Aspect-Oriented Software Development with Java
Aspect Components
Renaud Pawlak, Lionel Seinturier, Laurence Duchien, Laurent Martelli,
Fabrice Legond-Aubry, G´
erard Florin
To cite this version:
Renaud Pawlak, Lionel Seinturier, Laurence Duchien, Laurent Martelli, Fabrice Legond-Aubry,
et al.. Aspect-Oriented Software Development with Java Aspect Components. Mehmet
Ak-sit, Siobhan Clarke, Tzilla Elrad, Robert E Filman. Aspect-Oriented Software Development,
Addison-Wesley, 2004, 0-321-21976-7. <inria-00489501>
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https://hal.inria.fr/inria-00489501
Submitted on 5 Jun 2010
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Renaud Pawlak (1) , Lionel Seinturier (2) , Lauren e Du hien (3) LaurentMartelli (4) , Fabri e Legond-Aubry (2) , GérardFlorin (1) (1)
CNAM, Lab. CEDRIC, 292 rue Saint Martin, 75141Paris edex 03, Fran e (2)
Univ. Paris 6,Lab. LIP6, 4 pla e Jussieu,75252 Paris edex 05 (3)
Univ. Lille1, Lab. LIFL,Bâtiment M3,59655 Villeneuved'As q, Fran e (4)
AOPSYS, 5 rue Brown Séquard, 75015 Paris, Fran e
31st O tober 2002
Abstra t
Inthe last four years,our resear hproje t dealtwith separationof on erns for distributed programming environmentsandappli ations. Thisresear heortledtotheimplementationoftheJavaAspe tComponents (JAC) framework for aspe t-orientedprogramming (AOP)inJava. Amongthe manyrequirementsfor distri-bution, exibility andadaptability playastringentrole. Thehighvariabilityofexe uting onditions(interms of resour es, servers availability, faults, ...) also brings the needfor powerful programming paradigms. This led usto develop adynami modelofAOP whi h,unlikestati ally ompiledapproa hes, allows to on-the- y deploy and undeploy aspe ts ontop of running appli ations. This model omes with anUML notationand animplementation. AnIDE isprovidedwith JACtosupportall thedevelopmentstepsof anaspe toriented appli ation, fromitsdesign,toitsimplementation,andtoitsdeployment.
1 Introdu tion
In order to handle the omplexity of software development, separation of on erns [Par72℄[Dij76℄ distinguishes between fun tional andnon fun tionalrequirementsthat needsto beaddressed in an appli ation. It is assumed that the e ient handling of this issue is a key to software quality and reuse. Nevertheless, one should noti e that the frontierbetweenfun tional and non-fun tional properties may be movingdepending on the appli ation eld: features (e.g. time onstraints) maybepartofthe fun tionalrequirementsinsomedomains (e.g. real-time ontrol),andofnon-fun tionalonesinotherdomains(e.g. wordpro essing). Obje t-orientedprogramming(OOP) isapowerfultooltohandlefun tionalde omposition. Still,non-fun tionalpropertiesarespe i inthesensethat they an notalwaysbede omposed leanly from fun tional ones: mostof the time they anonlybe superposed totheoriginalfun tionalde omposition. Thisleadstothe odetanglingphenomenonwherea on erniss attered intomanydierentlo ations(i.e. pie esoffun tional ode),makingitsdevelopment,itsmaintenan e,anditsreuse di ult. Thisphenomenonhasbeenisolatedin[KLM
+
97℄andledtothedevelopmentofanewprogrammingstyle alled aspe t-orientedprogramming(AOP). Sin e then, several tools and ompilers havebeendeveloped (among them Aspe tJ [KHH
+
01℄), and loselyrelated te hniques havealso been improved(among themHyper/J[OT01℄ and omposition lters[BA01℄).
This arti le presents our programming environment alled Java Aspe t Components [JAC℄. The two main requirementsof this framework are to support dynami ity and distribution. Nevertheless, JAC is also ageneral purposeAOPenvironment. Asthis,it omeswithaprogrammingmodel,adesignnotationandanAPI.Previous papers des ribed the programmingmodel [PSDF01a℄ of JAC, itsaspe t omposition me hanism [PSDF01b℄, the rstelementsof ourUML notation[PDF
+
02℄, andthear hite turefordistribution[PDF +
℄. This arti lesumsup themainfeaturesofJACanddes ribesin detailsourUMLnotation.
Se tion 2 introdu es the programming model of JAC. The UML design notation is des ribed in se tion 3. Se tion 4reports on the ar hite ture of JAC for distribution support. Implementation details and performan e
te hnologies. Finally,se tion7 on ludes thisarti le.
2 JAC framework & programming model
The JAC framework is basedon the notionof ontainers. Mu h likein other omponent frameworks (e.g. EJB [Sun ℄),a ontainerisahostforsoftwareentities. JAC ontainershostbothbusiness omponent,andnonfun tional omponent( alledaspe t omponent). Aswewillseelaterinse tion5,whenworkingwith entralizedenvironment, the ontainerissimplya ustomizedJava lassloaderthatperformsbyte odeadaptationstogluethebusinessand aspe t omponentstogether. Wheneveradistribution on ernappearin theappli ation,these ontainersbe ome remotelya essible(eitherwithRMI orCORBA).
Programmingmodel
JAC identi es three dierent roles involved in the development of an aspe t-oriented appli ation. Appli ation programmer: thisroleis on ernedwiththe orebusinessoftheappli ation. S/heimplementsthesoftwareentities oming from the fun tional de omposition of the problem. Aspe t programmer: this role is on erned with the implementationofnonfun tionalservi es. Uptothisstage,theseservi esareindependentfromtheonesde nedby appli ationprogrammers. Softwareintegrator: thisroleputsappli ationandaspe t odetogether. Twoimportant tasksare under theresponsibility ofthis role: point ut de nitions andaspe t omposition. Forthese three roles, theprogrammingmodelofJACprovidesthefollowingsoftwareartifa ts:
1. Baseprogram: this isthesetofJavaobje tsthat implementsthe orefun tionalitiesofappli ations. These are regularJavaobje ts. Thisset ofobje tsisself su ientand anberun onaJVM(hen e, withoutany aspe t).
