分布式计算原理与应用(Distributed-Computing)第八章课件

AdvancedRemoteMethodInvocationsM.L.Liu12/3/20231DistributedComputing,M.L.LiuRMI–AdvancedtopicsTheJavaRMIAPIhasarichcollectionoffeatures.WewilllookatsomeofRMI’smoreinterestingadvancedfeatures,namely:

stubdownloadingsecuritymanagerclientcallback.Althoughthesefeaturesarenotinherenttothedistributedobjectparadigm,theyarehelpfulmechanismsandcanbeusefultoapplicationdevelopers.12/3/20232DistributedComputing,M.L.LiuTheJavaRMIArchitecture12/3/20233DistributedComputing,M.L.LiuJavaRMIClientServerInteraction12/3/20234DistributedComputing,M.L.LiuRMIStubDownloadingRMIisdesignedtoallowstubstobemadeavailabletotheclientdynamically.Doingsoallowschangestobemadeintheremotemethodswithoutaffectingtheclientprogram.ThestubcanbefiledwithanwebserverandbedownloadedusingHTTP.Securitymeasuresareneededtopreventboththeclientsideandtheserverside:Ajavasecuritypolicyfileneedstobesetontheserverhostandalsoontheclienthost.AJavaSecurityManagershouldbeinstantiatedinboththeclientandserverprograms.12/3/20235DistributedComputing,M.L.LiuStubdownloadingIfthestubistobedownloadedfromaremoteserver,transferthestubclasstotheappropriatedirectoryonthatserver,e.g.,/~mliu/www,andmakesurethattheaccesspermissiontothefileisset.Whenactivatingtheserver,specifycommandoptions:

12/3/20236DistributedComputing,M.L.LiuThejava.policyfileTheRMIsecuritymanagerdoesnotpermitnetworkaccess.Exceptionscanbemadeviathespecificationinajava.policyfile. grant{ //permitssocketaccesstoallcommonTCPports,includingthedefault//RMIregistryport(1099)–needforboththeclientandtheserver.permission.SocketPermission"*:1024-65535","connect,accept,resolve";//permitssocketaccesstoport80,thedefaultHTTPport–needed//byclienttocontactanHTTPserverforstubdownloadingpermission.SocketPermission"*:80","connect";};

Thisfilecanbefiledinthesamedirectoryastheserverclassfile.Whenactiviatingtheclient,ajava.policyfilealsoshouldbespecified:java-Djava.security.policy=java.policySomeClientSee/products/jdk/1.2/docs/guide/security/PolicyFiles.htmland/products/jdk/1.2/docs/guide/security/permissions.html12/3/20237DistributedComputing,M.L.LiuFilePlacements12/3/20238DistributedComputing,M.L.LiuRMISecurityManagerSinceRMIinvolvesaccessto/fromaforeignhost,andpossiblyobjectdownloading,itisimportantforboththeserverandtheclienttoprotectitssystemfrommaliciousaccess.TheRMISecurityManagerisaclassprovidedJava,andcanbeinstantiatedinboththeclientandtheserverforlimitingaccesspriviledges.Youcanwriteyourownsecuritymanager,ifsodesired.

try{System.setSecurityManager(newRMISecurityManager());}catch{…}12/3/20239DistributedComputing,M.L.LiuAlgorithmforbuildinganRMIApplicationServerside:Openadirectoryforallthefilestobegeneratedforthisapplication.Specifytheremote-serverinterface,andcompileittogeneratetheinterfaceclassfile.Buildtheremoteserverclassbyimplementingtheinterface,andcompileitusingjavac.Usermictoprocesstheserverclasstogenerateastub.classfileandaskelton.classfile:rmicSomeServerIfstubdownloadingisdesired,copythestubfiletoanappropriatedirectoryontheHTTPhost.ActivatetheRMIRegistry,ifithasnotalreadybeenactivated.Setupajava.policyfile.Activatetheserver,specifying(i)thecodebaseifstubdownloadingisdesired,(ii)theserverhostname,and(iii)thesecuritypolicyfile.

