Chapter 7 Debugging Your Code.docx

Chapter 7 Debugging Your Code.docx

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Chapter7DebuggingYourCode

Chapter7.DebuggingYourCode

Understandingtheoperationofcomplexsoftwareoftenrequiresintrospectionthatgoesbeyondthedisplayofloginformationusingprintln（）statementsandlog4joutput.Youneedtheabilitytostepthroughaprogramstatement-by-statementandviewormodifythestateoffields,methodparameters,andlocalvariables.

OurdebuggingactivitiesarefocusedontheproductcatalogprojectthatwasintroducedinChapter2,"HelloEclipse."WeinitiallyrantheapplicationwithinEclipseusingtheConsoleviewforcommand-lineinputandoutput.Nowwewillrunthesameprojectwithinthedebugger,whichenablesdetailedanalysisofitscontrolflowandstate.IfyouarenewtoJava,andespeciallyifyouarenewtoobject-orientedprogramming,steppingthroughyourcodeinthedebuggerisanexcellentwaytounderstandhowitruns.

Coursepoints

Inthischapter,we'llseehowto

∙Startandcustomizeadebuglaunchconfiguration

∙Usedebugstackframesthatprovideexecutioncontextforvariablesanddisplayexpressions

∙Setbreakpointsandstepthroughprogramexecution

∙Inspectandchangevaluesoffields,methodparameters,andlocalvariables

∙UsetheExpressionsviewtowatchvariablesduringdebugging

∙AssignDetailFormattersthatcustomizeobjectdisplaywhenexistingtoString（）methodsareinadequate

Reading

7.1.StartaDebugSession

Youcanusethedebuggingfeaturesinanyperspective;however,theDebugPerspectiveisdesignedspecificallytosupporttheseactivities.It'seasiesttostartanewdebugsessionfromtheJavaPerspectiveandthenswitchtotheDebugPerspective.Ofcourse,youcanmovefreelybetweenperspectivesatanytime.

Startadebugsessionbyright-clickingontheMain.javaentryfromthecom.eclipsedistilled.catalogpackageinthePackageExplorerviewandselectingDebugAs>JavaApplicationfromthecontextmenu.Alternatively,selectDebug>Debug…fromthecontextmenutoopenadialog,asshowninFigure7-1.PresstheNewbuttontocreateanewlaunchconfigurationforrunningtheMain.javaclass.PresstheDebugbuttontostartadebugsession.

Figure7-1.Createalaunchconfigurationforyourapplication.

OurMain.javaapplicationdoesnotrequireconfigurationbecausetheprojectbuildpathisusedbydefaultwhenrunningthisprogram.However,youcanlaunchanddebugmuchmorecomplexapplicationswithintheEclipseworkbench.AnythingthatcanberunasaJavaapplication（i.e.,anythingthathasamain（）method）canberuninthedebugger,suchasaSwing-basedGUIapplicationoracompleteapplicationserversuchasTomcatorJBoss.Thelaunchconfigurationdialogincludestabsforspecifyingcommand-linearguments,classpathlocations,andenvironmentvariablesneededtostarttheapplication.OtherdebugconfigurationsareshownontheleftsideofthisdialogforrunningJavaApplets,JUnittests,oraruntimeinstanceoftheEclipseworkbenchitselfusedwhendebuggingplug-ins.

Afteryouhaverunanapplication,itappearsinthelistofrecentlyrunconfigurationsundertheRunandDebugtoolbarmenus.TheDebugmenuisshowninFigure7-2withoneentryforMain（youcanrenamethisentryintheconfigurationdialog,forexampletoCatalogApp）.SelectMaintostartthedebugsession,orselectDebug…toopentheconfigurationdialogandmodifythelaunchparametersbeforestarting.YoucanalsopressF11todebugthepreviouslylaunchedconfiguration.

Figure7-2.Rerunorconfigureapreviousdebugsessionusingthetoolbarmenu.

Ifwerunthisapplicationbeforesettinganybreakpoints,itwilllookmuchlikethewayweranitinChapter2.TheConsoleviewdisplaysourmessagesandacceptsuserinputfromthecommandline.

NowswitchtotheDebugPerspectivebyselectingWindow>OpenPerspective>Debugfromtheworkbenchwindowmenu.Youcanusetheperspectivebartoswitchbetweenopenperspectives.

StartyourdebuggingworkbyusingtheConsoletoaddtwoproductstothecatalog.Now,presstheSuspendbuttonontheDebugviewtoolbarorpickRun>Suspendfromtheworkbenchmenu.Thedebugviewswillappear,asshowninFigure7-3.Theapplicationexecutionthreadissuspended,andyoucanselectanystackframefromthecallstackintheDebugview.TheVariablesviewshowsfields,localvariables,andmethodparametersfortheselectedframe.ThisfigureshowsanexpandedviewofthecatalogfieldfromaMainclassinstance.

Figure7-3.Suspendexecutionwhilewaitingforconsoleinput.

Youcanaddorremovebreakpointsinthesourcecodeatanytime.AddabreakpointwithintheJavaeditorbydouble-clickingintheleftrulerbesidealinewhereyouwantabreakpoint.Ablueballappearstomarkthebreakpoint.

Figure7-4.Createanewbreakpointbydouble-clickingintheleftruler.

