Software Engineering - Fall 1999

Lecture 8: Corba (Part 1) -- Corba and Java

Motivation

• Start playing with Corba over Java
• Know IDL
• Know Corba's event model
• Schmidt's Stock example in Java: push and pull models.

Hello World in Java/Corba

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

IDL to Java

% idltojava hello.idl
% ls -l HelloApp/*.java

-rw-r--r--   247	HelloApp/Hello.java
-rw-r--r--  1771	HelloApp/HelloHelper.java
-rw-r--r--   781	HelloApp/HelloHolder.java
-rw-r--r--  1452	HelloApp/_HelloImplBase.java
-rw-r--r--   960	HelloApp/_HelloStub.java

1. A Java package for each IDL module
2. A Java Interface for each IDL interface
3. A Helper Java class for each IDL interface
4. A Holder Java class for each IDL interface
5. A skeleton class (ImplBase) for each IDL interface to use on the server.
6. A stub (proxy) class for each IDL interface to use on the client.

A Java CORBA Client

1. Import the IDL generated package
2. Connect to the orb (ORB.init)
3. Connect to the Naming service (orb.resolve_initial_references)
4. Obtain an object reference from the naming service (ncRef.resolve)
5. Narrow the object reference to a stub interface (use the Helper class)
6. Use the object reference (use the Stub class)

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);
      }  
  }
}

A Java CORBA Server

1. Import the IDL generated package
2. Connect to the orb (ORB.init)
3. Create a servant object for each implemented IDL interface (new HelloServant)
4. Connect the servants with the orb (orb.connect)
5. Connect to the Naming service (orb.resolve_initial_references)
6. Register (bind) the servant objects with the naming service (ncRef.Rebind)
7. Enter in a loop to wait for client connections

If inheritance is used, the servant class extends the ImplBase class generated by idltojava and it only needs to implement the methods declared in the Java interface.

This is illustrated in the this program:

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


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

• Hello.idl
• HelloClient.java
• HelloServer.java

• Open a window on lead
• Make sure you define in your .cshrc (or .tcshrc):

  	setenv JAVA_HOME /usr/local/Java/LastJDK
  	alias java $JAVA_HOME/bin/java
  	alias javac $JAVA_HOME/bin/javac
  	alias tnameserv $JAVA_HOME/bin/tnameserv
      

• Start the name service:

      % tnameserv -ORBInitialPort 1050 &
      

• Compile the IDL:

      % idltojava Hello.idl
      

• Compile the client:

      % javac HelloClient.java HelloApp/*.java
      

• Compile the server:

      % javac HelloServer.java HelloApp/*.java
      

• Run the server:

      % java HelloServer -ORBInitialPort 1050 &
      

• Open another window on lead and run the client:

      % java HelloClient -ORBInitialPort 1050