2. Aspe t omponents: su h a omponentimplementsanonfun tional on ernthatwill lateron,bewovenon abaseprogram. An aspe t omponentde nesa ross utpoli y(i.e. themethodsofthebaseprogramwhose semanti s is modi ed by the non fun tional on ern) and someaspe t methods (advi es in Aspe tJ) that de nethesemanti almodi ations. Aspe tmethodsmaywrap(exe utebeforeand/orafter ode),repla eor extendthesemanti sof abasemethod.
3 Design notation
This se tion des ribes our UML pro le to support the design of aspe t with JAC. Stereotypes are proposed to qualify lassesimplementinganonfun tional on ern(3)andto qualifypoint utrelations3.2. Anexampleusing thesetwo on eptisgivenin se tion3.3. Se tion3.4goesastepfurtheranddrawssomesimilaritiesbetweenAOP andtheuse-providerelationship.
3.1 Aspe t omponent lasses
Aspe t Componentsare the entral pointof our AO framework. They are theimplementation units that de ne extra hara teristi sthat ross uta set of base obje ts. The key hara teristi sof JACis that the base obje ts that areinvolvedin a ross utarenotne essarilylo atedonasingle ontainer. Theyarede nedin lasses alled Aspe tComponent lasses(AC- lassesforshort).
An AC- lass is tagged with the aspe t stereotype. It ontains attributes and methods whose semanti s dier from regular methods. AC-methods are meant to extend the semanti s of regular lasses. The extension is performed on well de ned implementation points so that these points a tually use aspe t-servi es in order to integrate new on erns (e.g. a base lass an be made to use a Ca he interfa e if the aspe t implements some a hing on ern).
Ea h AC-method de nessome odeand extentsthesemanti sof somebase methods a ordingto amodality de nedbyastereotype. TheexistingstereotypesforanAC-methodm follow.
ofthebaseprogram.
after m(...): theAC-method m isexe utedafteragivenpointof there nedprogram.
around m(...): apartoftheAC-methodm isexe utedbeforeandanotherpartisexe utedafter agiven pointofthere nedprogram(thesetwoparts arede nedwithintheimplementationofm).
repla e m(...): the AC-method m modi es a given point of the extended program implementation by repla ingitbytheimplementationof m.
role m(...): theAC-methodm anbeinvokedontheobje tsthatareextendedbytheAC- lass;moreover, theAC-methodm an a esstheextended lassattributesandtheaspe t- lassattributes.
Forinstan e, gure1showsthe a hingAC- lassCa hing (withthe aspe t stereotype). Asitsnamesuggests it,thisAC- lassprovidesa a hingextensionme hanism. Thejobofstoringandretrievingvaluesfromthe a heis delegatedtotheCa he (regular) lass. ThewhenWritemethodoftheAC- lassCa hingistaggedwiththe after stereotype. It will beexe uted after any base method asso iatedwith whenWrite in the point ut de nition (see belowse tion3.2). ThewhenRead methodis tagged with around . Itwill beexe uted before and aftersome basemethods.
+ <<after>> whenWrite()
<<aspect>>
Caching
Cache
+ getValue(): Object
+ setValue(Object)
+ invalidate()
+ <<around>> whenRead()
Figure1: De nition ofa a hing on ernwithanAC- lass.
3.2 Point uts de nition
A point ut relation links an AC-method belonging to an AC- lass to aset of elements of abase program. The granularityoftheinvolvedelementsisthemethod: point utsinJAC annotgodeeperthanmethodstomodifythe base programsemanti s(e.g. we an notintrospe tmethodsbodiesto extendsomeparti ular odeinstru tions). Several arguments justify this feature. Firstly, for performan e reasons, reifying the whole ode stru ture hasa ost whi h ould not be bearable for real-lifeappli ations ( rst experimentswith a fully re e tive ompilersu h asOpenJava[Tat99℄ taughtusthat). Se ondly, extendingthe semanti sofanappli ationrequires before that,to understand itsoriginalsemanti s(we annotextend somethingthat is not learlystated). Most ofthetime this originalsemanti sisde nedthroughanAPI,e.g. throughmethods. Sobasemethodsarede nitivelythebestpla e toperformsomesemanti alextensions.
Two levelsof point ut de nition exist with JAC:either the point ut isde ned onaper- lass basis,ora per-instan ebasis.
Class levelpoint uts
Thislevelisverymu hsimilartotheonefoundinAspe tJ.Alltheinstan esofthe lassesinvolvedinthepoint ut areextendedbytheaspe t omponent. Inthis ase,apoint utrelationisanorientedasso iationfromanAC- lass towardsoneorseveral lasses. Theasso iationisstereotypedwith point ut . Theroleshavespe ialsemanti s: they mention whi h methods of the lient lass are extended and by whi h AC-methods. The semanti s of the elementsmentionedin gure2follows:
apoint utrelationpmustgofromanAC- lassAtoa lassC (ifseveral lassesareinvolvedinthepoint ut, severallinksaredrawnbetweentheAC- lassandthe lasses),
<<aspect>>
A
<<pointcut>>
r2
c2
p
tag
C
r1
c1
Figure2: Thepoint utasso iation: relatingaspe tsto lasses.
