachangetoanotherpartoftheapplication.HTMLdeveloperscanoftenchangethelookandfeelofanapplicationwithoutchanginghowtheback-officeservletswork.TheStrutsframeworkisbasedontheModel2architecture.Itprovidesacontrollerservlettohandlethenavigationalflowandspecialclassestohelpwiththedataaccess.AsubstantialcustomtaglibraryisbundledwiththeframeworktomakeStrutseasytousewithJSPpages.Summary:Inthisarticle,weintroducedStrutsasanapplicationframework.WeexaminedthetechnologybehindHTTP,theCommonGatewayInterface,Javaservlets,JSPs,andJavaBeans.WealsolookedattheModel2applicationarchitecturetoseehowitisusedtocombineservletsandJSPsinthesameapplication.框架（framework）是可重用的，半成品的應(yīng)用程序，可以用來產(chǎn)生專門的定制程序。象人一樣，軟件應(yīng)用的相似性比不同點要多。它們運行在相似的機器上，期望從相同的設(shè)備輸入信息，輸出到相同的顯示設(shè)備，并且將數(shù)據(jù)存儲到相同的硬盤設(shè)備。開發(fā)傳統(tǒng)桌面應(yīng)用的開發(fā)人員更習慣于那些可以涵蓋應(yīng)用開發(fā)同一性的工具包和開發(fā)環(huán)境。構(gòu)架在這些公共基礎(chǔ)上的應(yīng)用框架可以為開發(fā)人員提供可以為他們的產(chǎn)品提供可重用服務(wù)的基礎(chǔ)架構(gòu)。框架向開發(fā)人員提供一系列具有以下特征的骨架組件：1．已經(jīng)知道它們在其它程序上工作得很好；2．它們隨時可以在下一個項目中使用；3．它們可以被組織的其它團隊使用；對于框架是典型的構(gòu)建還是購買命題。如果你自己構(gòu)建它，在你完成時你就會理解它，但是在你被融入之前又將花費多長時間呢？如果要購買，你必須得克服學習曲線，同樣，在你可以用它進行工作之前又得花多長時間？這里沒有所謂正確答案，但許多觀察者都會同意，象Struts這樣的框架能提供比從頭開始開發(fā)更顯著的投資回報，特別是對于大型項目來說。其它類型的框架：框架的概念不僅用于應(yīng)用程序也可用于組件。在其它的資料里面，我們也介紹了一些可以和Struts一起使用的框架。這些包括Lucene搜索引擎，Scaffold工具包，Struts驗證器，以及Tiles標簽庫。與應(yīng)用框架一樣，這些工具也提供了一些半完成的版本，可以用在用戶的定制組件之中。某些框架被限制于專門的開發(fā)環(huán)境中。Struts以及本文中涉及的組件卻不是這樣。你可以在很多環(huán)境中來開發(fā)Struts：VisualAgeforJava,JBuilder,Eclipse,Emacs,甚至使用Textpad。對于你的工具，如果你可以用來開發(fā)Java,你就可以用它來開發(fā)Struts。使用的技術(shù)：使用Struts的應(yīng)用開發(fā)使用了大量的其他基礎(chǔ)技術(shù)。這些技術(shù)并不是專門針對Struts，而是所有Javaweb應(yīng)用都可以使用的。開發(fā)者使用Struts之類的框架是為了隱藏在諸如HTTP，CGI，以及JSP之類技術(shù)后面的繁瑣的細節(jié)。