A Swing Architecture Overview

摘要:MostSwingdevelopersknowbynowthatSwingcomponentshaveaseparablemodel-and-viewdesign.AndmanySwingusershaverunacrossarticlessayingthatSwingisbasedonsomethingcalleda"modifiedMVC(model-view-controller)architecture."

ButaccurateexplanationsofhowSwingcomponentsar...

TheInsideStoryonJFCComponentDesign

ByAmyFowler

MostSwingdevelopersknowbynowthatSwingcomponentshaveaseparablemodel-and-viewdesign.AndmanySwingusershaverunacrossarticlessayingthatSwingisbasedonsomethingcalleda"modifiedMVC(model-view-controller)architecture."
ButaccurateexplanationsofhowSwingcomponentsaredesigned,andhowtheirpartsallfittogether,havebeenhardtocomeby--untilnow.
Thesilenceendswiththepublicationofthisarticle,amajorwhitepaperonSwingcomponentdesign.ItprovidesacomprehensivetechnicaloverviewofSwing'smodifiedMVCstructureanddemystifiesmanyotherfacetsofSwingcomponentarchitectureaswell.

ThisdocumentpresentsatechnicaloverviewoftheSwingcomponentarchitecture.Inparticular,itcoversthefollowingareasindetail:

Designgoals

RootsinMVC

Separablemodelarchitecture

Pluggablelook-and-feelarchitecture

DesignGoals

TheoverallgoalfortheSwingprojectwas:
TobuildasetofextensibleGUIcomponentstoenabledeveloperstomorerapidlydeveloppowerfulJavafrontendsforcommercialapplications.
Tothisend,theSwingteamestablishedasetofdesigngoalsearlyintheprojectthatdrovetheresultingarchitecture.TheseguidelinesmandatedthatSwingwould:

BeimplementedentirelyinJavatopromotecross-platformconsistencyandeasiermaintenance.

ProvideasingleAPIcapableofsupportingmultiplelook-and-feelssothatdevelopersandend-userswouldnotbelockedintoasinglelook-and-feel.

Enablethepowerofmodel-drivenprogrammingwithoutrequiringitinthehighest-levelAPI.

AdheretoJavaBeansdesignprinciplestoensurethatcomponentsbehavewellinIDEsandbuildertools.

ProvidecompatibilitywithAWTAPIswherethereisoverlapping,toleveragetheAWTknowledgebaseandeaseporting.

RootsinMVC

Swingarchitectureisrootedinthemodel-view-controller(MVC)designthatdatesbacktoSmallTalk.MVCarchitecturecallsforavisualapplicationtobebrokenupintothreeseparateparts:

Amodelthatrepresentsthedatafortheapplication.

Theviewthatisthevisualrepresentationofthatdata.

Acontrollerthattakesuserinputontheviewandtranslatesthattochangesinthemodel.

Earlyon,MVCwasalogicalchoiceforSwingbecauseitprovidedabasisformeetingthefirstthreeofourdesigngoalswithintheboundsofthelattertwo.
ThefirstSwingprototypefollowedatraditionalMVCseparationinwhicheachcomponenthadaseparatemodelobjectanddelegateditslook-and-feelimplementationtoseparateviewandcontrollerobjects.

Thedelegate

Wequicklydiscoveredthatthissplitdidn'tworkwellinpracticaltermsbecausetheviewandcontrollerpartsofacomponentrequiredatightcoupling(forexample,itwasverydifficulttowriteagenericcontrollerthatdidn'tknowspecificsabouttheview).SowecollapsedthesetwoentitiesintoasingleUI(user-interface)object,asshowninthisdiagram:

(TheUIdelegateobjectshowninthispictureissometimescalledadelegateobject,orUIdelegate.TheUIdelegateusedinSwingisdescribedinmoredetailinthePluggablelook-and-feelsectionofthisarticle,underthesubheading"TheUIdelegate".)
Asthediagramillustrates,Swingarchitectureislooselybased--butnotstrictlybased--onthetraditionalMVCdesign.IntheworldofSwing,thisnewquasi-MVCdesignissometimesreferredtoaseparablemodelarchitecture.

Swing'sseparablemodeldesigntreatsthemodelpartofacomponentasaseparateelement,justastheMVCdesigndoes.ButSwingcollapsestheviewandcontrollerpartsofeachcomponentintoasingleUI(user-interface)object.

ToMVCornottoMVC?