Keywords Semanti s
ALL allthemethods
STATICS allthestati methods
CONSTRUCTORS allthe onstru tors
MODIFIERS allthemethodsthatmodifytheobje t'sstate, i.e. thatmodifyatleastoneofthe elds ACCESSORS allthemethodsthatread theobje t'sstate GETTERS[(...)℄ thegetters
SETTERS[(...)℄ thesetters
ADDERS[(...)℄ themethodsthataddanobje ttoa olle tion REMOVERS[(...)℄ the methods that remove an obje t to a
ol-le tion
FIELDGETTERS allthegettersforprimitive elds FIELDSETTERS allthesettersforprimitive elds REFGETTERS allthereferen egetters
REFSETTERS allthereferen esetters COLGETTERS allthe olle tiongetters COLSETTERS allthe olle tionsetters
Table2: Keywordsallowedinpoint ut expressions.
+ <<after>> whenWrite()
<<aspect>>
Caching
whenRead
whenWrite
0−1
0−1
1
1
Cache
+ getValue(): Object
+ setValue(Object)
+ invalidate()
<<pointcut>>
<<pointcut>>
?GETTERS
?SETTERS
Server
p1
p2
+ <<around>> whenRead()
is0-1),
ardinality 2 isthenumberofmembersofC that anbeinrelationwithoneinstan eofA(defaultis*for all),
role r1 is the name of an AC-method de ned in A that is applied at ea h base program point denoted by role r2,
roler2 de nesabaseprogram ross uti.e. asetofjoinpoints. r2 isalogi alexpression(withAND,ORand NOToperators)whereea htermis oftheform quali ermethodExpression.
Twomainsquali ersareused: ? whi hdesignatedamethodexe utionpoint,and! whi hdesignateda method invo ationpoint.
methodExpression is either afully-de ned methodprototype(e.g. get():int), orapartially-de ned one with GNU-likeregularexpressions (e.g. get.*(.*):int mat hesall methodswhose namestarts withget, returnaninteger,andtakeanyparameters),oranexpressionbasedonthekeywordde ned in table2. Forinstan e, GETTERS(a,b)mat hesthegettermethods for eldsaandb. Likein omponent frame-workssu has JavaBeans, naming onventionsare assumedonmethod names: getter/settershould be name get/set followed by the eld name (starting in upper ase). Adders/removersshould be named add/removeand takean obje tasan uniqueparameter. Ea h timea new lass is loaded in the JAC framework,someintrospe tionandbyte odeanalysis areperformed. A meta-model ofthe lassis on-stru tedonthe y(inadedi atedaspe t alledRTTIforRunTimeTypeInformation)withannotations that enable to a hievethe semanti sde ned in table 2. Forinstan e, ea h method byte ode is parsed to determine whethersome eldsare modi ed ornot. Ifso, themethod is taggedasaMODIFIER in the RTTI aspe t. As the pro ess of analyzing the byte ode of many lasses an be time onsuming, the lasses whi h areneverextendedbyanaspe t (i.e. that aresimply usedby baseobje tsoraspe t omponents), anbeex ludedfromthis analysisphase.
as any UML model element, thepoint ut relation anbetagged (tag) to express extrasemanti sthat an beused when implementing themodel towardsa on rete platform; somesemanti sexamplesare shownin further se tions.
Figure 3showstwopoint ut relationsthat implementa a hingaspe tbyusing theAC- lassde ned in gure1. Thisaspe tdiagrammustbereadasfollows.
After theexe ution ofanysetter(a method that hangestheobje tstate) of aServer obje t, theprogram mustexe utethewhenWrite AC-method.
Around(i.e. beforeandafter) theexe utionof anygetter(a methodthat readstheobje tstate) ofaserver obje t,theprogrammustexe utethewhenRead AC-method.
Instan e levelpoint uts
Besidesthepreviouslydes ribedme hanism,JACalsoallowsdeveloperstode nepoint utsonaper-instan ebasis. Theideahereistosele tivelyextendsthesemanti s ofsomeinstan esofa lass. Therationaleisthat inahighly dynami distributed environment,someserverobje tsmayneedto ustomized(e.g. repli ated),while othersmay needtostayunmodi edeveniftheybelongto thesame lass.
Oneofthedi ultyisthat, ontraryto lassesthatarestraightforwardlynamed,obje tsla kanydire tnaming s hemeinJava. ThesolutiontakeninJACistolettheframeworkatta hanuniquenametoea h reatedinstan e: the name is the on atenation of the lass name in lower ase and of an auto-in remented integer (e.g. server0 designates the rst reatedinstan e of lass Server). The framework provides anAPI to retrieveobje tsbased on theirname. Thisapproa h is atrade-o between generalityand simpli ity: it is learto us that this s heme is usableonly ifthe number of reatedinstan es for ea h lass stayssmall. This s heme alsorequires the aspe t programmertohaveadeepunderstandingoftheinstan e reationpro essgoingoninthebaseprogram.
anbeextendedwithaninstan enameoraregularexpressiononinstan enames. Forinstan e?SETTERS|server0 designatesthe exe utionpointsof thesetter methods of instan e server0, ?GETTERS|server[1-3℄ designatesthe exe utionpointsofthegettermethodsofinstan esserver1,server2 andserver3.
Whendistribution omesintoplay,point utde nitions analsobe lteredbasedon ontainernames(a ontainer namebeingaRMIorCORBAURLdependingonthe hosen ommuni ationproto olbetweenJACremote ontain-ers). Theideaistoletdesignersexpressbehaviorsthatwillbedependentonthe ontextintowhi h omponentsare deployed. Forinstan e,onemaywanttoinstallanauthenti ationaspe tonlyonspe i riti alhosts,whereasthe restoftheappli ationdeployedonotherhostsstaysunmodi ed,oronemayneedtoinstallsomeloggingaspe tonly onagiven ontainer. Toallowthis,point ut expressions anbeextendedwith ontainernamesorregular expres-sionson ontainernames. Mergedwiththepreviousextensionforinstan enames,thisleadstoa ompletes heme wherepoint ut expressionsareoftheform: quali er methodExpression|instan eExpression| ontainerExpression with instan eExpression and ontainerExpression beingoptional. For instan e ?ACCESSORS||rmi://myHost/s1 designatesthea essorsexe utionpointsofinstan eslo atedonJAC ontainerrmi://myHost/s1.
