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Unliketheearlierchapters,thischapterprovidesamorein-depthexplorationofacornerstoneelementoftheJSPstandard:
JSP’sExpressionLanguage,orELforshort.
Inthischapter,youwilldiscoverthefollowing:
❑WhyELisanindispensablepartofJSP2.0
❑HowtouseELnamedvariables
❑HowtoperformtypeconversionandcoercioninEL
❑HowtohandlenullvaluesinEL
❑Howtoworkwitharithmetic,logical,comparison,andotheroperatorsinEL
❑ThedifferentwaysofaccessingmembersofacollectioninEL
❑HowtoaccesspropertiesofobjectsinEL
❑HowtoaccessnestedobjectpropertiesinEL
❑HowtouseimplicitobjectsinEL
❑HowtodefineandaccessfunctionsinEL
❑HowtouseanamespacewhenreferencingELfunctions
Thischaptercoversalotofground,andincludesfiveseparateexamples.Bytheendofthechapter,youshouldbeveryfamiliarwithEL,andbecomfortableusingitinyourownapplications.
ELandItsVitalRoleinJSP
ELwasnotpartoftheJSPstandardpriorto2.0.OneofthemainreasonsforcreatingELwastoensurethatpresentation-levelJSPpagescouldbecreatedwithoutrelyingonscriptingelements.Chapter3,“JSPBasics2:
GeneralizedTemplatingandServerScripting,”coveredtheuseofscriptingelements（typicallycodewritteninJava）thatareembeddedwithinaJSPpage.
TheneedforscriptingelementswithinJSPistypicallydrivenbytherequirementsofanapplication.
Majorapplicationrequirementsthatdemandtheuseofscriptingelementsincludethefollowing:
❑Toprovidecontrol-flowforJSPexecution
❑Toset,andsubsequentlyaccess,variablesthatarelocaltotheJSPpage
❑TorenderavaluefromacomplexexpressionthatinvolvesJavaobjects
❑ToaccesspropertiesofanarbitraryJavaobject
❑TocallmethodsofJavaBeansorotherJavaobjects
Unfortunately,experiencehasshownthattheuseofscriptingelementsinJSPsmakeslargeprojectsdiffi-culttomaintainoverthelongterm.Italsoencouragesprogrammingpracticesthatmaytightlycouplethepresentation（userinterface）ofanapplicationtoitsbusinesslogic,reducingitsflexibilityandscala-bility.ThisisahighlyundesirablepracticeinthecreationofWebapplicationsandservices.Ideally,JSPsshouldbecreatedfreeofscriptingelementsifatallpossible.
InordertocreateaJSPversionthatcanworkcompletelyfreeofscriptingelements,itisessentialthatallfiveoftheapplicationrequirementscanbesatisfiedwithouttheuseofembeddedJavacoding.JSP2.0satisfiestheserequirements.ThefirsttwoitemsarehandledbyJSTL（exploredindetailinChapter6,“JSPTagLibrariesandJSTL”）,whilethelastthreerequirementsarehandledbyEL.
ELisindependentofJSP2.0.DespitethevitalroleofELinJSP2.0,theuseofELisnotexclusivetoJSPcontainers.TheELparserisalsonottightlyintegratedintotheJSPparser,butcanbedetachedandreusedforotherpurposes.Infact,ELisincorporatedintobothJavaServerFaces（JSF,aserver-sideGUIconstructionkittechnology;
seeChapter21,“JavaServerFaces,”formoreinformation）,andJSTL.Therefore,becomingfamiliarwithELwillsaveyoutimewhenexploringtheseothertechnologies.
ELNamedVariables
WhenworkingwithEL（andJSTL）,youwillfrequentlyworkwithnamedvariables.NamedvariablesinELrefertoattributesattachedtoJSPscopingobjects.Forexample,thefollowingELexpressionwillren-derthevalueofthenamedvariablecalledbearCount:
TheELparserwillsearchfortheattributethroughthevariousJSPscopingobjectsinthefollowingorder:
1Page
2.Request
Session（ifcurrentlyvalid）
Application
Inpractice,thisisequivalenttoacalltotheJavaPageContext.findAttribute（）method.IfyouaremaintaininglegacyJSPcodewithembeddedJava,youmayseethiscall.YoucanconsulttheJSPJavadocs（APIdocumentation）toseehowthismethodworks.
Figure5-1illustratesthesearchforanELnamedvariable.
Figure5-1:
OrderofsearchforanELnamedvariable
InFigure5-1,youcanseethesearchorderforanELnamedvariable:
FirstthesetofELimplicitobjectsissearchedforamatchingname（ELimplicitobjectsarecoveredlaterinthischapter）,andthenthefourscopes.Ifthenamedvariableisnotfound,someactionmaycreateit.
Iftheattributewiththespecifiednamecannotbelocatedinanyscope,anullvalueisreturned.Thenullvaluewillrenderasa“”（emptystring）intheoutput,andwillnottriggeranyerror.