NowpresstheResumebuttonontheDebugviewtoolbar.AddanewcatalogitemintheconsoleviewandpresstheEnterkey.Theexecutionwillsuspendwhenithitsthebreakpoint,displayingtheviewsshowninFigure7-5.

Figure7-5.Inspectvariableswhilesteppingthroughyourprogram.

Whenaprogramthreadissuspended,theDebugviewshowsoneormorestackframesforthatthread.Astackframeshowstheexecutioncontextcontaininglocalvariablesandargumentsforthemethodinthatpositionofthecallstack.

Asyoustepthroughaprogram'sexecution,thesourcecodelinesarehighlightedtotrackyourprogress.TwokindsofprogressstepsarehighlightedinFigure7-5.TheCurrentInstructionPointercontainsthestatementthatwillbeexecutednextwhenthethreadresumes.TheDebugCallStackmarksthelinewheretheselectedstackframewassuspended.Inthisexample,wehaveselectedapreviousstackframethatshowstheprogramstatebeforesteppingintothecurrentstatement,getItems（）.

7.1.1.SteppingThroughExecution

Afterexecutionhassuspendedonabreakpoint,thereareseveraloptionsforresumingorsteppingthroughtheprogramstatement-by-statement.Youcanterminateaprogramatanytimewhileit'srunningorsuspended.Figure7-6showsthemenuitemsavailableintheworkbenchRunmenuwhenaprogramthreadissuspended.TheSuspendcommandisdisabledforthatthreadbecauseitisalreadysuspended.

Figure7-6.Commandsusedtostepthroughtheexecutionofaprogram.

Thefirstthreecommandsprovidethemostgeneralcontrol:

Resume.Resumeexecutionofathreaduntiliteitherendsorencountersabreakpoint.

Suspend.Suspendanexecutingthread.

Terminate.TerminatetheJavaVM.

BehaviorofthestepcommandsisrelativetothestackframefortheselectedthreadintheDebugview.Forallcommands,thecurrentlineofexecutionintheselectedstackframeishighlightedintheeditor.

StepInto.Stepintothenextexecutedmethod.Steppingintoamethodcausesthestackframetogrowbyone.

StepOver.Finishexecutionofthecurrentlineandsuspendonthenextexecutableline.

StepReturn.Stepoutofthecurrentmethod.Executionresumesuntilareturnstatementisexecutedinthecurrentmethod.

RuntoLine.Resumeexecutionuptotheselectedline.Thisisaconvenientwaytosuspendexecutionatalinewithoutsettingabreakpoint.

UseStepFilters.Whenthiscommandistoggledon,eachofthestepcommandswillapplythesetofstepfiltersthataredefinedintheuserpreferencespageJava>Debug>StepFiltering.Whenastepactionisinvoked,steppingwillcontinueuntilanunfilteredlocationisreachedorabreakpointisencountered.

Ifabreakpointisencounteredwhileperformingastepoperation,theexecutionwillsuspendatthebreakpoint,andthestepoperationisended.

TheRuntoLinecommandisnotontheDebugviewtoolbar.Placeyourcursoronthelineatwhichyouwanttheprogramtobesuspended.SelectRuntoLinefromtheworkbenchRunmenuortheJavaeditorcontextmenu,orpressCtrl+R.Bydefault,RuntoLinestopsifabreakpointisencounteredbeforereachingtheselectedline（aswithotherstepoperations）.However,thereisasettingontheRun/DebugpreferencespagetoskipbreakpointsduringRuntoLineifyou'dliketoavoidthisbehavior.

7.2.InspectingandDisplayingState

TheVariablesviewshowsfields,localvariables,andmethodparametersfortheselectedstackframe.Thestackframeprovidestheexecutioncontextforthesevariables.SelectoneofthesevariablestodisplayitsvalueintheDetailspaneofthisviewnotethattheDetailspaneatthebottomofthisviewisdistinctfromtheDisplayview,althoughtheyhaveasimilarpurpose.Ifthevariablecontainsanobject,asopposedtoaprimitive,youcanalsoexpandthatobjecttodisplayitsmembervariables.RefertoFigures7-3and7-5,whichdisplayvaluesoftheCatalogmembervariables.TheDisplayviewisnotopenbydefault;openitwhileintheDebugperspectiveusingthemenuWindow>ShowView>Display.PleasebeawareofthedifferencebetweentheDetailspaneandDisplayviewinthefollowingdiscussion.

TheDetailspanecanalsobeusedtoevaluatecodesnippets.Thisevaluationoccurswithinthecontextoftheselectedvariable,orifnovariableisselected,thentheselectedstackframecontextisused.Youmustbeawareofthiscontextbecauseitdeterminesthevalueoftheidentifierthisandalsodetermineswhichothervariablenamesareavailablewithinthecodesnippet.InFigure7-7weenteredasnippet.forexample:

Catalogcatalog=newCatalog（）;

Stringitem="hello";

catalog.getItems（）.add（item）;

System.out.println（"Product:

"+item）;

Figure7-7.Executeacodesnippetduringdebugging.

Toexecutethiscodesnippet,dothefollowing:

1.SelectthedesiredCatalogIteminstancewithintheCatalogitemsvariable.

2.SelecttheentireexpressionintheDetailspane;alternat