CORBA/IDL

• Common Object Resource Broker Architecture (CORBA)

• Interface Definition Language (IDL)

• Object Management Group (OMG) (http://www.omg.org) Specification

• Java IDL (jdk1.2) use CORBA objects with Java Programming Language

Introduction• Technology for Distributed Objects

• Similar in function to Java RMI

• Not Java-centric

• Any language with IDL specification

• IDL is a language-neutral interface definition language

• Object Requst Broker (ORB) enables low-level communication between CORBA Objects

CORBA Architecture

CORBA Architecture

• Client and Server Relationships common to CORBA and RMI

• Server provides remote interface

• Client calls remote interface

• Object level interaction rather than application level interaction (sockets)

• Objects can fulfill both roles

Client Side

• Client has reference to remote object

• Object reference has a stub method

• Stand-in for remote method

• Stub wired into ORB

• Call on stub invokes ORB’s low-level communication routines

• ORB forwards invocation to server

Server Side

• ORB on Server Side uses Skeleton Code to translate remote invocation into call on the local object

• Skeleton transforms results or errors and returns to ORB for delivery to client

• ORB-ORB communication with IIOP (Internet Inter-ORB Protocol)

ORB

• Different Vendors

• IIOP based on TCP/IP by OMG

• ORB Services– Look up (JDK1.2)– Object Persistence– Transactions– Messaging

Java IDL Development

• Define interface to Remote Object with IDL

• idltojava compiler generates stub and skeleton source, and code to interface with ORB

• IDL interface can be implemented in any CORBA compliant language (C, C++, Smalltalk, COBOL, Ada)

• Compile remote interface (idltojava)

• Implement the server. It should start ORB and wait on invocations from clients, as well as implement remote methods.

• Implement client. Start ORB, look up server, obtain remote reference, and call remote method

• Start applications.

Example• Hello World has single remote method that

returns a string to be displayed.

• Client invokes sayHello on Hello server

• ORB transfers invocation to servant object registered for interface

• Servant’s sayHello runs, returns String• ORB transfers String back to client

• Client Displays String

Hello.idl

module HelloApp{ interface Hello { string sayHello(); };};

• CORBA module is name space that is a container for related interfaces and definitions.

• Like a Java Package

• module statement mapped to java package statement

• interface specifies contract object has with other objects

• interface to interface in java

• CORBA operations are behaviors that server promises to do on client’s behalf

• operation to method in java

• idltojava Hello.idl creates a HelloApp directory, and five files.

• _HelloImplBase.java: Abstract class is the server skeloton. It implements Hello.java. Server class will extend this class.

• _HelloStub.java: Client stub. Implements Hello.java

• Hello.java: Java version of IDL interface. Subclasses org.omg.CORBA.Object to provide base CORBA functionality.

• HelloHelper.java: Provides other functionality. narrow method cast CORBA object reference to proper type.

• HelloHolder.java: final class provides out and inout arguments (more later).

/* * File: ./HelloApp/Hello.java * From: Hello.idl * Date: Thu Sep 3 09:46:22 1998 * By: idltojava Java IDL 1.2 Nov 12 1997 12:23:47 */

package HelloApp;public interface Hello extends org.omg.CORBA.Object { String sayHello();}

/* * File: ./HelloApp/_HelloImplBase.java * From: Hello.idl * Date: Thu Sep 3 09:46:22 1998 * By: idltojava Java IDL 1.2 Nov 12 1997 12:23:47 */

package HelloApp;public abstract class _HelloImplBase extends org.omg.CORBA.DynamicImplementationimplements HelloApp.Hello { // Constructor public _HelloImplBase() { super(); } // Type strings for this class and its superclases private static final String _type_ids[] = { "IDL:HelloApp/Hello:1.0" };

public String[] _ids() { return (String[]) _type_ids.clone(); }

private static java.util.Dictionary _methods = new java.util.Hashtable(); static { _methods.put("sayHello", new java.lang.Integer(0)); } // DSI Dispatch call public void invoke(org.omg.CORBA.ServerRequest r) { switch (((java.lang.Integer) _methods.get(r.op_name())).intValue()) { case 0: // HelloApp.Hello.sayHello { org.omg.CORBA.NVList _list = _orb().create_list(0); r.params(_list); String ___result; ___result = this.sayHello(); org.omg.CORBA.Any __result = _orb().create_any(); __result.insert_string(___result); r.result(__result); } break; default: throw new org.omg.CORBA.BAD_OPERATION(0,org.omg.CORBA.CompletionStatus.COMPLETED_MAYBE); } }}

/* * File: ./HelloApp/_HelloStub.java * From: Hello.idl * Date: Thu Sep 3 09:46:22 1998 * By: idltojava Java IDL 1.2 Nov 12 1997 12:23:47 */

package HelloApp;public class _HelloStub

extends org.omg.CORBA.portable.ObjectImpl implements HelloApp.Hello {

public _HelloStub(org.omg.CORBA.portable.Delegate d) { super(); _set_delegate(d); }

private static final String _type_ids[] = { "IDL:HelloApp/Hello:1.0" };

public String[] _ids() { return (String[]) _type_ids.clone(); }

// IDL operations // Implementation of ::HelloApp::Hello::sayHello public String sayHello() { org.omg.CORBA.Request r = _request("sayHello");

r.set_return_type(org.omg.CORBA.ORB.init().get_primitive_tc(org.omg.CORBA.TCKind.tk_string)); r.invoke(); String __result; __result = r.return_value().extract_string(); return __result; }

