计算机外文翻译 XML与JSP联手.docx

计算机外文翻译 XML与JSP联手.docx

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计算机外文翻译XML与JSP联手

英文原文：

UsingXMLandJSPtogether

I'mgoingtoassumethat,likemostJavaprogrammers,youknowwhatJavaServerPages（JSP）andExtensibleMarkupLanguage（XML）are,butyoumaybealittleunclearonhowyoucanusethem.JSPuseisprettyeasytodefend.ItallowsyoutodesignaWebsitebuiltfromfilesthatlookandactalotlikeHTML.TheonlydifferenceisthatJSPsalsoactdynamically--forexample,theycanprocessformsorreaddatabases--usingJavaasaserver-sidescriptinglanguage.XMLuseismoredifficulttojustify.Whileitseemsasifeverynewproductsupportsit,eachoneseemstobeusingXMLforadifferentpurpose.

Inthisarticle,youwilllearntodesignasystemusingXMLinafairlymodestway.ManyWebsiteshavevastcollectionsofdatathataredisplayedinamoreorlessstandardway.IwilldesignasystemthatusesXMLfilestostoredataonaWebserverandJSPfilestodisplaythatdata.

XMLversusrelationaldatabases

"Butwait,"youmayask,"you'reusingXMLtostoredata?

Whynotuseadatabase?

"Goodquestion.Theansweristhatformanypurposes,adatabaseisoverkill.Touseadatabase,youhavetoinstallandsupportaseparateserverprocess,whichoftenalsorequiresinstallingandsupportingadatabaseadministrator.YoumustlearnSQL,andwriteSQLqueriesthatconvertdatafromarelationaltoanobjectstructureandbackagain.IfyoustoreyourdataasXMLfiles,youlosetheoverheadofanextraserver.Youalsogainaneasywaytoedityourdata:

justuseatexteditor,ratherthanacomplicateddatabasetool.XMLfilesarealsoeasiertobackup,tosharewithyourfriends,ortodownloadtoyourclients.Youcanalsoeasilyuploadnewdatatoyoursite,usingFTP.

AmoreabstractadvantageofXMListhat,beingahierarchicalratherthanarelationalformat,itcanbeusedinamuchmorestraightforwardmannertodesigndatastructuresthatfityourneeds.Youdon'tneedtouseanentityrelationshipeditornornormalizeyourschema.Ifyouhaveoneelementthatcontainsanotherelement,youcanrepresentthatdirectlyintheformat,ratherthanusingajointable.

Notethatformanyapplications,afilesystemwillnotsuffice.Ifyouhaveahighvolumeofupdates,afilesystemmaygetconfusedorcorruptedbysimultaneouswrites;databasesusuallysupporttransactions,whichallowconcurrencywithoutcorruption.Further,adatabaseisanexcellenttoolifyouneedtomakecomplicatedqueries,especiallyiftheywillvaryfromtimetotime.Databasesbuildindexes,andareoptimizedforkeepingtheindexesuptodatewithaconstantlychangingdataset.Relationaldatabasesalsohavemanyotheradvantages,includingarichquerylanguage,matureauthoringandschemadesigntools,provenscalability,fine-grainedaccesscontrol,andsoon.

（Note:

Youcanusesimplefilelockingtoprovideapoorman'stransactionserver.AndyoucanalsoimplementanXMLindex-and-searchtoolinJava,butthat'satopicforanotherarticle.）

Inthiscase,asinmostlow-to-mediumvolume,publishing-basedWebsites,youcanassumethefollowing:

mostofthedataaccessisreads,notwrites;thedata,thoughpotentiallylarge,isrelativelyunchanging;youwon'tneedtodocomplicatedsearches,butifyoudo,you'lluseaseparatesearchengine.TheadvantagesofusingamatureRDBMSfade,whiletheadvantageofusinganobject-orienteddatamodelcometothefore.

Finally,it'sentirelypossibletoprovideawrapperforyourdatabasethatmakesSQLqueriesandtranslatesthemintoXMLstreams,soyoucouldhaveitbothways.XMLbecomesamorerobust,programmer-friendlyfrontendtoamaturedatabaseforstoringandsearching.（Oracle'sXSQLservletisoneexampleofthistechnique.）

Theapplication:

Anonlinephotoalbum

Everybodylovesphotos!

Peopleloveshowingpicturesofthemselves,theirfriends,theirpets,andtheirvacations.TheWebistheultimatemediumforself-indulgentshutterbugs--theycanannoytheirrelativesfromthousandsofmilesaway.Whileafull-fledgedphotoalbumsitewouldrequireacomplicatedobjectmodel,I'llfocusondefiningasinglePictureobject.（ThesourcecodeforthisapplicationisavailableinResources.）Theobjectrepresentingapictureneedsfieldsrepresentingitstitle,thedateitwastaken,anoptionalcaption,and,obviously,apointertotheimagesource.

Animage,inturn,needsafewfieldsofitsown:

thelocationofthesourcefile（aGIForJPEG）andtheheightandwidthinpixels（toassistyouinbuildingtags）.Herethereisoneneatadvantagetousingthefilesystemasyourdatabase:

youcanstoretheimagefilesinthesamedirectoryasthedatafiles.

