Aspects in AspectJ - Télécom SudParisgibson/Teaching/CSC7336/L2-AspectJ.pdf · November 2017 CSC7336: AspectJ (J P Gibson) 1 Aspects in AspectJ •Motivation •Aspect Oriented

CSC7336:AspectJ(JPGibson)November2017 1

AspectsinAspectJ

•Motivation•AspectOrientedProgramming:abriefintroductiontoterminology•Installation•Experimentation•AspectJ–somedetails•AspectJ–thingsyoushouldknowaboutbutwedonthavetimetocoverindetail•AspectJ–profilingproblem


Motivation

“Abilityiswhatyou'recapableofdoing.Motivationdetermineswhatyoudo.Attitudedetermineshowwellyoudoit.”LouHoltz


Motivation

Evenwelldesignedsystemshaveaspectsthatcutacrossalargenumberofthecomponents

Forexample,considerApacheTomcat,wherecertainaspectsoffunctionalityare:

•XMLparsing•URLpatternmatching•Logging•SessionManagement

Question:whichofthesearewelllocalised?


Motivation

XMLparsing

URLpatternmatching

Logging

SessionManagement:differenttypes

•ApplicationSession•ServerSession•StandardSession•Managers

Sothecodeforeachsessionstageshouldbewelllocalised?


Motivation

SessionExpiration:codeisdistributedthroughlotsofdifferentclasses


Motivation

SessionExpiration:abetterdesign?

Unfortunately,ifwedothiswewillhavetocompromisebyredistributingpreviouslylocalisedcode.


Motivation

SeparationofConcernsisatime-honouredprincipleofSoftwaredesign

D.Parnas.OntheCriteriatoBeUsedinDecomposingSystemsintoModules.Comm.ACM15,12(December1972),1053-1058.

Designprinciplesfordecomposition:•Information-hidingmodules•Identifydesigndecisionsthatarelikelytochange.•Isolatetheseinseparatemodules(separationofconcerns)


Motivation

Cross-Cutting Concerns

Inthemotivation,

•XMLparsingandURLpatternmatchingfittheclasshierarchy,•LoggingandSessionManagementdonot.

Across-cuttingconcernisonethatneedstobeaddressedinmorethanoneofthemodulesinthehierarchicalstructureofthesoftware.

Cross-cuttingconcernsarealsocalledaspects.

Whatisanaspectdependsonthechosendecomposition!


Motivation

ProblemswithCross-CuttingConcerns

Cross-cuttingconcernsposeproblemsforstandard,e.g.OO,programmingtechniques:

•hardanderror-pronetointroduceinanexistingsystem•hardtochangeafterwards•hardtounderstand/explaintonewcomers

Cross-cuttingimplementationofcross-cuttingconcernsdoesnotprovideseparationofconcerns.


Motivation

Solutions

Possibletreatmentofcross-cuttingconcerns:

Refactorthemaway.Changethemodulehierarchysothattheaspectbecomesmodular,oftenthroughapplicationofadequatedesignpatterns.

But:•oftenperformancepenaltiesthroughindirection•oftenleavessomecross-cuttingboiler-plate•can'thopetocaptureallaspects


AspectOrientedProgramming:abriefintroduction

AprogrammingmethodologyiscalledAspect-Orientedifitprovidespossibilitiestocleanlyseparateconcernsthatwouldotherwisebecross-cutting.

TherearevariousAspect-Orientedmethods.Theydifferinthekindsofaspectstheycanaddressandinthewaysaspectsandtheirrelationtothechosenhierarchicaldecompositionareexpressed.



Don’tForgetThatGoodDesignHelpsAvoidCrossCutting:soonlyuseAspectsiftheyarereallyneeded,andnotjustbecausethedesignisbad!

ExampleofGoodDesign–TheLawofDemeter:

Anobjectshouldonlycallmethodsonthis,instancevariables,methodarguments.

nothis.getWife().getMother().getMaidenName()chains.

Preventsdependencyontoomanyotherclasses.

The"LawofDemeter"providesaclassicsolution,see:

Lieberherr,Karl.J.andHolland,I. Assuringgoodstyleforobject-orientedprograms IEEESoftware,September1989,pp38-48



JoinPoints

Analysecommonlyoccurringaspects.Cross-cuttingimplementationscanoftenbeformulatedintermslike:

•Before...iscalled,alwayscheckfor...•Ifanyof...throwsanexception,...•Everytime...getschanged,notify...

