Corba Intro

Introductionto CORBA

Alex ChaffeeandBruceMartin

1.0

JGuruTrainingby theMagelangInstitute

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Contents

Chapter 1. Intr oduction to CORBA .. .. .. .. .. .. .. .. .. .. .. .. .. 1

1.1. Introductionto CORBA .. .. .. .. .. .. .. .. .. .. .. .. .. .. 1

1.2. DistributedApplications .. .. .. .. .. .. .. .. .. .. .. .. .. .. 2

1.2.1. DistributedApplications .. .. .. .. .. .. .. .. .. .. .. 2

1.2.1.1.DataareDistributed .. .. .. .. .. .. .. .. .. 3

1.2.1.2.Computationis Distributed .. .. .. .. .. .. .. .. 3

1.2.1.3.UsersareDistributed .. .. .. .. .. .. .. .. .. 3

1.2.1.4.FundamentalRealitiesof DistributedSystems .. .. 3

1.2.1.5.DistributedObjectSystems .. .. .. .. .. .. .. 5

1.3. OMG CORBA Specification .. .. .. .. .. .. .. .. .. .. .. .. 5

1.3.1. Whatis CORBA? .. .. .. .. .. .. .. .. .. .. .. .. .. 5

1.3.1.1.TheOMG .. .. .. .. .. .. .. .. .. .. .. .. 5

1.3.1.2.CORBA Architecture .. .. .. .. .. .. .. .. .. 5

1.3.1.3.TheORB .. .. .. .. .. .. .. .. .. .. .. .. 6

1.3.1.4.CORBA asaStandardfor DistributedObjects .. .. 6

1.3.1.5.CORBA Services .. .. .. .. .. .. .. .. .. .. 7

1.3.1.6.CORBA Products .. .. .. .. .. .. .. .. .. .. 8

1.4. TheStockApplication .. .. .. .. .. .. .. .. .. .. .. .. .. .. 9

1.4.1. TheStockApplication .. .. .. .. .. .. .. .. .. .. .. 9

1.4.1.1.SomeObjectsin theStockApplication .. .. .. .. 11

1.5. ImplementingaClient .. .. .. .. .. .. .. .. .. .. .. .. .. .. 11

1.5.1. ImplementingaCORBA Client .. .. .. .. .. .. .. .. .. 12

1.5.1.1.CORBA ObjectsareDescribedby IDL Interfaces .. 12

1.5.1.2.ObjectReferencesandRequests .. .. .. .. .. .. 14

1.5.1.3. IDL TypeSystem .. .. .. .. .. .. .. .. .. .. 15

1.5.1.4. IDL to JavaBinding .. .. .. .. .. .. .. .. .. 19

1.5.1.5. IDL to JavaCompiler .. .. .. .. .. .. .. .. .. 20

1.5.1.6.ObtainingObjectReferences .. .. .. .. .. .. .. 20

1.5.1.7.TheClient’sModelof ObjectCreation .. .. .. .. 21

1.5.1.8.Exceptions .. .. .. .. .. .. .. .. .. .. .. .. 21

1.6. ImplementingaSimpleDistributedObject .. .. .. .. .. .. .. .. .. 22

1.6.1. ObjectImplementations .. .. .. .. .. .. .. .. .. .. .. 22

1.6.1.1.Providing anImplementation .. .. .. .. .. .. .. 23

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1.6.1.2. InterfaceversusImplementationHierarchies .. .. .. 23

1.6.1.3. ImplementationTypeChecking .. .. .. .. .. .. 25

1.6.1.4. ImplementingaServerUsingtheJava2 ORB .. .. 25

1.6.1.5. ImplementingaServerUsingVisiBroker3.x .. .. .. 27

1.6.1.6.DifferencesBetweenServer Implementations .. .. .. 28

1.6.1.7.PackagingObjectImplementations .. .. .. .. .. 28

1.7. ObjectAdapters .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 29

1.7.1. ObjectAdapters .. .. .. .. .. .. .. .. .. .. .. .. .. 29

1.7.1.1.Activation on Demandby the Basic Object Adapter(BOA) .. .. .. .. .. .. .. .. .. .. .. .. .. 29

1.7.1.2.PortableObjectAdapter(POA) .. .. .. .. .. .. 30

1.8. Resources .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 31

1.8.1. Resources .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 31

1.8.1.1.WebSites .. .. .. .. .. .. .. .. .. .. .. .. 31

1.8.1.2.DocumentationandSpecs .. .. .. .. .. .. .. .. 31

1.8.1.3.Books .. .. .. .. .. .. .. .. .. .. .. .. .. 32

1.8.1.4.Miscellaneous .. .. .. .. .. .. .. .. .. .. .. 32

1.9. Appendix: Java2 ORBNotes .. .. .. .. .. .. .. .. .. .. .. .. 32

1.9.1. AboutTheJava2 ORB .. .. .. .. .. .. .. .. .. .. .. 32

1.9.1.1. idltojavaNotes .. .. .. .. .. .. .. .. .. .. .. 33

1.9.1.2.SystemProperties .. .. .. .. .. .. .. .. .. .. 33

1.10. Appendix: VisiBroker3.x Notes .. .. .. .. .. .. .. .. .. .. .. 34

1.10.1.VisiBroker3.x .. .. .. .. .. .. .. .. .. .. .. .. .. 34

1.10.1.1.VisiBrokerTools .. .. .. .. .. .. .. .. .. .. 35

1.10.1.2.UsingVisiBrokerwith Java2 .. .. .. .. .. .. .. 35

1.10.1.3.PortableStubsandSkeletons .. .. .. .. .. .. .. 36

1.10.1.4.UsingtheBOA with VisiBroker .. .. .. .. .. .. 36

1.10.1.5.UsingtheVisiBrokerSmartAgent .. .. .. .. .. 37

Chapter 2. Intr oduction to CORBA : Magercises .. .. .. .. .. .. .. .. .. 39

2.1. Magercise:RuntheSimpleStockExample .. .. .. .. .. .. .. .. 40

2.2. Magercise:StockExample .. .. .. .. .. .. .. .. .. .. .. .. .. 42

2.3. Magercise:DynamicStockExample .. .. .. .. .. .. .. .. .. .. 46

2.4. Magercise:A MessageBox .. .. .. .. .. .. .. .. .. .. .. .. .. 50

2.5. Magercise:Writing Callbacks .. .. .. .. .. .. .. .. .. .. .. .. 53

Appendix A. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 57

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Chapter 1. Intr oduction to CORBA

1.1. Intr oduction to CORBA

The Java DeveloperConnectionSM (JDC)presentsa ShortCourseintroducingthe CommonObject RequestBroker Architecturewritten by Java Software licensee,the MageLangInstitute. A leadingprovider of Java technologytraining,MageLanghascontributedregularlyto theJDCsince1996.

The MageLangInstitute, since its founding in 1995, has beendedicatedto promoting thegrowth of the Java technologycommunityby providing excellenteducationandactingasanindependentresource.To find out moreaboutMageLang’s Java technologytraining,visit theMageLangwebsite(http://www.magelang.com/jdc/) .

The CommonObject RequestBroker Architecture(CORBA) from the Object ManagementGroup(OMG) providesa platform-independent,language-independentarchitecturefor writingdistributed,object-orientedapplications.CORBA objectscanresidein thesameprocess,onthesamemachine,downthehall,oracrosstheplanet.TheJavalanguageisanexcellentlanguageforwriting CORBA programs.Someof thefeaturesthataccountfor thispopularityincludetheclearmappingfromOMGIDL totheJavaprogramminglanguage,andtheJavaruntimeenvironment’sbuilt-in garbagecollection.

This introductorycoursewill give you an overall view of how CORBA works,how to writeCORBA applicationsin the Java programminglanguage,and illustratesSun’s Java 2 ORBimplementationof CORBA aswell asInprise’sVisiBrokerfor Java. TheapproachtopresentingCORBA takenin thiscourseis throughanexamplestocktradingapplication.

Objectives

After completingthismoduleyouwill understand:

• Thebasicstructureof aCORBA application

• Why theJavaprogramminglanguageis anideallanguagefor CORBA programming

• How to specifydistributedobjectinterfacesin IDL

• Whatareasof CORBA youshouldpursuefor furtherstudy

By theendof thismodulewill beableto:

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2 Chapter1. Introductionto CORBA

• Write simpleCORBA interfacesin IDL

• UsetheJavaclassesgeneratedby theIDL compiler

• CreateCORBA clientapplicationsin theJava language

• ImplementCORBA distributedobjectsin theJava language

• Finddistributedobjectsat run time

PrerequisitesA generalfamiliaritywith object-orientedprogrammingconceptsandtheJavaprogramminglan-guage.If youarenot familiarwith thesecapabilities,seetheJavaTutorial(http://java.sun.com/docs/books/tutorial/) . The Magercisesrequire the ability to modify and build simple Javaprograms.

About the AuthorsAlex Chaffee is a Software Guru with MageLangInstitute. As the Director of SoftwareEngineeringfor EarthWeb,Alex co-createdGamelan(http://www.gamelan.com/

BruceMartin is oneof the pioneersof distributedobjectsystemsandprogramming.At SunMicrosystemshewasoneof Sun’sCORBA architectsandwastheprimaryauthorof fiveof theOMG’sCORBA Servicesspecifications.At InpriseCorporation,Brucecontributedto Inprise’sfirst CORBA-basedApplicationServer. Now, Bruceis a SoftwareGuruwith MageLangInsti-tute. Brucehaswrittenmany papersandgiventalksondistributedsystems,advancedtransactionmodels,object-orientedprogramming,anddistributedobjecttechnologiesatbothacademiccon-ferencesandindustrialevents.

1.2. Distrib uted Applications

1.2.1. Distrib uted Applications

CORBA productsprovidea framework for thedevelopmentandexecutionof distributedappli-cations. But why wouldonewantto developadistributedapplicationin thefirst place?As youwill seelater, distribution introducesa wholenew setof difficult issues.However, sometimesthereisnochoice;someapplicationsby theirverynaturearedistributedacrossmultiplecomput-ersbecauseof oneor moreof thefollowing reasons:

• Thedatausedby theapplicationaredistributed

• Thecomputationis distributed

• Theusersof theapplicationaredistributed

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1.2. DistributedApplications 3

1.2.1.1.DataareDistributed

Someapplicationsmustexecuteon multiple computersbecausethe datathat the applicationmustaccessexist on multiplecomputersfor administrativeandownershipreasons.Theownermaypermitthedatato beaccessedremotelybut not storedlocally. Or perhapsthedatacannotbeco-locatedandmustexist onmultipleheterogeneoussystemsfor historicalreasons.