3.3 A rst simple example
Thisse tionillustratestheprogrammingmodelofJACbasedontheCa hing aspe tof gure3. Thedetailsofthe APIandsometutorials an befoundontheJACwebsite[JAC℄.
Figure4givesthe odeoftheCa hing aspe t. Anaspe t omponentmustextendtheja . ore.Aspe tComponent lass. Amongotherthingsthis lassprovidesapoint ut methodthatletprogrammersexpressapoint ut. The pa-rametersare: thebase lassthispoint utdesignates,thequali ermethodExpressionasastring,the lass ontaining theAC-method,theAC-methodinvolvedinthepoint ut. Heretwosu hpoint utsarede ned. Additionalpoint ut methodsareavailablewheninstan eExpression and ontainerExpression aretobeasso iatedtothepoint ut.
import ja . ore.Aspe tComponent; import ja . ore.Wrapper;
import ja . ore.Intera tion;
publi lass Ca hing extends Aspe tComponent { publi Ca hing() {
point ut("Server","?SETTERS",Ca hingWrapper. lass,"whenWrite"); point ut("Server","?GETTERS",Ca hingWrapper. lass,"whenRead"); }
publi lass Ca hingWrapper extends Wrapper { private Ca he a he = new Ca he();
publi void whenWrite( Intera tion i ) { pro eed();
Obje t value = i.arg[0℄; a he.setValue(value); }
publi Obje t whenRead( Intera tion i ) { Obje t value = a he.getValue(); if ( value == null )
value = pro eed(); a he.setValue(value); return value;
} } }
prototypeismandatoryforallAC-methods: theya eptonlyoneparameterthatisaja . ore.Intera tion instan e. Theymayreturnanyparameters. Therationalebehindthis onstraintisthatAC-methodsareup alledbytheJAC frameworkwhenevera alltothebasemethodtheyextendisissuedorexe uted(i.e. wheneverthe allmat hesthe point ut expression). An Intera tion obje ti provides dataabout the urrent all: argumentsof the all(in the arg array),areferen eto thebaseobje t(i.wrappee),and somemethods tostoreandretrieve ontextparameters (forinstan e,parametersthat an beaddedbyanAC-methodonthe allerside,andthat anlateronretrieveon there eiversidebyanotherAC-method).
3.4 Extended design notation for distribution
The group paradigm
Intheprevious a hingexample,thesemanti smodi ationintrodu edbythe a hing on ernintotheappli ation is quite symmetri . Con retely, it means that all the obje tsthat are modi edto implement a hes (the Server obje ts) anbeseenasmodi edbythe sameabstra t transformationrule. However,onemaywantto weavethe Ca hing aspe ttodierent lasses. Thus,anotherdesignation me hanismisneededto expressthefa t that aset ofwell-de nedobje tsimplementsthesame on ern.
Thisneedforanewkindofstru turedelementsbringsustofo usonthegroupnotion. Ifwelookatthegroup notionvery arefully, we annoti e thatit istightlylinkedto aspe ts. Indeed, ontrarytoa lassthat abstra tly representsaset ofinstan esrealizingthesamefun tional hara teristi s,agroupis, inourde nition,anabstra t representationofasetofinstan esthatdonotne essaryhavehomogeneousfun tionaltypesbut thatarelogi ally groupedtogetherbe ausetheyimplementthesameservi e(servergroups)orusethesameone( lientgroups).
Figure 5representstheappli ationofthe a hingaspe tonagroupofserversthatimplementstheserverpart ofasimple lient/serverappli ation. Weuseaninstan ediagramsothatitbe omesobviousthatthegrouponthe topofthe gureisanon-uniform setofinstan es (thethree instan esa,b, and belongto threedierent lasses A, B and C). As shown on this gure, the appli ation of the a hing aspe t reates anew group that ontains instan es of a Ca he lass that provides the a hing fun tionality. In other words,we an say that these Ca he instan esbelongtoaservergroupthatprovidesa a hingfun tionalityforthe lientgroupformedbythea,b,and servers.
A group-based de nitionof aspe ts
Itappearsthattheintrodu tionofthe a hing on ernwithintheoriginal lient/serverappli ationisabstra tlydone bytheuseof theservi estheCa he groupinterfa eprovidesto theserversgroup. This anbeeasily represented in UMLbyusingthe use relationasrepresentedin gure6. Inthegeneral ase,implementinganew on ern mayrequirethe useof several interfa es. Inthese ases, several lients anbe related to several serversthrough some use relations.
Finally,asimplebut su ientde nition ofanaspe twithinthis ontextisthefollowing.
De nition: an aspe t is the implementation of one ormany use-provide relationship(s) between one ormany lientgroup(s)and oneormanyservergroups.
The model of gure6 learly bringsup ause-providerelationshipbetweena lient-group(the servers), anda servergroup(the a hes)that de nesthe grouplevelservi esgetValue(),setValue(Obje t) and invalidate(). This relationshipimplementationrequirestheappli ationtomodifythe lientgroupmemberobje tsimplementationto introdu ethe a hing on ernwithintheappli ation. Ifthis on ernimplementationismodularized(i.e. ifthe ode that implements the a hing on ern introdu tion is lo ally de ned), then the implementation te hnique follows theAOPguidelinesandwe an alltheobtainedmoduleanaspe t.
At theanalysis level,to express the fa t that ause-providerelationshipis implementedin anaspe t-oriented fashion, the appli ation designer an add a tagged value aspe t:aspe tName to all the use relationships implementedbytheaspe t alledaspe tName (see gure6).
