ssd参考.docx

ssd参考.docx

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ssd参考

Unit1TheWorldWideWeb

ThisunitofthecourseintroducesyoutotheWorldWideWebanditsworkings.YouwilllearnhowtousetheWebasaneffectivetoolforfindinginformation.YouwillalsolearnhowtocreateyourownWebpagesusingHyperTextMark-upLanguage（HTML）.

∙1.1UsingtheWeb

∙1.2What'sintheWeb?

∙1.3IntroductiontoHTMLFormsandServlets

1.1UsingtheWeb

∙1.1.1SurfingtheWeb

∙1.1.2YourWebPages

∙1.1.3 Clients,Servers,andURLs

∙1.1.4 SearchingtheWeb

∙1.1.5 CommerceontheWeb

∙1.1.6 SomeEthicalConsiderations

1.1.1SurfingtheWeb

Let'sstartwithsomebasicdefinitions.Incaseyoudonotalreadyknow,theInternetisacomputernetworkthatconnectsmillionsofcomputersacrossanumberofcountries.ThereisnocentralauthoritythatcontrolstheInternet;differentorganizationsowndifferentpiecesofit.TheInternetwasoriginallyconceivedofbytheAdvancedResearchProjectAgency（ARPA）oftheU.S.governmentinthe1960s.TheWorldWideWeb（or"theWeb"forshort）referstothatportionofthecomputersontheInternetthatcancommunicatewitheachotherusingacomputer-networkprotocolcalledHTTP.AllbrowsersuseHTTPtorequestandreceiveWebpagesfromothercomputers.

Onlyafewyearsagotheexpression"surfin'theWeb"hadmeaningforonlyasmallpercentageofthepopulation. Todaythatisnolongertrue,andifyouhaven'tyetexperiencedsurfingtheWeb,youshouldtryit. Surfing（orbrowsing）theWebcantakeyoutomanydifferentplaceswithonlytheclickofthemouse. However,ifyoudoso,lookingforinformationinahaphazardfashion,youmayfindtheinformationquickly.Or,youmayfindyourselfspendinganentireeveningwanderinghitherandthither,frompagetopage,findingatallsortsofinterestingmaterial—butnotwhatyouwereactuallylookingfor. So,inordertousetheWebeffectively,youneedtoknowhowtosearchtheWebefficiently.Thattopiciscoveredinalaterpage. 

Asyoursearchskillsimprove,youwillfindthatyoucanlocatealotofinformationveryquickly.But,bewarned:

allthatmeansisthatyouarenowabletosearchthatmanymoreplaces!

 Again,don'tbesurprisedifentireeveningsslipawaywhileyoumeanderaroundtheWeb.Bytheway,wehaveusedthewordspageandplaceinterchangeablywhenreferringtotheWeb.Anothersynonymoustermissite,asinWebsite.AllthreetermsrefertolocationsontheWebthatyoucanvisitandviewthroughabrowser.Youare,infact,familiarwithatleastoneWebsite:

theiCarnegiesiteyouvisitedtologontothiscourse.

TheWebisaninterestingplace. Thereisverylittle"law"ontheWeb—andithasbeendescribedasaplaceof"controlledanarchy." EffortstoregulateorgovernithavebeenvigorouslyresistedbyWebusers. OnewaytolookattheWebistocompareittotheearlydaysoftheWildWest,wherejustaboutanythingcouldanddidhappen. Whiletherearefewrules,thereare,however,twomajorareasofcontrol. OnehastodowiththenamingofasiteontheWebandtheotherwiththerulesofanInternetServiceProvider（ISP）.（AnISPisanyoneofanumberofcompaniesthatenablepeoplelikeyouandmenotonlytoconnecttotheInternetandsurftheWebbutalsotopublishWebpages.） Withrespecttothefirstareaofcontrol,allWebsitesmusthaveuniquenamesoraddresses—onepermachine.Therefore,ifyouwanttosetupaWebsite,youhavetoapplytoregisterasitenameandpayasmallfee;afterthat,nooneelsecanuseyoursitename.Concerningthesecondareaofcontrol,ifyouwishtopublishWebpages,youmustdosothroughanISP,andISPsoftenhaverulesthatyoumustfollow—rulesthatmaygovern,forexample,thetypeofcontentWebpagesmaycontain,wherethepagescanbestored,andsoon.

Otherthantheseconstraints,theWebisaprettywide-openplace. However,ontheWebyouarestillsubjecttosocietalrulesandcouldfindyourselfinlegaltroubleifyouslandersomeone,displayinformationthatiscopyrightedbysomeoneelse,orcommitfraud. WhenbrowsingtheWeb,youalsoneedtounderstandthatthepeoplewhousetheWebandcreateWebpagesrepresentacrosssectionofsociety. Assuch,thereisbothgoodandbadmaterialontheweb（"bad"materialrangesfrompornographytoinaccurateormisleadinginformation）. And,becausethereisnoonepersonororganizationincontroloftheWeb,thereisnoonetocertifythattheinformationpresentedontheWebisaccurate,correct,andup-to-date.

