Getting Started with Java IDL: Developing a Client Application

This topic introduces the basics of writing a CORBA client application. To create a CORBA client applet, follow the link. Included in this lesson are:

Creating HelloClient.java
Understanding HelloClient.java
Compiling HelloClient.java

To see a completed version of HelloClient.java, follow the link.

Understanding HelloClient.java

This section explains each line of HelloClient.java, describing what the code does, as well as why it is needed for this application.

Performing Basic Setup

The basic shell of a CORBA client is the same as many Java applications: You import required library packages, declare the application class, define a main method, and handle exceptions.

Importing Required Packages

First, we import the packages required for the client 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.

Declaring the Client Class

The next step is to declare the client class:

public class HelloClient
{
  // The main() method goes here.
}

Defining a `main()` Method

Every Java application needs a main() method. It is declared within the scope of the HelloClient class, as follows:

  public static void main(String args[])
  {
    // The try-catch block goes here.
  }

Handling CORBA System Exceptions

Because all CORBA programs can throw CORBA system exceptions at runtime, all of the main() functionality is placed within a try-catch block. CORBA programs throw system exceptions whenever trouble occurs during any of the processes (marshaling, unmarshaling, upcall) involved in invocation.

Our exception handler simply prints the name of the exception and its stack trace to standard output so you can see what kind of thing has gone wrong.

The try-catch block is set up inside main(),

    try{
    
      // Add the rest of the HelloClient code here.
    
    } catch(Exception e) {
        System.out.println("ERROR : " + e);
        e.printStackTrace(System.out);
      }

Creating an ORB Object

A CORBA client needs a local ORB object to perform all of its marshaling and IIOP work. Every client instantiates an org.omg.CORBA.ORB object and initializes it by passing to the object certain information about itself.

The ORB variable is declared and initialized inside the try-catch block.

      ORB orb = ORB.init(args, null);

The call to the ORB's init() method passes in your application's command line arguments, allowing you to set certain properties at runtime.

Finding the Hello Server

Now that the application has an ORB, it can ask the ORB to locate the actual service it needs, in this case the Hello server. There are a number of ways for a CORBA client to get an initial object reference; our client application will use the COS Naming Service specified by OMG and provided with Java IDL. See Using Stringified Object References for information on how to get an initial object reference when there is no naming service available.

Obtaining the Initial Naming Context

The first step in using the naming service is to get the initial naming context. In the try-catch block, below your ORB initialization, you call orb.resolve_initial_references() to get an object reference to the name server:

      org.omg.CORBA.Object objRef = 
            orb.resolve_initial_references("NameService");

The string "NameService" is defined for all CORBA ORBs. When you pass in that string, the ORB returns the initial naming context, an object reference to the name service.

Narrowing the Object Reference

As with all CORBA object references, objRef is a generic CORBA object. To use it as a NamingContext object, you must narrow it to its proper type.

      NamingContext ncRef = NamingContextHelper.narrow(objRef);

Here we see the use of an idlj-generated helper class, similar in function to HelloHelper. The ncRef object is now an org.omg.CosNaming.NamingContext and you can use it to access the naming service and find other services. You will do that in the next step.

Finding a Service in Naming

Names can have different structures depending upon the implementation of the naming service. Consequently, CORBA name servers handle complex names by way of NameComponent objects. Each NameComponent holds a single part, or element, of the name. An array of NameComponent objects can hold a fully specified path to an object on any computer file or disk system.

To find the Hello server, you first need a NameComponent to hold an identifying string for the Hello server.

      NameComponent nc = new NameComponent("Hello", "");

This statement sets the id field of nc to "Hello" and the kind field to an empty string. Be sure this is an empty string, do not enter a space between "".

Because the path to the Hello object has just one element, we have created a single-element array out of nc. The NamingContext.resolve() method requires this array for its work:

      NameComponent path[] = {nc};

Finally, we pass path to the naming service's resolve() method to get an object reference to the Hello server and narrow it to a Hello object:

      Hello helloRef = HelloHelper.narrow(ncRef.resolve(path));

Here you see the HelloHelper helper class at work. The resolve() method returns a generic CORBA object as you saw above when locating the name service itself. Therefore, you immediately narrow it to a Hello object, which is the object reference you need to perform the rest of your work.

Invoking the `sayHello()` Operation

CORBA invocations look like a method call on a local object. The complications of marshaling parameters to the wire, routing them to the server-side ORB, unmarshaling, and placing the upcall to the server method are completely transparent to the client programmer. Because so much is done for you by generated code, invocation is really the easiest part of CORBA programming.

      String Hello = helloRef.sayHello();

Finally, we print the results of the invocation to standard output:

      System.out.println(Hello);

Compiling HelloClient.java

Now we will compile HelloClient.java so that we can correct any errors before continuing with this tutorial.

Windows users note that you should substitute backslashes (\) for the slashes (/) in all paths in this document.

To compile HelloClient.java,

Change to the Hello directory.
Run the Java compiler on HelloClient.java:
```
javac HelloClient.java HelloApp/*.java
```
Correct any errors in your file and recompile if necessary. (You can copy the file from HelloClient.java if you have trouble finding your typographical errors).
The HelloClient.class is generated to the Hello directory.

Running the Client Application

We need to create and compile the Hello server before we can successfully run the Hello client application. Running the Hello World application is covered in Running the Hello World Application.

For More Information

Developing Clients: Covers topics of interest to CORBA client programmers
Exceptions: System Exceptions: Explains how CORBA system exceptions work and provides details on the minor codes of Java IDL's system exceptions
Initialization: System Properties: Explains what properties can be passed to the ORB at initialization
Naming Service: Covers the COS Naming Service in greater detail

Previous: Writing the IDL interface
Next: Developing a Client Applet or Next: Developing the Hello World Server
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