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24/12/2014 ASwingArchitectureOverview

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ASwingArchitectureOverview

TheInsideStoryonJFCComponentDesignByAmyFowler

MostSwingdevelopersknowbynowthatSwingcomponentshaveaseparablemodelandviewdesign.AndmanySwingusershaverunacrossarticlessayingthatSwingisbasedonsomethingcalleda"modifiedMVC(modelviewcontroller)architecture."

ButaccurateexplanationsofhowSwingcomponentsaredesigned,andhowtheirpartsallfittogether,havebeenhardtocomebyuntilnow.

Thesilenceendswiththepublicationofthisarticle,amajorwhitepaperonSwingcomponentdesign.ItprovidesacomprehensivetechnicaloverviewofSwing'smodifiedMVCstructureanddemystifiesmanyotherfacetsofSwingcomponentarchitectureaswell.

ThisdocumentpresentsatechnicaloverviewoftheSwingcomponentarchitecture.Inparticular,itcoversthefollowingareasindetail:

DesigngoalsRootsinMVCSeparablemodelarchitecturePluggablelookandfeelarchitecture

DesignGoalsTheoverallgoalfortheSwingprojectwas:

TobuildasetofextensibleGUIcomponentstoenabledeveloperstomorerapidlydeveloppowerfulJavafrontendsforcommercialapplications.

Tothisend,theSwingteamestablishedasetofdesigngoalsearlyintheprojectthatdrovetheresultingarchitecture.TheseguidelinesmandatedthatSwingwould:

BeimplementedentirelyinJavatopromotecrossplatformconsistencyandeasiermaintenance.ProvideasingleAPIcapableofsupportingmultiplelookandfeelssothatdevelopersandenduserswouldnotbelockedintoasinglelookandfeel.EnablethepowerofmodeldrivenprogrammingwithoutrequiringitinthehighestlevelAPI.AdheretoJavaBeansdesignprinciplestoensurethatcomponentsbehavewellinIDEsandbuildertools.ProvidecompatibilitywithAWTAPIswherethereisoverlapping,toleveragetheAWTknowledgebaseandeaseporting.

RootsinMVCSwingarchitectureisrootedinthemodelviewcontroller(MVC)designthatdatesbacktoSmallTalk.MVCarchitecturecallsforavisualapplicationtobebrokenupintothreeseparateparts:

Amodelthatrepresentsthedatafortheapplication.Theviewthatisthevisualrepresentationofthatdata.Acontrollerthattakesuserinputontheviewandtranslatesthattochangesinthemodel.Earlyon,MVCwasalogicalchoiceforSwingbecauseitprovidedabasisformeetingthefirstthreeofourdesigngoalswithintheboundsofthelattertwo.

ThefirstSwingprototypefollowedatraditionalMVCseparationinwhicheachcomponenthadaseparatemodelobjectanddelegateditslookandfeelimplementationtoseparateviewandcontrollerobjects.

Thedelegate

Wequicklydiscoveredthatthissplitdidn'tworkwellinpracticaltermsbecausetheviewandcontrollerpartsofacomponentrequiredatightcoupling(forexample,itwasverydifficulttowriteagenericcontrollerthatdidn'tknowspecificsabouttheview).SowecollapsedthesetwoentitiesintoasingleUI(userinterface)object,asshowninthisdiagram:

(TheUIdelegateobjectshowninthispictureissometimescalledadelegateobject,orUIdelegate.TheUIdelegateusedinSwingisdescribedinmoredetailinthePluggablelookandfeelsectionofthisarticle,underthesubheading"TheUIdelegate".)

Asthediagramillustrates,SwingarchitectureislooselybasedbutnotstrictlybasedonthetraditionalMVCdesign.IntheworldofSwing,thisnewquasiMVCdesignissometimesreferredtoaseparablemodelarchitecture.

Swing'sseparablemodeldesigntreatsthemodelpartofacomponentasaseparateelement,justastheMVCdesigndoes.ButSwingcollapsestheviewandcontrollerpartsofeachcomponentintoasingleUI(userinterface)object.

ToMVCornottoMVC?

Onenoteworthypointisthatasanapplicationdeveloper,youshouldthinkofacomponent'sview/controllerresponsibilitiesasbeinghandledbythegenericcomponentclass(suchas.JButton,JTree,andsoon).ThecomponentclassthendelegatesthelookandfeelspecificaspectsofthoseresponsibilitiestotheUIobjectthatisprovidedbythecurrentlyinstalledlookandfeel.

Forexample,thecodethatimplementsdoublebufferedpaintingisinSwing'sJComponentclass(the"mother"ofmostSwingcomponentclasses),whilethecodethatrendersaJButton'slabelisinthebutton'sUIdelegateclass.Theprecedingdiagramillustratesthissubtle(andoftenconfusing)point:

SoSwingdoeshaveastrongMVClineage.Butit'salsoimportanttoreiteratethatourMVCarchitectureservestwodistinctpurposes:

First,separatingthemodeldefinitionfromacomponentfacilitatesmodeldrivenprogramminginSwing.Second,theabilitytodelegatesomeofacomponent'sview/controllerresponsibilitiestoseparatelookandfeelobjectsprovidesthebasisforSwing'spluggablelookandfeelarchitecture.AlthoughthesetwoconceptsarelinkedbytheMVCdesign,theymaybetreatedsomewhatorthogonallyfromthedeveloper'sperspective.Theremainderofthisdocumentwillcovereachofthesemechanismsingreaterdetail.

