分布式计算原理与应用(DistributedComputing)第八章课件

AdvancedRemoteMethodInvocationsM.L.Liu7/20/20231DistributedComputing,M.L.LiuRMI–AdvancedtopicsTheJavaRMIAPIhasarichcollectionoffeatures.WewilllookatsomeofRMI’smoreinterestingadvancedfeatures,namely:

stubdownloadingsecuritymanagerclientcallback.Althoughthesefeaturesarenotinherenttothedistributedobjectparadigm,theyarehelpfulmechanismsandcanbeusefultoapplicationdevelopers.7/20/20232DistributedComputing,M.L.LiuTheJavaRMIArchitecture7/20/20233DistributedComputing,M.L.LiuJavaRMIClientServerInteraction7/20/20234DistributedComputing,M.L.LiuRMIStubDownloadingRMIisdesignedtoallowstubstobemadeavailabletotheclientdynamically.Doingsoallowschangestobemadeintheremotemethodswithoutaffectingtheclientprogram.ThestubcanbefiledwithanwebserverandbedownloadedusingHTTP.Securitymeasuresareneededtopreventboththeclientsideandtheserverside:Ajavasecuritypolicytobesetontheserverhostandalsoontheclienthost.AJavaSecurityManagershouldbeinstantiatedinboththeclientandserverprograms.7/20/20235DistributedComputing,M.L.LiuStubdownloadingIfthestubistobedownloadedfromaremoteserver,transferthestubclasstotheappropriatedirectoryonthatserver,e.g.,,andmakesurethattheaccesspermissiontotheset.Whenactivatingtheserver,specifycommandoptions:

7/20/20236DistributedComputing,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";};

Thisbefiledinthesamedirectoryastheserverclassfile.Whenactiviatingtheclient,ajava.policyshouldbespecified:java-Djava.security.policy=java.policySomeClientSeeand7/20/20237DistributedComputing,M.L.Liu7/20/20238DistributedComputing,M.L.LiuRMISecurityManagerSinceRMIinvolvesaccessto/fromaforeignhost,andpossiblyobjectdownloading,itisimportantforboththeserverandtheclienttoprotectitssystemfrommaliciousaccess.TheRMISecurityManagerisaclassprovidedJava,andcanbeinstantiatedinboththeclientandtheserverforlimitingaccesspriviledges.Youcanwriteyourownsecuritymanager,ifsodesired.

try{System.setSecurityManager(newRMISecurityManager());}catch{…}7/20/20239DistributedComputing,M.L.LiuAlgorithmforbuildinganRMIApplicationServerside:Openadirectoryforallthefilestobegeneratedforthisapplication.Specifytheremote-serverinterface,andcompileittogeneratetheinterfaceclassfile.Buildtheremoteserverclassbyimplementingtheinterface,andcompileitusingjavac.Usermictoprocesstheserverclasstogenerateastub.classaskelton.classfile:

rmicSomeServerIfstubdownloadingisdesired,copythestubanappropriatedirectoryontheHTTPhost.ActivatetheRMIRegistry,ifithasnotalreadybeenactivated.Setupajava.policyfile.Activatetheserver,specifying(i)thecodebaseifstubdownloadingisdesired,(ii)theserverhostname,and(iii)thesecuritypolicyfile.

7/20/202310DistributedComputing,M.L.LiuAlogorithmforbuildinganRMIApplicationClientside:Openadirectoryforallthefilestobegeneratedforthisapplication.Implementtheclientprogramorapplet,andcompileittogeneratetheclientclass.Ifstubdownloadingisnotineffect,copytheserverinterfacestubclasshand.Setupajava.policyfile.Activatetheclient,specifying(i)theserverhostname,and(ii)thesecuritypolicyfile.