1.2.1.2.Computationis Distributed

Someapplicationsexecuteonmultiplecomputersin orderto takeadvantageof multipleproces-sorscomputingin parallelto solve someproblem.Otherapplicationsmayexecuteon multiplecomputersin orderto takeadvantageof someuniquefeatureof aparticularsystem.Distributedapplicationscantakeadvantageof thescalabilityandheterogeneityof thedistributedsystem.

1.2.1.3.UsersareDistributed

Someapplicationsexecuteonmultiplecomputersbecauseusersof theapplicationcommunicateandinteractwith eachothervia theapplication.Eachuserexecutesa pieceof thedistributedapplicationonhisor hercomputer, andsharedobjects,typicallyexecuteononeor moreservers.A typicalarchitecturefor thiskind of applicationis illustratedbelow.

shared objects

Priortodesigningadistributedapplication,it isessentialtounderstandsomeof thefundamentalrealitiesof thedistributedsystemonwhich it will execute.

1.2.1.4.FundamentalRealitiesof DistributedSystems

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Distributedapplicationdevelopersmustaddressanumberof issuesthatcanbetakenfor grantedin alocalprogramwhereall logicexecutesin thesameoperatingsystemprocess.Thefollowingtablesummarizessomeof thebasicdifferencesbetweenobjectsthatareco-locatedin thesameprocess,andobjectsthatinteractacrossprocessor machineboundaries.

Co-located Distrib utedCommunication Fast SlowFailur es Objectsfail together Objectsfail separately

Network canpartitionConcurrent access Only with multiplethreads YesSecure Yes No

The communicationbetweenobjectsin the sameprocessis ordersof magnitudefasterthancommunicationbetweenobjectson different machines.The implication of this is that youshouldavoid designingdistributedapplicationsin which two or moredistributedobjectshaveverytight interactions.If they dohavetight interactions,they shouldbeco-located.

Whentwo objectsareco-located,they fail together;if theprocessin which they executefails,bothobjectsfail. Thedesignerof theobjectsneednot beconcernedwith thebehavior of theapplicationif oneof theobjectsis availableandtheotheroneis not. But if two objectsaredis-tributedacrossprocessboundaries,theobjectscanfail independently. In thiscase,thedesigneroftheobjectsmustbeconcernedwith eachof theobject’sbehavior in theeventtheotherobjecthasfailed. Similarly, in adistributedsystemthenetwork canpartitionandbothobjectscanexecuteindependentlyassumingtheotherhasfailed.

Thedefault modefor mostlocal programsis to operatewith a singlethreadof control. Singlethreadedprogrammingiseasy. Objectsareaccessedin awell-definedsequentialorderaccordingto theprogram’salgorithms,andyouneednotbeconcernedwith concurrentaccess.

If youdecideto introducemultiplethreadsof controlwithin a localprogram,youmustconsiderthepossibleorderingsof accessto objectsandusesynchronizationmechanismsto controlcon-currentaccessto sharedobjects.But at leastyouhavea choiceof introducingmultiple threadsof control. In a distributedapplication,therearenecessarilymultiple threadsof control. Eachdistributedobjectisoperatingin adifferentthreadof control. A distributedobjectmayhavemul-tipleconcurrentclients.Asthedeveloperof theobjectandthedeveloperof theclients,youmustconsiderthisconcurrentaccessto objectsandusethenecessarysynchronizationmechanisms.

Whentwo objectsareco-locatedin thesameprocess,youneednotbeconcernedaboutsecurity.Whentheobjectsareondifferentmachines,youneedtousesecuritymechanismstoauthenticatetheidentityof theotherobject.

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1.2. DistributedApplications 5

1.2.1.5.DistributedObjectSystems

Distributedobjectsystemsaredistributedsystemsin which all entitiesaremodeledasobjects.Distributedobjectsystemsarea popularparadigmfor object-orienteddistributedapplications.Sincetheapplicationis modeledasa setof cooperatingobjects,it mapsvery naturallyto theservicesof thedistributedsystem.

In spiteof the naturalmappingfrom object-orientedmodelingto distributedobjectsystems,donot forgettherealitiesof distributedsystemsdescribedabove. Processboundariesreallydomatterandthey will impactyourdesign.

That said, the next section of this coursediscussesthe CORBA standardfor distributedobjectsystems.

1.3. OMG CORBA Specification

1.3.1. What is CORBA?

CORBA, or CommonObjectRequestBroker Architecture,is a standardarchitecturefor dis-tributedobjectsystems.It allowsa distributed,heterogeneouscollectionof objectsto interop-erate.

1.3.1.1.TheOMG

TheObjectManagementGroup(OMG)isresponsiblefor definingCORBA. TheOMGcompris-esover700companiesandorganizations,includingalmostall themajorvendorsanddevelopersof distributedobjecttechnology, includingplatform,database,andapplicationvendorsaswell assoftwaretool andcorporatedevelopers.

1.3.1.2.CORBAArchitecture

CORBA definesanarchitecturefor distributedobjects.ThebasicCORBA paradigmis thatofarequestfor servicesof adistributedobject. Everythingelsedefinedby theOMG is in termsofthisbasicparadigm.

Theservicesthatanobjectprovidesaregivenby its interface. Interfacesaredefinedin OMG’sInterfaceDefinition Language(IDL). Distributed objectsare identified by object references,whicharetypedby IDL interfaces.

Thefigurebelow graphicallydepictsarequest.A clientholdsanobjectreferencetoadistributedobject. Theobjectreferenceis typedby an interface. In thefigurebelow theobjectreferenceis typedby theRabbit interface.TheObjectRequestBroker, or ORB,deliverstherequesttotheobjectandreturnsany resultsto theclient. In thefigure,a jump requestreturnsanobjectreferencetypedby theAnotherObject interface.

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ORB

client�

object�object�

implementation

Rabbit

interface Rabbit {:

AnotherObject jump(in long how_high);:

};�

1.3.1.3.TheORB

TheORBis thedistributedservicethat implementstherequestto theremoteobject. It locatestheremoteobjecton thenetwork, communicatestherequestto theobject,waitsfor theresultsandwhenavailablecommunicatesthoseresultsbackto theclient.

TheORBimplementslocationtransparency. Exactlythesamerequestmechanismisusedby theclientandtheCORBA objectregardlessof wheretheobjectis located.It mightbein thesameprocesswith theclient,down thehall or acrosstheplanet.Theclientcannottell thedifference.

TheORBimplementsprogramminglanguageindependencefor therequest.Theclient issuingthe requestcanbe written in a differentprogramminglanguagefrom the implementationoftheCORBA object. TheORBdoesthenecessarytranslationbetweenprogramminglanguages.Languagebindingsaredefinedfor all popularprogramminglanguages.

1.3.1.4.CORBAasa Standard for DistributedObjects

Oneof the goalsof the CORBA specificationis that clientsandobject implementationsareportable. The CORBA specificationdefinesan applicationprogrammer’s interface(API) forclientsof adistributedobjectaswell asanAPI for theimplementationof aCORBA object.Thismeansthatcodewritten for onevendor’sCORBA productcould,with a minimumof effort, berewritten to work with a differentvendor’sproduct. However, thereality of CORBA productson themarket todayis thatCORBA clientsareportablebut objectimplementationsneedsomerework to port from oneCORBA productto another.

CORBA 2.0addedinteroperabilityasa goalin thespecification.In particular, CORBA 2.0de-finesa network protocol,calledIIOP (InternetInter-ORBProtocol),thatallowsclientsusingaCORBA productfrom any vendorto communicatewith objectsusinga CORBA productfromany othervendor. IIOP worksacrosstheInternet,or moreprecisely, acrossany TCP/IPimple-mentation.

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1.3. OMGCORBASpecification 7

Interoperabilityis moreimportantin adistributedsystemthanportability. IIOP is usedin othersystemsthatdo not evenattemptto provide theCORBA API. In particular, IIOP is usedasthetransportprotocolfor aversionof Java RMI (socalled"RMI overIIOP").SinceEJBisdefinedin termsof RMI, it toocanuseIIOP.VariousapplicationserversavailableonthemarketuseIIOPbut donot exposetheentireCORBA API. Becausethey all useIIOP, programswritten to thesedifferentAPI’scaninteroperatewith eachotherandwith programswrittento theCORBA API.

1.3.1.5.CORBAServices

Anotherimportantpartof theCORBA standardis thedefinitionof a setof distributedservicesto supporttheintegrationandinteroperationof distributedobjects.As depictedin thegraphicbelow, the services,known asCORBA Servicesor COS,aredefinedon top of the ORB.Thatis, they aredefinedasstandardCORBA objectswith IDL interfaces,sometimesreferredto as"ObjectServices."

Chicago Chicago Fred

Sue

ORB

...

Factory NamingContext EventChannel

ThereareseveralCORBA services.Thepopularonesaredescribedin detailin anothermoduleof thiscourse.Below is abrief descriptionof each:

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Service DescriptionObject life cycle Defines how CORBA objects are created,removed, moved,

andcopiedNaming Defineshow CORBA objectscanhavefriendly symbolicnamesEvents DecouplesthecommunicationbetweendistributedobjectsRelationships Provides arbitrary typed n-ary relationshipsbetweenCORBA

objectsExternalization Coordinatesthe transformationof CORBA objectsto andfrom

externalmediaTransactions Coordinatesatomicaccessto CORBA objectsConcurrencyControl Providesa lockingservicefor CORBA objectsin orderto ensure

serializableaccessProperty Supports the associationof name-value pairs with CORBA

objectsTrader Supportsthe finding of CORBA objectsbasedon properties

describingtheserviceofferedby theobjectQuery Supportsqueriesonobjects

1.3.1.6.CORBAProducts

CORBA is a specification;it is a guidefor implementingproducts.Several vendorsprovideCORBA productsfor variousprogramminglanguages.TheCORBA productsthatsupporttheJavaprogramminglanguageinclude:

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1.3. OMGCORBASpecification 9

ORB DescriptionThe Java 2 ORB TheJava 2 ORBcomeswith Sun’s Java 2 SDK. It is

missingseveralfeatures.VisiBroker for Java A popularJava ORB from InpriseCorporation.Vis-

iBroker is alsoembeddedin otherproducts.For ex-ample,it is theORBthatis embeddedin theNetscapeCommunicatorbrowser.

OrbixW eb A popularJavaORBfrom IonaTechnologies.WebSphere A popular application serverwith anORBfrom IBM.NetscapeCommunicator Netscapebrowsershave a versionof VisiBroker em-

beddedin them. Appletscanissuerequeston COR-BA objectswithoutdownloadingORBclassesinto thebrowser. They arealreadythere.

Various fr eeor sharewareORBs CORBA implementationsfor variouslanguagesareavailable for download on the web from varioussources.

