Assignment 5 Discussion: 4. July 2007

Transcription

1 Assignment 5 Discussion: 4. July 2007 Exercise 5.1: IDL A basic concept of CORBA is the separation between interface and implementation. An interface is meant to describe an object s functionality, i.e. everything that is needed to develop a client that makes use of the remote service. A special language the interface definition language (IDL) has been defined to syntactically describe an object s interface. You are asked to implement a CORBA-based database server with the following properties: Clients are able to read, write, and request the number of currently stored items. A write operation needs two parameters: an identification of the item to be (over-)written and the new value for this item. Analogously, a read operation needs two parameters: the identification of the item to be read and a variable to store the item s value. The return value of the read operation is an error code indicating whether the operation was processed successfully or not. The number of currently stored data items can be read by clients but it shall be impossible to manipulate it from outside of the database. a) Give an IDL definition of an interface that describes this functionality. b) How can the interface definition of part a) be mapped to java? 1

2 a) A possible description of this interface in IDL: typedef long Item; typedef double Value; enum ErrorCode {OK, NoDataItem, UnknownError; interface DB { void write( in Item it, in Value val ); ErrorCode read( in Item it, out Value val ); readonly attribute long itemcount; ;

3 b) A possible mapping to java: typedef long Item; typedef double Value; enum ErrorCode {OK, NoDataItem, UnknownError; interface DB { void write( in Item it, in Value val ); ErrorCode read( in Item it, out Value val ); readonly attribute long itemcount; ; public interface DB { void write (int it, double val ); ErrorCode read (int it, DoubleHolder val ); int itemcount (); public final class DoubleHolder { public double value = 0.0; public DoubleHolder () { public DoubleHolder (double initialvalue ) { value = initialvalue ; public class ErrorCode { private static int size = 3; private static ErrorCode [] array = new ErrorCode [ size ]; public static final int _OK = 0; public static final ErrorCode OK = new ErrorCode (_OK ); public static final int _NoDataItem = 1; public static final ErrorCode NoDataItem = new ErrorCode ( _NoDataItem ); public static final int _UnknownError = 2; public static final ErrorCode UnknownError = new ErrorCode ( _UnknownError ); private int value; protected ErrorCode (int value) { value = value; array[ value] = this; public int value () { return value;

4 Exercise 5.2: CORBA The goal of this exercise is to port the time server of exercise 1.3 to CORBA: The time server provides a method gettime() that returns the server s local time as a string object. Additionally, the server provides the method shutdown() that internally calls ORB.shutdown(false) and enables a client to shut down the time service. Implement the time server and a corresponding client. For this J2SE 5.0 from Sun can be used, which is available for free and well documented: 2

5 Define the interface of the service using IDL * Timeservice.idl interface Timeservice { string gettime (); oneway void shutdown (); ;

6 Generate proxies and helper classes idlj -fall Timeservice.idl generates the following files: TimeservicePOA.java Server skeleton (basic CORBA functionality for the server) TimeserviceStub.java Client stub (CORBA functionality for the client) TimeserviceOperations.java IDL-to-Java mapping (shared by both the stubs and skeletons) * TimeserviceOperations.java public interface TimeserviceOperations { String gettime (); void shutdown (); Timeservice.java Interface that merges TimeserviceOperations and some CORBA interfaces (used as an object reference for a CORBA Timeservice object) * Timeservice.java public interface Timeservice extends TimeserviceOperations, org.omg.corba.object, org.omg.corba.portable.idlentity { TimeserviceHelper.java Auxiliary functionality (e.g. narrow() to cast CORBA object references) TimeserviceHolder.java Holder class (see previous exercise)

7 Define the servant class, which implement the service * MyTimeservice.java import org.omg.corba.*; import java. text. DateFormat ; public class MyTimeservice extends TimeservicePOA { private ORB orb; public void setorb( ORB orb_val ) { orb = orb_val; public String gettime () { DateFormat myformater = DateFormat. getdatetimeinstance (); String mytime = myformater. format( new java. util. Date () ); return mytime; public void shutdown () { orb. shutdown( false );

8 The server * MyServer.java import org. omg. CosNaming.*; import org. omg. CosNaming. NamingContextPackage.*; import org.omg.corba.*; import org. omg. PortableServer.*; import org.omg. PortableServer.POA; public class MyServer { public static void main( String args [] ) { try { // create and initialize an ORB object ORB orb = ORB. init( args, null); // get a reference to the root POA and activate the POAManager POA rootpoa = POAHelper. narrow( orb. resolve_initial_references (" RootPOA") ); rootpoa. the_poamanager (). activate (); // create instance of the servant MyTimeservice MyTimeservice instance = new MyTimeservice (); instance. setorb( orb ); // get an object reference associated with the servant org. omg. CORBA. Object ref = rootpoa. servant_to_reference ( instance ); Timeservice href = TimeserviceHelper. narrow( ref ); // obtain the initial naming context org. omg. CORBA. Object objref = orb. resolve_initial_references (" NameService "); NamingContextExt ncref = NamingContextExtHelper. narrow( objref ); // register the servant s object reference with the name server String name = " TimeService "; NameComponent path [] = ncref. to_name( name ); ncref. rebind( path, href ); // wait for invocations from clients System. out. println(" TimeServer ready and waiting..."); orb.run (); catch ( Exception e) { System.err.println("ERROR: " + e); e. printstacktrace (System.out ); System.out.println( "Server exiting." );

9 The client * MyClient.java import org. omg. CosNaming.*; import org. omg. CosNaming. NamingContextPackage.*; import org.omg.corba.*; public class MyClient { public static void main( String args [] ) { try { // create and initialize an ORB object ORB orb = ORB. init( args, null); // obtain the initial naming context org. omg. CORBA. Object objref = orb. resolve_initial_references (" NameService "); NamingContextExt ncref = NamingContextExtHelper. narrow( objref ); // resolve the Object Reference in Naming String name = " TimeService "; Timeservice timeservice = TimeserviceHelper. narrow( ncref. resolve_str ( name )); // invoke gettime on server System. out. println(" Obtained a handle on server object: " + timeservice ); System. out. println(" Java client calls gettime: " + timeservice. gettime () ); // shutdown System.out.println( " Shutting down orb." ); timeservice. shutdown (); catch ( Exception e) { System. out. println(" ERROR : " + e) ; e. printstacktrace (System.out );