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SeparablemodelarchitectureItisgenerallyconsideredgoodpracticetocenterthearchitectureofanapplicationarounditsdataratherthanarounditsuserinterface.Tosupportthisparadigm,Swingdefinesaseparatemodelinterfaceforeachcomponentthathasalogicaldataorvalueabstraction.ThisseparationprovidesprogramswiththeoptionofpluggingintheirownmodelimplementationsforSwingcomponents.

ThefollowingtableshowsthecomponenttomodelmappingforSwing.

Component ModelInterface ModelTypeJButton ButtonModel GUI

JToggleButton ButtonModel GUI/data

JCheckBox ButtonModel GUI/data

JRadioButton ButtonModel GUI/data

JMenu ButtonModel GUI

JMenuItem ButtonModel GUI

JCheckBoxMenuItem ButtonModel GUI/data

JRadioButtonMenuItem ButtonModel GUI/data

JComboBox ComboBoxModel data

JProgressBar BoundedRangeModel GUI/data

JScrollBar BoundedRangeModel GUI/data

JSlider BoundedRangeModel GUI/data

JTabbedPane SingleSelectionModel GUI

JList ListModel data

JList ListSelectionModel GUI

JTable TableModel data

JTable TableColumnModel GUI

JTree TreeModel data

JTree TreeSelectionModel GUI

JEditorPane Document data

JTextPane Document data

JTextArea Document data

JTextField Document data

JPasswordField Document data

GUIstatevs.applicationdatamodelsThemodelsprovidedbySwingfallintotwogeneralcategories:GUIstatemodelsandapplicationdatamodels.

GUIstatemodels

GUIstatemodelsareinterfacesthatdefinethevisualstatusofaGUIcontrol,suchaswhetherabuttonispressedorarmed,orwhichitemsareselectedinalist.GUIstatemodelstypicallyarerelevantonlyinthecontextofagraphicaluserinterface(GUI).WhileitisoftenusefultodevelopprogramsusingGUIstatemodelseparationparticularlyifmultipleGUIcontrolsarelinkedtoacommonstate(suchasinasharedwhiteboardprogram),orifmanipulatingonecontrolautomaticallychangesthevalueofanothertheuseofGUIstatemodelsisnotrequiredbySwing.ItispossibletomanipulatethestateofaGUIcontrolthroughtoplevelmethodsonthecomponent,withoutanydirectinteractionwiththemodelatall.Intheprecedingtable,GUIstatemodelsinSwingarehighlightedinblue.

Applicationdatamodels

Anapplicationdatamodelisaninterfacethatrepresentssomequantifiabledatathathasmeaningprimarilyinthecontextoftheapplication,suchasthevalueofacellinatableortheitemsdisplayedinalist.ThesedatamodelsprovideaverypowerfulprogrammingparadigmforSwingprogramsthatneedacleanseparationbetweentheirapplicationdata/logicandtheirGUI.FortrulydatacentricSwingcomponents,suchasJTreeandJTable,interactionwiththedatamodelisstronglyrecommended.Applicationdatamodelsarehighlightedinredinthetablepresentedatthebeginningofthissection.

Ofcoursewithsomecomponents,themodelcategorizationfallssomewhereinbetweenGUIstatemodelsandapplicationdatamodels,dependingonthecontextinwhichthemodelisused.ThisisthecasewiththeBoundedRangeModelonJSliderorJProgressBar.Thesemodelsarehighlightedinpurpleintheprecedingtable.

Swing'sseparablemodelAPImakesnospecificdistinctionsbetweenGUIstatemodelsandapplicationdatamodelshowever,wehaveclarifiedthisdifferenceheretogivedevelopersabetterunderstandingofwhenandwhytheymightwishtoprogramwiththeseparablemodels.

SharedmodeldefinitionsReferringagaintothetableatthebeginningofthissection,noticethatmodeldefinitionsaresharedacrosscomponentsincaseswherethedataabstractionforeachcomponentissimilarenoughtosupportasingleinterfacewithoutovergenericizingthatinterface.Commonmodelsenableautomaticconnectabilitybetweencomponenttypes.Forexample,becausebothJSliderandJScrollbarusetheBoundedRangeModelinterface,asingleBoundedRangeModelinstancecouldbepluggedintobothaJScrollbarandaJSliderandtheirvisualstatewouldalwaysremaininsync.

TheseparablemodelAPISwingcomponentsthatdefinemodelssupportaJavaBeansboundpropertyforthemodel.Forexample,JSliderusestheBoundedRangeModelinterfaceforitsmodeldefinition.Consequently,itincludesthefollowingmethods:

publicBoundedRangeModelgetModel()publicvoidsetModel(BoundedRangeModelmodel)AllSwingcomponentshaveonethingincommon:Ifyoudon'tsetyourownmodel,adefaultiscreatedandinstalledinternallyinthecomponent.Thenamingconventionforthesedefaultmodelclassesistoprependtheinterfacenamewith"Default."ForJSlider,aDefaultBoundedRangeModelobjectisinstantiatedinitsconstructor:

publicJSlider(intorientation,intmin,



intmax,intvalue){checkOrientation(orientation)this.orientation=orientationthis.model=newDefaultBoundedRangeModel(value,0,min,max)this.model.addChangeListener(changeListener)updateUI()}

IfaprogramsubsequentlycallssetModel(),thisdefaultmodelisreplaced,asinthefollowingexample:

JSliderslider=newJSlider()BoundedRangeModelmyModel=newDefaultBoundedRangeModel(){publicvoidsetValue(intn){System.out.println("SetValue:"+n)super.setValue(n)}})slider.setModel(myModel)

Formorecomplexmodels(suchasthoseforJTableandJList),anabstractmodelimplementationisalsoprovidedtoenabledeveloperstocreatetheirownmodelswithoutstartingfromscratch.Theseclassesareprependedwith"Abstract".