Onenoteworthypointisthatasanapplicationdeveloper,youshouldthinkofacomponent'sview/controllerresponsibilitiesasbeinghandledbythegenericcomponentclass(suchas.JButton,JTree,andsoon).Thecomponentclassthendelegatesthelook-and-feel-specificaspectsofthoseresponsibilitiestotheUIobjectthatisprovidedbythecurrentlyinstalledlook-and-feel.
Forexample,thecodethatimplementsdouble-bufferedpaintingisinSwing'sJComponentclass(the"mother"ofmostSwingcomponentclasses),whilethecodethatrendersaJButton'slabelisinthebutton'sUIdelegateclass.Theprecedingdiagramillustratesthissubtle(andoftenconfusing)point:

SoSwingdoeshaveastrongMVClineage.Butit'salsoimportanttoreiteratethatourMVCarchitectureservestwodistinctpurposes:

First,separatingthemodeldefinitionfromacomponentfacilitatesmodel-drivenprogramminginSwing.

Second,theabilitytodelegatesomeofacomponent'sview/controllerresponsibilitiestoseparatelook-and-feelobjectsprovidesthebasisforSwing'spluggablelook-and-feelarchitecture.

AlthoughthesetwoconceptsarelinkedbytheMVCdesign,theymaybetreatedsomewhatorthogonallyfromthedeveloper'sperspective.Theremainderofthisdocumentwillcovereachofthesemechanismsingreaterdetail.

Separablemodelarchitecture

Itisgenerallyconsideredgoodpracticetocenterthearchitectureofanapplicationarounditsdataratherthanarounditsuserinterface.Tosupportthisparadigm,Swingdefinesaseparatemodelinterfaceforeachcomponentthathasalogicaldataorvalueabstraction.ThisseparationprovidesprogramswiththeoptionofpluggingintheirownmodelimplementationsforSwingcomponents.
Thefollowingtableshowsthecomponent-to-modelmappingforSwing.

Component	ModelInterface	ModelType
JButton	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

GUI-statevs.application-datamodels

ThemodelsprovidedbySwingfallintotwogeneralcategories:GUI-statemodelsandapplication-datamodels.

GUI-statemodels

GUIstatemodelsareinterfacesthatdefinethevisualstatusofaGUIcontrol,suchaswhetherabuttonispressedorarmed,orwhichitemsareselectedinalist.GUI-statemodelstypicallyarerelevantonlyinthecontextofagraphicaluserinterface(GUI).WhileitisoftenusefultodevelopprogramsusingGUI-statemodelseparation--particularlyifmultipleGUIcontrolsarelinkedtoacommonstate(suchasinasharedwhiteboardprogram),orifmanipulatingonecontrolautomaticallychangesthevalueofanother--theuseofGUI-statemodelsisnotrequiredbySwing.ItispossibletomanipulatethestateofaGUIcontrolthroughtop-levelmethodsonthecomponent,withoutanydirectinteractionwiththemodelatall.Intheprecedingtable,GUI-statemodelsinSwingarehighlightedinblue.

Application-datamodels

Anapplication-datamodelisaninterfacethatrepresentssomequantifiabledatathathasmeaningprimarilyinthecontextoftheapplication,suchasthevalueofacellinatableortheitemsdisplayedinalist.ThesedatamodelsprovideaverypowerfulprogrammingparadigmforSwingprogramsthatneedacleanseparationbetweentheirapplicationdata/logicandtheirGUI.Fortrulydata-centricSwingcomponents,suchasJTreeandJTable,interactionwiththedatamodelisstronglyrecommended.Application-datamodelsarehighlightedinredinthetablepresentedatthebeginningofthissection.
Ofcoursewithsomecomponents,themodelcategorizationfallssomewhereinbetweenGUIstatemodelsandapplication-datamodels,dependingonthecontextinwhichthemodelisused.ThisisthecasewiththeBoundedRangeModelonJSliderorJProgressBar.Thesemodelsarehighlightedinpurpleintheprecedingtable.
Swing'sseparablemodelAPImakesnospecificdistinctionsbetweenGUIstatemodelsandapplication-datamodels;however,wehaveclarifiedthisdifferenceheretogivedevelopersabetterunderstandingofwhenandwhytheymightwishtoprogramwiththeseparablemodels.

Sharedmodeldefinitions

Referringagaintothetableatthebeginningofthissection,noticethatmodeldefinitionsaresharedacrosscomponentsincaseswherethedataabstractionforeachcomponentissimilarenoughtosupportasingleinterfacewithoutover-genericizingthatinterface.Commonmodelsenableautomaticconnectabilitybetweencomponenttypes.Forexample,becausebothJSliderandJScrollbarusetheBoundedRangeModelinterface,asingleBoundedRangeModelinstancecouldbepluggedintobothaJScrollbarandaJSliderandtheirvisualstatewouldalwaysremaininsync.

