S. Monaghan CSEE, University of Essex. September 21, Client Program and Server Program 3

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1 CSEE University of Essex CE806 - Distributed Computing ( ) Laboratory 6 Java IDL (Script/code checked and working in Computer Laboratory 1 on 21/9/2010) S. Monaghan CSEE, University of Essex September 21, 2010 Contents 1 Introduction 1 2 Compilation of the IDL 2 3 Client Program and Server Program 3 4 Running the Application 6 5 Investigations 6 6 Summary 6 7 Reference 7 1 Introduction This laboratory is the sixth in a number of weekly self-service laboratory activities to practically support the module CE806 - Distributed Computing. The laboratory is designed to be straightforward and should take no more than one hour to accomplish. Further thought is required though to understand properly the implications of the laboratory work. Although these laboratories are self-service in nature, it is important to carry them out because they build into the coursework assignment associated with the module. The fortnightly CE806 classes will also be concerned with these laboratories.

2 2 COMPILATION OF THE IDL In previous weeks laboratories you ran programs written using the fundamental distributing computing technologies developed 25 years ago - Berkeley Sockets and RPC. With these technologies distributed applications can be built in the C programming language (i.e., they provide two ways to implement the arrows in system diagrams of distributed applications). You also programmed a distributed application using Java RMI, which provides many advantages for distributing computing because of its virtual machine architecture. However, the problem with Java RMI is that it works only for Java. There are many important legacy applications that will not be ported to Java because they are core and critical applications to businesses that have been thoroughly debugged and optimised over the years. The business case for a high cost and risky port to Java might be very hard to make, especially if it means taking away resources from other projects (perhaps written in Java!) that are highly likely to increase revenues and profits. Sun have recognised this by including into their enterprise software J2EE an implementation of CORBA. With such a CORBA implementation, it then becomes possible to interface Java RMI applications to legacy applications through the industry standard CORBA middleware. As we will see this implementation is very similar to the OmbiORB C++ implementation we discussed in the lectures. As this may be the only CORBA code you get to write yourself in CE806 we use the same IDL example as used in the OmniORB demo. 2 Compilation of the IDL Sun s Java implementation of CORBA (there may be other implementations) is called Java IDL. As such it has to use exactly the same IDL as all other implementations on all other languages. To illustrate the use of Java IDL we use the IDL example discussed in the lectures on OmniORB. Here the remote object interface is described by #ifndef ECHO_IDL #define ECHO_IDL interface Echo { string echostring(in string mesg); ; #endif // ECHO_IDL The remote object has an interface with only one (public) method. The client calls this method with a string argument and the method returns the same string back to the client. In Windows create a directory M:\CE806LAB \LAB6 \JavaIDL CSEE, University of Essex 2 September 21, 2010

3 3 CLIENT PROGRAM AND SERVER PROGRAM and copy the above IDL code into the file echo.idl. (If you use Notepad to do this, remember to remove the.txt extension, e.g., by using the rename command on the command line.) Open a command window in the above directory and run Java s CORBA compiler with the command :- idlj -fall echo.idl idlj is the name of Sun s most recent CORBA IDL compiler. The switch tells the compiler to generate both the server and client stub code. Problem. Count the number of java files created by idlj. Briefly look at the classes they contain. Of course in Java, one of the classes has to have the same name as the file. 3 Client Program and Server Program As with all the other RPC-like middleware technologies, the developer needs to carry out two tasks to complete the distributed client-server application. The developer has to write the client program that uses the (automatically created) client stub to call the remote server. The developer also has to write the server functionality to be accessed via the server stub. In the context of CORBA this means that the client program uses an IOR to contact the server and the developer has to implement the servant class derived from a server stub class. Cut and paste into the file EchoServer.java the following server code (remember to remove any junk such as page headers and numbers also copied):- import org.omg.cosnaming.*; import org.omg.cosnaming.namingcontextpackage.*; import org.omg.corba.*; import org.omg.portableserver.*; import org.omg.portableserver.poa; import java.util.properties; class EchoImpl extends EchoPOA { private ORB orb; public void setorb(orb orb_val) { orb = orb_val; // implement echostring() method public String echostring(string mesg) { return mesg; CSEE, University of Essex 3 September 21, 2010