ApplyingEL
Asthenameimplies,theExpressionLanguageworkswithexpressions.Itisaprogramminglanguageusedintheconstructionofexpressions.AllELexpressionsareenclosedinthe${...}notation.TheexpressionisevaluatedbeforetherestoftheJSPisevaluated.YoucanplaceELexpressionsintwoplaceswithinaJSP:
❑Inlinewithtemplatedata
❑WithinattributesofJSPactions,standardorcustom
UsingELexpressionsinlinewithtemplatedata
ThemosttypicaluseofELexpressionsistorenderatextualstringinlinewithtemplatedata.Forexample,considerthefollowingJSPfragment:
<
b>
Thereare${5+1}bearsinthecave.<
/b>
Inthiscase,theELexpressionisplacedinlinewiththeHTMLtemplatedata.AtJSPprocessingtime,theexpression${5+1}willevaluateto6,andbeplacedatthesamepositionastheoriginalexpression.ThiswillresultinthefollowingHTML:
Thereare6bearsinthecave.<
UsingELexpressionsinattributevalues
TheotherplacewhereyouwillseeELexpressionsusediswithintheattributesofcertaintags,includingJSTLtags,standardJSPactions,andcustomtags.
Forexample,considerthefollowingJSPfragmentusingJSTL:
c:
iftest=”${salary>
100000}”>
Richcousin!
br/>
/c:
if>
Inthiscase,theELexpression${salary>
100000}isusedtoconditionallyrenderthebodyoftheJSTLtag.Itisplacedinthetestattributeofthe<
customJSTLaction.ThiswillcausetheELexpressiontobeevaluatedbeforetheJSTLcustomactionisevaluated.TheHTML<
willberenderedonlyifthenamedvariablecalledsalaryisgreaterinvaluethan100,000.
Forexample,considerthefollowingJSPfragmentfeaturingtheJSTL<
set>
tag:
setvar=”datetime”val=’${dateNow}-${timeNow}’/>
IfthedateNownamedvariablecontainsthestring“May1,2005”andtimeNowcontains“11:
00AM”,thentheresultingdatetimenamedvariablewillcontain“May1,2005-11:
00AM”.
Asshownintheprecedingsection,anELexpressioncanbeanarithmeticexpression,oranexpressionfeaturingacomparison.ThefollowingTryItOutsectionexploressomeoftheseELexpressions.
Totryoutthefirstexample,deploych05.wartoyourTomcat5server.ReviewChapter1ifyoudonotrememberhowtodeployaWARfile.
AccessthefollowingURLusingyourbrowsertoseetheexample’soutput:
http:
//localhost:
8080/ch05/example1/
YoushouldseesomethingsimilartowhatisshowninFigure5-2.
Figure5-2:
OutputresultofELexpressions
TheoutputinFigure5-2istheresultofacollectionofdifferentELexpressions.ThefollowingHowItWorkssectionexaminestheELexpressionsusedtogeneratethisoutput.
HowItWorks
TheJSPusedtogeneratetheoutputshowninFigure5-2islocatedat<
TomcatInstallation
Notethatoperatorprecedencehascausedthemultiplicationanddivisiontobeexecutedfirst.Furthermore,thecalculationhascausedadecimaltoappearintheoutput.YoucanusethenumericformattingtagsinJSTLtoeliminatethis.Forexample,thefollowingJSPfragmentinindex.jsprendersthesameresultwith-outthedecimalpoint.
TheformatNumberJSTLtagisusedtoformatthenumericaloutput,usingapatternthatspecifiesnodecimalpointshouldbedisplayed.Thisresultsinthefollowingoutput.
Comparisonoperators
ELsupportstheentirerangeofcomparisonoperatorsfoundinmostprogramminglanguages.Eachoftheoperatorshasasymbolicformandanabbreviatedtextualform.Thefollowingtableenumeratestheavailableoperators.
NotethatinthepreviouscomparisonELalwaysattemptstoconvertthetypeofthedatabeingcom-pared“intherightway.”Thatis,thecharacterdataisconvertedtonumericvaluebeforebeingcom-pared,exceptinthecasewheretwocharactersarecompared.
Inthepreviousexample,theemptystring“”isconsideredemptywhileanyotherstringvalueisnot.Anonexistentvariable,Junk,isgivenanullvalueinEL.Thisnullvalueisconsideredemptywhentestedwiththeemptyoperator.Ontheotherhand,theguessvariableexists（usedearlier）andthereforeisnotempty.Inotherwords,dependingonthedatatypeoftheoperand,theemptyoperatorcanalsobeusedtotestfortheconditionsshowninthefollowingtable.
ThenamedvariableStrVarisinitiallysettothestringtrue.ThisisavariableoftypeString.However,becausethetestattributeoftheJSTL<
tagrequiresaBooleanexpression,itisautomaticallycon-vertedtoaBooleanbyELbeforeuse.Thiscausestheprintingofthe“equal!
”message.
Finally,thecomparisonoperatorisbeingusedonthespecialnullvalue.Itisshown
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