Theseparable-modelAPI

SwingcomponentsthatdefinemodelssupportaJavaBeansboundpropertyforthemodel.Forexample,JSliderusestheBoundedRangeModelinterfaceforitsmodeldefinition.Consequently,itincludesthefollowingmethods:

publicBoundedRangeModelgetModel()

publicvoidsetModel(BoundedRangeModelmodel)

AllSwingcomponentshaveonethingincommon:Ifyoudon'tsetyourownmodel,adefaultiscreatedandinstalledinternallyinthecomponent.Thenamingconventionforthesedefaultmodelclassesistoprependtheinterfacenamewith"Default."ForJSlider,aDefaultBoundedRangeModelobjectisinstantiatedinitsconstructor:

publicJSlider(intorientation,intmin, intmax,intvalue) { checkOrientation(orientation); this.orientation=orientation; this.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.

Modelchangenotification

Modelsmustbeabletonotifyanyinterestedparties(suchasviews)whentheirdataorvaluechanges.SwingmodelsusetheJavaBeansEventmodelfortheimplementationofthisnotification.TherearetwoapproachesforthisnotificationusedinSwing:

Sendalightweightnotificationthatthestatehas"changed"andrequirethelistenertorespondbysendingaquerybacktothemodeltofindoutwhathaschanged.Theadvantageofthisapproachisthatasingleeventinstancecanbeusedforallnotificationsfromaparticularmodel--whichishighlydesirablewhenthenotificationstendtobehighinfrequency(suchaswhenaJScrollBarisdragged).

Sendastatefulnotificationthatdescribesmorepreciselyhowthemodelhaschanged.Thisalternativerequiresaneweventinstanceforeachnotification.Itisdesirablewhenagenericnotificationdoesn'tprovidethelistenerwithenoughinformationtodetermineefficientlywhathaschangedbyqueryingthemodel(suchaswhenacolumnofcellschangevalueinaJTable).

Lightweightnotification

ThefollowingmodelsinSwingusethelightweightnotification,whichisbasedontheChangeListener/ChangeEventAPI:

Model	Listener	Event
BoundedRangeModel	ChangeListener	ChangeEvent
ButtonModel	ChangeListener	ChangeEvent
SingleSelectionModel	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()); } });

Statefulnotification

ModelsthatsupportstatefulnotificationprovideeventListenerinterfacesandeventobjectsspecifictotheirpurpose.Thefollowingtableshowsthebreakdownforthosemodels:

Model	Listener	Event
ListModel	ListDataListener	ListDataEvent
ListSelectionModel	ListSelectionListener	ListSelectionEvent
ComboBoxModel	ListDataListener	ListDataEvent
TreeModel	TreeModelListener	TreeModelEvent
TreeSelectionModel	TreeSelectionListener	TreeSelectionEvent
TableModel	TableModelListener	TableModelEvent
TableColumnModel	TableColumnModel- Listener	TableColumnModel- Event
Document	DocumentListener	DocumentEvent
Document	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.

Ignoringmodelscompletely

Asmentionedpreviously,mostcomponentsprovidethemodel-definedAPIdirectlyinthecomponentclasssothatthecomponentcanbemanipulatedwithoutinteractingwiththemodelatall.Thisisconsideredperfectlyacceptableprogrammingpractice(especiallyfortheGUI-statemodels).Forexample,followingisJSlider'simplementationofgetValue(),whichinternallydelegatesthemethodcalltoitsmodel:

publicintgetValue(){

returngetModel().getValue();
}

Andsoprogramscansimplydothefollowing:

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

Separablemodelsummary

Sowhileit'susefultounderstandhowSwing'smodeldesignworks,itisn'tnecessarytousethemodelAPIforallaspectsofSwingprogramming.Youshouldcarefullyconsideryourapplication'sindividualneedsanddeterminewherethemodelAPIwillenhanceyourcodewithoutintroducingunnecessarycomplexity.
Inparticular,werecommendtheusageoftheApplication-DatacategoryofmodelsforSwing(modelsforJTable,JTree,andthelike)becausetheycangreatlyenhancethescalabilityandmodularityofyourapplicationoverthelongrun.