Thus group-oriented modeling allowsthe designer to expli itin a omprehensivewaywhat parts of the (dis-tributed) appli ation areaspe tsand what parts are not. Infa t, for ea h modeled groupleveluse-provide rela-tionship,aspe t-orientedte hniques anbeusedto separate on ernswithin the nalimplementation.
a:A
a:Cache
a:A
a group
client:Client
b:B
c:C
b:Cache
c:Cache
client:Client
b:B
c:C
application of the caching aspect
Figure 5: Relatingaspe tstogroup.
b:B
c:C
a:A
+ getValue(): Object
+ invalidate()
+ setValue(Object)
Server
Cache
aspect: caching
<<use>>
s/he anaskher/himselfifanaspe twouldbewellsuitedinthis ase. Despitetheuseofanaspe tornotismainly relatedtothedesignerexperien eand hoi es,we angivesome luesonwhenanaspe twillbebettersuitedthan a lassi aldesign.
The lientgroupis heterogenous: it meansthat theuse oftheserverservi esare spreadalloverthe lient-groupmember lasses. Thisisbyessen ea ross uting on ernandsomeextradesignisne essaryto leanly modularizealltheinvolveddependen ies(forinstan e, we anuseinheritan ethat impliesbad omposition with other on erns or we an use some delegation related pattern whi h leads to a more omplex design model). In this ase, theuse of anaspe t angreatlysimplify theprogrammer's task andwill ensuregood maintenabilityandevolutivityofthe nalimplementation, evenifsome on ernsareaddedafterwards.
Severalhomogeneous lientgroupsusethesameservergroup: thisis exa tlythesamesituation that above sin eseveralhomogeneousgroups anbemodeledintooneheterogenousgroup.
Several lientgroupsuseseveralservergroupsbutitseemsthatthe nalpurposeoftheseuse-providerelations entersintoonesame on ernfor the nal appli ation: thisis amore ontextual hoi e that depends onthe knowledgeofthemodeleddomain.
Figuresumsupthenotionintrodu edinthisse tionandproposesaUMLmetamodelwhereadditionsintrodu ed byJACaredrawnwithboldlines.
orientation:int
startCardinality:String
startRole:String
endCardinality:String
endRole:String
RoledLink
Link
isAggregation:boolean
RelationLink
packagePath:String
Type
TypedElement
Instance
Member
Field
body:String
Method
Constructor
Parameter
InheritanceLink
name:String
description:String
ModelElement
start
end
endingLinks
*
PointcutLink
methodPCD:String
hostPCD:String
aspectRole:String
Aspect
Class
superClass
*
relationLinks
pointcutLinks
*
*
fields
methods
*
*
parameters
*
aspectMethods
AspectMethod
classPCD:String
objectPCD:String
Group
Figure7: TheUMLextensionmeta-model.
4 JAC ar hite ture for distribution
4.1 Aspe ts deployment and distribution
This se tion sums upthe features of the ar hite ture set up in JACto handle distribution. This ar hite ture is alledAODA(Aspe t OrientedDistributedAr hite ture)andisdes ribedinmoredetails in[PDF
+ ℄.
naturaland onsistent ohabitationbetweendistributionandaspe ts. Todothis,theAODAprovides orefeatures tosupport distributedaspe ts. Figure8showshowtheAODA managesdistributed aspe ts. Thetopofthe gure is a simple appli ation omposed of a set of omponents. The middle of the gure shows the same appli ation, but extendedby asample aspe t. Finally, the bottom part depi ts the appli ation deployed by theAODA. We annoti ethat ea h ontainer ontainsalo alinstan eoftheoriginalaspe t,hen e,theaspe tisapplied onea h ontainerinthesameway. Thesetof ontainerswheretheaspe tispresentis alledanaspe t-spa esothatit an nallyberegardedasasinglebutdistributedaspe t.
AnAspect
aspectMethod()
AnAspect
aspectMethod()
AnAspect
aspectMethod()
AnAspect
aspectMethod()
a pointcut relation
instance
the distribution aspect is woven
a distributed
pointcut relation
instance
a distributed
aspect−space
the aspect−extended
program
the original
program
an aspect instance is woven
container 1
container 2
container 3
Figure8: AODA:distributed supportofaspe ts.
Ourmotivationfordistributedaspe tssupportistoallowtheaspe tprogrammertoexpress globaland de en-tralizedprogramproperties. Indeed,ithappensquiteoftenthatanon-fun tionalproperty ross utsasetofobje ts that are notlo atedon thesame ontainer. Forinstan e, when addingan authenti ation on ern,the apa ities maybe he kedonseveralserver ontainerssothatitisveryusefulto modularizealltheauthenti ationde nition inoneuniqueaspe tde nitionthatisseamlesslyapplied tothewholedistributedappli ation.
This se tionpresentsasimpleexampleofadistributed appli ationwith JAC.Readersinterestedin readingmore detailedexamplesofdistributedprogrammingwithJAC anreferto[Paw02℄where,amongotherthings,arepli ated load-balan edserverisdes ribed.
Letustakethesimpleexampleofathreenoderingwhi hprovidesasimplefun tionalitytopassatokenbetween membersofaring. Thefun tionalbaseprogramis omposedofthreeobje tsthatarethenodesofthering;Sothe rststepistodevelopsomebaselevel lasses(this odesamples analsobefoundin theJACversionthat anbe downloadedfrom[JAC℄).
publi lass Ring {
publi stati void main( String[℄ args ) { RingElement element0 = new RingElement(); RingElement element1 = new RingElement(); RingElement element2 = new RingElement(); element0.setPrevious( element2 );
element1.setPrevious( element0 ); element2.setPrevious( element1 ); element2.roundTrip( 9 );
} }
publi lass RingElement {
publi RingElement previousElement; publi RingElement() {}
publi RingElement( RingElement previousElement ) { this.previousElement = previousElement;
}
publi void setPrevious( RingElement previousElement ) { this.previousElement = previousElement;
}
publi void roundTrip( int step ) {
if( step > 0 ) previousElement.roundTrip( step-1 );
} }
For nowon, one andevelopanaspe t omponentthat will deploythe three reatedobje tson JAC ontainers, or use the existing DeploymentAC aspe t omponent provided with JAC. Ea h aspe t omponent woven to an appli ation anbe asso iated with a on guration le that gives, with a s ript-like syntax, the steps needed to on gureit. Ea hstep orrespondsto allingamethodoftheaspe t omponent. Forinstan e,thefollowings ript instru tstheinstan e ofDeploymentAC woventothepreviousbase program,to:
1. remotelyinstall(AC-methoddeploy)instan esringelement0,ringelement1,ringelement2on ontainersbound to, respe tively,theRMInamermi://host0/s0,rmi://host1/s1,rmi://host2/s2.