Forexample,JList'smodelinterfaceisListModel,whichprovidesbothDefaultListModelandAbstractListModelclassestohelpthedeveloperinbuildingalistmodel.

ModelchangenotificationModelsmustbeabletonotifyanyinterestedparties(suchasviews)whentheirdataorvaluechanges.SwingmodelsusetheJavaBeansEventmodelfortheimplementationofthisnotification.TherearetwoapproachesforthisnotificationusedinSwing:

Sendalightweightnotificationthatthestatehas"changed"andrequirethelistenertorespondbysendingaquerybacktothemodeltofindoutwhathaschanged.Theadvantageofthisapproachisthatasingleeventinstancecanbeusedforallnotificationsfromaparticularmodelwhichishighlydesirablewhenthenotificationstendtobehighinfrequency(suchaswhenaJScrollBarisdragged).Sendastatefulnotificationthatdescribesmorepreciselyhowthemodelhaschanged.Thisalternativerequiresaneweventinstanceforeachnotification.Itisdesirablewhenagenericnotificationdoesn'tprovidethelistenerwithenoughinformationtodetermineefficientlywhathaschangedbyqueryingthemodel(suchaswhenacolumnofcellschangevalueinaJTable).

LightweightnotificationThefollowingmodelsinSwingusethelightweightnotification,whichisbasedontheChangeListener/ChangeEventAPI:

Model Listener EventBoundedRangeModel ChangeListener ChangeEventButtonModel ChangeListener ChangeEventSingleSelectionModel ChangeListener ChangeEvent

TheChangeListenerinterfacehasasinglegenericmethod:

publicvoidstateChanged(ChangeEvente)TheonlystateinaChangeEventistheevent"source."Becausethesourceisalwaysthesameacrossnotifications,asingleinstancecanbeusedforallnotificationsfromaparticularmodel.ModelsthatusethismechanismsupportthefollowingmethodstoaddandremoveChangeListeners:

publicvoidaddChangeListener(ChangeListenerl)publicvoidremoveChangeListener(ChangeListenerl)Therefore,tobenotifiedwhenthevalueofaJSliderhaschanged,thecodemightlooklikethis:

JSliderslider=newJSlider()BoundedRangeModelmodel=slider.getModel()model.addChangeListener(newChangeListener(){publicvoidstateChanged(ChangeEvente){//needtoquerythemodel//togetupdatedvalue...BoundedRangeModelm=(BoundedRangeModel)e.getSource()System.out.println("modelchanged:"+m.getValue())



}})

Toprovideconvenienceforprogramsthatdon'twishtodealwithseparatemodelobjects,someSwingcomponentclassesalsoprovidetheabilitytoregisterChangeListenersdirectlyonthecomponent(sothecomponentcanlistenforchangesonthemodelinternallyandthenpropagatesthoseeventstoanylistenersregistereddirectlyonthecomponent).Theonlydifferencebetweenthesenotificationsisthatforthemodelcase,theeventsourceisthemodelinstance,whileforthecomponentcase,thesourceisthecomponent.

Sowecouldsimplifytheprecedingexampleto:

JSliderslider=newJSlider()slider.addChangeListener(newChangeListener(){publicvoidstateChanged(ChangeEvente){//thesourcewillbe//thesliderthistime..JSliders=(JSlider)e.getSource()System.out.println("valuechanged:"+s.getValue())}})

StatefulnotificationModelsthatsupportstatefulnotificationprovideeventListenerinterfacesandeventobjectsspecifictotheirpurpose.Thefollowingtableshowsthebreakdownforthosemodels:

Model Listener EventListModel ListDataListener ListDataEventListSelectionModel ListSelectionListener ListSelectionEventComboBoxModel ListDataListener ListDataEventTreeModel TreeModelListener TreeModelEventTreeSelectionModel TreeSelectionListener TreeSelectionEventTableModel TableModelListener TableModelEvent

TableColumnModel TableColumnModelListenerTableColumnModelEvent

Document DocumentListener DocumentEventDocument UndoableEditListener UndoableEditEvent

TheusageoftheseAPIsissimilartothelightweightnotification,exceptthatthelistenercanquerytheeventobjectdirectlytofindoutwhathaschanged.Forexample,thefollowingcodedynamicallytrackstheselectediteminaJList:

Stringitems[]={"One","Two","Three")JListlist=newJList(items)ListSelectionModelsModel=list.getSelectionModel()sModel.addListSelectionListener(newListSelectionListener(){publicvoidvalueChanged(ListSelectionEvente){//getchangeinformationdirectly//fromtheeventinstance...if(!e.getValueIsAdjusting()){System.out.println("selectionchanged:"+e.getFirstIndex())}}})

AutomaticViewUpdates

Amodeldoesnothaveanyintrinsicknowledgeoftheviewthatrepresentsit.(Thisrequirementiscriticaltoenablemultipleviewsonthesamemodel).Instead,amodelhasonlyalistoflistenersinterestedinknowingwhenitsstatehaschanged.ASwingcomponenttakesresponsibilityforhookinguptheappropriatemodellistenersothatitcanappropriatelyrepaintitselfasthemodelchanges(ifyoufindthatacomponentisnotupdatingautomaticallywhenthemodelchanges,itisabug!).Thisistruewhetheradefaultinternalmodelisusedorwhetheraprograminstallsitsownmodelimplementation.

