Remote Method Invocation with Java-RMI

Client

Stub- Marshalling

Skeleton- Demarshalling- Dispatch

ServerServerI Network

Automatically generated

Contains only functionality

What does the client see?

• A remote object (proxy, stub) can– be casted to all interface the server implements, – be accessed only via interface methods.

• On a remote invocation– non-remote arguments are copied (using serialization),– remote arguments are given as reference,

different to local invocation– java.rmi.RemoteException can always be thrown.

• Remote Objects always support the interface Remote. (Tag interface)

How to build the server?

Remote RemoteObject

RemoteServer

UnicastRemoteObject

ServerImplServerI

- every method which is remotely invocable must be declared to throw RemoteException.- Skeletons/Stubs are created with the JDK tool rmic.

Provided by the JDK

Class diagram:

Activatable

Provided by the programer

Coordination with a naming service

• The naming service (rmiregistry)– is a remote Java object, – manages relationships between Strings (i.e. names) and remote

Java objects (on the same host).

• remote Java objects register themselves with java.rmi.Naming.bind(String name,Remote obj); java.rmi.Naming.rebind(String name,Remote obj);

• the name has the form: “//host:port/identifier“(default: host=localhost, port=1099)

• clients use the name to receive a stub:Remote Naming.lookup(String name);

Examplary Server

Server interface:interface ServerI extends Remote{ public void doSomeService() throws RemoteException;}

Server implementation:class Server extends UnicastRemoteObject implements ServerI{ public Server() throws RemoteException { super();}

public void doSomeService() throws RemoteException { System.out.println("Working hard..");} ....

Initialisation+Registration

... public static void main(String args[]) {try { // create instance

Server myServer = new Server(); // bind it to the naming service Naming.rebind(“//elfe:4711/HardServer",

myServer); // done} catch(Exception ex) { ex.printStackTrace();

System.exit(5); }

}}

Remote Invocation of the Server

public class Client { static public void main(String args[]) { try { // locate Server

Remote rs = Naming.lookup(“//elfe:4711/HardServer"); ServerI server = (ServerI) rs; // use service server.doSomeService(); // done} catch(Exception ex) { ex.printStackTrace(); System.exit(5); }

} }

Compilation, Configuration, Startup

Compile classes

jedi> javac Server.java ServerI.java Client.java

Create Skeletons and Stubs

jedi> rmic Server

Start naming service on elfe

elfe> rmiregistry 4711 &

Start server on elfe

elfe> java Server &

Start client somewhere

jedi> java Client

Working hard.. on elfe

Installation of JacORB

• Source code with examples and documentation:http://www.inf.fu-berlin.de/~brose/jacorb

• local installation in the Sun pool:setenv JACO /import/public/opt/JacORB1_1setenv PATH $JACO/bin:$PATHsetenv CLASSPATH $JACO/lib/idl.jar:$JACO/lib/jacorb.jar:$CLASSPATHcp $JACO/jacorb_properties.template ~/.jacorb_propertiesedit ~/.jacorb_properties(an install script is on the WWW)

• adapt the URL settings in the property file; at least this entry:

jacorb.NameServerURL=file:/....

ServerServer

Overview about CORBA

ClientClient

Stub

ORB

Servant

POA

Servant

Servant

Skeleton

Impl.IDL

Interface

POA

Class Hierarchie for CORBA Servers for the Interface Foo

FooImpl

FooPOA

<<interface>>FooOperations

<<interface>>Foo

Servant

Servants are no CORBA objectsServants are no CORBA objects

FooPOATie

FooDelegateImpl

Delegate allows use of inheritence:new FooPOATie(new FooDelegateImpl())Delegate allows use of inheritence:new FooPOATie(new FooDelegateImpl())

Delegates to

The CORBA COS Naming Service

• A naming context – stores relationships between names and Objects,– is itself a CORBA object (allows hierarchical, potentially cyclic

name spaces).