ImplementationsofaspectsareattachedtocertainpointsintheImplementation,eg:

•methodcalls•constructorcalls•fieldaccess(read/write)•Exceptions

Thesecorrespondtopointinthedynamicexecutionoftheprogram.Suchpointsarecalledjoinpoints



CodeExample:FigureEditor(fromhttp://eclipse.org/aspectj/doc/released/progguide/)


AspectOrientedProgramming:abriefintroductionCodeExample:FigureEditor

A join point is a point in the control flow of a program

https://en.wikipedia.org/wiki/Control_flow

https://en.wikipedia.org/wiki/Computer_program



CodeExample:FigureEditor–joinpointsformoveBy

In aspect-oriented programming a set of join points is called a pointcut

Forexample,allcallsandreturnsfrommoveBymethods

https://en.wikipedia.org/wiki/Aspect-oriented_programming

https://en.wikipedia.org/wiki/Set_(mathematics)

https://en.wikipedia.org/wiki/Pointcut



Apointcutdesignatesasetofjoinpointsforanyprogramexecution.

Atexecution,anyjoinpointmayormaynotbeselectedbyapointcut.

Examples:•allcallstopublicmethodsofthePointclass•everyexecutionofaconstructorwithoneintargument•everywriteaccesstoapublicfield

Membershipofajoinpointmaybedeterminedatruntime



Advice

Adviceiscodethatshouldbeinsertedbefore,afteroreveninsteadofexistingcodeatsomesetofjoinpoints.

`Mainstream'AOP:

•Designatesetsofjoinpointsusingsomespecificationlanguageforpointcuts.•Declareadvicetobeexecutedbefore/after/insteadofthecalls/methodexecutions/fieldaccessesetc.selectedbythepointcut.



Example

DisplayupdatingintheFigureEditor:

AftereverychangeinaFigureElement'sstate,updatethedisplay..

AOPimplementation:•pointcuttoselecteverystate-changingmethodinFigureElementclasses.•`after'-advicewhichcallsdisplayupdatecode.

Question:whichdesignpatterncouldyouuse(insteadofAOP)?



Weaving:

Aprogramcalledanaspectweaverisusedtoweavetheadvicecodeintothemainprogramcode.

Often,butnotalways,joinpointmembershipinapointcutcanbedecidedstatically.=>noneedtoinsertcodeateverypossiblejoinpoint.

Modernsystems:•somedoweavingandcompilationinonestep•somecandoweavingatruntime(e.g.onJavabyte-code)



AspectJ

ExtensionsoftheJavalanguagefor•pointcuts•attachingadvice•staticcross-cutting

OriginallydevelopedatXeroxPARCFirstversionsinSpringof2000Hostedbyeclipse.orgsinceDecember2002


Installation

DownloadtheEclipseAJDTpluginfromhttp://eclipse.org/ajdt/downloads/

http://eclipse.org/ajdt/downloads/


Installation

IfAspectJisalreadytherethenyoushouldnotneedtoaddit(butmayneedtoupdateit)


Installation

TryOutTheHelloWorldExampleDemo:

http://www.eclipse.org/ajdt/demos/



LetslookatthistogetherinEclipse


Installation


package p_helloworld;

public class Hello {

public static void main(String[] args) { sayHello(); }

public static void sayHello() { System.out.print("Hello"); }

}


Installation



public aspect World {

pointcut greeting() : execution(* Hello.sayHello(..));

before() : greeting() { System.out.println(" World!"); }

}


Installation

Eclipseprovidesausefulcrossreferencewindow:

advisesHello.sayHello()


Experimentation

ExperimentwiththeDemo:whatwill/shouldhappenifyouaddanotheraspecttotheproject?


public aspect MethodCounter {

int counter =0;

pointcut publicMethodCall() : execution(public * *.*(..)); pointcut afterMain(): execution(public * *.main(..));

after() returning() : publicMethodCall() { counter++; }

after() returning() : afterMain() { System.out.println("Method call counter = "+counter); } }


Experimentation

ExperimentwiththeDemo:canyouwriteanAspectthatwilltimetheexecutionofeachmethodcall?


public aspect Profiler{

}

Thisshouldbeeasytocomplete.Istheon-linedocumentationgoodenough?


Experimentation

ExperimentwiththeDemo:canyouwriteanAspectthatwilltimetheexecutionofeachmethodcall?