作為一個Struts開發(fā)者，你并不需要知曉所有的相關(guān)知識，但是這些基本技術(shù)的工作原理可能有助于你針對棘手問題設(shè)計出創(chuàng)造性的方案。超文本傳輸協(xié)議(HTTP)：當兩個國家之間進行調(diào)解時，外交官們總是遵循一定的正式協(xié)議。外交協(xié)議主要設(shè)計來避免誤解，以及防止談判破裂。同樣，當計算機間需要對話，它們也遵循一個正式的協(xié)議。這個協(xié)議定義數(shù)據(jù)是如何傳輸，以及它們到達后如何進行解碼。Web應(yīng)用程序就是使用HTTP協(xié)議在運行瀏覽器的計算機和運行的服務(wù)器的程序間傳輸數(shù)據(jù)。很多服務(wù)器應(yīng)用程序使用HTTP之外的其他協(xié)議。他們在計算機之間維護一個持久性的連接。應(yīng)用服務(wù)器可以清楚的知道是誰連接上來，而且何時中斷連接。因為它們知道每一個連接的狀態(tài)，以及每一個使用它的人。這稱之為狀態(tài)協(xié)議。相反，HTTP是一個無狀態(tài)協(xié)議。HTTPServer可以接受來自于各種客戶的各種請求，并提供各種響應(yīng)，即使是這個響應(yīng)僅僅是說No。沒有大量的協(xié)商和連接持久性，無狀態(tài)協(xié)議可以處理大量的請求。這也是Internet可以擴展到很多計算機的原因。HTTP成為通用標準的原因是其簡單性。HTTP請求看起來就像一個平常的文本文檔。這使應(yīng)用程序很容易創(chuàng)建HTTP請求。你甚至可以通過標準的程序如Telnet來手動傳遞一個HTTP請求。當HTTP響應(yīng)返回時，它也是一個開發(fā)者可以直接閱讀的平面文本。HTTP請求的第一行包含方法，其后是請求的來源地址和HTTP版本。HTTP請求頭跟在首行后面，可以沒有也可以有多個。HTTP頭向服務(wù)器提供額外的信息?？梢园g覽器的種類和版本，可接受的文檔類型，瀏覽器的cookies等等。7種請求方法中，GET和POST是用得最多的。一旦服務(wù)器接收到請求，它就要產(chǎn)生一個HTTP響應(yīng)。響應(yīng)的第一行稱為狀態(tài)行，包含了HTTP協(xié)議的版本，數(shù)字型狀態(tài)，以及狀態(tài)的簡短描述。狀態(tài)行后，服務(wù)器將返回一個HTTP響應(yīng)頭，類似于HTTP請求頭。如上所述，HTTP并不在請求間保持狀態(tài)信息。服務(wù)器接受請求，發(fā)出響應(yīng)，并且繼續(xù)愉快地處理文本請求。因為簡單和效率，無狀態(tài)協(xié)議不適合于需要跟蹤用戶狀態(tài)的動態(tài)應(yīng)用。Cookies和URL重寫是兩個在請求間跟蹤用戶狀態(tài)的方式。cookie是一種特殊的信息包，存儲于用戶的計算機中。URL重寫是在頁面地址中存儲一個特殊的標記，Java服務(wù)器可以用它來跟蹤用戶。這兩種方法都不是無縫的，是用哪一個都意味著在開發(fā)時都要進行額外的工作。對其本身來說，標準的HTTPweb服務(wù)器并不傳輸動態(tài)內(nèi)容。它主要是使用請求來定位文件資源，并在響應(yīng)中返回此資源。通常這里的文件使用HypertextMarkupLanguage(HTML)[W3C,HTML]格式化，以使瀏覽器可以顯示它們。HTML頁面通常包含一些到其他頁面的超文本連接，也可以顯示其他一些內(nèi)容比如圖像和視頻等等。用戶點擊連接將產(chǎn)生另一個請求，就開始一個新的處理過程。標準web服務(wù)器處理靜態(tài)內(nèi)容處理得很好，但處理動態(tài)內(nèi)容時則需要額外的幫助手段了。定義靜態(tài)內(nèi)容直接來自于文本或數(shù)據(jù)文件，比如HTML或者JPEG文件。這些文件可以隨時改變，但通過瀏覽器請求時，卻不能自動改變。相反，動態(tài)內(nèi)容是臨時產(chǎn)生的，典型地，它是針對瀏覽器的個別請求的響應(yīng)。