2. reatea lientstub(AC-method reateAsyn hronousStubsFor)forringelement0 ons2,a lientstubfor ringele-ment1 ons1,a lientstub forringelement2 ons2. Thestubdelegatesmethod allstoremoteinstan es. By this way,remote ommuni ationdetailsarehiddentoringelementobje ts.
deploy "ringelement0" "rmi://host0/s0"
reateAsyn hronousStubsFor "ringelement0" "rmi://host0/s0" "rmi://host2/s2" deploy "ringelement1" "rmi://host1/s1"
reateAsyn hronousStubsFor "ringelement1" "rmi://host1/s1" "rmi://host0/s0" deploy "ringelement2" "rmi://host2/s2"
reateAsyn hronousStubsFor "ringelement2" "rmi://host2/s2" "rmi://host1/s1"
local reference
remote reference
ringelement0
ringelement1
ringelement2
The centralized version
s0
s1
s2
aspect
actual object
deployment
The same application after the deployment aspect is woven
A
A
A
A
asynchronous stub
Figure9: Deploymentoftheringappli ation.
Adding a tra ing aspe t to the ring
Letusassumethatwenowwanttoseetheround-tripprogressiononthedierent ontainers(inotherwords,where thetokenis). Of ourse,we ouldmodifytheRingElement.roundTrip methodimplementationtoaddaprintln all sothat theround-trippingeventsareloggedelsewhere. However,thiste hniquehasmanydrawba ks.
1. Itisnotdynami : on ethetra ehandlingisthere,youmustremoveitfromthe odeand ompileitagainto freethe omponentsfromthetra emanagement.
2. It is not lean: theRingElement.roundTrip ode is lesseasy to read for an external eye sin eit handles a on ernthat isnotpurelyrelatedtothering orefun tionalities.
3. Itislessreusable: whathappensifyoureusearingprogramthathasbeenprovidedbyanotherprogrammer and thatyoudonothavethesour e ode? Whathappensifyouwantyourringreused? Doyoufurnishthe tra e-freeversionorthetra edone?
4. It isnotsafe: thetra eexampleissimple,but imaginethatyouintrodu eabugoraregressionjust be ause youwanttoaddanewte hni al on ern(forinstan e,youlogthetra esintoa leand,somewhereinallthe linesyouaddintotheinitialprogram,youforgetto at hthediskfull orpermissiondenied ex eptionsso that theprogramstops be auseofthetra esyouadded). Finalusersmaybeupsetbythis. Usinganaspe t allowsyouto modularizethetra ingme hanismsothat itismu heasierto ontrolandtoensurethat your modi ationsdonot auseanyregression.
5. It is not sosimplein adistributed environment: ifyouwant yourtra es entralized in oneuniquestorage, youmayneedto installatra ingserver... on ernsassimpleasdebuggingorloggingbe omemore omplex whentheprogramisdistributed.
Forallthesereasonsandmanyothers,youmaywanttoimplementthistra ingfeaturewithinanaspe t. When theappli ation be omesmoreand more omplex, youwill takefull advantageof this approa h. Figure 10shows thetra ingaspe tdesign. Thefollowing odeisthestraightforwardJACimplementationofthismodel.
!roundTrip
<<pointcut>>
<<pointcut>>
tokenArrived
tokenPassed
?roundTrip
<<aspect>>
Tracing
trace
Trace
RingElement
+ <<before>> tokenPassed()
+ <<before>> tokenArrived()
+ trace(String message)
Figure10: Asimpletra ingaspe tfortheringsample.
publi lass Tra ingAC extends Aspe tComponent { Tra e tra e = new Tra e();
Tra ingAC() { point ut(RingElement, !roundTrip(int):void, Tra ingAC.Tra ingWrapper, tokenPassed); point ut(RingElement, ?roundTrip(int):void, Tra ingAC.Tra ingWrapper, tokenArrived); }
lass Tra ingWrapper extends Wrapper {
publi Obje t tokenPassed( Intera tion i ) {
tra e.tra e(The token has been passed by +i.wrappee); return pro eed();
}
publi Obje t tokenArrived( Intera tion i ) {
tra e.tra e(The token has arrived in +i.wrappee); return pro eed();
} } }
Asyou anseeonthe gureorwithintheJAC ode,thereisnomentionofdistribution. Thismeansthattheaspe t also works ifthe ring is running in a entralized or in a distributed mode (and for any sort of distribution that weprovide). Byusing AODA,wehave ompletelyseparatedthedistribution on ernfrom thetra ingonebutwe havealsomadetheaspe tsandtheirpoint utsemanti sinherentlydistributed. Thisdistributedsemanti sgreatly reinfor es the AOP expressiveness byallowing the modularized de nition of extensionsthat ross utdistributed appli ations.
thetra ingaspe t). Forinstan e,ifyouwantallthetra estobe entralizedonthes0 ontainer,thenjust on gure adistributionaspe tasthefollowssothatallthe alls tothetra efeaturesareforwardedons0.
deploy "tra e0" "s0"
reateStubsFor "tra e0" "s0" ".*"
The nalringappli ationar hite tureis givenin gure11.
tracing aspect
tracing aspect
tracing aspect
s0
s1
s2
ringelement0
ringelement1
ringelement1
ringelement2
ringelement2
ringelement0
trace0
trace0
trace0
tokenPassed()
tokenArrived()
tokenArrived()
tokenPassed()
tokenPassed()
tokenArrived()
pointcut instance
S
S
A
A
A
Figure11: Theringexample ompletedwiththedistributionandtra ingaspe ts.