World! Hello void p_helloworld.Hello.sayHello() took 26042892 nanoseconds

Method call counter = 2

void p_helloworld.Hello.main(String[]) took 32298490 nanoseconds

Whenyourunthecodewithall3aspectswoventogether,youshouldgetoutputofthefollowingform(moreorless):


AspectJ–somedetails

Reconsiderthefigureeditor




Primitivepointcuts:

call(void Point.setX(int))

eachjoinpointthatisacalltoamethodthathasthesignaturevoidPoint.setX(int)

Alsoforinterfacesignatures:

call(void FigureElement.moveBy(int,int))

EachcalltothemoveBy(int,int)methodinaclassthatimplementsFigureElement




Pointcutscanbejoinedusingbooleanoperators&&,||,!.

call(void Point.setX(int)) || call(void Point.setY(int))

callstothesetXandsetYmethodsofPoint.

Joinpointsfromdifferenttypespossible:

call(void FigureElement.moveBy(int,int)) || call(void Point.setX(int)) || call(void Point.setY(int)) || call(void Line.setP1(Point)) || call(void Line.setP2(Point))

Anycalltoastate-changingmethodinthegivenFigureElementclasses




NamedPointcutsPointcutscanandshouldbedeclaredtogivethemaname:

pointcut stateChange() : call(void FigureElement.moveBy(int,int)) || call(void Point.setX(int)) || call(void Point.setY(int)) || call(void Line.setP1(Point)) || call(void Line.setP2(Point));

AnalogoustomethoddeclarationortypedefinC.

Afterdeclaration,stateChange()canbeusedwhereverapointcutisexpected.




Wildcards

Methodsignaturescancontainwildcards:

call(void java.io.PrintStream.println(*))

anyPrintStreammethodnamedprintlnreturningvoidandtakingexactlyoneargumentofanytype.

call(public * Figure.*(..))

anypublicmethodinFigure.

call(public * Line.set*(..))

anymethodinLinewithanamestartingwithset.




Example

Thepointcutfrombefore,usingwildcards:

pointcut stateChange() : call(void FigureElement.moveBy(int,int)) || call(* Point.set*(*)) || call(* Line.set*(*));




AdviceinAspectJ

Advicecanbeattachedtojoinpoints:

before(): stateChange() { System.out.println("about to change state"); }

after() returning: stateChange() { System.out.println("just successfully changed state"); }



//Aftereverystatechangingcall,updatethedisplay.

public aspect DisplayUpdating { pointcut stateChange() : call(void FigureElement.moveBy(int,int)) || call(* Point.set*(*)) || call(* Line.set*(*)); after() returning : stateChange() { Display.update(); } }

QUESTION:DoesthislooklikeagooduseofAspects?







Example:DiagonalMoves

Defineapointcutformoveswithequaldxanddy.

pointcut diagHelp(int dx,int dy) : call(void FigureElement.moveBy(int,int)) && args(dx,dy) && if(dx==dy);

pointcut diagMove(int dxy) : diagHelp(dxy,int);



AboutConditionals

AspectJspecification:

Thebooleanexpressionusedcanonlyaccessstaticmembers,variablesexposedbytheenclosingpointcutoradvice

Butstill...staticmethodsmaybecalled,whichmayhavesideeffects!

Question:whataretheconsequencesofthis?



BeforeandAfter

`Before'advice:before(formal parameters) : Pointcut { . . . advice body. . . }

Theadvicebodygetsexecutedeverytimejustbeforetheprogramflowentersajoinpointmatchedbythepointcut.Theformalparametersreceivevaluesfromthepointcut

`After'advice:after(formal parameters) returning : Pointcut {...} Theadvicebodygetsexecutedeverytimejustaftertheprogramflowexitsajoinpointmatchedbythepointcutbyreturningnormally.

Capturereturnvalue:after(...) returning (int ret): Pointcut {...}



Whataboutexceptions?

Adviceafterthrowinganexception:after(formal parameters) throwing : Pointcut {...}

Capturethrownexception:after(...) throwing (Exception e): Pointcut {...}

Matchnormalandabruptreturn:after(...) : Pointcut {...}



AroundAdvice

Runadviceinsteadoforiginalcode:

Type around(. . . ) : . . . { . . . proceed(. . . ); . . . }

•runadvicebodyinsteadoforiginalcall,fieldaccess,methodbody,etc.•useproceedtousetheoriginaljoinpoint,ifneeded.