7/20/202311DistributedComputing,M.L.LiuRMICallbacks7/20/202312DistributedComputing,M.L.LiuIntroductionIntheclientservermodel,theserverispassive:theIPCisinitiatedbytheclient;theserverwaitsforthearrivalofrequestsandprovidesresponses.Someapplicationsrequiretheservertoinitiatecommunicationuponcertainevents.Examplesapplicationsare:monitoringgamesauctioningvoting/pollingchat-toommessage/bulletinboardgroupware7/20/202313DistributedComputing,M.L.LiuPollingvs.CallbackIntheabsenceofcallback,aclientwillhavetopollapassiveserverrepeatedlyifitneedstobenotifiedthataneventhasoccurredattheserverend.7/20/202314DistributedComputing,M.L.LiuTwo-waycommunicationsSomeapplicationsrequirethatbothsidesmayinitiateIPC.Usingsockets,duplexcommunicationcanbeachievedbyusingtwosocketsoneitherside.Withconnection-orientedsockets,eachsideactsasbothaclientandaserver.7/20/202315DistributedComputing,M.L.LiuRMICallbacksAcallbackclientregistersitselfwithanRMIserver.Theservermakesacallbacktoeachregisteredclientupontheoccurrenceofacertainevent.7/20/202316DistributedComputing,M.L.LiuCallbackClient-ServerInteractions7/20/202317DistributedComputing,M.L.LiuCallbackapplicationfiles7/20/202318DistributedComputing,M.L.LiuRMICallback7/20/202319DistributedComputing,M.L.LiuTheHelloApplicationwithCallback

7/20/202320DistributedComputing,M.L.LiuRMICallbackInterfaceTheserverprovidesaremotemethodwhichallowsaclienttoregisteritselfforcallbacks.ARemoteinterfaceforthecallbackisneeded,inadditiontotheserver-sideinterface.Theinterfacespecifiesamethodforacceptingacallbackfromtheserver.TheclientprogramisasubclassofRemoteObjectandimplementsthecallbackinterface,includingthecallbackmethod.Theclientregistersitselfforcallbackinitsmainmethod.Theserverinvokestheclient’sremotemethodupontheoccurrenceoftheanticipatedevent.7/20/202321DistributedComputing,M.L.LiuRemoteInterfaceforServerpublicinterfaceHelloInterfaceextendsRemote{//remotemethodpublicStringsayHello()throwsjava.rmi.RemoteException;

//methodtobeinvokedbyaclienttoadditselftothecallbacklistpublicvoidaddCallback(HelloCallbackInterfaceCallbackObject)throwsjava.rmi.RemoteException;}7/20/202322DistributedComputing,M.L.LiuRemoteInterfaceforCallbackClient//aninterfacespecifyingacallbackmethodpublicinterfaceHelloCallbackInterfaceextendsjava.rmi.Remote{

//methodtobecalledbytheserveroncallbackpublicvoidcallMe(Stringmessage)throwsjava.rmi.RemoteException;}7/20/202323DistributedComputing,M.L.LiuHelloServer,withcallbackpublicclassHelloServerextendsUnicastRemoteObjectimplementsHelloInterface{staticintRMIPort;

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

//instantiateaVectorobjectforstoringcallbackobjectscallbackObjects=newVector();}

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

7/20/202324DistributedComputing,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);…7/20/202325DistributedComputing,M.L.LiuAlogorithmforbuildinganRMICallbackApplicationServerside:Openadirectoryforallthefilestobegeneratedforthisapplication.Specifytheremote-serverinterface,andcompileittogeneratetheinterfaceclassfile.Buildtheremoteserverclassbyimplementingtheinterface,andcompileitusingjavac.Usermictoprocesstheserverclasstogenerateastub.classaskelton.classfile:

rmicSomeServerIfstubdownloadingisdesired,copythestubanappropriatedirectoryontheHTTPhost.ActivatetheRMIRegistry,ifithasnotalreadybeenactivated.Setupajava.policyfile.Activatetheserver,specifying(i)thecodebaseifstubdownloadingisdesired,(ii)theserverhostname,and(iii)thesecuritypolicyfile.ObtaintheCallbackInterface.Compileitwithjavac,thenusermic

togeneratethestubthecallback.

7/20/202326DistributedComputing,M.L.LiuAlogorithmforbuildinganRMICallbackApplicationClientside:Openadirectoryforallthefilestobegeneratedforthisapplication.Implementtheclientprogramorapplet,andcompileittogeneratetheclientclass.Ifstubdownloadingisnotineffect,copytheserverinterfacestubclasshand.Implementthecallbackinterface.Compileitusing

javac,thenusing

rmic

togenerateastubclassandaskeletonclassforit.Setupajava.policyfile.Activatetheclient,specifying(i)theserverhostname,and(ii)thesecuritypolicyfile.

7/20/202327DistributedComputing,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}//endHelloClientclass7/20/202328DistributedComputing,M.L.LiuHelloServer,HelloClient7/20/202329DistributedComputing,M.L.LiuSummary-1Clientcallback:Clientcallbackisusefulforanapplicationwheretheclientsdesiretobenotifiedbytheserverof