};

/* * File: ./HelloApp/HelloHelper.java * From: Hello.idl * Date: Thu Sep 3 09:46:22 1998 * By: idltojava Java IDL 1.2 Nov 12 1997 12:23:47 */

package HelloApp;public class HelloHelper { // It is useless to have instances of this class private HelloHelper() { }

public static void write(org.omg.CORBA.portable.OutputStream out,HelloApp.Hello that) { out.write_Object(that); } public static HelloApp.Hello read(org.omg.CORBA.portable.InputStream in) { return HelloApp.HelloHelper.narrow(in.read_Object()); } public static HelloApp.Hello extract(org.omg.CORBA.Any a) { org.omg.CORBA.portable.InputStream in = a.create_input_stream(); return read(in); } public static void insert(org.omg.CORBA.Any a, HelloApp.Hello that) { org.omg.CORBA.portable.OutputStream out = a.create_output_stream(); write(out, that); a.read_value(out.create_input_stream(), type()); } private static org.omg.CORBA.TypeCode _tc; synchronized public static org.omg.CORBA.TypeCode type() { if (_tc == null) _tc = org.omg.CORBA.ORB.init().create_interface_tc(id(), "Hello"); return _tc; } public static String id() { return "IDL:HelloApp/Hello:1.0"; } public static HelloApp.Hello narrow(org.omg.CORBA.Object that)

throws org.omg.CORBA.BAD_PARAM { if (that == null) return null; if (that instanceof HelloApp.Hello) return (HelloApp.Hello) that;

if (!that._is_a(id())) { throw new org.omg.CORBA.BAD_PARAM();}

org.omg.CORBA.portable.Delegate dup =((org.omg.CORBA.portable.ObjectImpl)that)._get_delegate(); HelloApp.Hello result = new HelloApp._HelloStub(dup); return result; }}

/* * File: ./HelloApp/HelloHolder.java * From: Hello.idl * Date: Thu Sep 3 09:46:22 1998 * By: idltojava Java IDL 1.2 Nov 12 1997 12:23:47 */

package HelloApp;public final class HelloHolder implements org.omg.CORBA.portable.Streamable{ // instance variable public HelloApp.Hello value; // constructors public HelloHolder() {

this(null); } public HelloHolder(HelloApp.Hello __arg) {

value = __arg; }

public void _write(org.omg.CORBA.portable.OutputStream out) { HelloApp.HelloHelper.write(out, value); }

public void _read(org.omg.CORBA.portable.InputStream in) { value = HelloApp.HelloHelper.read(in); }

public org.omg.CORBA.TypeCode _type() { return HelloApp.HelloHelper.type(); }}

Client

• Needs ORB object to do marshaling and IIOP

• Instantiate ORB object and pass it information

• Asks ORB for service it needs

• Gets object reference from COS naming service provided with Java IDL

• Get reference to naming service

• All CORBA object references returned as generic CORBA object

• Must narrow it to its proper type

• NameComponent for complex names, including fully qualified paths

• resolve naming context

• narrow to proper type

• invoke method

import HelloApp.*; // The package containing our stubs.import org.omg.CosNaming.*; // HelloClient will use the naming service.import org.omg.CORBA.*; // All CORBA applications need these classes.

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

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

// Get the root naming context org.omg.CORBA.Object objRef =orb.resolve_initial_references("NameService"); NamingContext ncRef = NamingContextHelper.narrow(objRef);

// Resolve the object reference in naming NameComponent nc = new NameComponent("Hello", ""); NameComponent path[] = {nc}; Hello helloRef = HelloHelper.narrow(ncRef.resolve(path));

// Call the Hello server object and print results String Hello = helloRef.sayHello(); System.out.println(Hello);

} catch(Exception e) { System.out.println("ERROR : " + e); e.printStackTrace(System.out); } }}

Server

• Servant object implements the interface

• Create ORB

• COS Naming: get instance and narrow

• Register servant with name server

• WAIT

// The package containing our stubs.import HelloApp.*;

// HelloServer will use the naming service.import org.omg.CosNaming.*;

// The package containing special exceptions thrown by the name service.import org.omg.CosNaming.NamingContextPackage.*;// All CORBA applications need these classes.import org.omg.CORBA.*;public class HelloServer{ public static void main(String args[]) { try{ // Create and initialize the ORB ORB orb = ORB.init(args, null); // Create the servant and register it with the ORB HelloServant helloRef = new HelloServant(); orb.connect(helloRef); // Get the root naming context org.omg.CORBA.Object objRef =orb.resolve_initial_references("NameService"); NamingContext ncRef = NamingContextHelper.narrow(objRef); // Bind the object reference in naming NameComponent nc = new NameComponent("Hello", ""); NameComponent path[] = {nc}; ncRef.rebind(path, helloRef); // Wait for invocations from clients java.lang.Object sync = new java.lang.Object(); synchronized(sync){ sync.wait(); } } catch(Exception e) { System.err.println("ERROR: " + e); e.printStackTrace(System.out); } }}class HelloServant extends _HelloImplBase{ public String sayHello() { return "\nHello world!!\n";

}}

Running the Example

• Compile the files generated by idltojava• Compile client and server

• start name service: tnameserv -ORBInitialPort 1050

• start client and serverjava HelloServer -ORBInitialPort 1050java HelloClient -ORBInitialPort 1050