Hint:thismayhelpyoutowritethetimerAspectaskedforearlier



TracingAspect(firsttry)

package tracing;

public aspect TraceAllCalls { pointcut pointsToTrace() : call(* *.*(..)) ;

before() : pointsToTrace() { System.err.println("Enter " + thisJoinPoint); }

after() : pointsToTrace() { System.err.println("Exit " + thisJoinPoint);

} }

QUESTION:Whereistheproblem?



TracingAspect(firsttry)

package tracing; public aspect TraceAllCalls { pointcut pointsToTrace() : call(* *.*(..)) && !within(TraceAllCalls);

before() : pointsToTrace() { System.err.println("Enter " + thisJoinPoint); }

after() : pointsToTrace() { System.err.println("Exit " + thisJoinPoint); } }

QUESTION:Whydoweneedthewithin?


AspectJ–thingsyoushouldknowabout

ExceptionsinAdvice

Advicebodymaythrowexceptions:

before() : doingIO() { openOutputFile(); }

If openOutputFile() throws java.io.IOException: before() throws java.io.IOException : doingIO() { openOutputFile(); }

=>IOExceptionmustbedeclared/handledatallplaceswherepointcutapplies.



Aspectsthrowingexceptions

Sometimes,anaspectcanchangethebehaviourofmethods,sothatnewcheckedexceptionsarethrown:

•Addsynchronization:InterruptedException•Executecallsremotely:RemoteException

=>Twopossibilities:•catchandhandleexceptiondirectlyinadvice.Mightnotbeappropriate.•passexceptionoutofadvice.Needslotsofdeclarations.





AspectPrecedence(cont.)

Syntaxtodeclareaspectprecedence:

declare precedence : TypePattern1, TypePattern2, . . . ;

Mayoccurinanyaspect.Saysthatanythingmatchingtypepattern1hashigherprecedencethananythingmatchingtypepattern2,etc.

aspect CyclicPrecedence { declare precedence : AspectA, AspectB; declare precedence : AspectB, AspectA; }

OKiffaspectssharenojoinpoints.



AspectPrecedence(cont.)

Ifnotdeclared,implicitruleforinheritance:

IfAspectAextendsAspectB,thenAspectAhashigherpriority.

⇒possibletooverruleadvicefromsuper-aspect.

Ifstillnotdeclared,implicitruleforadvicewithinoneaspect:•Ifeitherareafteradvice,thentheonethatappearslaterintheaspecthasprecedenceovertheonethatappearsearlier.•Otherwise,thentheonethatappearsearlierintheaspecthasprecedenceovertheonethatappearslater.

=>firstdosomethinginthesameorderastheyappearinthesource



AbstractAspects

Reminder:aspectscan•extendclasses•extendabstractaspects•implementinterfaces

Abstractaspectsmaycontain

•abstractmethods,likeabstractclasses•abstractpointcutdeclarations

Abstractaspectsarethekeytowritingreusableaspects.



AspectInstantiation

Atruntime,aspectshavefieldslikeobjects.

Whendotheygetinstantiated?Usually:Instantiateanaspectonceperprogramexecution.

aspect Id {...} aspect Id issingleton {...}

Implementedassingleton=>staticfieldinaspectclass.

NOTE:ThingsareactuallymuchmorecomplicatedwhenweconsiderallthedifferentwaysinwhichJavaobjects(includingAspects)areinstantiated



PrivilegedAspects

Usually,advicecodehasnoaccesstoprivatemembersofadvisedclasses.

(Notethatmatchinginpointcutsdoesseeprivatemembers)

Buttheprivilegedkeywordcanhelp:

privileged public aspect A { before(MyFriend f) : this(f) && ... { System.out.println("My best friends secret: " + f._privateField); } }


AspectJ–profilingproblem

Returntotheprofilingproblemwelookedatinpreviousclass

TODO:Implement3ormoreinterestingprofilingaspectsandtestondifferentprograms

PracticalWorkForNextClass:InvariantTesting

Writeanaspectthatteststheinvariantofaclasseverytimeamethodisexecuted,andwritestoaninvariantlogfilewhetherthetestpassesorfails.

Aspects in AspectJ - Télécom SudParisgibson/Teaching/CSC7336/L2-AspectJ.pdf · November 2017 CSC7336: AspectJ (J P Gibson) 1 Aspects in AspectJ •Motivation •Aspect Oriented

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