Providing detailedinformationaboutall of theseproductsis beyondthescopeof this introduc-tory course.Thiscoursewill just useexamplesfrom bothSun’sJava 2 ORBandInprise’sVisi-Broker3.x for Javaproducts.

1.4. The StockApplication

1.4.1. The StockApplication

ThestocktradingapplicationisadistributedapplicationthatillustratestheJava programminglanguageandCORBA. In thisintroductorymoduleonly asmallsimplesubsetof theapplicationis used.Feelfreeto expandupontheapplicationto enhanceit onceyou aremorecomfortablewith CORBA.

Thestockapplicationallowsmultipleuserstowatchtheactivity of stocks.Theuserispresentedwith a list of availablestocksidentifiedby theirstocksymbols.Theusercanselectastockandthenpressthe"view" button.

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Selectingthe "view" button resultsin a report about the stock, indicating the nameof thecompany, thestocksymbol,thecurrentprice,the last time it wasupdated,thetradingvolume,anda graphthatshowsthestockpriceover someinterval. Thisreportis automaticallyupdatedasnew stockdatabecomesavailable.

Thestockreportalsoletstheusersetanalarmby pressingthe"Alarm" button. Thealarmcanbesetto activatewhenthe priceof the stockfalls below a certainpriceor whenit exceedsacertainprice.

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1.4. TheStock Application 11

Whenthepriceof thestocksatisfiesthealarm’scondition,it activatesandtheuseris notified.

Latertheapplicationcouldbeextendedto allow usersto buy andsellstocks.

1.4.1.1.SomeObjectsin theStock Application

From the above description,you can easily identify the following distributed objects intheapplication.

Stock A distributedobjectthatrepresentsaparticularstock.StockPresentation A distributedobjectin theGUI thatpresentsthestockdatatotheuser

for aparticularstock.Alarm A distributedobjectthatrepresentsthealarmsetby theuser.AlarmPr esentation A distributedobjectin theGUI thatpresentsthealarmgoingoff to

theuser.

TheStockobjectis now usedto illustratetheCORBA distributedobjectmodel.

1.5. Implementing a Client

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1.5.1. Implementing a CORBA Client

Thissectioncoverswhatyouneedtoknow touseCORBA objectsfromtheJava programminglanguage.It examinesOMG IDL interfaces,theJava programminglanguagebindingfor IDLinterfaces,objectreferences,andrequests,how to obtainobjectreferences,andhow, asa client,tocreatedistributedobjects.After readingthissectionandcompletingtheexercises,youshouldbeabletowriteaclientusingtheJavaprogramminglanguage.Again,thestockexampleisusedto illustratetheclient’smodelof CORBA.

1.5.1.1.CORBAObjectsareDescribedbyIDL Interfaces

TheOMGInterfaceDefinitionLanguageIDL supportsthespecificationof objectinterfaces.Anobjectinterfaceindicatestheoperationstheobjectsupports,but not how they areimplemented.Thatis, in IDL thereis nowayto declareobjectstateandalgorithms.Theimplementationof aCORBA objectis providedin astandardprogramminglanguage,suchastheJavaprogramminglanguageor C++.An interfacespecifiesthecontractbetweencodeusingtheobjectandthecodeimplementingtheobject. Clientsonly dependon theinterface.

IDL interfacesareprogramminglanguageneutral. IDL defineslanguage bindingsfor manydifferentprogramminglanguages.Thisallowsanobjectimplementorto choosetheappropriateprogramminglanguagefor theobject. Similarly, it allowsthedeveloperof theclient to choosetheappropriateandpossiblydifferentprogramminglanguagefor theclient. Currently, theOMGhasstandardizedonlanguagebindingsfor theC,C++,Java,Ada,COBOL,Smalltalk,ObjectiveC,andLisp programminglanguages.

So by using OMG IDL, the following can be describedwithout regards to any particularprogramminglanguage:

• Modularizedobjectinterfaces

• Operationsandattributesthatanobjectsupports

• Exceptionsraisedby anoperation

• Datatypesof anoperationreturnvalue,its parameters,andanobject’sattributes

TheIDL datatypesare:

• Basicdatatypes( long , short , string , float …)

• Constructeddatatypes( struct , union , enum, sequence )

• Typedobjectreferences

• Theany type,adynamicallytypedvalue

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1.5. Implementinga Client 13

Again,IDL saysnothingaboutobjectimplementations.Here’stheIDL interfacefor theexamplestockobjects:

module StockObjects {

struct Quote {string symbol;long at_time;double price;long volume;

};

exception Unknown{};

interface Stock {

// Returns the current stock quote.Quote get_quote() raises(Unknown);

// Sets the current stock quote.void set_quote(in Quote stock_quote);

// Provides the stock description,// e.g. company name.readonly attribute string description;

};

interface StockFactory {

Stock create_stock(in string symbol,in string description

);};

};

Note that the above example definesan IDL module namedStockObjects , which con-tainsthe:

• DatastructureQuote

• ExceptionUnknown

• InterfaceStock

• InterfaceStockFactory

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Themoduledefinesa scopefor thesenames.Within themodule,a datastructureQuote andanexceptionUnknown aredefinedandthenusedin theStock interface.TheStock interfaceis usedin the definition of the StockFactory interface. Also note that the parameterstooperationsaretaggedwith thekeywordsin , out , or inout . Thein keyword indicatesthedataarepassedfrom theclient to theobject. Theout keyword indicatesthat thedataarereturnedfrom theobjectto theclient,andinout indicatesthatthedataarepassedfrom theclient to theobjectandthenreturnedto theclient.

IDL declarationsarecompiledwith anIDL compilerandconvertedto theirassociatedrepresen-tationsin thetargetprogramminglanguagesaccordingto thestandardlanguagebinding. (ThiscourseusestheJava languagebindingin all of theexamples.Lateryouwill seetheJavabindingin moredepth.)

1.5.1.2.ObjectReferencesandRequests

Clientsissuea requestonaCORBA objectusinganobjectreference. An objectreferenceiden-tifies thedistributedobjectthatwill receive therequest.Here’s a Java programminglanguagecodefragmentthatobtainsaStock objectreferenceandthenit usesit toobtainthecurrentpriceof thestock. NotethatthecodefragmentdoesnotdirectlyuseCORBA types;insteadit usestheJava typesthathavebeenproducedby theIDL to Javacompiler.

Stock theStock = …try {

Quote current_quote = theStock.get_quote();} catch (Throwable e) {}

Object referencescan be passedaroundthe distributed object system,i.e. as parameterstooperationsandreturnedasresultsof requests.For example,noticethat the StockFactory

interfacedefinesa create() operationthat returnsan instanceof a Stock . Here’s a Javaclient codefragmentthat issuesa requeston thefactoryobjectandreceivestheresultingstockobjectreference.

StockFactory factory = …Stock theStock = …try {

theStock = factory.create("GII","Global Industries Inc.");

} catch (Throwable e) {}

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NotethatissuingarequestonaCORBA objectisnotall thatdifferentfrom issuingarequestonaJavaobjectin alocalprogram.Themaindifferenceis thattheCORBA objectscanbeanywhere.The CORBA systemprovideslocationtransparency, which implies that the client cannottellif therequestis to anobjectin thesameprocess,on thesamemachine,down thehall,or acrosstheplanet.

AnotherdifferencefromalocalJavaobjectisthatthelife timeof theCORBA objectisnottiedtotheprocessin whichtheclientexecutes,norto theprocessin whichtheCORBA objectexecutes.Objectreferencespersist;they canbesavedasastringandrecreatedfrom astring.

Thefollowing JavacodeconvertstheStock objectreferenceto astring:

String stockString =orb.object_to_string(theStock);

Thestringcanbestoredor communicatedoutsideof thedistributedobjectsystem.Any clientcanconvert thestringbackto anobjectreferenceandissuea requeston thedistributedobject.

ThisJavacodeconvertsthestringbackto a Stock objectreference:

org.omg.CORBA.Object obj =orb.string_to_object(stockString);

Stock theStock = StockHelper.narrow(obj);

Notethattheresultingtypeof thestring_to_object() methodis Object , notStock . Thesecondline narrows the type of the objectreferencefrom Object to Stock . IDL supportsahierarchy of interfaces;thenarrow() methodcall is anoperationon thehierarchy.

1.5.1.3.IDL TypeSystem

IDL interfacescanbedefinedin termsof otherIDL interfaces.You previously saw a Stock

interfacethatrepresentsthebasicbehavior of astockobject.

ConsideranotherIDL module:

module ReportingObjects {

exception EventChannelFailure{};

interface Reporting {

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// Receive events in push modeCosEventComm::PushSupplier push_events(

in CosEventComm::PushConsumer consumer)raises(EventChannelFailure);

// Receive events in pull modeCosEventComm::PullSupplier pull_events(

in CosEventComm::PullConsumer consumer)raises(EventChannelFailure);

};

};

TheReporting interfacesupportstheregistrationof interestin events.(Don’t worryaboutthedetailsof usingtheCORBA EventService.)

Given thedefinitionof theStock interfaceanda Reporting interface,it is now possibletodefineanew ReportingStock interfacein termsof Reporting andStock .

interface ReportingStock: Reporting, Stock {};

A ReportingStock supportsall of theoperationsandattributesdefinedby theReporting

interfaceaswell asall of thosedefinedbytheStock interface.TheReportingStock interfaceinheritstheStock interfaceandtheReporting interface.Graphicallythisis representedas:

Reporting Stock�

ReportingStock

All CORBA interfacesimplicitly inherit theObject interface.They all supporttheoperationsdefinedfor Object . Inheritanceof Object is implicit; thereis noneedto declareit.

Object referencesare typedby IDL interfaces. In a Java programyou could type an objectreferenceto bea ReportingStock .

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ReportingStock theReportingStock;

Clients can passthis object referenceto an operationexpectinga supertype.For exampleassumethereisanEventManager interfacethathasa register operationthattakesanobjectreferencetypedby theReporting interface.

interface EventManager {:

void register(in Reporting event_supplier);:

};

Thefollowing is a legal requestbecausea ReportingStock is a Reporting .

EventManager manager = …ReportingStock theReportingStock = …manager->register(theReportingStock); // ok

However, thefollowing is nota legal requestbecausea Stock is not a Reporting :

EventManager manager = …Stock theStock = …manager->register(theStock); // type error

IDL TypeOperations

Given that IDL interfacescan be arrangedin a hierarchy, a small numberof operationsaredefinedon that hierarchy. The narrow() operationcastsan object referenceto a morespe-cific type:

org.omg.CORBA.Object obj = …Stock theStock = StockHelper.narrow(obj);

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Theis_a() operation,determinesif anobjectreferencesupportsaparticularinterface:

if (obj._is_a(StockHelper.id()) …

The id() operationdefinedon the helperclassreturnsa repositoryid for the interface. Therepositoryid is astringrepresentingtheinterface.For thestockexample,therepositoryid is:

IDL:StockObjects/Stock:1.0

Finally, it is possibleto widenanobjectreference,thatis castit to a lessspecificinterface:

Stock theStock = theReportingStock;

Therearenospecialoperationstowidenanobjectreference.It isaccomplishedexactlyasin theJavaprogramminglanguage.

