Chapter 7 Debugging Your CodeWord格式.docx

Chapter 7 Debugging Your CodeWord格式.docx

《Chapter 7 Debugging Your CodeWord格式.docx》由会员分享，可在线阅读，更多相关《Chapter 7 Debugging Your CodeWord格式.docx（12页珍藏版）》请在冰点文库上搜索。

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