IDL Models

1. IDL Models • The Inheritance Model: Using the Inheritance Model, you implement the IDL interface using an implementation class that also extends the compiler-generated skeleton. • The OMG-standard, POA. Given an interface My defined in My.idl, the file MyPOA.java is generated by the idlj compiler. You must provide the implementation for My and it must inherit from MyPOA, a stream-based skeleton that extends org.omg.PortableServer.Servant, which serves as the base class for all POA servant implementations.

2. The Delegation ModelUsing the Delegation Model, you implement the IDL interface using two classes: • An IDL-generated Tie class that inherits from the compiler-generated skeleton, but delegates all calls to an implementation class. • A class that implements the IDL-generated operations interface (such as HelloOperations), which defines the IDL function. • The Delegation model is also known as the Tie model, or the Tie Delegation model. It inherits from either the POA or ImplBase compiler-generated skeleton, so the models will be described as POA/Tie or ImplBase/Tie models in this document.

3. Portable Object Adaptor • An object adapter is the mechanism that connects a request using an object reference with the proper code to service that request. The Portable Object Adapter, or POA, is a particular type of object adapter that is defined by the CORBA specification.

4. POA • Allow programmers to construct object implementations that are portable between different ORB products. • Provide support for objects with persistent identities. • Provide support for transparent activation of objects. • Allow a single servant to support multiple object identities simultaneously.

5. Create and Use a POA • Get the root POA • The root POA is managed by the ORB and provided to the application using the ORB initialization interface under the initial object name "RootPOA". • Define the POA's Policies • control over the object's identity, state, storage, and life cycle

6. Create the POA • Activate the POAManager • Each POA object has an associated POAManager object that controls the processing state of the POAs with which it is associated • Activate the servants • At any point in time, a CORBA object may or may not be associated with an active servant. • Create the object reference • Explicitly activate a servant and associate it with an object reference. • Server application directly creates a reference. • Server application causes a servant to implicitly activate itself.

7. IDL and Java module HelloApp { interface Hello { string sayHello(); oneway void shutdown(); }; };

8. A CORBA module is a namespace that acts as a container for related interfaces and declarations. It corresponds closely to a Java package. Each module statement in an IDL file is mapped to a Java package statement. • Like Java interfaces, CORBA interfaces declare the API contract an object has with other objects. Each interface statement in the IDL maps to a Java interface statement when mapped. • CORBA operations are the behavior that servers promise to perform on behalf of clients that invoke them. Each operation statement in the IDL generates a corresponding method statement in the generated Java interface.

9. Hello.java is the signature interface and is used as the signature type in method declarations when interfaces of the specified type are used in other interfaces. • the IDL-to-Java mapping puts all of the operations defined on the IDL interface in the operations interface, HelloOperations.java.

10. import HelloApp.*;//stubs import org.omg.CosNaming.*;//Naming Service import org.omg.CosNaming.NamingContextPackage.*;//Naming Exceptions import org.omg.CORBA.*;//CORBA import org.omg.PortableServer.*;//Portable Server Model import org.omg.PortableServer.POA; import java.util.Properties;//To initiate ORB class HelloImpl extends HelloPOA {//Servant inheritance private ORB orb; public void setORB(ORB orb_val) { orb = orb_val; } // implement sayHello() method public String sayHello() { return "\nHello world !!\n"; } // implement shutdown() method public void shutdown() { orb.shutdown(false); } }

11. public class HelloServer { public static void main(String args[]) { try{ // create and initialize the ORB ORB orb = ORB.init(args, null); // get reference to rootpoa & activate the POAManager POA rootpoa = POAHelper.narrow(orb.resolve_initial_references("RootPOA")); rootpoa.the_POAManager().activate(); // create servant and register it with the ORB HelloImpl helloImpl = new HelloImpl(); helloImpl.setORB(orb); // get object reference from the servant org.omg.CORBA.Object ref = rootpoa.servant_to_reference(helloImpl); Hello href = HelloHelper.narrow(ref); // get the root naming context org.omg.CORBA.Object objRef = orb.resolve_initial_references("NameService"); // Use NamingContextExt which is part of the Interoperable // Naming Service (INS) specification. NamingContextExt ncRef = NamingContextExtHelper.narrow(objRef);

12. // bind the Object Reference in Naming String name = "Hello"; NameComponent path[] = ncRef.to_name( name ); ncRef.rebind(path, href); System.out.println("HelloServer ready and waiting ..."); // wait for invocations from clients orb.run(); } catch (Exception e) { System.err.println("ERROR: " + e); e.printStackTrace(System.out); } System.out.println("HelloServer Exiting ..."); } }