RequestTypeChecking

TheIDL compilerfor Java programminglanguagegeneratesclient-sidestubs,which representthe CORBA object locally in the Java programminglanguage. The generatedcode alsorepresentsin theJava programminglanguageall of the IDL interfacesanddatatypesusedtoissuerequests.Theclientcodethusdependson thegeneratedJavacode.

client code�

stubs�IDL generated

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As you previously saw, passinganobjectreferencetypedby theStock interfaceto theeventmanagerwouldbeillegalbecausetheStock interfacedoesnotinherittheReporting interface.TheJavacompiler, not theIDL compiler, wouldcatchthiserrorat compiletime.

1.5.1.4.IDL to JavaBinding

TheJavabindingfor IDL mapsthevariousIDL constructstocorrespondingJavaconstructs.Thefollowingtableshowshow theIDL constructsarerepresentedin theJavaprogramminglanguage.For comparison,theC++bindingis alsoshown.

IDL Java C++module package namespaceinterface interface abstractclassoperation method memberfunctionattribute pairof methods pairof functionsexception exception exception

Eachof theIDL datatypesarerepresentedin theJavaprogramminglanguageasfollows:

IDL Type JavaTypeboolean booleanchar / wchar charoctet byte

short / unsigned short short

long / unsigned long int

long long / unsigned long long longfloat floatdouble doublestring / wstring String

Whendiscussingdatatype mapping,oneterm you run acrossfrequentlyis marshaling. Mar-shalingis theconversionof a language-specificdatastructureinto theCORBA IIOP streamingformat. IIOP datacanthenbetransmittedover a network to its destination,whereit is thenun-

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marshaledfrom IIOP backinto a language-dependentdatastructure.

1.5.1.5.IDL to JavaCompiler

CORBA productsprovide an IDL compiler that converts IDL into the Java programminglanguage.The IDL compiler available for the Java 2 SDK is called idltojava . The IDLcompilerthatcomeswith VisiBroker for Java is calledidl2java .

For the stock example,the command" idltojava Stock.idl " generatesthe files listedbelow. (The VisiBroker ORB generatesthe samefiles with the exceptionthat the stubfile iscalled_st_Stock.java , ratherthan_StockStub.java .)

Stock.java TheIDL interfacerepresentedasaJava interfaceStockHelper.java Implementsthetypeoperationsfor theinterfaceStockHolder.java Usedfor out andinout parameters_StockStub.java Implementsa localobjectrepresentingtheremoteCORBA object.

This objectforwardsall requeststo theremoteobject. Theclientdoesnotusethisclassdirectly.

The developer compilesthe IDL using the IDL compiler and then compilesthe generatedcodeusing the Java compiler. The compiledcodemust be on the classpathof the runningJavaprogram.

1.5.1.6.ObtainingObjectReferences

Youmayhavenoticedthattherearethreefundamentalmechanismsin whichapieceof codecanobtainanobjectreference:

• It canbecanbepassedto it asaparameter

• It canbereturnedastheresultof issuinga request

• It canbeobtainedby convertingastringinto anobjectreference

Thesefundamentalmechanismsare supportedby the ORB. Using thesemechanisms,it ispossibleto definehigherlevel servicesfor locatingobjectsin thedistributedobjectsystem.

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1.5.1.7.TheClient’sModelof ObjectCreation

Youmaydecidetoexporttheability tocreateanobjectto thedistributedobjectsystem.Youcanaccomplishthisby definingafactoryfor theobject. Factoriesaresimplydistributedobjectsthatcreateotherdistributedobjects.

Thereis nothingspecialabouta factory. It is just anotherdistributedobject: It hasan IDLinterface,it is implementedin someprogramminglanguage,andclientsissuestandardCORBArequestson factoryobjects.

Thereis nostandardinterfacefor a factory. Recallin theStockObjects example,thefactoryinterfaceis:

interface StockFactory {Stock create_stock(

in string stock_symbol,in string stock_description);

};

To createastockobject,aclientsimply issuesa requeston thefactory.

Anotherobjectimplementorcoulddefineanobjectfactorydifferently.

1.5.1.8.Exceptions

As youhaveseenin thestockexample,CORBA hasaconceptof exceptionsthatis verysimilarto that of the Java programminglanguage;naturally, CORBA exceptionsaremappedto Javaexceptions.WhenyouissueaCORBA request,youmustusetheJavaprogramminglanguage’stry andcatch keywords.

Therearetwotypesof CORBA exceptions,SystemExceptionsandUserExceptions.SystemEx-ceptionsarethrown whensomethinggoeswrongwith thesystem–forinstance,if yourequestamethodthatdoesn’t exist ontheserver, if there’sacommunicationproblem,or if theORBhasn’tbeeninitialized correctly. The Java classSystemException extendsRuntimeException ,sothecompilerwon’t complainif you forget to catchthem. You needto explicitly wrapyourCORBA callsin try…catch blocksin orderto recovergracefullyfrom SystemExceptions.

CORBA SystemExceptionscancontain"minor codes"whichmayprovideadditionalinforma-tion aboutwhatwentwrong. Unfortunately, thesearevendor-specific,soyouneedto tailor yourerrorrecoveryroutinesto theORByou’reusing.

User Exceptionsare generatedif somethinggoeswrong inside the executionof the remotemethoditself. ThesearedeclaredinsidetheIDL definitionfor theobject,andareautomatically

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generatedby theidltojava compiler. In thestockexample,Unknown is auserexception.

SinceUserExceptionsaresubclassesof java.lang.Exception , thecompilerwill complainif youforgetto trapthem(andthisis asit shouldbe).

1.6. Implementing a SimpleDistrib uted Object

1.6.1. Object Implementations

NOTE: theprevioussectiondiscussedtheclient’sview of CORBA, thatis,how aJava client issuesarequestonaCORBA object. Theclient’sviewis standardacrossmostCORBA products.Basically, thestandardworkedand thereareonly minor differences.Unfortunately, the sameis not thecasefor the implementationview of CORBA. As such,someof thedetailsgivenheremightnotmatchaparticularCORBA product.NotesondifferentCORBA productsappearasappendices.

Thissectiondescribeswhatyouneedto know to implementasimpleCORBA objectin theJavaprogramminglanguage.It examinestheJavaserver-sidelanguagebindingfor IDL, implement-ing objectsandservers,implementationpackagingissues,andCORBA objectadaptors.Aftercompletingthis section,you shouldbeableto write a simpleCORBA objectandserver in theJava programminglanguage.Again,thestockexampleis usedto illustratetheimplementationmodelof CORBA.

CORBA object implementationsare completelyinvisible to their clients. A client can onlydependontheIDL interface.In theJavaprogramminglanguage,orC++,thisisnotthecase.Theuserof anobjectdeclaresvariablesby a classname;doingsomakesthecodedependon muchmorethan just the interface. The client dependson the object implementationprogramminglanguage,the nameof the class,the implementationclasshierarchy, and, in C++, even theobjectlayout.

Thecompleteencapsulationfor CORBA objectsmeanstheobjectimplementorhasmuchmorefreedom. Object implementationscan be provided in a numberof supportedprogramminglanguages.This is not necessarilythe sameone the clientsarewritten in. (Of course,hereeverythingis in theJavaprogramminglanguage,but CORBA doesnot requirethis.)

The sameinterfacecan be implementedin multiple ways. There is no limit. In the stockexample,thefollowing arepossibleimplementationsof theStock interface:

• A stockimplementationclasswritten in theJava programminglanguagethatobtainsvaluesfrom acommercialfeed

• A stockimplementationclasswritten in C++thataccessesadatabaseon theInternet

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1.6. Implementinga SimpleDistributedObject 23

• A stockimplementationwritten in Smalltalkthatguessesstockprices

1.6.1.1.ProvidinganImplementation

RecallthatgivenanIDL file, theIDL compilergeneratesvariousfilesfor aCORBA client. In ad-dition to thefilesgeneratedfor aclient,it alsogeneratesaskeletonclassfor theobjectimplemen-tation. A skeletonis theentrypoint into thedistributedobject. It unmarshalstheincomingdata,callsthemethodimplementingtheoperationbeingrequested,andreturnsthemarshaledresults.Theobjectdeveloperneedonly compiletheskeletonandnotbeconcernedwith theinsidesof it.Theobjectdevelopercanfocusonproviding theimplementationof theIDL interface.

To implementaCORBA objectin theJavaprogramminglanguage,thedevelopersimplyimple-mentsaJavaclassthatextendsthegeneratedskeletonclassandprovidesamethodfor eachoper-ationin theinterface.In theexample,theIDL compilergeneratestheskeletonclass_StockIm -plBase for theStock interface.A possibleimplementationof theStock interfaceis:

public class StockImpl extends StockObjects._StockImplBase {

private Quote _quote=null;private String _description=null;

public StockImpl(String name, String description) {super();_description = description;

}

public Quote get_quote() throws Unknown {if (_quote==null) throw new Unknown();return _quote;

}

public void set_quote(Quote quote) {_quote = quote;

}

public String description() {return _description;

}}

1.6.1.2.InterfaceversusImplementationHierarchies

Notice that therearetwo separatehierarchies:an interfacehierarchy andan implementationhierarchy. Recallthattheinterfacehierarchy for theexampleof a ReportingStock is:

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Reporting Stock�

ReportingStock

In IDL thisis representedas:

interface ReportingStock: Reporting, Stock {};

Now supposethereisanimplementationof aReportingStock ,namedReportingStockIm -pl , thatinheritstheIDL generatedskeletons_ReportingStockImplBase , delegatessomeofits stockmethodsto StockImpl , andimplementstheReporting operationsdirectly. Graphi-cally:

_ReportingStockImplBase

ReportingStockImpl

delegates toStockImpl

In theJavaprogramminglanguage,thisclasshierarchy is representedas:

class ReportingStockImpl implements ReportingStockextends _ReportingStockImplBase {

…}

Since the Java programminglanguageonly supportssingle inheritanceof implementationclasses,implementationsoftencreateaninstanceof anotherclassanddelegateto it. In theaboveexample,theReportingStockImpl delegatesto theStockImpl classfor theimplementationof someof its methods.