Pluggablelook-and-feelarchitecture

Swing'spluggablelook-and-feelarchitectureallowsustoprovideasinglecomponentAPIwithoutdictatingaparticularlook-and-feel.TheSwingtoolkitprovidesadefaultsetoflook-and-feels;however,theAPIis"open"--adesignthatadditionallyallowsdeveloperstocreatenewlook-and-feelimplementationsbyeitherextendinganexistinglook-and-feelorcreatingonefromscratch.Althoughthepluggablelook-and-feelAPIisextensible,itwasintentionallydesignedatalevelbelowthebasiccomponentAPIinsuchawaythatadeveloperdoesnotneedtounderstanditsintricatedetailstobuildSwingGUIs.(Butifyouwanttoknow,readon...)
Whilewedon'texpect(oradvise)themajorityofdeveloperstocreatenewlook-and-feelimplementations,werealizePL&Fisaverypowerfulfeatureforasubsetofapplicationsthatwanttocreateauniqueidentity.Asitturnsout,PL&FisalsoideallysuitedforuseinbuildingGUIsthatareaccessibletouserswithdisabilities,suchasvisuallyimpairedusersoruserswhocannotoperateamouse.
Inanutshell,pluggablelook-and-feeldesignsimplymeansthattheportionofacomponent'simplementationthatdealswiththepresentation(thelook)andevent-handling(thefeel)isdelegatedtoaseparateUIobjectsuppliedbythecurrentlyinstalledlook-and-feel,whichcanbechangedatruntime.

Thepluggablelook-and-feelAPI

Thepluggablelook-and-feelAPIincludes:

SomesmallhooksintheSwingcomponentclasses.

Sometop-levelAPIforlook-and-feelmanagement.

AmorecomplexAPIthatactuallyimplementslook-and-feelsinseparatepackages.

Thecomponenthooks

EachSwingcomponentthathaslook-and-feel-specificbehaviordefinesanabstractclassintheswing.plafpackagetorepresentitsUIdelegate.Thenamingconventionfortheseclassesistotaketheclassnameforthecomponent,removethe"J"prefix,andappend"UI."Forexample,JButtondefinesitsUIdelegatewiththeplafclassButtonUI.
TheUIdelegateiscreatedinthecomponent'sconstructorandisaccessibleasaJavaBeansboundpropertyonthecomponent.Forexample,JScrollBarprovidesthefollowingmethodstoaccessitsUIdelegate:

publicScrollBarUIgetUI()

publicvoidsetUI(ScrollBarUIui)

ThisprocessofcreatingaUIdelegateandsettingitasthe"UI"propertyforacomponentisessentiallythe"installation"ofacomponent'slook-and-feel.
EachcomponentalsoprovidesamethodwhichcreatesandsetsaUIdelegateforthe"default"look-and-feel(thismethodisusedbytheconstructorwhendoingtheinstallation):

publicvoidupdateUI()

Alook-and-feelimplementationprovidesconcretesubclassesforeachabstractplafUIclass.Forexample,theWindowslook-and-feeldefinesWindowsButtonUI,aWindowsScrollBarUI,andsoon.WhenacomponentinstallsitsUIdelegate,itmusthaveawaytolookuptheappropriateconcreteclassnameforthecurrentdefaultlook-and-feeldynamically.ThisoperationisperformedusingahashtableinwhichthekeyisdefinedprogrammaticallybythegetUIClassID()methodinthecomponent.Theconventionistousetheplafabstractclassnameforthesekeys.Forexample,JScrollbarprovides:

publicStringgetUIClassID(){

return"ScrollBarUI";
}

Consequently,thehashtableintheWindowslook-and-feelwillprovideanentrythatmaps"ScrollBarUI"to
"com.sun.java.swing.plaf.windows.WindowsScrollBarUI"

Look-and-feelmanagement

SwingdefinesanabstractLookAndFeelclassthatrepresentsalltheinformationcentraltoalook-and-feelimplementation,suchasitsname,itsdescription,whetherit'sanativelook-and-feel--andinparticular,ahashtable(knownasthe"DefaultsTable")forstoringdefaultvaluesforvariouslook-and-feelattributes,suchascolorsandfonts.
Eachlook-and-feelimplementationdefinesasubclassofLookAndFeel(forexample,swing.plaf.motif.MotifLookAndFeel)toprovideSwingwiththenecessaryinformationtomanagethelook-and-feel.
TheUIManageristheAPIthroughwhichcomponentsandprogramsaccesslook-and-feelinformation(theyshouldrarely,ifever,talkdirectlytoaLookAndFeelinstance).UIManagerisresponsibleforkeepingtrackofwhichLookAndFeelclassesareavailable,whichareinstalled,andwhichiscurrentlythedefault.TheUIManageralsomanagesaccesstotheDefaultsTableforthecurrentlook-and-feel.