5 Implementation & performan e issues for JAC
5.1 Implementation of JAC
JACisentirelywritteninJava. Theaspe tweavingisperformedat lassloadtimeusingthebyte odeengineering libraryBCEL.Thisleavesustheabilityto weaveexistingappli ationswhosesour e odeisnotavailable. Insu h a ase, software integrators need only to knowthe methods on erned by the point ut de nitions. The urrent distributionofJACprovidesaset ofprede ned aspe ts fordistribution (eitherwith RMIorCORBASOAP to omein futurereleases),persisten e(JDBCor lesystem),GUI(Swing),authenti ation,transa tion, onsisten y, loadbalan ing,and broad asting. A GUI onsoleis provided to on-the- yweaveorunweaveaspe tson topofa runningappli ation. ACASEtoolimplementingtheUMLdesignnotationde nedinse tion2isalsoavailable(see gure12foras reenshot).
Thejoinpoints onsideredinJACaremethodinvo ationsandexe utions. Hooksareintrodu edtowardswoven aspe ts wheneverthese eventsare generated. The ideais notnewand has been proposed bymany authors (e.g. [Chi95℄) to implement MOPs. It onsists in introdu ing a stub method for ea h method of a base lass. These stub methodsintrodu tionsaredonebytranslatingtheoriginal lassessothattheirinstan es ansupport aspe t weaving.
Weinvestigated several te hniques to perform this translation. One of these is to use ompile-time re e tion by using anopen ompiler su h asOpenJava [Tat99℄. OpenJavais verypowerfulsin e itallows allkind of ode manipulation at ompile-time (ittakesJava odeand produ esa translatedJava ode). However, sin eitrei es the whole syntax tree of the program it is quite slow and it is not very well suited to our simple problem (we under-use OpenJavafor su h asimple translation). Another solutionfor us is to perform the translation at the
byte odelevel. Several byte odetranslatorsareavailableandmostofthem anworkat lassload-time. Theidea istousea ustomized lass-loaderthatreadsthe lass leandmodi esthestream ontentsbeforea tuallyde ning and registering the new lass within the JVM. This solution is verywell suited for us. First, our translation is verysimpleand an beperformedwith very littleoverhead. Se ond,we antranslatethe lasses omingfrom a third-partyprogramorfrom externallibrarieswithnoneedoftheJavasour e ode.
We implemented the translation with two dierent byte ode translators. Javassist 1.0 [Chi00℄ is a re e tive high-leveltranslatorthat hidesthe omplexityofthebyte odeformatbyinstantiating aload-timemetamodel. It isquitefastandeasyto use. However,be auseofitshigh-levelAPI, Javassistintrodu essomerestri tionsonthe byte ode manipulations that an bedone (for instan e, onstru tors, stati s, and method invo ations annot be orre tlytranslated). Asawork-around,Javassist2.0proposesalow-levelAPIin additiontothehigh-levelone.
BCEL [Dah99℄ is the most popular byte ode translator. It proposes a verylow-level byte ode manipulation interfa ethatmakesitverypowerful(allkindoftranslations anbeperformed). Thetranslationweimplemented withBCELismore omplexandslowerthanwithJavassistbutthebyte odeprodu ed isofbetterquality.
5.2 Performan e measurements
The riti alpointoftheJACframeworkintermsofperforman esisthedynami wrappersinvo ationme hanism. Sin ethisinvo ationreliesonre e tioninordertoa hievedynami addingorremovingofaspe ts,theperforman e overheadofJACmainly omesfromthere e tive allsoverhead. Table3showstheperforman esofemptymethod allsonregularobje tsandonJACwrappableobje ts. Thesetestsareperformedwithaben hprogramthat alls severalmethod withdierentprototypesandthat isavailablein theJACdistribution [JAC℄. Theben hprogram wasrun underLinuxwith aPentiumIII 600MHzwith 256KBof a heand withtheSUN'sJavaHotSpot Client VMversion1.4.
(A)regularobje t alls 6,000,000 55 9.16ns
-(B)re e tive alls 60,000 47 0.78s (A)x85
(C) JAC obje ts alls (0 wrapper) 60,000 61 1s (A)x 111 (B)x1.29 JAC(1wrapper) 60,000 85 1.41s (C)+41% JAC(2wrappers) 60,000 110 1.83s (C)+83% JAC(3wrappers) 60,000 130 2.16s (C)+116%
Table3: Comparativeperforman emeasurementsforJavaandJAC.
(withanoverhead of29%). Ea htime awrapperisadded, anoverheadofabout40%ofthe initialtimeis added (notethattheben haddsemptywrappersthat just allpro eed in theirimplementations).
Finally,thepri etopayforadaptabilityisquitehigh(asforre e tion) omparedto ompiledapproa hessu h asAspe tJ.However,withreal-wordaspe tsandespe ially whentheappli ationisdistributed,this ostbe omes negligible omparedtotheadded ostofremote alls. Forthemoment, theJACapproa histhusmoresuitedfor middle grainedwrappableobje ts(onlybusiness obje tsare madewrappable in real-wordappli ations, te hni al omponentsthat needperforman esarenotaspe tized)andfordistributed andadaptableprogramming.
6 Related te hnologies and tools for AOP
Thisse tion omparesJACwithexistingapproa hesforAOPor loselyrelatedte hnologies.