IgnoringmodelscompletelyAsmentionedpreviously,mostcomponentsprovidethemodeldefinedAPIdirectlyinthecomponentclasssothatthecomponentcanbemanipulatedwithoutinteractingwiththemodelatall.Thisisconsideredperfectlyacceptableprogrammingpractice(especiallyfortheGUIstatemodels).Forexample,followingisJSlider'simplementationofgetValue(),whichinternallydelegatesthemethodcalltoitsmodel:

publicintgetValue(){



returngetModel().getValue()}Andsoprogramscansimplydothefollowing:

JSliderslider=newJSlider()intvalue=slider.getValue()//what'sa"model,"anyway?

SeparablemodelsummarySowhileit'susefultounderstandhowSwing'smodeldesignworks,itisn'tnecessarytousethemodelAPIforallaspectsofSwingprogramming.Youshouldcarefullyconsideryourapplication'sindividualneedsanddeterminewherethemodelAPIwillenhanceyourcodewithoutintroducingunnecessarycomplexity.

Inparticular,werecommendtheusageoftheApplicationDatacategoryofmodelsforSwing(modelsforJTable,JTree,andthelike)becausetheycangreatlyenhancethescalabilityandmodularityofyourapplicationoverthelongrun.

PluggablelookandfeelarchitectureSwing'spluggablelookandfeelarchitectureallowsustoprovideasinglecomponentAPIwithoutdictatingaparticularlookandfeel.TheSwingtoolkitprovidesadefaultsetoflookandfeelshowever,theAPIis"open"adesignthatadditionallyallowsdeveloperstocreatenewlookandfeelimplementationsbyeitherextendinganexistinglookandfeelorcreatingonefromscratch.AlthoughthepluggablelookandfeelAPIisextensible,itwasintentionallydesignedatalevelbelowthebasiccomponentAPIinsuchawaythatadeveloperdoesnotneedtounderstanditsintricatedetailstobuildSwingGUIs.(Butifyouwanttoknow,readon...)

Whilewedon'texpect(oradvise)themajorityofdeveloperstocreatenewlookandfeelimplementations,werealizePL&Fisaverypowerfulfeatureforasubsetofapplicationsthatwanttocreateauniqueidentity.Asitturnsout,PL&FisalsoideallysuitedforuseinbuildingGUIsthatareaccessibletouserswithdisabilities,suchasvisuallyimpairedusersoruserswhocannotoperateamouse.

Inanutshell,pluggablelookandfeeldesignsimplymeansthattheportionofacomponent'simplementationthatdealswiththepresentation(thelook)andeventhandling(thefeel)isdelegatedtoaseparateUIobjectsuppliedbythecurrentlyinstalledlookandfeel,whichcanbechangedatruntime.

ThepluggablelookandfeelAPIThepluggablelookandfeelAPIincludes:

SomesmallhooksintheSwingcomponentclasses.SometoplevelAPIforlookandfeelmanagement.AmorecomplexAPIthatactuallyimplementslookandfeelsinseparatepackages.Thecomponenthooks

EachSwingcomponentthathaslookandfeelspecificbehaviordefinesanabstractclassintheswing.plafpackagetorepresentitsUIdelegate.Thenamingconventionfortheseclassesistotaketheclassnameforthecomponent,removethe"J"prefix,andappend"UI."Forexample,JButtondefinesitsUIdelegatewiththeplafclassButtonUI.

TheUIdelegateiscreatedinthecomponent'sconstructorandisaccessibleasaJavaBeansboundpropertyonthecomponent.Forexample,JScrollBarprovidesthefollowingmethodstoaccessitsUIdelegate:

publicScrollBarUIgetUI()publicvoidsetUI(ScrollBarUIui)ThisprocessofcreatingaUIdelegateandsettingitasthe"UI"propertyforacomponentisessentiallythe"installation"ofacomponent'slookandfeel.

EachcomponentalsoprovidesamethodwhichcreatesandsetsaUIdelegateforthe"default"lookandfeel(thismethodisusedbytheconstructorwhendoingtheinstallation):

publicvoidupdateUI()AlookandfeelimplementationprovidesconcretesubclassesforeachabstractplafUIclass.Forexample,theWindowslookandfeeldefinesWindowsButtonUI,aWindowsScrollBarUI,andsoon.WhenacomponentinstallsitsUIdelegate,itmusthaveawaytolookuptheappropriateconcreteclassnameforthecurrentdefaultlookandfeeldynamically.ThisoperationisperformedusingahashtableinwhichthekeyisdefinedprogrammaticallybythegetUIClassID()methodinthecomponent.Theconventionistousetheplafabstractclassnameforthesekeys.Forexample,JScrollbarprovides:

publicStringgetUIClassID(){return"ScrollBarUI"}Consequently,thehashtableintheWindowslookandfeelwillprovideanentrythatmaps"ScrollBarUI"to

"com.sun.java.swing.plaf.windows.WindowsScrollBarUI"

Lookandfeelmanagement

SwingdefinesanabstractLookAndFeelclassthatrepresentsalltheinformationcentraltoalookandfeelimplementation,suchasitsname,itsdescription,whetherit'sanativelookandfeelandinparticular,ahashtable(knownasthe"DefaultsTable")forstoringdefaultvaluesforvariouslookandfeelattributes,suchascolorsandfonts.

EachlookandfeelimplementationdefinesasubclassofLookAndFeel(forexample,swing.plaf.motif.MotifLookAndFeel)toprovideSwingwiththenecessaryinformationtomanagethelookandfeel.

TheUIManageristheAPIthroughwhichcomponentsandprogramsaccesslookandfeelinformation(theyshouldrarely,ifever,talkdirectlytoaLookAndFeelinstance).UIManagerisresponsibleforkeepingtrackofwhichLookAndFeelclassesareavailable,whichareinstalled,andwhichiscurrentlythedefault.TheUIManageralsomanagesaccesstotheDefaultsTableforthecurrentlookandfeel.