The'default'lookandfeel

TheUIManageralsoprovidesmethodsforgettingandsettingthecurrentdefaultLookAndFeel:

publicstaticLookAndFeel getLookAndFeel() publicstaticvoid setLookAndFeel(LookAndFeelnewLookAndFeel) publicstaticvoid setLookAndFeel(StringclassName)

Asadefaultlook-and-feel,Swinginitializesthecross-platformJavalookandfeel(formerlyknownas"Metal").However,ifaSwingprogramwantstosetthedefaultLook-and-Feelexplicitly,itcandothatusingtheUIManager.setLookAndFeel()method.Forexample,thefollowingcodesamplewillsetthedefaultLook-and-FeeltobeCDE/Motif:

UIManager.setLookAndFeel(

"com.sun.java.swing.plaf.motif.MotifLookAndFeel");

Sometimesanapplicationmaynotwanttospecifyaparticularlook-and-feel,butinsteadwantstoconfigurealook-and-feelinsuchawaythatitdynamicallymatcheswhateverplatformithappenstoberunningon(forinstance,the.Windowslook-and-feelifitisrunningonWindowsNT,orCDE/MotififitrunningonSolaris).Or,perhaps,anapplicationmightwanttolockdownthelook-and-feeltothecross-platformJavalookandfeel.
TheUIManagerprovidesthefollowingstaticmethodstoprogrammaticallyobtaintheappropriateLookAndFeelclassnamesforeachofthesecases:

publicstaticString
getSystemLookAndFeelClassName()
publicstaticString
getCrossPlatformLookAndFeelClassName()

So,toensurethataprogramalwaysrunsintheplatform'ssystemlook-and-feel,thecodemightlooklikethis:

UIManager.setLookAndFeel(

UIManager.getSystemLookAndFeelClassName());

DynamicallyChangingtheDefaultLook-and-Feel

WhenaSwingapplicationprogrammaticallysetsthelook-and-feel(asdescribedabove),theidealplacetodosoisbeforeanySwingcomponentsareinstantiated.ThisisbecausetheUIManager.setLookAndFeel()methodmakesaparticularLookAndFeelthecurrentdefaultbyloadingandinitializingthatLookAndFeelinstance,butitdoesnotautomaticallycauseanyexistingcomponentstochangetheirlook-and-feel.
RememberthatcomponentsinitializetheirUIdelegateatconstructtime,therefore,ifthecurrentdefaultchangesaftertheyareconstructed,theywillnotautomaticallyupdatetheirUIsaccordingly.Itisuptotheprogramtoimplementthisdynamicswitchingbytraversingthecontainmenthierarchyandupdatingthecomponentsindividually.(NOTE:SwingprovidestheSwingUtilities.updateComponentTreeUI()methodtoassistwiththisprocess).
Thelook-and-feelofacomponentcanbeupdatedatanytimetomatchthecurrentdefaultbyinvokingitsupdateUI()method,whichusesthefollowingstaticmethodonUIManagertogettheappropriateUIdelegate:

publicstaticComponentUIgetUI(JComponentc)

Forexample,theimplementationofupdateUI()fortheJScrollBarlookslikethefollowing:

publicvoidupdateUI(){

setUI((ScrollBarUI)UIManager.getUI(this));
}

Andsoifaprogramneedstochangethelook-and-feelofaGUIhierarchyafteritwasinstantiated,thecodemightlooklikethefollowing:

//GUIalreadyinstantiated,wheremyframe //istop-levelframe try{ 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)); }

Managinglook-and-feeldata

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

publicstaticLookAndFeelInfo[] getInstalledLookAndFeels() publicstaticvoid setInstalledLookAndFeels(LookAndFeelInfo[]infos) throwsSecurityException publicstaticvoid installLookAndFeel(LookAndFeelInfoinfo) publicstaticvoid installLookAndFeel(Stringname,StringclassName)

Thesemethodscanbeusedtoprogrammaticallydeterminewhichlook-and-feelimplementationsareavailable,whichisusefulwhenbuildinguserinterfaceswhichallowtheend-usertodynamicallyselectalook-and-feel.

Thelook-and-feelpackages

TheUIdelegateclassesprovidedinswing.plaf(ButtonUI,ScrollBarUI,andsoon)definethepreciseAPIthatacomponentcanusetointeractwiththeUIdelegateinstance.(NOTE:Interfaceswereoriginallyusedhere,buttheywerereplacedwithabstractclassesbecausewefelttheAPIwasnotmatureenoughtowithstandtheconcretecastingofaninterface.)TheseplafAPIsaretherootofalllook-and-feelimplementations.
Eachlook-and-feelimplementationprovidesconcretesubclassesoftheseabstractplafclasses.Allsuchclassesdefinedbyaparticularlook-and-feelimplementationarecontainedinaseparatepackageundertheswing.plafpackage(forexample,.swing.plaf.motif,swing.plaf.metal,andsoon).Alook-and-feelpackagecontainsthefollowing:

TheLookAndFeelsubclass(forinstance,MetalLookAndFeel).