12/3/202310DistributedComputing,M.L.LiuAlogorithmforbuildinganRMIApplicationClientside:Openadirectoryforallthefilestobegeneratedforthisapplication.Implementtheclientprogramorapplet,andcompileittogeneratetheclientclass.Ifstubdownloadingisnotineffect,copytheserverinterfacestubclassfilebyhand.Setupajava.policyfile.Activatetheclient,specifying(i)theserverhostname,and(ii)thesecuritypolicyfile.

12/3/202311DistributedComputing,M.L.LiuRMICallbacks12/3/202312DistributedComputing,M.L.LiuIntroductionIntheclientservermodel,theserverispassive:theIPCisinitiatedbytheclient;theserverwaitsforthearrivalofrequestsandprovidesresponses.Someapplicationsrequiretheservertoinitiatecommunicationuponcertainevents.Examplesapplicationsare:monitoringgamesauctioningvoting/pollingchat-toommessage/bulletinboardgroupware12/3/202313DistributedComputing,M.L.LiuPollingvs.CallbackIntheabsenceofcallback,aclientwillhavetopollapassiveserverrepeatedlyifitneedstobenotifiedthataneventhasoccurredattheserverend.12/3/202314DistributedComputing,M.L.LiuTwo-waycommunicationsSomeapplicationsrequirethatbothsidesmayinitiateIPC.Usingsockets,duplexcommunicationcanbeachievedbyusingtwosocketsoneitherside.Withconnection-orientedsockets,eachsideactsasbothaclientandaserver.12/3/202315DistributedComputing,M.L.LiuRMICallbacksAcallbackclientregistersitselfwithanRMIserver.Theservermakesacallbacktoeachregisteredclientupontheoccurrenceofacertainevent.12/3/202316DistributedComputing,M.L.LiuCallbackClient-ServerInteractions12/3/202317DistributedComputing,M.L.LiuCallbackapplicationfiles12/3/202318DistributedComputing,M.L.LiuRMICallbackfileplacements12/3/202319DistributedComputing,M.L.LiuTheHelloApplicationwithCallback

12/3/202320DistributedComputing,M.L.LiuRMICallbackInterfaceTheserverprovidesaremotemethodwhichallowsaclienttoregisteritselfforcallbacks.ARemoteinterfaceforthecallbackisneeded,inadditiontotheserver-sideinterface.Theinterfacespecifiesamethodforacceptingacallbackfromtheserver.TheclientprogramisasubclassofRemoteObjectandimplementsthecallbackinterface,includingthecallbackmethod.Theclientregistersitselfforcallbackinitsmainmethod.Theserverinvokestheclient’sremotemethodupontheoccurrenceoftheanticipatedevent.12/3/202321DistributedComputing,M.L.LiuRemoteInterfaceforServerpublicinterfaceHelloInterfaceextendsRemote{//remotemethodpublicStringsayHello()throwsjava.rmi.RemoteException;

//methodtobeinvokedbyaclienttoadditselftothecallbacklistpublicvoidaddCallback(HelloCallbackInterfaceCallbackObject)throwsjava.rmi.RemoteException;}12/3/202322DistributedComputing,M.L.LiuRemoteInterfaceforCallbackClient//aninterfacespecifyingacallbackmethodpublicinterfaceHelloCallbackInterfaceextendsjava.rmi.Remote{

//methodtobecalledbytheserveroncallbackpublicvoidcallMe(Stringmessage)throwsjava.rmi.RemoteException;}12/3/202323DistributedComputing,M.L.LiuHelloServer,withcallbackpublicclassHelloServerextendsUnicastRemoteObjectimplementsHelloInterface{staticintRMIPort;

//vectorforstorelistofcallbackobjectsprivatestaticVectorcallbackObjects;publicHelloServer()throwsRemoteException{super();