• name = sequence<name_components>,name_component=(string id, string kind)

• ORB provides bootstrapping with the root context NameServer(ORB uses jacorb.NameServerURL; it points to an IOR)

• bind an object in a subcontextNamingContext.bind(Name n, Object o);

• resolve an object Object Object NamingContext.resolve(Name n);

• create a new sub contextNamingContext NamingContext.new_context();NamingContext.bind_new_context(Name n);

• bug in JDK use NamingContextExt !

Examplary Server now in CORBA

Server IDL interface:interface Server { void doSomeService();};

Server implementation:class ServerImpl extends ServerPOA{ public void doSomeService() { System.out.println("Working hard..");} // main method follows

Initialisation+Registrationstatic public void main(String args[]) { try { // access CORBA, activate POA ORB orb = org.omg.CORBA.ORB.init(args, null); POA poa = POAHelper.narrow(orb.resolve_initial_references("RootPOA")); poa.the_POAManager().activate(); // from now on requests are handled !!! // get name server NamingContextExt nc = NamingContextExtHelper.narrow(orb.resolve_initial_references("NameService")); // generate a server and register it at the POA org.omg.CORBA.Object cob = poa.servant_to_reference(new ServerImpl()); // generate a name NameComponent [] name = new NameComponent[1]; name[0] = new NameComponent( "HardServer", "some kind"); // bind server to name nc.bind(name, cob); // done System.out.println("Server is ready."); orb.run(); } catch(Exception ex){ex.printStackTrace();}} } // main, class

Remote Invocation of the Server

class Client{ static void main(String args[]) { try{ // get ORB and naming service ORB orb = org.omg.CORBA.ORB.init(args, null); NamingContextExt nc = NamingContextExtHelper.narrow(orb.resolve_initial_references("NameService")); // generate the name NameComponent [] name = new NameComponent[1]; name[0] = new NameComponent( "HardServer", "some kind"); // query naming service, cast the stub org.omg.CORBA.Object cob = nc.resolve(name); Server server = ServerHelper.narrow(cob); // use the generated Helper class // use service server.doSomeService(); } catch(Exception ex){ ex.printStackTrace(); }} }

Compile the Example

• Generate skeleton,stubs, helper classes from the IDL> idl server.idl– Skeletons: ServerPOA.java,ServerPOATie.java

– Stub: _ServerStub.java– Helpers: ServerHelper.java, ServerHolder.java

– Interfaces: Server.java, ServerOperations.java

• compile the classes> javac *.java(take care of your CLASSPATH!)

Startup your Scenario

• Start the naming service> ns NSIORNSIOR should be: file(jacorb.NameServerURL)E.g.: file:/home/tnfink/ns.ior /home/tnfink/ns.ior

• start the serverjedi> jaco ServerImplServer is ready.

• Start the clienttroll> jaco ClientWorking hard. (auf jedi)

Miscellaneous Methods

• Convert CORBA object references to Java Strings:String ORB.object_to_string(org.omg.CORBA.Object obj);org.omg.CORBA.Object ORB.string_to_object(String ior);

• test for existance of an CORBA object (i.e. stub)boolean Object._non_existent();

• test if two CORBA objects are equalboolean Object._is_equivalent(Object other);a return of false does not guarantee inequality!

• Duplicate a CORBA object (reference)Object Object.duplicate();

Miscellaneous Tools, Parameters

• naming service GUI>nmg

• analysis of IORs> dior -f ns.ior------IOR components-----TypeId : IDL:omg.org/CosNaming/NamingContextExt:1.0Profile Id : TAG_INTERNET_IOPIIOP Version : 1.1Host : 160.45.110.127Port : 1281...

• Parameter for debug output (0=none, 1=important, ... 10)jacorb.verbosity = 0

Java RMI CORBA

• Java RMI– is easy to learn and use,– does not support heterogeneity, scalability, reliability

• CORBA – needs effort for learning– is a sophisticated industrial tool for the real world.