The'default'lookandfeel

TheUIManageralsoprovidesmethodsforgettingandsettingthecurrentdefaultLookAndFeel:

publicstaticLookAndFeelgetLookAndFeel()



publicstaticvoidsetLookAndFeel(LookAndFeelnewLookAndFeel)publicstaticvoidsetLookAndFeel(StringclassName)

Asadefaultlookandfeel,SwinginitializesthecrossplatformJavalookandfeel(formerlyknownas"Metal").However,ifaSwingprogramwantstosetthedefaultLookandFeelexplicitly,itcandothatusingtheUIManager.setLookAndFeel()method.Forexample,thefollowingcodesamplewillsetthedefaultLookandFeeltobeCDE/Motif:

UIManager.setLookAndFeel("com.sun.java.swing.plaf.motif.MotifLookAndFeel")Sometimesanapplicationmaynotwanttospecifyaparticularlookandfeel,butinsteadwantstoconfigurealookandfeelinsuchawaythatitdynamicallymatcheswhateverplatformithappenstoberunningon(forinstance,the.WindowslookandfeelifitisrunningonWindowsNT,orCDE/MotififitrunningonSolaris).Or,perhaps,anapplicationmightwanttolockdownthelookandfeeltothecrossplatformJavalookandfeel.

TheUIManagerprovidesthefollowingstaticmethodstoprogrammaticallyobtaintheappropriateLookAndFeelclassnamesforeachofthesecases:

publicstaticStringgetSystemLookAndFeelClassName()publicstaticStringgetCrossPlatformLookAndFeelClassName()So,toensurethataprogramalwaysrunsintheplatform'ssystemlookandfeel,thecodemightlooklikethis:

UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName())DynamicallyChangingtheDefaultLookandFeel

WhenaSwingapplicationprogrammaticallysetsthelookandfeel(asdescribedabove),theidealplacetodosoisbeforeanySwingcomponentsareinstantiated.ThisisbecausetheUIManager.setLookAndFeel()methodmakesaparticularLookAndFeelthecurrentdefaultbyloadingandinitializingthatLookAndFeelinstance,butitdoesnotautomaticallycauseanyexistingcomponentstochangetheirlookandfeel.

RememberthatcomponentsinitializetheirUIdelegateatconstructtime,therefore,ifthecurrentdefaultchangesaftertheyareconstructed,theywillnotautomaticallyupdatetheirUIsaccordingly.Itisuptotheprogramtoimplementthisdynamicswitchingbytraversingthecontainmenthierarchyandupdatingthecomponentsindividually.(NOTE:SwingprovidestheSwingUtilities.updateComponentTreeUI()methodtoassistwiththisprocess).

ThelookandfeelofacomponentcanbeupdatedatanytimetomatchthecurrentdefaultbyinvokingitsupdateUI()method,whichusesthefollowingstaticmethodonUIManagertogettheappropriateUIdelegate:

publicstaticComponentUIgetUI(JComponentc)Forexample,theimplementationofupdateUI()fortheJScrollBarlookslikethefollowing:

publicvoidupdateUI(){setUI((ScrollBarUI)UIManager.getUI(this))}AndsoifaprogramneedstochangethelookandfeelofaGUIhierarchyafteritwasinstantiated,thecodemightlooklikethefollowing:

//GUIalreadyinstantiated,wheremyframe//istoplevelframetry{

UIManager.setLookAndFeel("com.sun.java.swing.plaf.motif.MotifLookAndFeel")myframe.setCursor(Cursor.getPredefinedCursor(Cursor.WAIT_CURSOR))SwingUtilities.updateComponentTreeUI(myframe)myframe.validate()

}catch(UnsupportedLookAndFeelExceptione){}finally{myframe.setCursor(Cursor.getPredefinedCursor(Cursor.DEFAULT_CURSOR))}

Managinglookandfeeldata

TheUIManagerdefinesastaticclass,namedUIManager.LookAndFeelInfo,forstoringthehighlevelname(suchas."Metal")andparticularclassname(suchas"com.sun.java.swing.plaf.MetalLookAndFeel")foraLookAndFeel.ItusestheseclassesinternallytomanagetheknownLookAndFeelobjects.ThisinformationcanbeaccessedfromtheUIManagerviathefollowingstaticmethods:

publicstaticLookAndFeelInfo[]



getInstalledLookAndFeels()

publicstaticvoidsetInstalledLookAndFeels(LookAndFeelInfo[]infos)throwsSecurityException

publicstaticvoidinstallLookAndFeel(LookAndFeelInfoinfo)

publicstaticvoidinstallLookAndFeel(Stringname,StringclassName)

Thesemethodscanbeusedtoprogrammaticallydeterminewhichlookandfeelimplementationsareavailable,whichisusefulwhenbuildinguserinterfaceswhichallowtheendusertodynamicallyselectalookandfeel.

Thelookandfeelpackages

TheUIdelegateclassesprovidedinswing.plaf(ButtonUI,ScrollBarUI,andsoon)definethepreciseAPIthatacomponentcanusetointeractwiththeUIdelegateinstance.(NOTE:Interfaceswereoriginallyusedhere,buttheywerereplacedwithabstractclassesbecausewefelttheAPIwasnotmatureenoughtowithstandtheconcretecastingofaninterface.)TheseplafAPIsaretherootofalllookandfeelimplementations.

