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Example 1: Hello World

This page contains:

• The IDL for a simple "Hello World" program
• A transient server that creates an object and registers it in a naming context
• An application client that, knowing the object's name, retrieves a reference for it from the naming context, and invokes it
• An applet that does the same

Instructions for compiling and running the example are provided, and source code is also included.

Interface Definition (Hello.idl)

The following example OMG IDL describes a CORBA object whose single sayHello() operation returns a string.

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

Compile the IDL interface with the command:

        idltojava  Hello.idl

_HelloImplBase.java

This abstract class is the server skeleton, providing basic CORBA functionality for the server. It implements the Hello.java interface. The server class HelloServant extends _HelloImplBase.

_HelloStub.java

This class is the client stub, providing CORBA functionality for the client. It implements the Hello.java interface.

Hello.java

This interface contains the Java version of our IDL interface. It contains the single method sayHello(). The Hello.java interface extends org.omg.CORBA.Object, providing standard CORBA object functionality as well.

HelloHelper.java

This final class provides auxiliary functionality, notably the narrow() method required to cast CORBA object references to their proper types.

HelloHolder.java

This final class holds a public instance member of type Hello. It provides operations for out and inout arguments, which CORBA has but which do not map easily to Java's semantics.

The example server consists of two classes, the servant and the server. The servant, HelloServant, is the implementation of the Hello IDL interface; each Hello instance is implemented by a HelloServant instance. The servant is a subclass of _HelloImplBase, which is generated by the idltojava compiler from the example IDL. The servant contains one method for each IDL operation, in this example, just the sayHello() method. Servant methods are just like ordinary Java methods; the extra code to deal with the ORB, with marshaling arguments and results, and so on, is provided by the server and the stubs.

The server class has the server's main() method, which:

• Creates an ORB instance
• Creates a servant instance (the implementation of one CORBA Hello object) and tells the ORB about it
• Gets a CORBA object reference for a naming context in which to register the new CORBA object
• Registers the new object in the naming context under the name "Hello"
• Waits for invocations of the new object

// Copyright and License 

import HelloApp.*;
import org.omg.CosNaming.*;
import org.omg.CosNaming.NamingContextPackage.*;
import org.omg.CORBA.*;
 
class HelloServant extends _HelloImplBase 
{
    public String sayHello()
    {
	return "\nHello world !!\n";
    }
}

 
public class HelloServer {
 
    public static void main(String args[])
    {
	try{
	    // create and initialize the ORB
	    ORB orb = ORB.init(args, null);
 
	    // create 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);
	}
    }
}
 

The example application client that follows:

• Creates an ORB
• Obtains a reference to the naming context
• Looks up "Hello" in the naming context and receives a reference to that CORBA object
• Invokes the object's sayHello() operation and prints the result

// Copyright and License 
 
import HelloApp.*;
import org.omg.CosNaming.*;
import org.omg.CORBA.*;
 
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);
	}
    }
}

 

The example applet client that follows:

• Creates an ORB
• Obtains a reference to the naming context
• Looks up "Hello" in the naming context and receives a reference to that CORBA object
• Invokes the object's sayHello() operation and prints the result

// Copyright and License 

import HelloApp.*;
import org.omg.CosNaming.*;
import org.omg.CosNaming.NamingContextPackage.*;
import org.omg.CORBA.*;
import java.awt.Graphics;
 
public class HelloApplet extends java.applet.Applet 
{
    public void init() {
	try {
	    // create and initialize the ORB
            ORB orb = ORB.init(this, 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
            message = helloRef.sayHello();
 
	} catch (Exception e) {
	    System.out.println("HelloApplet exception: " + e.getMessage());
	    e.printStackTrace(System.out);
        }
    }
 

    public void paint(Graphics g) 
    {
	g.drawString(message, 25, 50);
    }
 
    String message = "";
}
 

The source code for the Hello World example is located in the examples/hello directory. The following instructions assume you can use port 1050 for the Java IDL name server. Substitute a different port if necessary. Note that for ports below 1024, you need root access on UNIX machines, and administrator privileges on Windows95 and NT. Note also that the instructions use a slash (/) for path names. Windows95 and NT users should subtitute a backslash (\).

NOTE: You must have downloaded the idltojava compiler before you can build and run Hello World. It is available free from the JDC (Java Developer Connection) web page. If you are not already a member, you must register with the JDC to access this page, but there is no cost. The download page is

    http://developer.java.sun.com/developer/earlyAccess/jdk12/idltojava.html

• Change to the directory that contains the file Hello.idl

  	cd docs/guide/idl/examples/hello
  

• Run idltojava on the IDL file to create stubs and skeletons.
  Note: idltojava is hard-coded to use a default preprocessor. On Windows machines, it uses the MS Visual C++ preprocessor. On Solaris machines, it uses the path /usr/ccs/lib/cpp when looking for the preprocessor. You can change the preprocessor that idltojava uses by setting two environment variables: CPP and CPARGS. Set CPP to the full pathname of the preprocessor you want to use. Set CPARGS to the complete list of arguments to be passed to the preprocessor. You can also turn off the preprocessor by adding -fno-cpp to the idltojava command line. More information is available in the idltojava reference page that is downloaded with the idltojava compiler.

  	idltojava  Hello.idl
  

• Compile the .java files, including the stubs and skeletons (which are in the directory HelloApp):

  	javac *.java HelloApp/*.java
  

• Make sure the name server is running:

  	tnameserv -ORBInitialPort 1050&
  

• Start the Hello server:

  	java HelloServer -ORBInitialPort 1050
  

• Run the Hello application client from a different shell than the server:

  	java HelloClient -ORBInitialPort 1050
  

• Go to the shell where you started the server and kill the process. (It will just wait for invocations until you kill it or shut down your machine.)