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Otherclasshierarchiesimplementingthe sameinterfacehierarchy arepossible.Furthermore,if you needto changethe classhierarchy of the implementationin someway, the clientsarenotaffected.

1.6.1.3.ImplementationTypeChecking

Justastypecheckingis doneat theclient for therequestto adistributedobject,typecheckingisalsodonefor theobjectimplementation.

The IDL compiler for the Java programminglanguagegeneratesobject skeletonsand Javacodeto representall of theIDL interfacesanddatatypesusedin the interfacedefinition. Theimplementationcodethusdependson thegeneratedJavacode.

object implementation�

skeletons�IDL generated

If thereareany typeerrorsin theobjectimplementation,theJavacompiler,nottheIDL compiler,catchesthe errorsat compiletime. Thus,in the example,supposethe developererroneouslyimplementedthe get_quote() operationto returna doubleinsteadof the structurethat isdeclaredin theIDL:

Quote StockImpl.get_quote() {double price = …;return price;

}

TheJavacompilerwoulddetectthiserrorat compiletime.

1.6.1.4.Implementinga ServerUsingtheJava2 ORB

Youpreviouslysaw how toprovideanimplementationof aCORBA objectin theJavaprogram-ming language.The remainingtaskis to definea server that whenrun makestheservicesofits objectsavailableto clients. A server thatwill run with theJava 2 ORBneedsto do thefol-lowing:

• Defineamainmethod

• Initialize theORB

• Instantiateat leastoneobject

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• Connecteachobjectto theorb

• Wait for requests

Theservermustinstantiateat leastoneobjectsinceobjectsaretheonly wayto offer servicesinCORBA systems.

Here’sanimplementationof thestockobjectsserver. Thiscodedependson theJava2 ORB.

public class theServer {public static void main(String[] args) {

try {// Initialize the ORB.org.omg.CORBA.ORB orb =

org.omg.CORBA.ORB.init(args,null);

// Create a stock object.StockImpl theStock =

new StockImpl("GII","Global Industries Inc.");

// Let the ORB know about the objectorb.connect(theStock);

// Write stringified object reference to a filePrintWriter out =

new PrintWriter(new BufferedWriter(new FileWriter(args[0])));

out.println( orb.object_to_string(theStock) );out.close();

// wait for invocations from clientsjava.lang.Object sync = new java.lang.Object();synchronized (sync) {

sync.wait();}

} catch (Exception e) {System.err.println("Stock server error: " + e);e.printStackTrace(System.out);

}}

}

Noticethat theserver doesa new on theStockImpl classimplementingtheStock interfaceandthenpassesit to theORBusingtheconnect() call, indicatingthat theobjectis readytoacceptrequests.Finally, theserverwaitsfor requests.

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1.6.1.5.Implementinga ServerUsingVisiBroker 3.x

You previously saw how to provide a server usingtheJava 2 ORB. If you areusingInprise’sVisiBroker3.x for JavaORByouneedto dothefollowing:

• Defineamainmethod

• Initialize theORBandtheBOA (page29)(basicobjectadapter)

• Instantiateat leastoneobject

• Let theBOA know thattheobjectis readyto provideservice

• Let theBOA know thattheserver is ready

Theservermustinstantiateat leastoneobjectsinceobjectsaretheonly wayto offer servicesinCORBA systems.

Here’sanimplementationof thestockobjectsserver. ThiscodedependsonVisiBroker3.x:.

public class theServer {public static void main(String[] args) {

try {// Initialize the ORB.org.omg.CORBA.ORB orb =

org.omg.CORBA.ORB.init(args,null);

// Initialize the BOA.org.omg.CORBA.BOA boa =

((com.visigenic.vbroker.orb.ORB)orb).BOA_init();

// Create a stock object.StockImpl theStock =

new StockImpl("GII","Global Industries Inc.");

// Write stringified object reference to a filePrintWriter out =

new PrintWriter(new BufferedWriter(new FileWriter(args[0])));

out.println( orb.object_to_string(theStock) );out.close();

// Tell the BOA that the object is ready to// receive requests.boa.obj_is_ready(theStock);

// Tell the boa that the server is ready. This// call blocks.boa.impl_is_ready();

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} catch (Exception e) {System.err.println("Stock server error: " + e);e.printStackTrace(System.out);

}}

}

Noticethat theserver doesa new on theStockImpl classimplementingtheStock interfaceandthenpassesit to theBOA, indicatingthattheobjectis readyto acceptrequests.Finally, theservercallstheBOA to indicatethatit is ready. At thispoint,theimplementationwill becalledwhenrequestsarrive.

1.6.1.6.DifferencesBetweenServerImplementations

The following summarizesthe differencesbetweenimplementinga transientCORBA serverusingtheJava2 ORBandimplementinga transientserverusingInprise’sVisiBroker3.x:

Java 2 ORB VisiBroker 3.x for JavaInitialization Justinitialize theORB Initialize both theORBand

theBOAObject export orb.connect(theStock) boa.obj_is_ready(theStock)

�

Indicate server ready forrequests

Suspendmain threaddoinga wait()

boa.impl_is_ready()

Thesearetheonly differencesfor transientobjectservers. TherearefurtherAPI differencesofCORBA productsdueto persistenceandautomaticactivationof servers.

1.6.1.7.PackagingObjectImplementations

As illustratedabove,you shouldseparatetheimplementationsof your objectsfrom theimple-mentationof theserver. Thisallowsyou to mix andmatchobjectimplementationsin a server.Theobjectimplementationdoesnotdependontheserver. Theserver, of coursedependsontheobjectimplementationsthatit contains.

Anotheradvantageof carefullyisolatingobjectimplementationcodefrom servercodeis porta-bility. Mostof theproduct-specificcodeexistsin theserver, not in theobjectimplementation.

A goodstrategy is to packageanobjectimplementationwith its generatedstubsandskeletonsas a JavaBeancomponent.This allows the implementationto be manipulatedby JavaBean

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designtools.

1.7. Object Adapters

1.7.1. Object Adapters

The CORBA specificationdefinesthe conceptof an object adapter. An object adapteris aframework for implementingCORBA objects.It providesanAPI thatobjectimplementationsusefor variouslow level services.Accordingto theCORBA specification,anobjectadapterisresponsiblefor thefollowing functions:

• Generationandinterpretationof objectreferences

• Methodinvocation

• Securityof interactions

• Objectandimplementationactivationanddeactivation

• Mappingobjectreferencesto thecorrespondingobjectimplementations

• Registrationof implementations

The architecturesupportsthe definition of many kinds of objectadapters.The specificationincludesthedefinitionof thebasicobjectadapter(BOA). In theprevioussection(page27), yousaw someserver codethatusestheservicesof VisiBroker’s implementationof theBOA. TheBOA hasbeenimplementedin variousCORBA products.Unfortunately,sincethespecificationof theBOA wasnotcomplete,thevariousBOA implementationsdiffer in somesignificantways.Thishascompromisedserverportability.

To addressthis shortcoming,an entirely new object adapterwasadded,the portable objectadapter(POA). Unfortunately, thePOA isnotyetsupportedin many products.In any event,theBOA andthePOA aredescribedhere.

1.7.1.1.ActivationonDemandbytheBasicObjectAdapter(BOA)

Oneof the main tasksof the BOA is to supporton-demandobjectactivation. Whena clientissuesa request,theBOA determinesif theobjectis currentlyrunningandif so,it deliverstherequestto theobject. If theobjectis not running,theBOA activatestheobjectandthendeliverstherequest.

TheBOA definesfour differentmodelsfor objectactivation:

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Sharedserver Multiple active objectssharethe sameserver. The server servicesrequestsfrom multiple clients. Theserver remainsactive until it isdeactivatedor exits.

Unsharedserver Only oneobject is active in the server. The server exits whentheclient thatcausedits activationexits.

Server-per-method Eachrequestresultsin thecreationof aserver. Theserverexitswhenthemethodcompletes.

Persistentserver Theserver isstartedby anentityotherthantheBOA (you,operatingservices,etc.).Multiple activeobjectssharetheserver.

1.7.1.2.PortableObjectAdapter(POA)

According to the specification,"The intent of the POA, as its namesuggests,is to providean object adapterthat canbe usedwith multiple ORB implementationswith a minimum ofrewriting neededto deal with different vendors’implementations."However, most CORBAproductsdonotyetsupportthePOA.

The POA is alsointendedto allow persistentobjects– at least,from the client’s perspective.Thatis,asfarastheclient is concerned,theseobjectsarealwaysalive,andmaintaindatavaluesstoredin them,eventhoughphysically, theserver mayhave beenrestartedmany times,or theimplementationmaybeprovidedby many differentobjectimplementations.

The POA allows the object implementora lot morecontrol. Previously, the implementationof theobjectwasresponsibleonly for thecodethatis executedin responseto methodrequests.Now, additionally, the implementorhasmorecontrol over the object’s identity, state,storage,andlifecycle.

ThePOA hassupportfor many otherfeatures,includingthefollowing:

• Transparentobjectactivation

• Multiple simultaneousobjectidentities

• Transientobjects

• ObjectID namespaces

• Policiesincludingmultithreading,security, andobjectmanagement

• Multiple distinctPOAs in asingleserverwith differentpoliciesandnamespaces

For moredetailon thePOA, pleaseseethespecification.

A wordonmultithreading.EachPOA hasathreadingpolicy thatdetermineshow thatparticular

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1.7. ObjectAdapters 31

POA instancewill dealwith multiple simultaneousrequests.In the single threadmodel,allrequestsare processedone at a time. The underlyingobject implementationscan thereforebe lazy andthread-unsafe.Of course,this canleadto performanceproblems.In thealternateORB-controlledmodel,theORBis responsiblefor creatingandallocatingthreadsandsendingrequestsin to the object implementationsefficiently. The programmerdoesn’t needto worryabout threadmanagementissues;however, the programmerdefinitely hasto make suretheobjectsareall thread-safe.

1.8. Resources

1.8.1. Resources

1.8.1.1.WebSites

• TheOMG website: http://www.omg.org

1.8.1.2.DocumentationandSpecs

• The CORBA/IIOP Specificationcan be obtained from the OMG web site (http://www.omg.org/library/c2indx.html)

• Objects By Value Proposal can be obtained from the OMG FTP site:ftp://ftp.omg.org/pub/docs/orbos/98-01-18.pdf

• TheVisiBroker For JavaReferenceis availablefrom Inprise: http://www.inprise.com

• The VisiBroker For Java Programmers Guide is available from Inprise:http://www.inprise.com

• JDK 1.2CORBA documentationhttp://java.sun.com/products/jdk/1.2/docs/guide/idl/

• The Java 2 Documentation has extensive information on JavaIDL at theorg.omg.CORBA packagedocumentationandtheJavaIDL Guidedocumentation.