Eachlookandfeelimplementationprovidesconcretesubclassesoftheseabstractplafclasses.Allsuchclassesdefinedbyaparticularlookandfeelimplementationarecontainedinaseparatepackageundertheswing.plafpackage(forexample,.swing.plaf.motif,swing.plaf.metal,andsoon).Alookandfeelpackagecontainsthefollowing:

TheLookAndFeelsubclass(forinstance,MetalLookAndFeel).Alllookandfeel'sUIdelegateclasses(forexample,MetalButtonUI,MetalTreeUI,andthelike).Anylookandfeelutilityclasses(MetalGraphicsUtils,MetalIconFactory,andsoon).Otherresourcesassociatedwiththelookandfeel,suchasimagefiles.InimplementingthevariousSwinglookandfeels,wesoondiscoveredthattherewasalotofcommonalityamongthem.Wefactoredoutthiscommoncodeintoabaselookandfeelimplementation(called"basic")whichextendstheplafabstractclassesandfromwhichthespecificlookandfeelimplementations(motif,windows,andsoon.)extend.Thebasiclookandfeelpackagesupportsbuildingadesktoplevellookandfeel,suchasWindowsorCDE/Motif.

Thebasiclookandfeelpackageisjustoneexampleofhowtobuildapluggablelookandfeelthearchitectureisflexibleenoughtoaccommodateotherapproachesaswell.

Theremainderofthisdocumentwillshowhowalookandfeelpackageworksatthegenericlevel,leavingthedetailsonthebasicpackageforafuturedocument.

WARNING:AllAPIsdefinedbelowtheswing.plafpackagearenotfrozeninthe1.0.XversionofSwing.WearecurrentlycleaningupthoseAPIsfortheversionofSwingthatwillshipwithJDK1.2beta4,atwhichtimetheywillbecomefrozen.Soifyouaredevelopingyourownlookandfeelimplementationusingthe1.0.1API,thisislikelytoaffectyou.

TheLookAndFeelSubclass

TheLookAndFeelclassdefinesthefollowingabstractmethods,whichallsubclassesmustimplement:

publicStringgetName()publicStringgetID()publicStringgetDescription()publicbooleanisNativeLookAndFeel()publicbooleanisSupportedLookAndFeel()ThegetName(),getID(),andgetDescription()methodsprovidegenericinformationaboutthelookandfeel.

TheisNativeLookAndFeel()methodreturnstrueifthelookandfeelisnativetothecurrentplatform.Forexample,MotifLookAndFeelreturnstrueifitiscurrentlyrunningontheSolarisplatform,andreturnsfalseotherwise.

TheisSupportedLookAndFeel()methodreturnswhetherornotthislookandfeelisauthorizedtorunonthecurrentplatform.Forexample,WindowsLookAndFeelreturnstrueonlyifitisrunningonaWindows95,Windows98,orWindowsNTmachine.

ALookAndFeelclassalsoprovidesmethodsforinitializationanduninitialization:

publicvoidinitialize()publicvoiduninitialize()Theinitialize()methodisinvokedbytheUIManagerwhentheLookAndFeelismadethe"default"usingtheUIManager.setLookAndFeel()method.uninitialize()isinvokedbytheUIManagerwhentheLookAndFeelisabouttobereplacedasthedefault.

TheDefaultsTable

Finally,theLookAndFeelclassprovidesamethodtoreturnthelookandfeel'simplementationoftheDefaultsTable:

publicUIDefaultsgetDefaults()TheDefaultsTableisrepresentedbytheUIDefaultsclass,adirectextensionofjava.util.Hashtable,whichaddsmethodsforaccessingspecifictypesofinformationaboutalookandfeel.ThistablemustincludealltheUIClassIDtoclassnamemappinginformation,aswellasanydefaultvaluesforpresentationrelatedproperties(suchascolor,font,border,andicon)foreachUIdelegate.Forexample,followingisasampleofwhatafragmentofgetDefaults()mightlooklikeforahypotheticallookandfeelinapackagecalled"mine":

publicUIDefaultsgetDefaults(){UIDefaultstable=newUIDefaults()



Object[]uiDefaults={

"ButtonUI","mine.MyButtonUI","CheckBoxUI","mine.MyCheckBoxUI","MenuBarUI","mine.MyMenuBarUI",..."Button.background",newColorUIResource(Color.gray),"Button.foreground",newColorUIResource(Color.black),"Button.font",newFontUIResource("Dialog",Font.PLAIN,12),"CheckBox.background",newColorUIResource(Color.lightGray),"CheckBox.font",newFontUIResource("Dialog",Font.BOLD,12),

...}table.putDefaults(uiDefaults)returntable

}

WhenthedefaultlookandfeelissetwithUIManager.setLookAndFeel(),theUIManagercallsgetDefaults()onthenewLookAndFeelinstanceandstoresthehashtableitreturns.SubsequentcallstotheUIManager'slookupmethodswillbeappliedtothistable.Forexample,aftermaking"mine"thedefaultLookandFeel:

UIManager.get("ButtonUI")=>"mine.MyButtonUI"TheUIclassesaccesstheirdefaultinformationinthesameway.Forexample,ourexampleButtonUIclasswouldinitializetheJButton's"background"propertylikethis:

button.setBackground(UIManager.getColor("Button.background")Thedefaultsareorganizedthiswaytoallowdeveloperstooverridethem.MoredetailaboutSwing'sDefaultsmechanismwillbepublishedinafuturearticle.

DistinguishingbetweenUIsetandappsetproperties

Swingallowsapplicationstosetpropertyvalues(suchascolorandfont)individuallyoncomponents.Soit'scriticaltomakesurethatthesevaluesdon'tgetclobberedwhenalookandfeelsetsupits"default"propertiesforthecomponent.