公共網(wǎng)關(guān)接口(CGI)：第一個普遍用來產(chǎn)生動態(tài)內(nèi)容的標準是通用網(wǎng)關(guān)接口（CommonGatewayInterface(CGI)）。CGI使用標準的操作系統(tǒng)特征，比如環(huán)境變量和標準輸入輸出，在Web服務(wù)器間以及和主機系統(tǒng)間創(chuàng)建橋接和網(wǎng)關(guān)。其他程序可以看到webserver傳遞過來的請求，并創(chuàng)建一個定制的響應(yīng)。當web服務(wù)器接收到一個對CGI程序的請求時，它便運行這個程序并向其提供它請求里面所包含的信息。CGI程序運行，并將輸出返回給Webserver，webserver則將輸出響應(yīng)給瀏覽器。CGI定義了一套關(guān)于什么信息將作為環(huán)境變量傳遞，以及它希望怎樣使用標準輸入和輸出的慣例。與HTTP一樣，CGI是靈活和易于實現(xiàn)的，并且已經(jīng)有大量現(xiàn)成的CGI程序。CGI的主要缺點是它必須為每個請求運行一個程序。這是一個相對昂貴的處理方法，對大容量站點來說，每分鐘有數(shù)千個請求，有可能使站點癱瘓。CGI程序的另一個缺點是平臺依賴性，一個平臺上開發(fā)的程序不一定在另一個平臺上能運行。Javaservlet：Sun公司的JavaServlet平臺直接解決了CGI程序的兩個主要缺點：首先，servlet比常規(guī)CGI程序提供更好的性能和資源利用。其次，一次編寫，隨處運行的JAVA特性意味著servlet在有JVM的操作系統(tǒng)間是輕便的可移動的。Servlet看起來好像是一個微小的webserver。它接受請求并產(chǎn)生響應(yīng)。但，和常規(guī)webserver不同，servletAPI是專門設(shè)計來幫助Java開發(fā)人員創(chuàng)建動態(tài)應(yīng)用的。Servlet本身是要編譯成字節(jié)碼的Java類，就像其他Java對象一樣。Servlet訪問HTTP特定服務(wù)的API，但它仍然是一個運行于程序之中的Java對象，并可以利用所有的Java資產(chǎn)。為了使常規(guī)webservers能訪問servlet，servlet被安插在一個容器之中。Servlet容器連接到Web服務(wù)器。每servlet都可以聲明它可以處理何種樣式的URL。當符合所注冊樣式的請求到達，webserver將請求傳遞給容器，容器則調(diào)用響應(yīng)的servlet。但和CGI程序不同，并不是針對每個請求都要創(chuàng)建一個新的servlet。一旦容器實例化了一個servlet，它就僅為每個新的請求創(chuàng)建一個新的線程。Java線程可比使用CGI程序的服務(wù)器處理開銷小多了。一旦servlet被創(chuàng)建，使用它處理額外的請求僅帶來很小的額外開銷。Servlet開發(fā)人員可以使用init()方法保持對昂貴資源的引用，比如到數(shù)據(jù)庫或者EJBHome接口的連接,以便它們可以在不同的請求之間進行共享。獲得這些資源要耗費數(shù)秒時間，這比大多數(shù)沖浪者愿意等的時間要長些。Servlet的另一個好處是，它是多線程的，servlet開發(fā)人員必須特別注意確保它們的servlet是線程安全的。學習servlet編程，我們推薦JavaServletsbyExample,作者AlanR.Williamson[Williamson]。JavaServerPages：雖然servlets對CGI程序來說前進了一大步，但它也不是萬能靈藥。為了產(chǎn)生響應(yīng)，開發(fā)人員不得不使用大量的println語句來生成HTML。比如這樣的代碼：out.println("<P>OnelineofHTML.</P>");out.println("<P>AnotherlineofHTML.</P>");在產(chǎn)生HTTP