Alllook-and-feel'sUIdelegateclasses(forexample,MetalButtonUI,MetalTreeUI,andthelike).

Anylook-and-feelutilityclasses(MetalGraphicsUtils,MetalIconFactory,andsoon).

Otherresourcesassociatedwiththelook-and-feel,suchasimagefiles.

InimplementingthevariousSwinglook-and-feels,wesoondiscoveredthattherewasalotofcommonalityamongthem.Wefactoredoutthiscommoncodeintoabaselook-and-feelimplementation(called"basic")whichextendstheplafabstractclassesandfromwhichthespecificlook-and-feelimplementations(motif,windows,andsoon.)extend.Thebasiclook-and-feelpackagesupportsbuildingadesktop-levellook-and-feel,suchasWindowsorCDE/Motif.
Thebasiclook-and-feelpackageisjustoneexampleofhowtobuildapluggablelook-and-feel;thearchitectureisflexibleenoughtoaccommodateotherapproachesaswell.
Theremainderofthisdocumentwillshowhowalook-and-feelpackageworksatthegenericlevel,leavingthedetailsonthebasicpackageforafuturedocument.
WARNING:AllAPIsdefinedbelowtheswing.plafpackagearenotfrozeninthe1.0.XversionofSwing.WearecurrentlycleaningupthoseAPIsfortheversionofSwingthatwillshipwithJDK1.2beta4,atwhichtimetheywillbecomefrozen.Soifyouaredevelopingyourownlook-and-feelimplementationusingthe1.0.1API,thisislikelytoaffectyou.

TheLookAndFeelSubclass

TheLookAndFeelclassdefinesthefollowingabstractmethods,whichallsubclassesmustimplement:

publicStringgetName();

publicStringgetID();

publicStringgetDescription();

publicbooleanisNativeLookAndFeel();

publicbooleanisSupportedLookAndFeel();

ThegetName(),getID(),andgetDescription()methodsprovidegenericinformationaboutthelook-and-feel.
TheisNativeLookAndFeel()methodreturnstrueifthelook-and-feelisnativetothecurrentplatform.Forexample,MotifLookAndFeelreturnstrueifitiscurrentlyrunningontheSolarisplatform,andreturnsfalseotherwise.
TheisSupportedLookAndFeel()methodreturnswhetherornotthislook-and-feelisauthorizedtorunonthecurrentplatform.Forexample,WindowsLookAndFeelreturnstrueonlyifitisrunningonaWindows95,Windows98,orWindowsNTmachine.
ALookAndFeelclassalsoprovidesmethodsforinitializationanduninitialization:

publicvoidinitialize()

publicvoiduninitialize()

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

TheDefaultsTable

Finally,theLookAndFeelclassprovidesamethodtoreturnthelook-and-feel'simplementationoftheDefaultsTable:

publicUIDefaultsgetDefaults()

TheDefaultsTableisrepresentedbytheUIDefaultsclass,adirectextensionofjava.util.Hashtable,whichaddsmethodsforaccessingspecifictypesofinformationaboutalook-and-feel.ThistablemustincludealltheUIClassID-to-classnamemappinginformation,aswellasanydefaultvaluesforpresentation-relatedproperties(suchascolor,font,border,andicon)foreachUIdelegate.Forexample,followingisasampleofwhatafragmentofgetDefaults()mightlooklikeforahypotheticallook-and-feelinapackagecalled"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; }

Whenthedefaultlook-and-feelissetwithUIManager.setLookAndFeel(),theUIManagercallsgetDefaults()onthenewLookAndFeelinstanceandstoresthehashtableitreturns.SubsequentcallstotheUIManager'slookupmethodswillbeappliedtothistable.Forexample,aftermaking"mine"thedefaultLook-and-Feel:

UIManager.get("ButtonUI")=>"mine.MyButtonUI"

TheUIclassesaccesstheirdefaultinformationinthesameway.Forexample,ourexampleButtonUIclasswouldinitializetheJButton's"background"propertylikethis:

button.setBackground(

UIManager.getColor("Button.background");

Thedefaultsareorganizedthiswaytoallowdeveloperstooverridethem.MoredetailaboutSwing'sDefaultsmechanismwillbepublishedinafuturearticle.