• JavaTransactions

♦ http://java.sun.com/products/jta/

♦ http://java.sun.com/products/jts/

♦ ftp://www.omg.org/pub/docs/formal/97-12-17.pdf

• JavaIDL NamingServicedocumentation

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1.8.1.3.Books

• Orfali & Harkey, Client/ServerProgrammingwith JavaandCORBA (Wiley)

• Orfali, Harkey & Edwards,InstantCORBA (Wiley)

• Vogel,AndreasandKeithDuddy, JavaProgrammingwith CORBA(Wiley)

• Perdricketal,Programmingwith VisiBroker (Wiley).

1.8.1.4.Miscellaneous

If youdon’t havetheidltojava compiler, youcanfind it atSun’sJavaIDL website.

RMI over IIOP compileris availableathttp://java.sun.com/products/rmi-iiop/.

JacORB- FreeJavaORB,includingPOA, DII, DSI,CosNaming

OrbixWebfrom Iona

WebSphereApplicationServer from IBM

A nicelist of ORBsonaniceCORBA infomationsite

1.9. Appendix: Java 2 ORB Notes

1.9.1. About The Java 2 ORB

TheJava IDL ORB that shipswith theJava 2 platformallows applicationsto run eitherasstand-aloneJava applicationsor asappletswithin Java-enabledbrowsers. It usesIIOP asitsnativeprotocol.

The SunJava ORB is fairly generic. This is good,becausetherearefew surprises;however,therearemany advancedfeaturesof CORBA thataremissing.Thereis noInterfaceRepository(thoughJava IDL clientscanaccessan InterfaceRepositoryprovidedby anotherJava or C++ORB),TransactionService,or POA, for example. For a completelist of theseunimplementedfeatures,seetheCORBAPackage JavaDoc Comments – scrolldown to find thesectionnearthebottomof thepagedescribingtheseshortcomings.

Java IDL is structuredwith a "pluggableORB" architecture,which allows you to instantiateORBsfrom othervendorsfrom within theJavaVirtual Machine.Thisis accomplishedthroughsettingenvironmentvariables,or systemproperties, or at run time throughtheuseof a Prop -erties or String[ ] object.SeetheCORBAPackage JavaDoc Comments for moredetails(scrolldown pastthelist of classesto find theappropriatesections).

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1.9. Appendix:Java2 ORBNotes 33

1.9.1.1.idltojavaNotes

If youdon’t havetheidltojava compiler, youcanfind it at theJava IDL website.

By default, idltojava triesto run a C preprocessoron the IDL files beforecompilingthem.Unfortunately, if youdonot havea C preprocessorinstalledon your system,or if idltojava

cannotfind it, youwill seecrypticerrormessage:

Bad command or file nameCouldn’t open temporary fileidltojava: fatal error: cannot preprocess input;No such file or directory

If you get this message,it meansyou must invoke idltojava with the -fno-cpp option,asfollows:

idltojava -fno-cpp foo.idl

1.9.1.2.SystemProperties

TheORB.init() methodcanreadin its configurationparametersfrom a numberof differentsources: from the application parameters(the first argument to ORB.init() ), from anapplication-specificProperties object(the secondargumentto ORB.init() ), or from theSystemProperties(definedon thecommandline by -D flags).

Quotedverbatimfrom theJava IDL guide:

Currently, thefollowingconfigurationpropertiesaredefinedfor all ORBim-plementations:

org.omg.CORBA.ORBClass

The name of a Java class that implements theorg.omg.CORBA.ORB interface. Applets andapplications do not need to supply this propertyunless they must have a particular ORB imple-mentation. The value for the Java IDL ORB iscom.sun.CORBA.iiop.ORB .

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org.omg.CORBA.ORBSingletonClass

The name of a Java class that implements theorg.omg.CORBA.ORB interface.Thisis theobjectre-turnedby a call to orb.init() with noarguments.Itis usedprimarily to createtypecodeinstancesthancanbesharedacrossuntrustedcode(suchasunsignedap-plets)in asecuredenvironment.Thevaluefor theJavaIDL ORBis com.sun.CORBA.iiop.ORB .

In additionto thestandardpropertieslistedabove,Java IDL alsosupportsthefollowing properties:

org.omg.CORBA.ORBInitialHost

Thehostnameof amachinerunningaserverordaemonthatprovidesinitial bootstrapservices,suchasa nameservice.Thedefault valuefor this propertyis local -host for applications.For appletsit is theapplethost,equivalentto getCodeBase().getHost() .

org.omg.CORBA.ORBInitialPort

Theport the initial namingservicelistensto. Thede-fault valueis 900 .

1.10. Appendix: VisiBroker 3.x Notes

1.10.1. VisiBroker 3.x

Herearesomeadditionaldetailsfor the VisiBroker 3.x implementationof CORBA. Seetheproductdocumentation(http://www.inprise.com/visibroker/) for moredetails.

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1.10. Appendix:VisiBroker 3.xNotes 35

1.10.1.1.VisiBroker Tools

VisiBroker for Java shipswith a numberof tools. Somearereplacementsor wrappersfor thestandardJava compilerandinterpreter. Othersarespecificto theVisiBroker product. Theimportantonesare:

• vbj is awrapperfor java whichsetssomepropertiesandaddsto yourclasspath

• vbjc is awrapperfor javac whichsetssomepropertiesandaddsto yourclasspath

• idl2java is an IDL compiler that can produceproprietaryor portablestubsandskeletons

• osagent launchestheproprietarySmartAgentbindingservice

1.10.1.2.UsingVisiBroker with Java2

To make VisiBroker for Java 3.4 work with the Java 2 platform, a numberof changesarenecessary, involving bothcodeandconfiguration.

The Java 2 platform ships with a standardimplementationof CORBA classesin theorg.omg.CORBA.* package.Theseclassesaresomewhatdifferentfrom theCORBA classesincludedwith VisiBroker. TheVisiBrokerclasseshaveseveralnonstandardextensionsto COR-BA; someof thesenonstandardextensionsarerequiredfor successfuloperation.Toaccessthesefunctions,youmustchangeyoursourcecodetocasttheJavaIDL ORBtoaVisiBrokerORB.Forexample:

org.omg.CORBA.ORB orb =org.omg.CORBA.ORB.init(args, null);

org.omg.CORBA.BOA boa =((com.visigenic.vbroker.orb.ORB)orb).BOA_init();

Thisexampleappliesto theservercode.

For thiscastto work, you mustalsoguaranteethat theORBreturnedby theORB.init() callis indeedaVisiBrokerORB,andnot thestandardJavaIDL ORBfrom Sun. For that,youneedtodefinetwoJavasystempropertiesbeforeyoulaunchtheJVM.Thesepropertiesareautomaticallysetif youusevbj insteadof java to launchyourprograms.But if youusejava , youmustsetthesepropertiesasfollows(whereeachliststhepropertynamefirst andvaluesecond):

• org.omg.CORBA.ORBClass - com.visigenic.vbroker.orb.ORB

• org.omg.CORBA.ORBSingletonClass - com.visigenic.vbroker.orb.ORB

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1.10.1.3.PortableStubsandSkeletons

By default, VisiBroker for Java createsstub and skeletoncodethat is interoperablebut notportable.Thismakessensein theVisiBrokerworld,sincethenon-portablecodeismoreefficientandslightly smaller. However, if yourcodeneedsto runonseveraldifferentORBs,youcanusethecommand

idl2java -portable -no_bind Foo.idl

andthestubsandskeletonswill beportable.

Therearea few reasonsfor this. Oneis if you’rewriting anappletthatwill run insidea remotewebbrowserenvironment,suchasNetscapeCommunicatoror theJavaPlug-in. ThelatterusesJavaIDL; theformermayberunninganolderversionof VisiBroker.

What’s the differencebetweenportableand proprietaryversions?A portablestub usesDII(Dynamic InvocationInterface)to marshalthe object request;a portableskeletonusesDSI(DynamicSkeletonInterface).The proprietaryversionsmake direct calls (to the ORB or theimplementation),andhencedonot have to go throughtheoverheadof creatingandparsingthevariousDII andDSI objects.

Note that your code doesn’t need to change–this all hap-pens behind the sceneswith idl2java . The only difference in the portable codeis that _FooImplBase extends org.omg.CORBA.DynamicImplementation in-stead of com.inprise.vbroker.CORBA.portable.Skeleton , and that the stubclass is named _portable_stub_Foo.java instead of _st_Foo.java . (Notealso that if you really want to, you can switch stubs on the fly by usingFooHelper.setProxyClass(_portable_stub_Foo.class) – though that would bekind of weird).

1.10.1.4.UsingtheBOA with VisiBroker

The VisiBroker BOA usesa slightly modified version of the standardBOA initializationsequence.For VisiBroker, follow thefollowing boilerplatecode.

// create and initialize the ORBORB orb = ORB.init(args, null);

The VisiBroker BOA is a customized,proprietary implementationof the CORBA.BOAinterface.It hasseveralmethodsthatarenotpartof thestandardinterface.In orderto usethese

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1.10. Appendix:VisiBroker 3.xNotes 37

proprietarymethods,youmustcasttheORBto aVisiBrokerclass,asfollows.

// Initialize the BOA.// Must cast to VBJ ORB for Java 2 compatibilityorg.omg.CORBA.BOA boa =((com.inprise.vbroker.CORBA.ORB)orb).BOA_init();

VisiBrokerobjectsareusuallypersistent. In VisiBrokerterms,thismeansthatthey areinitializedwith aname.Thisis notneededwith aportable,non-VisiBrokerobjectimplementation.

// create object and register it with the ORBStock theStock = new StockImpl(name);

The VisiBroker BOA skips the boa.create() phase and jumps straight toobj_is_ready() .

// Export the newly created object.boa.obj_is_ready(theStock);

Theimpl_is_ready() methodwaitsindefinitely.

// Wait for incoming requestsboa.impl_is_ready();

1.10.1.5.UsingtheVisiBroker SmartAgent

VisiBroker for Javashipswith its own locationservice,calledthesmartagent.Thesmartagentisadistributedlocationservice.It collaborateswith othersmartagentsrunningonthenetwork tolocateasuitableimplementationof anobject. If thereismorethanoneimplementationavailable;thesmartagentselectsone. Thisprovidesa degreeof fault toleranceandloadbalancing.If amachinegoesdown, thesmartagentwill automaticallyfind anotherimplementationonanothermachineto servicetherequest.Theclient is unawareof this.

If youcreateapersistentobject,bypassingin anamewhenyoucall itsconstructor, thentheBOA

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will automaticallyinform thesmartagent.Thenameyoupassedin theconstructorwill becomethenameit is known by on thesmartagentnetwork.