4 3 CLIENT PROGRAM AND SERVER PROGRAM // implement shutdown() method public void shutdown() { orb.shutdown(false); public class EchoServer { 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 EchoImpl echoimpl = new EchoImpl(); echoimpl.setorb(orb); // get object reference from the servant org.omg.corba.object ref = rootpoa.servant_to_reference(echoimpl); Echo href = EchoHelper.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); // bind the Object Reference in Naming String name = "Echo"; NameComponent path[] = ncref.to_name( name ); ncref.rebind(path, href); System.out.println("EchoServer ready and waiting..."); // wait for invocations from clients orb.run(); catch (Exception e) { System.err.println("ERROR: " + e); CSEE, University of Essex 4 September 21, 2010

5 3 CLIENT PROGRAM AND SERVER PROGRAM e.printstacktrace(system.out); System.out.println("EchoServer Exiting..."); and cut and paste into the file echoclient.java the code import org.omg.cosnaming.*; import org.omg.cosnaming.namingcontextpackage.*; import org.omg.corba.*; public class echoclient { static Echo echoimpl; 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"); // Use NamingContextExt instead of NamingContext. This is // part of the Interoperable naming Service. NamingContextExt ncref = NamingContextExtHelper.narrow(objRef); // resolve the Object Reference in Naming String name = "Echo"; echoimpl = EchoHelper.narrow(ncRef.resolve_str(name)); System.out.println("Obtained a handle on server object: " + echoimpl); System.out.println(echoImpl.echoString("Hello")); //echoimpl.shutdown(); catch (Exception e) { System.out.println("ERROR : " + e) ; e.printstacktrace(system.out); CSEE, University of Essex 5 September 21, 2010

6 6 SUMMARY 4 Running the Application In the directory javaidl run the command line javac *.java to compile all the java files present in that directory. With the version of compiler used to develop the example a warning is given about one of the files automatically generated by idlj. Ignore the warning as the code will still work. In this example the COS naming service is used so this service has to be started first with the command start orbd -ORBInitialPort 1308 where the last number is the port number on which the name server will run. (If a firewall window pops up, press the cancel button.) Another window will pop up representing the naming service. Note also that a database directory orb.db is created to store the services to be advertised by the COS naming service. With the naming service running, the server can be started so that it registers with the naming service. Using the same window in which the naming service was started, start the server with the command start java EchoServer -ORBInitialPort ORBInitialHost localhost The same command windown (in the javaidl directory) can also be used to start the client program with the command java echoclient -ORBInitialPort ORBInitialHost localhost Explain the output. Look at the code to do this. Note. Here are two cases where the program arguments use CORBA arguments that are used to initialise the ORB in the client and the ORB in the server. This feature was not used in the OmniORB C++ examples shown in the CE806 lectures. 5 Investigations Having now got a CORBA application to work using Java there are now two tasks to carry out. To check you can use Java IDL, modify the above program so that it computes π using Monte Carlo simulation. Proof that you have a working program is that it gives a good approximation to π. If you do not know how already, you will have to find out how to get a supply of random numbers in Java (you should have already done this in the Java RMI laboratory script). The second task is to compare the above Java CORBA application with its OmnORB C++ counterpart. Note especially the similarities in the two programs. 6 Summary In this laboratory you compiled and ran a CORBA distributed application by making use of Sun s Java IDL implementation of the CORBA standard for distributed computing. Java IDL is important to J2EE because it provides the CSEE, University of Essex 6 September 21, 2010

7 7 REFERENCE interface to legacy enterprise code that would be otherwise difficult to access from Java applications. 7 Reference For more information on Java IDL and the code on which this tutorial is based see End of Laboratory CSEE, University of Essex 7 September 21, 2010