DistinguishingbetweenUI-setandapp-setproperties

Swingallowsapplicationstosetpropertyvalues(suchascolorandfont)individuallyoncomponents.Soit'scriticaltomakesurethatthesevaluesdon'tgetclobberedwhenalook-and-feelsetsupits"default"propertiesforthecomponent.
ThisisnotanissuethefirsttimeaUIdelegateisinstalledonacomponent(atconstructtime)becauseallpropertieswillbeuninitializedandlegallysettablebythelook-and-feel.Theproblemoccurswhentheapplicationsetsindividualpropertiesaftercomponentconstructionandthensubsequentlysetsanewlook-and-feel(thatis,dynamiclook-and-feelswitching).Thismeansthatthelook-and-feelmustbeabletodistinguishbetweenpropertyvaluessetbytheapplication,andthosesetbyalook-and-feel.
Thisissueishandledbymarkingallvaluessetbythelook-and-feelwiththeplaf.UIResourceinterface.Theplafpackageprovidesasetof"marked"classesforrepresentingthesevalues,ColorUIResource,FontUIResource,andBorderUIResource.TheprecedingcodeexampleshowstheusageoftheseclassestomarkthedefaultpropertyvaluesforthehypotheticalMyButtonUIclass.

TheUIdelegate

ThesuperclassofallUIDelegateclassesisswing.plaf.ComponentUI.Thisclasscontainstheprimary"machinery"formakingthepluggablelook-and-feelwork.ItsmethodsdealwithUIinstallationanduninstallation,andwithdelegationofacomponent'sgeometry-handlingandpainting.
ManyoftheUIDelegatesubclassesalsoprovideadditionalmethodsspecifictotheirownrequiredinteractionwiththecomponent;however,thisdocumentfocusesprimarilyonthegenericmechanismimplementedbyComponentUI.

UIinstallationanddeinstallation

Firstoff,theComponentUIclassdefinesthesemethodsmethodsforUIdelegateinstallationanduninstallation:

publicvoidinstallUI(JComponentc)

publicvoiduninstallUI(JComponentc)

LookingattheimplementationofJComponent.setUI()(whichisalwaysinvokedfromthesetUImethodonJComponentsubclasses),wecanclearlyseehowUIdelegateinstallation/de-installationworks:

protectedvoidsetUI(ComponentUInewUI){ if(ui!=null){ ui.uninstallUI(this); } ComponentUIoldUI=ui; ui=newUI; if(ui!=null){ ui.installUI(this); } invalidate(); firePropertyChange("UI",oldUI,newUI); }

UIinstallationillustrated

Thisarticlecomeswithagiantposter-sizechartthatillustratestheprocessinstallingaUIdelegate.Itcanprovideyouwithavaluableoverviewofthedelegate-installationprocess.
Tofoldoutthechart,justfollowthislink

TheUIdelegate'sinstallUI()methodisresponsibleforthefollowing:

Setdefaultfont,color,border,andopacitypropertiesonthecomponent.

Installanappropriatelayoutmanageronthecomponent.

Addanyappropriatechildsubcomponentstothecomponent

Registeranyrequiredeventlistenersonthecomponent.

Registeranylook-and-feel-specifickeyboardactions(mnemonics,etc.)forthecomponent.

Registerappropriatemodellistenerstobenotifiedwhentorepaint.

Initializeanyappropriateinstancedata.

Forexample,theinstallUI()methodforanextensionofButtonUImightlooklikethis:

protectedMyMouseListenermouseListener; protectedMyChangeListenerchangeListener; publicvoidinstallUI(JComponentc){ AbstractButtonb=(AbstractButton)c; //Installdefaultcolors&opacity Colorbg=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); //Installlisteners mouseListener=newMyMouseListener(); c.addMouseListener(mouseListener); c.addMouseMotionListener(mouseListener); changeListener=newMyChangeListener(); b.addChangeListener(changeListener); }

Conventionsforinitializingcomponentproperties

Swingdefinesanumberofconventionsforinitializingcomponentpropertiesatinstall-time,includingthefollowing:

Allvaluesusedforsettingcolors,font,andborderpropertiesshouldbeobtainedfromtheDefaultstable(asdescribedinthesubsectionontheLookAndFeelsubclass).

Color,fontandborderpropertiesshouldbesetif--andonlyif--theapplicationhasnotalreadysetthem.

TofacilitateconventionNo1,theUIManagerclassprovidesanumberofstaticmethodstoextractpropertyvaluesofaparticulartype(forinstance,thestaticmethodsUIManager.getColor(),UIManager.getFont(),andsoon).
ConventionNo.2isimplementedbyalwayscheckingforeitheranullvalueoraninstanceofUIResourcebeforesettingtheproperty.
TheComponentUI'suninstall()methodmustcarefullyundoeverythingthatwasdoneintheinstallUI()methodsothatthecomponentisleftinapristinestateforthenextUIdelegate.Theuninstall()methodisresponsiblefor:

ClearingtheborderpropertyifithasbeensetbyinstallUI().