The omposition lterobje tmodel (CFOM) [BA01℄is anextension to the onventional obje tmodel where inputandoutput lters anbede nedtohandlesendingandre eivingofmessages. Thismodelisimplementedfor several languages,in luding Smalltalk,C++ and Java. The latterimplementation isan extension to theregular Javasyntaxwhere keywordsareaddedtode lare,forinstan e, ltersatta hed to lasses. Thegoalsofthis model andoursare rathersimilar: to handleseparationof on erns atameta level. Nevertheless,JACdoesnotrequire anylanguageextension.
Aspe tJ[KHH +
01℄isapowerfullanguagethatprovidessupportfortheimplementationof ross utting on erns throughpoint uts( olle tionsofprin iplepointsintheexe utionofaprogram),andadvi es(method-likestru tures atta hed to point uts). Pre eden erules are de ned when morethan oneadvi e applyat a join point. Inmany features(e.g. point utsde nition)Aspe tJhasari handvastsemanti s.Nevertheless,wearguethatinmany ases thatwehavestudied,simples hemessu hasthewrappingte hniqueproposedbyJACaresu ienttoimplement abroadrangeofsolutionsdealingwithseparationof on erns.
Aspe tual omponents [LLM99℄ and their dire t prede essors adaptative plug and play omponents [ML98, MSL00℄ de ne patterns of intera tion, alled parti ipant graphs(PG), that implement aspe ts for appli ations. PGs ontainparti ipantsroles(e.g. publishersandsubs ribersin apublish/subs ribeintera tionmodel)that,(1) expe tfeatures aboutthe lassesuponwhi htheywillbemapped,(2)mayreimplementfeatures, and(3)provide somelo al features. PGsare then mapped onto lassgraphswith entities alled onne tors, that de ne the way aspe ts and lasses are omposed. Aspe tual omponents anbe omposed by onne ting part of the expe ted interfa eof one omponentto partof the provided interfa eof another. Nevertheless, itseemsthat bydoingso, thede nition ofthe omposition ross utsthede nition oftheaspe ts, loosing bythiswaytheexpe ted bene ts ofAOP.
Subje t oriented programming[HO93℄[OKH +
95℄ (SOP) and its dire t su essorthe Hyper/J tool [TOHS99℄, provide theability tohandledierentsubje tiveperspe tives, alled subje ts,ontheproblem tomodel. Subje ts an be omposed using orresponden e rules (spe ifying the orresponden es between lasses, methods, elds of dierentsubje ts), ombinationrules(givingthewaytwosubje ts anbegluedtogether,and orresponden e-and- ombinationrulesthatmixbothapproa hes. PrototypeimplementationsofSOPforC++andSmalltalkexist,and amorere ent versionfor Java alled Hyper/Jis available. This lattertoolimplementsthe notionof hyperspa e [OT01℄thatpermitsthe expli it identi ation ofany on ernsof importan e.
programmingsystemontheOzlanguageandusesobje t/ omponent-orientation,de larative,logi ,and onstraint programming to support the separation of the fun tional and of the distribution on erns. It provides a good separationof on ernsdegreewithasupportformultipleparadigms. Despiteits ompletenature,the oreMozart does not take the full advantageof new AO programming on epts su h as aspe t- lasses or point uts. In our opinion, it is therefore moredi ult to apprehend and less exible sin ethe provided on erns are built-in (but on gurable)withinthesystem.
Superimpositions [SK02℄ are also an approa h for separationof on erns in distributed environments. It is a theoreti alworkthatfurnishesalanguagethat anbeappliedtoAOP.Weare urrentlyworkingonusingsomeof thefundamental on eptsofsuperimpositionsinouraspe ts.
Finally, several proje tssu h asLasagne[TVJ +
01℄, JMangler[KCA01℄, orPROSE [PGA02℄ provide dynami weaving/unweavingofaspe tsthat makesthem losefromtheJACimplementation. However,noneof themfully handletheautomati distributionoftheaspe tswhentheappli ationisdistributed.
7 Con lusion
JACisaframeworkforaspe t-orientedprogramming(AOP)inJava. Itprovidesageneralprogrammingmodeland anumberofartifa tstoletprogrammersdevelopaspe t-orientedappli ationsinaregularJavasyntax(i.e. without any synta ti al languageextensions). Themain elementsmanaged bytheframeworkareaspe t omponents(AC for short). Theyarethe pie e of ode that apturea ross- utting on ern. Likein others AOPapproa hes,the ideaisto modularizethis on ernto easeitsmaintenan eandits evolution. JACprovides ontainerstohostAC andbusiness(also alledbase) omponents. Inthe urrentversionofJAC(downloadablefromourwebsite[JAC℄), these ontainersareremotelya essibleeitherwithRMI,orwithCORBA.Furtherdevelopmentsareunderwayfor theSOAP ommuni ationproto ol.
ACsde ne twomain elements: point ut relationsand AC-methods. Point ut relationsarethemethods ofthe baseprogramwhosesemanti sismeanttobeextendedbytheAC.AC-methodsaretheblo ksof odethatperform theextension. Theoriginalityof point ut relationswith JAC isthat they anbe de nedon aper- lassbasis(all instan es of some given lasses are equally extended), or on a per-instan e basis (only given instan es of some lassesareextended). Toa hievethisfeature,anamings hemeisprovidedforea hbaseinstan emanagedbythe framework. A languageforpoint utsde nitionis providedthatletdevelopers lterinstan esbasedontheirname oronthenameofthe ontainerhostingthem. TheAC-methodsprovide odethat anberunbeforeand/orafter, orrepla ethemethodsdesignatedbythepoint ut.
An UML notation has been proposed in se tion 3. The stereotypes provided enable designers to express all theabovementioned elements on erning AC, point ut relations, andAC-method. Se tion 3.4investigatedsome moreadvan ed on eptswhereAOPis omparedtotheuse-providerelationshipandtosomenotionsof groupsof heterogeneous lasses. ThenotationissupportedbyaCASEtool.
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