ThisisnotanissuethefirsttimeaUIdelegateisinstalledonacomponent(atconstructtime)becauseallpropertieswillbeuninitializedandlegallysettablebythelookandfeel.Theproblemoccurswhentheapplicationsetsindividualpropertiesaftercomponentconstructionandthensubsequentlysetsanewlookandfeel(thatis,dynamiclookandfeelswitching).Thismeansthatthelookandfeelmustbeabletodistinguishbetweenpropertyvaluessetbytheapplication,andthosesetbyalookandfeel.

Thisissueishandledbymarkingallvaluessetbythelookandfeelwiththeplaf.UIResourceinterface.Theplafpackageprovidesasetof"marked"classesforrepresentingthesevalues,ColorUIResource,FontUIResource,andBorderUIResource.TheprecedingcodeexampleshowstheusageoftheseclassestomarkthedefaultpropertyvaluesforthehypotheticalMyButtonUIclass.

TheUIdelegateThesuperclassofallUIDelegateclassesisswing.plaf.ComponentUI.Thisclasscontainstheprimary"machinery"formakingthepluggablelookandfeelwork.ItsmethodsdealwithUIinstallationanduninstallation,andwithdelegationofacomponent'sgeometryhandlingandpainting.

ManyoftheUIDelegatesubclassesalsoprovideadditionalmethodsspecifictotheirownrequiredinteractionwiththecomponenthowever,thisdocumentfocusesprimarilyonthegenericmechanismimplementedbyComponentUI.

UIinstallationanddeinstallation

Firstoff,theComponentUIclassdefinesthesemethodsmethodsforUIdelegateinstallationanduninstallation:

publicvoidinstallUI(JComponentc)publicvoiduninstallUI(JComponentc)LookingattheimplementationofJComponent.setUI()(whichisalwaysinvokedfromthesetUImethodonJComponentsubclasses),wecanclearlyseehowUIdelegateinstallation/deinstallationworks:

protectedvoidsetUI(ComponentUInewUI){if(ui!=null){ui.uninstallUI(this)



}ComponentUIoldUI=uiui=newUIif(ui!=null){ui.installUI(this)}invalidate()firePropertyChange("UI",oldUI,newUI)}

UIinstallationillustratedThisarticlecomeswithagiantpostersizechartthatillustratestheprocessinstallingaUIdelegate.Itcanprovideyouwithavaluableoverviewofthedelegateinstallationprocess.

Tofoldoutthechart,justfollowthislink

TheUIdelegate'sinstallUI()methodisresponsibleforthefollowing:

Setdefaultfont,color,border,andopacitypropertiesonthecomponent.Installanappropriatelayoutmanageronthecomponent.AddanyappropriatechildsubcomponentstothecomponentRegisteranyrequiredeventlistenersonthecomponent.Registeranylookandfeelspecifickeyboardactions(mnemonics,etc.)forthecomponent.Registerappropriatemodellistenerstobenotifiedwhentorepaint.Initializeanyappropriateinstancedata.Forexample,theinstallUI()methodforanextensionofButtonUImightlooklikethis:

protectedMyMouseListenermouseListenerprotectedMyChangeListenerchangeListener

publicvoidinstallUI(JComponentc){AbstractButtonb=(AbstractButton)c//Installdefaultcolors&opacityColorbg=c.getBackground()if(bg==null||bginstanceofUIResource){c.setBackground(UIManager.getColor("Button.background"))}Colorfg=c.getForeground()if(fg==null||fginstanceofUIResource){c.setForeground(UIManager.getColor("Button.foreground"))}c.setOpaque(false)//InstalllistenersmouseListener=newMyMouseListener()c.addMouseListener(mouseListener)c.addMouseMotionListener(mouseListener)changeListener=newMyChangeListener()b.addChangeListener(changeListener)}

Conventionsforinitializingcomponentproperties

Swingdefinesanumberofconventionsforinitializingcomponentpropertiesatinstalltime,includingthefollowing:

Allvaluesusedforsettingcolors,font,andborderpropertiesshouldbeobtainedfromtheDefaultstable(asdescribedinthesubsectionontheLookAndFeelsubclass).Color,fontandborderpropertiesshouldbesetifandonlyiftheapplicationhasnotalreadysetthem.TofacilitateconventionNo1,theUIManagerclassprovidesanumberofstaticmethodstoextractpropertyvaluesofaparticulartype(forinstance,thestaticmethodsUIManager.getColor(),UIManager.getFont(),andsoon).

ConventionNo.2isimplementedbyalwayscheckingforeitheranullvalueoraninstanceofUIResourcebeforesettingtheproperty.



TheComponentUI'suninstall()methodmustcarefullyundoeverythingthatwasdoneintheinstallUI()methodsothatthecomponentisleftinapristinestateforthenextUIdelegate.Theuninstall()methodisresponsiblefor:

ClearingtheborderpropertyifithasbeensetbyinstallUI().RemovethelayoutmanagerifithadbeensetbyinstallUI().RemoveanysubcomponentsaddedbyinstallUI().Removeanyevent/modellistenersthatwereaddedbyinstallUI().RemoveanylookandfeelspecifickeyboardactionsthatwereinstalledbyinstallUI().Nullifyanyinitializedinstancedata(toallowGCtocleanup).Forexample,anuninstall()methodtoundowhatwedidintheaboveexampleinstallationmightlooklikethis:

publicvoiduninstallUI(JComponentc){AbstractButtonb=(AbstractButton)c//Uninstalllistenersc.removeMouseListener(mouseListener)c.removeMouseMotionListener(mouseListener)mouseListener=nullb.removeChangeListener(changeListener)changeListener=null}

Defininggeometry

IntheAWT(andthusinSwing)acontainer'sLayoutManagerwilllayoutthechildcomponentsaccordingtoitsdefinedalgorithmthisisknownas"validation"ofacontainmenthierarchy.TypicallyLayoutManagerswillquerythechildcomponents'preferredSizeproperty(andsometimesminimumSizeand/ormaximumSizeaswell,dependingonthealgorithm)inordertodeterminepreciselyhowtopositionandsizethosechildren.