Finally,let'sextendthepicturerecordwithanelementdefiningasetofthumbnailimagesforuseinthetableofcontentsorelsewhere.HereIusethesameconceptofimageIdefinedearlier.

TheXMLrepresentationofapicturecouldlooksomethinglikethis:

1999-08-08

Tryinginvaintogetatan

alex-beach.jpg

340

200

alex-beach-sm.jpg

72

72

alex-beach-med.jpg

150

99

NotethatbyusingXML,youputalltheinformationaboutasinglepictureintoasinglefile,ratherthanscatteringitamongthreeorfourseparatetables.Let'scallthisa.pixfile--soyourfilesystemmightlooklikethis:

summer99/alex-beach.pix

summer99/alex-beach.jpg

summer99/alex-beach-sm.jpg

summer99/alex-beach-med.jpg

summer99/alex-snorkeling.pix

etc.

Techniques

There'smorethanonewaytoskinacat,andthere'smorethanonewaytobringXMLdataontoyourJSPpage.Hereisalistofsomeofthoseways.（Thislistisnotexhaustive;manyotherproductsandframeworkswouldserveequallywell.）

DOM:

YoucanuseclassesimplementingtheDOMinterfacetoparseandinspecttheXMLfile

XMLEntryList:

YoucanusemycodetoloadtheXMLintoajava.util.Listofname-valuepairs

XPath:

YoucanuseanXPathprocessor（likeResin）tolocateelementsintheXMLfilebypathname

XSL:

YoucanuseanXSLprocessortotransformtheXMLintoHTML

Cocoon:

YoucanusetheopensourceCocoonframework

Rollyourownbean:

YoucanwriteawrapperclassthatusesoneoftheothertechniquestoloadthedataintoacustomJavaBean

NotethatthesetechniquescouldbeappliedequallywelltoanXMLstreamyoureceivefromanothersource,suchasaclientoranapplicationserver.

JavaServerPages

TheJSPspechashadmanyincarnations,anddifferentJSPproductsimplementdifferent,incompatibleversionsofthespec.IwilluseTomcat,forthefollowingreasons:

Itsupportsthemostup-to-dateversionsoftheJSPandservletspecs

It'sendorsedbySunandApache

YoucanrunitstandalonewithoutconfiguringaseparateWebserver

It'sopensource

（FormoreinformationonTomcat,seeResources.）

YouarewelcometouseanyJSPengineyoulike,butconfiguringitisuptoyou!

BesurethattheenginesupportsatleasttheJSP1.0spec;thereweremanychangesbetween0.91and1.0.TheJSWDK（JavaServerWebDevelopmentKit）willworkjustfine.

TheJSPstructure

WhenbuildingaJSP-drivenWebsite（alsoknownasaWebapp）,Iprefertoputcommonfunctions,imports,constants,andvariabledeclarationsinaseparatefilecalledinit.jsp,locatedinthesourcecodeforthisarticle.

IthenloadthatfileintoeachJSPfileusing<%@includefile="init.jsp"%>.The<%@include%>directiveactsliketheClanguage's#include,pullinginthetextoftheincludedfile（here,init.jsp）andcompilingitasifitwerepartoftheincludingfile（here,picture.jsp）.Bycontrast,the

include>tagcompilesthefileasaseparateJSPfileandembedsacalltoitinthecompiledJSP.

Findingthefile

WhentheJSPstarts,thefirstthingitneedstodoafterinitializationisfindtheXMLfileyouwant.Howdoesitknowwhichofthemanyfilesyouneed?

TheanswerisfromaCGIparameter.TheuserwillinvoketheJSPwiththeURLpicture.jsp?

file=summer99/alex-beach.pix（orbypassingafileparameterthroughanHTMLform）.

However,whentheJSPreceivestheparameter,you'restillonlyhalfwaythere.Youstillneedtoknowwhereonthefilesystemtherootdirectorylies.Forexample,onaUnixsystem,theactualfilemaybeinthedirectory/home/alex/public_html/pictures/summer99/alex-beach.pix.JSPsdonothaveaconceptofacurrentdirectorywhileexecuting,soyouneedtoprovideanabsolutepathnametothejava.iopackage.

TheServletAPIprovidesamethodtoturnaURLpath,relativetothecurrentJSPorServlet,intoanabsolutefilesystempath.ThemethodServletContext.getRealPath（String）doesthetrick.EveryJSPhasaServletContextobjectcalledapplication,sothecodewouldbe:

Stringpicturefile=

application.getRealPath（"/"+request.getParameter（"file"））;

or

Stringpicturefile=

getServletContext（）.getRealPath（"/"+request.getParameter（"file"））;

whichalsoworksinsideaservlet.（Youmustappenda/becausethemethodexpectstobepassedtheresultsofrequest.getPathInfo（）.）

Oneimportantnote:

wheneveryouaccesslocalresources,beverycarefultovalidatetheincomingdata.Ahacker,oracarelessuser,cansendbogusdatatohackyoursite.Forinstance,considerwhatwouldhappenifthevaluefile=../../../../etc/passwdwereentered.Theusercouldinthiswayreadyourserver'spasswordfile.

TheDocumentObjectModel

DOMstandsfortheDocumentObjectModel.ItisastandardAPIforbrowsingXMLdocuments,developedbytheWorld