RemovethelayoutmanagerifithadbeensetbyinstallUI().

RemoveanysubcomponentsaddedbyinstallUI().

Removeanyevent/modellistenersthatwereaddedbyinstallUI().

Removeanylook-and-feel-specifickeyboardactionsthatwereinstalledbyinstallUI().

Nullifyanyinitializedinstancedata(toallowGCtocleanup).

Forexample,anuninstall()methodtoundowhatwedidintheaboveexampleinstallationmightlooklikethis:

publicvoiduninstallUI(JComponentc){ AbstractButtonb=(AbstractButton)c; //Uninstalllisteners c.removeMouseListener(mouseListener); c.removeMouseMotionListener(mouseListener); mouseListener=null; b.removeChangeListener(changeListener); changeListener=null; }

Defininggeometry

IntheAWT(andthusinSwing)acontainer'sLayoutManagerwilllayoutthechildcomponentsaccordingtoitsdefinedalgorithm;thisisknownas"validation"ofacontainmenthierarchy.TypicallyLayoutManagerswillquerythechildcomponents'preferredSizeproperty(andsometimesminimumSizeand/ormaximumSizeaswell,dependingonthealgorithm)inordertodeterminepreciselyhowtopositionandsizethosechildren.
Obviously,thesegeometrypropertiesaresomethingthatalook-and-feelusuallyneedstodefineforagivencomponent,soComponentUIprovidesthefollowingmethodsforthispurpose:

publicDimension getPreferredSize(JComponentc) publicDimension getMinimumSize(JComponentc) publicDimension getMaximumSize(JComponentc) publicboolean contains(JComponentc,intx,inty)

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

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

EventhoughtheboundingboxforallcomponentsisaRectangle,it'spossible
tosimulateanon-rectangularcomponentbyoverridingtheimplementationofthecontains()methodfromjava.awt.Component.(Thismethodisusedforthehit-testingofmouseevents).But,liketheothergeometrypropertiesinSwing,theUIdelegatedefinesitsownversionofthecontains()method,whichisalsodelegatedtobyJComponent.contains():

publicbooleancontains(JComponentc,intx,inty){

return(ui!=null)?
ui.contains(this,x,y)
:

super.contains(x,y);
}

SoaUIdelegatecouldprovidenon-rectangular"feel"bydefiningaparticularimplementationofcontains()(forexample,ifwewantedourMyButtonUIclasstoimplementabuttonwithroundedcorners).

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:

publicstaticComponentUI

createUI(JComponentc)

It'stheimplementationofthismethodthatdetermineswhetherthedelegateisstatelessorstateful.That'sbecausetheUIManager.getUI()methodinvokedbythecomponenttocreatetheUIdelegateinternallyinvokesthiscreateUImethodonthedelegateclasstogettheinstance.
TheSwinglook-and-feelimplementationsusebothtypesofdelegates.Forexample,Swing'sBasicButtonUIclassimplementsastatelessdelegate:

//SharedUIobject protectedstaticButtonUIbuttonUI; publicstaticComponentUIcreateUI(JComponentc) if(buttonUI==null){ buttonUI=newBasicButtonUI(); } returnbuttonUI; }

WhileSwing'sBasicTabbedPaneUIusesthestatefulapproach:
publicstaticComponentUIcreateUI(JComponentc) returnnewBasicTabbedPaneUI(); }

PluggableLook-and-Feelsummary

Thepluggablelook-and-feelfeatureofSwingisbothpowerfulandcomplex(whichyouunderstandifyou'vegottenthisfar!).Itisdesignedtobeprogrammedbyasmallsubsetofdeveloperswhohaveaparticularneedtodevelopanewlook-and-feelimplementation.Ingeneral,applicationdevelopersonlyneedtounderstandthecapabilitiesofthismechanisminordertodecidehowtheywishtosupportlook-and-feels(suchaswhethertolock-downtheprogramtoasinglelook-and-feelorsupportlook-and-feelconfigurationbytheuser).Swing'sUIManagerprovidestheAPIforapplicationstomanagethelook-and-feelatthislevel.
Ifyou'reoneofthosedeveloperswhoneeds(orwants)todevelopacustomlook-and-feel,it'scriticaltounderstandtheseunderpinningsbeforeyouwriteasinglelineofcode.We'reworkingonprovidingbetterdocumentationtohelpwiththisprocess--startingwiththisdocument,andcontinuingwithothersthatwillfollowsoon.