Obviously,thesegeometrypropertiesaresomethingthatalookandfeelusuallyneedstodefineforagivencomponent,soComponentUIprovidesthefollowingmethodsforthispurpose:

publicDimensiongetPreferredSize(JComponentc)publicDimensiongetMinimumSize(JComponentc)publicDimensiongetMaximumSize(JComponentc)publicbooleancontains(JComponentc,intx,inty)

JComponent'sparallelmethods(whichareinvokedbytheLayoutManagerduringvalidation)thensimplydelegatetotheUIobject'sgeometrymethodsifthegeometrypropertywasnotexplicitlysetbytheprogram.BelowistheimplementationofJComponent.getPreferredSize()whichshowsthisdelegation:

publicDimensiongetPreferredSize(){if(preferredSize!=null){returnpreferredSize}Dimensionsize=nullif(ui!=null){size=ui.getPreferredSize(this)}return(size!=null)?size:super.getPreferredSize()}

EventhoughtheboundingboxforallcomponentsisaRectangle,it'spossibletosimulateanonrectangularcomponentbyoverridingtheimplementationofthecontains()methodfromjava.awt.Component.(Thismethodisusedforthehittestingofmouseevents).But,liketheothergeometrypropertiesinSwing,theUIdelegatedefinesitsownversionofthecontains()method,whichisalsodelegatedtobyJComponent.contains():

publicbooleancontains(JComponentc,intx,inty){return(ui!=null)?ui.contains(this,x,y):super.contains(x,y)}SoaUIdelegatecouldprovidenonrectangular"feel"bydefiningaparticularimplementationofcontains()(forexample,ifwewantedourMyButtonUIclasstoimplementabuttonwithroundedcorners).



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Painting

Finally,theUIdelegatemustpaintthecomponentappropriately,henceComponentUIhasthefollowingmethods:

publicvoidpaint(Graphicsg,JComponentc)publicvoidupdate(Graphicsg,JComponentc)Andonceagain,JComponent.paintComponent()takescaretodelegatethepainting:

protectedvoidpaintComponent(Graphicsg){if(ui!=null){GraphicsscratchGraphics=SwingGraphics.createSwingGraphics(g.create())try{ui.update(scratchGraphics,this)}finally{scratchGraphics.dispose()}}}

SimilarlytothewayinwhichthingsaredoneinAWT,theUIdelegate'supdate()methodclearsthebackground(ifopaque)andtheninvokesitspaint()method,whichisultimatelyresponsibleforrenderingthecontentsofthecomponent.

Statelessvs.statefuldelegates

AllthemethodsonComponentUItakeaJComponentobjectasaparameter.ThisconventionenablesastatelessimplementationofaUIdelegate(becausethedelegatecanalwaysquerybacktothespecifiedcomponentinstanceforstateinformation).StatelessUIdelegateimplementationsallowasingleUIdelegateinstancetobeusedforallinstancesofthatcomponentclass,whichcansignificantlyreducethenumberofobjectsinstantiated.

ThisapproachworkswellformanyofthesimplerGUIcomponents.Butformorecomplexcomponents,wefounditnottobea"win"becausetheinefficiencycreatedbyconstantstaterecalculationswasworsethancreatingextraobjects(especiallysincethenumberofcomplexGUIcomponentscreatedinagivenprogramtendstobesmall).

TheComponentUIclassdefinesastaticmethodforreturningadelegateinstance:

publicstaticComponentUIcreateUI(JComponentc)It'stheimplementationofthismethodthatdetermineswhetherthedelegateisstatelessorstateful.That'sbecausetheUIManager.getUI()methodinvokedbythecomponenttocreatetheUIdelegateinternallyinvokesthiscreateUImethodonthedelegateclasstogettheinstance.

TheSwinglookandfeelimplementationsusebothtypesofdelegates.Forexample,Swing'sBasicButtonUIclassimplementsastatelessdelegate:

//SharedUIobjectprotectedstaticButtonUIbuttonUIpublicstaticComponentUIcreateUI(JComponentc)if(buttonUI==null){buttonUI=newBasicButtonUI()}returnbuttonUI}

WhileSwing'sBasicTabbedPaneUIusesthestatefulapproach:

publicstaticComponentUIcreateUI(JComponentc)returnnewBasicTabbedPaneUI()}

PluggableLookandFeelsummaryThepluggablelookandfeelfeatureofSwingisbothpowerfulandcomplex(whichyouunderstandifyou'vegottenthisfar!).Itisdesignedtobeprogrammedbyasmallsubsetofdeveloperswhohaveaparticularneedtodevelopanewlookandfeelimplementation.Ingeneral,applicationdevelopersonlyneedtounderstandthecapabilitiesofthismechanisminordertodecidehowtheywishtosupportlookandfeels(suchaswhethertolockdowntheprogramtoasinglelookandfeelorsupportlookandfeelconfigurationbytheuser).Swing'sUIManagerprovidestheAPIforapplicationstomanagethelookandfeelatthislevel.

Ifyou'reoneofthosedeveloperswhoneeds(orwants)todevelopacustomlookandfeel,it'scriticaltounderstandtheseunderpinningsbeforeyouwriteasinglelineofcode.We'reworkingonprovidingbetterdocumentationtohelpwiththisprocessstartingwiththisdocument,andcontinuingwithothersthatwillfollowsoon.



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