Fraudisaproblem,too. TherehavebeenseveralrecentpressreleasesfromthefederalgovernmentwarningpeopletobecautiousaboutbuyingunsolicitedstocksandothersecuritiesovertheWeb. Understand,however,thatwhenfraudisdetected,thoseresponsiblecanbeprosecuted,andthepenaltiescanbesevere. Nevertheless,whenshoppingontheWeb,youwoulddowelltoremembertheoldsaying,"afoolandhismoneyaresoonparted."Thisisnottosaythatthereisn'talotofgoodinformationontheWebandthatyoucan'tfindgoodbargains;asurferjusthastobecarefulandevaluateanyinformationtakenfromtheWeb.

OnethingthattheWebhasfacilitatedistheformationofinterestgroupsofvarioustypes. Thesegroupscanrangefromsupportgroupsforspecificillnesses,togroupsinterestedinhobbiessuchasstampcollectingorflyingradio-controlledmodelplanes.TheWebprovidesaconvenientmeetingplaceforpeoplewithsimilarinterests.Becausedistanceisnotaproblem,peoplecaneasilycommunicateeventhoughtheyareseparatedbyvastdistances.SomefamiliesthataregeographicallyseparatedusetheWebtocommunicate. Peoplecanseepicturesoftheirgrandchildalmostassoonastheyaretaken. Theparentshaveadigitalcamera（costabout$250）andtheyputthepicturesontheirWebpage. ThegrandparentscanaccesstheWebpageandseethem.

IfyouhavenotyettriedsearchingtheWebforinformation,youshoulddoso. Youmighttrylookingforinformationonafavoritetopicusingasearchengine.AsearchengineisaprogramthatallowsonetosearchforkeywordsinfilesatoneormoreInternetsites.PopularsearchenginesincludeLycos,Excite,andAltaVista.Totrylookingforinformationusingasearchengine,youmight,first,pickatopic,thenvisitasearchenginesuchasoneoftheonesmentionedaboveandreadthehelppagestolearnhowtouseit.Thenyouarereadytostartsearchingforinformationonyourchosentopic. However,aftereveryfifteenminutesorsoofbrowsing,youshouldstopandseewhereyouare.Don'tbesurprisedifyoufindyourselffarfromthetopicyouchoseoriginally,havingbecomesidetrackedbysomethingelsethatcaughtyoureyealongtheway. Also,besuretorememberthatthereisnoonecertifyingthatwhatyoureadisaccurate.OneimportantruleforWebsurferstokeepinmindis"caveatemptor"or"buyerbeware."

Wediscusssearchenginesinmoredetailinthecoursepage1.1.4 SearchingtheWeb.

1.1.2YourWebPages

Aswehaveseen,thereisnoagencyorgroupthatcertifiestheinformationontheWebtobeaccurate,valid,andup-to-date. Thereis,however,theideaofthe"ReasonablePersonPrinciple"thatmanypeoplefollow. ThisideaissimplytheGoldenRuleatapracticallevel. IfyouaregoingtoprovideWebpagesforotherstoview,thenyoushouldkeepyourWebpagesinaconditionthatmirrorswhatyouwouldliketofindwhenyouvisitotherpeople'spages.

Ataminimum,theinformationshouldbecorrectandup-to-date.Ifyouremailaddresschanges,thenyoushouldedityourWebpagetoreflectthechange. Ifyoudon'twantalotofmail,don'tincludeyouremailintheWebpage. Puttingyouremailaddressonthepageimpliesthatyouwillrespondtomail. Asacorollarytothe"ReasonablePersonPrinciple,"ifyouhaveanydoubtaboutdisplayinginformation,donotdisplayit.

RememberthattheusersoftheWebrepresentacrosssectionofsocietywithitsgoodandbadelements. Becarefulaboutthepersonalinformationconcerningyourself,yourfamily,andothersthatyouputontheWeb. IfyousurftheWeb,youwillnoticethatmanypersonalsitesdonotincludehomeaddressesandphonenumbers. Manyuserskeepthecommunicationatanelectroniclevel—probablynotabadidea.

IfyoudecidetojointheWebsociety,startsmallandsimple. KeepyourgoalsforyourWebpageswithinyourgrasp. Asyourskillsgrow,youcanenhanceyourWebpagestomatchyourskills. Ifyoubuildyourpagesinthismanner,youwillprobablyfindyoureffortsmorerewarding.

1.1.3Clients,Servers,andURLs

∙Clients

∙Servers

∙URLs

∙HowitWorks—theBasicModel

∙BeyondtheBasicModel

∙QuestionstoConsider

Clients

Clientandserveraretwotermsthatarecommonincomputingcontexts,buttheirmeaningsarenotalwaysunderstood.Inaclient-serversetup,aclientapplicationrequestsinformationfromaserverapplicationoraskstheservertoperformsometaskonitsbehalf.IfyourunaWebbrowseronyourcomputerandusethatbrowsertoviewWebpagesfromothercomputers,yourbrowserisconsideredaclient. Whenyou,throughabrowser,requestinformationfromanothersiteontheWeb,thesitethatsuppliestheinformation（say,aWebpageyouwishtoview）isconsideredaserver. 

Servers

Serverapplicationsaretypicallyrunonpowerfulcomputers,sincetheyneedtobeabletoserviceconcurrentrequestsfromanumberofclients.Ontheotherhand,clientapplicationsaretypicallyrunonlesspowerfulcomputers,suchasPCsorworkstations.Youprobablydonotuseyourhomecomputerasaserver,andifyoupublishpagesontheWeb,youprobablydosothroughyourschool,yourcompany,orthroughanindependentISP,whichoperatesthe