13. Persistent Server persistent object An object that can survive the process or thread that created it. A persistent object exists until it is explicitly deleted. PersistentHello.idl module Persistent { interface Hello { string sayHello( ); oneway void shutdown(); }; };

14. import java.util.Properties; import org.omg.CORBA.Object; import org.omg.CORBA.ORB; import org.omg.CosNaming.NameComponent; import org.omg.CosNaming.NamingContextExt; import org.omg.CosNaming.NamingContextExtHelper; import org.omg.CORBA.Policy; import org.omg.PortableServer.POA; import org.omg.PortableServer.*; import org.omg.PortableServer.Servant; public class PersistentServer { public static void main( String args[] ) { Properties properties = System.getProperties(); properties.put( "org.omg.CORBA.ORBInitialHost", "localhost" ); properties.put( "org.omg.CORBA.ORBInitialPort", "1050" ); try { // Step 1: Instantiate the ORB ORB orb = ORB.init(args, properties); // Step 2: Instantiate the servant PersistentHelloServant servant = new PersistentHelloServant(orb);

15. // Step 3 : Create a POA with Persistent Policy // ******************* // Step 3-1: Get the rootPOA POA rootPOA = POAHelper.narrow(orb.resolve_initial_references("RootPOA")); // Step 3-2: Create the Persistent Policy Policy[] persistentPolicy = new Policy[1]; persistentPolicy[0] = rootPOA.create_lifespan_policy( LifespanPolicyValue.PERSISTENT); // Step 3-3: Create a POA by passing the Persistent Policy POA persistentPOA = rootPOA.create_POA("childPOA", null, persistentPolicy ); // Step 3-4: Activate PersistentPOA's POAManager, Without this // All calls to Persistent Server will hang because POAManager // will be in the 'HOLD' state. persistentPOA.the_POAManager().activate( ); // *********************** // Step 4: Associate the servant with PersistentPOA persistentPOA.activate_object( servant );

16. // Step 5: Resolve RootNaming context and bind a name for the // servant. // NOTE: If the Server is persistent in nature then using Persistent // Name Service is a good choice. Even if ORBD is restarted the Name // Bindings will be intact. To use Persistent Name Service use // 'NameService' as the key for resolve_initial_references() when // ORBD is running. org.omg.CORBA.Object obj = orb.resolve_initial_references( "NameService" ); NamingContextExt rootContext = NamingContextExtHelper.narrow( obj ); NameComponent[] nc = rootContext.to_name( "PersistentServerTutorial" ); rootContext.rebind( nc, persistentPOA.servant_to_reference( servant ) ); // Step 6: We are ready to receive client requests orb.run(); } catch ( Exception e ) { System.err.println( "Exception in Persistent Server Startup " + e ); } } }

17. import org.omg.CORBA.ORB; public class PersistentHelloServant extends Persistent.HelloPOA { private ORB orb; public PersistentHelloServant( ORB orb ) { this.orb = orb; } /** * sayHello() method implementation returns a simple message. */ public String sayHello( ) { return "Hello From Persistent Server..."; } /** * shutdown() method shuts down the Persistent Server. * See NOTE below. */ public void shutdown( ) { orb.shutdown( false ); } }

18. import java.util.Properties; import org.omg.CORBA.ORB; import org.omg.CORBA.OBJ_ADAPTER; import org.omg.CosNaming.NamingContext; import org.omg.CosNaming.NamingContextHelper; import org.omg.CosNaming.NameComponent; import org.omg.PortableServer.POA; import Persistent.HelloHelper; import Persistent.Hello; public class PersistentClient { public static void main(String args[]) { try { // Step 1: Instantiate the ORB ORB orb = ORB.init(args, null); // Step 2: Resolve the PersistentHelloServant by using INS's // corbaname url. The URL locates the NameService running on // localhost and listening on 1050 and resolve // 'PersistentServerTutorial' from that NameService org.omg.CORBA.Object obj = orb.string_to_object( "corbaname::localhost:1050#PersistentServerTutorial"); Hello hello = HelloHelper.narrow( obj );

19. // Step 3: Call the sayHello() method every 60 seconds and shutdown // the server. Next call from the client will restart the server, // because it is persistent in nature. while( true ) { System.out.println( "Calling Persistent Server.." ); String helloFromServer = hello.sayHello(); System.out.println("Message From Persistent Server: " + helloFromServer ); System.out.println( "Shutting down Persistent Server.." ); hello.shutdown( ); Thread.sleep( 60000 ); } } catch ( Exception e ) { System.err.println( "Exception in PersistentClient.java..." + e ); e.printStackTrace( ); } } }