Ontheclientside,theproprietaryHelperobjectdefinesamethodbind() thatfetchesanobjectreferencefor you,bypassingtheneedto convert a stringinto anobjectreference.Thebind()

methodis notpartof thestandardCORBA-Javamapping.

// "bind" (actually lookup) the object referenceStock theStock = StockHelper.bind(orb, "GII");

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Chapter 2. Intr oduction to CORBA :Magercises

Welcometo the MageLangInstitute Magercises(page57) for the Introduction to CORBAshortcourse.

TheseMagerciseswill giveyousomeinitial experiencein writing andrunningsimpleCORBAapplicationsusingtheJava 2 ORBandVisiBroker for Java from Inprise.

Whenyoufinish theseMagercises,youwill know thebasicstepsfor designing,compiling,andrunningCORBA applicationsusingtheJava2 ORBandVisiBroker.

A help (page57) documentprovidesa quick explanationof the conceptsbehindthe Mager-cises.

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40 Chapter2. Introductionto CORBA: Magercises

2.1. Magercise: Run the SimpleStockExample

This simpleMagercisetakesyou throughthe stepsof runningthe simplestockserverandaclient. Basicallyit teststheinstallationof yourORB.ThisMagercisecanbecompletedwith eitherSun’s Java IDL ORBor Inprise’sVisiBroker3.x for Java.

Educationalgoal(s):

• RunasimpleCORBA application.

Prerequisites

None.

SkeletonFiles

• stockClient.jar

• stockServer.jar

Solution To run theserver you needto downloadthestockServer.jar

file. To run theclientyouneedto downloadthestockServer.jar file.

• stockClient.jar

• stockServer.jar

Intr oduction

In thisMagercise,you will run a CORBA server andclient thathave beenprovided. Basicallyit teststheinstallationof yourORB.

This Magercisecan be completedwith either Sun’s Java IDL ORB orInprise’sVisiBroker3.x for Java.

Perform the following tasks:

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2.1. Magercise: RuntheSimpleStock Example 41

1. Download the Jar files for the client and the for the server. (See"SkeletonCode"above.) To runtheserveryouneedto downloadthestockServer.jar file. To runtheclientyouneedto downloadthestockServer.jar file.

2. Runtheprogramsin theordergivenbelow:

run the server java StockServer.theServergii.ior

run the client java StockClient.theClientgii.ior

Notes Your classpathneedsto include the jar file containingtheStockServer or StockClient. For VisiBroker you also needto in-clude the files vbjorb.jar and vbjapp.jar on the classpath.Alternatively, you can usethe vbj programto run your programsinsteadof using java directly. The vbj commandaddsthe vb -jorb.jar and vbjapp.jar files to the classpathappropriate-ly and then invokesJava. If you areusingVisiBroker and Java 2,then you needto define the following systempropertiesas well:org.omg.CORBA.ORBClass=com.visigenic.vbroker.orb.ORB

3. org.omg.CORBA.ORBSingletonClass=com.visigenic.vbroker.orb.ORB

Conclusion

This Magercisehaswalked you throughthe stepsof runninga CORBAserver anda CORBA client. In futureexcercisesyou will build your ownCORBA programsandrun themanalogously.

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2.2. Magercise: StockExample

ThisMagercisereviewstheorderof tasksin developingaCORBA applica-tion usingaminimalexample.

Educationalgoal(s):

• Learnto modify, build andrunasimpleCORBA application.

Prerequisites

RuntheSimpleStockExample(page40)

SkeletonFiles

• stock.idl

• StockClient.java

• StockServer.java

• StockImpl.java

Solution

Thefollowing Javasourcefilesrepresentasolutionto thisMagercise:

• stock.idl

• StockClient.java

• StockServer.java

• StockImpl.java

Intr oduction

In thisexercise,you will addanoperationto thestockIDL file, implementit, generateJava code from the IDL file and then compile and run thedistributedstockapplication.

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2.2. Magercise: Stock Example 43

The order in which you will edit thesefiles is the sameorder that youwill follow for normalCORBA applicationdevelopment.Thesestepsareindicatedby noteswithin squarebracketsat thebeginningof eachtask.

ThisexercisecanbecompletedusingeitherSun’sJavaIDL ORBor Inprise’sVisiBroker3.x for Java. Noteson theseproductsareavailable:

• SunJava IDL ORB Implementation (page32)

• Inprise�

VisiBroker for Java (page34)

If youareusingtheJava2 platform,seethenotesonVisiBrokervs. Java2 (page35).


1. [ Modify the IDL file] Firstedit theIDL file. Thereis alreadyadefi-nition for themoduleandinterface.Theinterface,however, doesnothaveany wayof settingapricequoteof astock. Within theinterfacedefinition,addan operationnamedsetQuote() . It shouldtake anin parameterof typeQuote .RememberthattheseareIDL types,notJava types.

2. [ Run IDL Compiler] At thecommandline,runtheIDL compilertogenerateJavacodefrom theIDL file.

SunJavaIDL idltojava -fno-cpp stock.idl

Inprise VisiBroker idl2java stock.idl

3. [ CompilethegeneratedJavacode] Now compilethegeneratedJavacodeusingthestandardJava compiler. Compileit in the generatedStockObjects directory.

SunJavaIDL javac *.java

Inprise VisiBroker vbjc *.java

4. [ Examine the generatedJava files] All thefiles will begeneratedintotheStockObjects directorybecausetheIDL file definesamod-uleStockObjects.(Thepurposeof eachfile isdescribedin moredetailin thecoursenotes(page20).)Look carefullyatthefile Stock.java

- - thisis theinterfacethatyouwill haveto implement.

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5. [ Create"Impl" Classes] Theskeletoncodefile wehavegivenyou,StockImpl.java hasmost of the implementationyou will need.To finish,adda bodyto themethodsetQuote() . Note: Besurenotto confusethe skeletonfiles that Magelangprovidesaspart of outMagerciseswith CORBA skeletonfiles. TheMagelangfiles appearaslinksat thebeginningof theMagercise,andarefilesthatyouaretouseasastartingpointfor coding.CORBA skeletonfilesaregeneratedfrom IDL files,endwith thenameImplBase andareusedfor creat-ing classesthatimplementCORBA interfaces.

6. [ CreateServer Class] Now look at the file StockServer.java .This classis a java applicationthat startsup the orb, instantiatesaStockImpl instance,connectsit to theORB,andprintsoutastringi-fied referenceto it. Thereis nothingfor you to doexceptlook at thefile. Look carefullyateachline.

7. [ CreateClient Class] In thefile StockClient.java , youwill findcodeto instantiatetheorb,readin astringifiedreferenceto theremoteobject,andcall thegetQuote operationonit. Again,thereisnothingfor youtodoexceptlook atthefile. Lookcarefullyateachline. Note:Most of the Magercisesareapplicationsthat have separate"server"and"client" portions. Theserver portionsinstantiatedistributedob-jects,theclientportionsusethem. Thisisnotarequirementof COR-BA programs,any applicationthat is partof a CORBA environmentcanbothinstantiateandusedistributedobjects.Theclientandserverpatternis usedby the Magercisesfor convenienceandconsistency.SomelaterMagercisesdispensewith thisconventionentirely.

8. [ Compiletheprogram]Now compiletheprogramusingthestandardJavacompiler.

SunJavaIDL javac -classpath . *.java


9. [ Run the Server program] The server needsto run asa separateprocess.In DOS/Windows, you will usethe start command.InUNIX, youwill usethe& (ampersand).TheServeroutputsanIOR (orstringifiedreference)to a file andto theconsolefor theStockobjectit creates.Thenameof thefile is passedasthefirst argumentto theserver. To run theserver, usethenormaljava command.If youareusingVisiBroker, youcanusethevbj commandinstead.vbj is justlike java , exceptit setsuptheCLASSPATH variablesautomatically.Puttingit all together:

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2.2. Magercise: Stock Example 45

SunJavaIDL

java -cp . StockServergii.ior

Inprise VisiBroker

vbj StockServer gii.ior

10. [ Run the Client program] The client readsthe stringifiedobjectreferenceof thestockobjectcreatedbyyourserver. A file containingthestringifiedobjectreferenceis passedasanargumentto theclient.

SunJavaIDL java -cp . StockClient gii.ior

Inprise VisiBroker vbj StockClient gii.ior

11. [Kill the Server] Don’t forgetto kill theserver.

Conclusion

This exercisehaswalkedyou throughthestepsof writing, compiling,andrunninga portableCORBA application.Most otherexerciseswill follow asimilarpattern.

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2.3. Magercise: Dynamic StockExample

This Magerciseaddsdynamiccreationof stock objectsto the previousstockserver.

Educationalgoal(s):

• Learnaboutfactoriesin asimpleCORBA application.

Prerequisites

StockExample(page42)

SkeletonFiles

• stock.idl

• StockFactoryImpl.java

• StockImpl.java

• DynamicStockServer.java

• DynamicStockClient.java

Solution


• stock.idl

• StockFactoryImpl.java

• StockImpl.java

• DynamicStockServer.java

• DynamicStockClient.java

Intr oduction

In this Magercise,you will add a factory interfacewith a single create

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2.3. Magercise: DynamicStock Example 47

operationto thestockIDL file, implementit, generateJava codefrom theIDL file andthencompileandrun thedistributedstockapplication.

This Magercisecan be completedusing either Sun’s Java IDL ORB orInprise’sVisiBroker3.x for Java. Noteson theseproductsareavailable:

• SunJavaIDL ORB Implementation (page32)

• Inprise VisiBroker for Java (page34)

If youareusingtheJava2 platform,seethenotesonUsingVisiBrokerwith Java2 (page35).


1. [ Modify IDL file] First edit theIDL file. TheStockObjectsmodulein the IDL file definesthe interfaceto a stock object. But thereiscurrentlyno way for a client to createstock objects. You needtoaddanotherinterface,StockFactory thathasa singleoperationtocreatea stockandreturna referenceto thecreatedstock. Thecreateoperationshouldtake two string parameters:one giving the stocksymbolandtheothergiving adescriptionof thecompany.

2. [ Run IDL Compiler] At thecommandline,runtheIDL compilertogenerateJavacodefrom theIDL file.

SunJavaIDL idltojava -fno-cpp stock.idl

Inprise VisiBroker idl2java stock.idl

3. [ CompilethegeneratedJavacode] Now compilethegeneratedJavacodeusingthestandardJava compiler. Compileit in the generatedStockObjects directory.

SunJavaIDL javac *.java


4. [ Examine the generatedJava files] All thefiles will begeneratedintotheStockObjects directorybecausetheIDL file definesamod-uleStockObjects.(Thepurposeof eachfile isdescribedin moredetailin thecoursenotes(page20).) Look carefullyat thefile StockFac -tory.java - - thisis theinterfacethatyouwill haveto implement.