//instantiateaVectorobjectforstoringcallbackobjectscallbackObjects=newVector();}

//methodforclienttocalltoadditselftoitscallbackpublicvoidaddCallback(HelloCallbackInterfaceCallbackObject){//storethecallbackobjectintothevectorSystem.out.println("Servergotan'addCallback'call.");callbackObjects.addElement(CallbackObject);}

12/3/202324DistributedComputing,M.L.LiuHelloServer,withcallback-2publicstaticvoidmain(Stringargs[]){…registry=LocateRegistry.createRegistry(RMIPort);…

callback();…}//endmainprivatestaticvoidcallback(){…

for(inti=0;i<callbackObjects.size();i++){System.out.println("Nowperformingthe"+i+"thcallback\n");//convertthevectorobjecttoacallbackobject

HelloCallbackInterfaceclient=(HelloCallbackInterface)callbackObjects.elementAt(i);…client.callMe("Servercallingbacktoclient"+i);…12/3/202325DistributedComputing,M.L.LiuAlogorithmforbuildinganRMICallbackApplicationServerside:Openadirectoryforallthefilestobegeneratedforthisapplication.Specifytheremote-serverinterface,andcompileittogeneratetheinterfaceclassfile.Buildtheremoteserverclassbyimplementingtheinterface,andcompileitusingjavac.Usermictoprocesstheserverclasstogenerateastub.classfileandaskelton.classfile:rmicSomeServerIfstubdownloadingisdesired,copythestubfiletoanappropriatedirectoryontheHTTPhost.ActivatetheRMIRegistry,ifithasnotalreadybeenactivated.Setupajava.policyfile.Activatetheserver,specifying(i)thecodebaseifstubdownloadingisdesired,(ii)theserverhostname,and(iii)thesecuritypolicyfile.ObtaintheCallbackInterface.Compileitwithjavac,thenusermictogeneratethestubfileforthecallback.

12/3/202326DistributedComputing,M.L.LiuAlogorithmforbuildinganRMICallbackApplicationClientside:Openadirectoryforallthefilestobegeneratedforthisapplication.Implementtheclientprogramorapplet,andcompileittogeneratetheclientclass.Ifstubdownloadingisnotineffect,copytheserverinterfacestubclassfilebyhand.Implementthecallbackinterface.Compileitusingjavac,thenusingrmictogenerateastubclassandaskeletonclassforit.Setupajava.policyfile.Activatetheclient,specifying(i)theserverhostname,and(ii)thesecuritypolicyfile.

12/3/202327DistributedComputing,M.L.LiuHelloClient,withcallback

HelloClient(){//constructorSystem.setSecurityManager(newRMISecurityManager());//exportthisobjectasaremoteobjectUnicastRemoteObject.exportObject(this);//…Registryregistry=LocateRegistry.getRegistry("localhost",RMIPort);h=(HelloInterface)registry.lookup("helloLiu");h.addCallback(this);//…}//endconstructor

//callbackmethod-thisdisplaysthemessagesentbytheserverpublicvoidcallMe(Stringmessage){System.out.println("Callbackreceived:"+message);}publicstaticvoidmain(Stringargs[]){//…

HelloClientclient=newHelloClient();//…

while(true){;}//endwhile}//endmain}//endHelloClientclass12/3/202328DistributedComputing,M.L.LiuHelloServer,HelloClient12/3/202329DistributedComputing,M.L.LiuSummary-1Clientcallback:Clientcallbackisusefulforanapplicationwheretheclientsdesiretobenotifiedbytheserveroftheoccurrenceofsomeevent.Clientcallbackallowsanobjectservertomakeremotemethodcalltoaclientviaareferencetoaclientremoteinterface.12/3/202330DistributedComputing,M.L.LiuSummary-2Clientcallback:Toprovideclientcallback,theclient-sidesoftwaresuppliesaremoteinte