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5. [ Implement the stockfactory] Theskeletoncodefile wehavegivenyou, StockFactoryImpl.java hasmost of the implementationyouwill need.To finish,addabodyto themethodcreate_stock .

6. [ CreateServer Class] Now look at thefile DynamicStockServ -er.java .Thisclassisajavaapplicationthatstartsuptheorb,instan-tiatesa StockFactoryImpl instance,connectsit to theORB,andprintsout a stringifiedreferenceto it. Thereis nothingfor you to doexceptlook at thefile. Look carefullyateachline.

7. [ Create Client Class] In the file DynamicStockClient.java ,youwill find codeto instantiatetheorb,readin astringifiedreferenceto the remotefactory, andcall the create_stock operationon it.Again, thereis nothingfor you to do except look at the file. Lookcarefullyateachline.

8. [ Compiletheprogram]Now compiletheprogramusingthestandardJavacompiler.

SunJavaIDL javac -classpath . *.java


9. [ Run theServer program] Theserverneedsto runasaseparatepro-cess.In DOS/Windows,youwill usethestart command.In UNIX,youwill usethe& (ampersand).TheServeroutputsanIOR(orstringi-fiedreference)to afile andto theconsolefor theStockfactoryobjectit creates.Thenameof thefile is passedasthefirst argumentto theserver. To run theserver, usethenormaljava command.If youareusingVisiBroker, youcanusethevbj commandinstead.vbj is justlike java , exceptit setsuptheCLASSPATH variablesautomatically.Puttingit all together:

SunJavaIDL java -cp . DynamicStockServerstockfactory.ior

Inprise VisiBroker vbj DynamicStockServer stock -factory.ior

10. [ Run theClient program] Theclientreadsthestringifiedobjectref-erenceof thestockfactoryobjectcreatedby yourserver. An file con-tainingthestringifiedobjectreferenceis passedasthefirst argumentto theclient. Thesecondandthird argumentspassedto theclient isthestocksymbolandcompany descriptionof thestockto becreated.Notethefollowing commandsareeachonasingleline.

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2.3. Magercise: DynamicStock Example 49

SunJavaIDL java -cp . DynamicStockClientstockfactory.ior gii "GlobalIndustries Inc."

Inprise VisiBroker vbj DynamicStockClient stock -factory.ior gii "Global Indus -tries Inc."

11. [Kill the Server] Don’t forgetto kill theserver.

Conclusion

This Magercisehas walked you through the stepsof adding dynamicdistributedobjectcreationby meansof a factoryobject. As you probablyobserved,a factoryobjectis just likeany otherCORBA object.

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2.4. Magercise: A MessageBox

In this Magerciseyou will createthe implementationclassfor an IDL in-terface.

Educationalgoal(s):

• Learnhow to write interfaceimplementationclasses.

• Call methodsonadistributedobject.

• Throw andcatchCORBA UserExceptions

This Magerciseis meantfor usewith the VisiBroker for Java ORB fromInprise.

Prerequisites


SkeletonFiles

• messageBox.idl

• MBClient.java

• MBServer.java

• MessageBoxImpl.java

Solution


• solution/messageBox.idl

• solution/MBClient.java

• solution/MBServer.java

• solution/MessageBoxImpl.java

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2.4. Magercise: A MessageBox 51

Intr oduction

Theapplicationin thisMagerciseis asimplemessagebox,likea telephoneansweringmachine.TheMessageBoxinterfacehasanoperationfor leavingmessages,anotherfor theownertogetthemessages,andanattributefor set-ting thereplythatis sentoutwhenamessageis left. Hereis theIDL file:

module MessageModule {

typedef sequence<string> MessageSeq;

interface MessageBox {attribute string reply;

string leaveMessage(in string msg)raises (boxFull);

MessageSeq getMessages();};

};

Yourtaskis tocreateaMessageBoxImpl classto implementthisinterface.Youwill alsoaddmethodstotheMBClient classtotestyourMessageBox -Impl .


1. FirstgeneratetheIDL file.

2. Examine the generated Java interface in MessageMod-ule/MessageBox.java . Using the given skeletonfile, Message -BoxImpl.java , implementthenecessarymethods.Don’t forgettoaddaconstructor. It shouldtakeastringargumentasaname,andcallthesuperclassconstructorto setthenameof theobject. Noticehowthesequenceis generated.No MessageSeq classis created;insteadyouusetheJavatypeString[] .Thereare,however,helperandhold-erclassesgeneratedfor MessageSeq .

3. Modify the MBServer class to instantiatea MessageBoxImpl

with a nameof yourchoosing.Remember, youwantthisnameto beuniqueto avoid conflictswith otherORBsonyournetwork.

4. Look at theMBClient sourcecodeprovidedfor you. Makesureyouunderstandhow thecommand-lineparametersareparsed.Theusageof MBClient is:

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vbj MBClient servername command paramvbj MBClient servername leave messagevbj MBClient servername reply reply-messsagevbj MBClient servername get

5. Finishtheskeletonversionof MBClient by addingcodeasspecifiedbythecomments.Youshouldcall thereply settertosetanappropri-atereply, call leaveMessage to leave somemessages,andretrieveyourmessagesusinggetMessages sothey canbeprintedout.

6. Now compiletheprogram,usingvbjc .

7. Run the server: start vbj MBServer . Checkthat it is runningok.

8. Run theclient: vbj MBClient servername command param.Checktheoutputof yourprogram.Seetheexpectedbehaviorsectionfor moredetailon thecorrectoutputfor yourprogram.

9.

If youhappentohavemultiplemachines,runtheprogramoneachandleaveandretrievemessageson theothermessageboxes.

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2.4. Magercise: A MessageBox 53

2.5. Magercise: Writing Callbacks

This Magerciseshows how to createa pair of distributed objectsthatcommunicatewith eachotheraspeers.

Educationalgoal(s):

• Write codeto instantiateORB-awareobjects.

• Createcallbacksfrom onedistributedobjectto another.

Prerequisites


SkeletonFiles

• pingpong.idl

• PingObjectImpl.java

• PongObjectImpl.java

• PingServer.java

• PongServer.java

Solution


• solution/pingpong.idl

• solution/PingObjectImpl.java

• solution/PongObjectImpl.java

• solution/PingServer.java

• solution/PongServer.java

Intr oduction

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Unlikea client/serversystem,theobjectsin CORBA applicationarepeers:noneis intrinsically more important than any other. You may decidetodesignyour programusingvariouskindsof Client/Server metaphors,butthatdecisionis yoursto make. TheORBjust seesobjectscommunicatingwith eachotheroveranetwork.

In thisMagercisethereisapairof peerobjects,PingObject andPongOb-ject , for whichJavaimplementationshavealreadybeenwritten: PingOb -jectImpl andPongObjectImpl . Their behavior is suchthat whenthePingObject is pinged,it pongsthegivenPongObject . WhenPongOb-ject is ponged,it returnsthefavor by pingingthegivenPingObject . Tokeepthingsfrom gettingout of hand,thePingObject keepstrackof thenumberof timesit hasbeenpinged,andstopsafter a maximumnumberof pings.

Your task is to createtwo Java classes,PingServer and PongServer

whosemain methodswill instantiateeithera PingObject or a PongOb-ject respectively, andcall theappropriatemethodsto startthepingsandpongsgoing.


1. First takea look at theIDL file, pingpong.idl , andgenerateit.

2. Examinethe two implementationclasses,PingObjectImpl andPongObjectImpl to seehow they work andhow to usethem.

3. Using the skeleton code file as a startingpoint, modify the classPingServer to addall thecodenecessaryto startup theORBandBOA, createa PingObject instance,setthe maximumnumberofpings,etc. Therearecommentsto guideyour way. Besureto pick auniquenamefor your PingObject instance.

4. Now fill out the PongServer class.The codeneededherewill bevery similar to that for PingServer , but you will alsoinitiate theping-pongexchange.

5. Compileyourprogramandrun it to seewhathappens.

Conclusion

Thetiming informationprintedoutwhenthesolutionis rungivesyousomeideaof the latency involvedwhencalling methodson a distributedobject.For extracredit,youcanmodify theimplementationclassesto print out thetimedifferencebetweenpingsandpongs.

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2.5. Magercise: Writing Callbacks 55

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Appendix A

About Magercises

A Magerciseisaflexibleexercisedesignedtoprovidehelpaccordingto theneedsof thestudent.For example,somestudentswill simply completethe exercisegiven the informationandthetasklist in theMagercisebody;somestudentsmaywanta few hintswhile othersmaywantastep-by-stepguidetosuccessfullycompleteaparticularMagercise.Studentsmayuseasmuchoraslittle helpasthey needperMagercise.Moreover, sincecompletesolutionsarealsoprovided,studentscanskipa few Magercisesandstill beableto completefutureMagercisesrequiringtheskippedones.

The Anatomy of A Magercise

EachMagercisehasa list of any prerequisiteMagercises,a list of skeletoncodefor youto startwith, links to necessaryAPI pages,andatext descriptionof theMagercisegoal. In addition,thefollowing informationis availablevia fivebuttons:

• Expected behavior : Launchesan applet illustrating the desiredbehavior fromyourapplet.

• Table of contents: Brings up the tableof contentsfor the coursenotesandthe listof magercises.

• Help: Givesyouhelpor hintson thecurrentMagercise(anannotatedsolution).

• Solution: The< applet > tagandJavasourceresultingin theexpectedbehavior.

• API Documentation: A link directlyto theonlineAPI documentation.

MagerciseDesignGoals

Therearethreefundamentalmagercisetypes:

"Blank screen"Theprogrammerisconfrontedwith a"blankscreen";i.e.,theprogrammercreatestheentiredesiredfunctionality.

ExtensionThe programmerextends the functionality of an existing, correctly-workingprogram.

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58 AppendixA

RepairTheprogrammerrepairsundesirablebehavior in anexistingprogram.

Where possible,the programmer shall be relieved fr om chores that are irr elevant orunrelatedto the techniqueor conceptunder examination.

Wherereasonable,a commonthr eadshall run thr oughthe magercisesfor eachlabsection.

Giventheconstraintsof thetechniqueor conceptunderexamination,themagercisesshall bemadeasinterestingor usefulaspossiblewithoutpresentinganoverly-complex programmingproblemto thestudent.

Magercisesshall executevia the webunlessaparticularconceptrelatedto non-webexecutionis requiredor the browserdoesnot supportthe capabilitiesyet. In addition,magercisesthatmustaccessJava featuresor library elementscausingwebsecurityviolationsarenot executedon theweb.

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Corba Intro

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Transcript of Corba Intro