Creating CORBA Client Applications

Creating CORBA Client Applications

This topic includes the following sections:

Summary of the Development Process for CORBA C++ Client Applications

The steps for creating a CORBA C++ client application are as follows:

Each step in the process is explained in detail in the following sections.

The BEA Tuxedo development environment for CORBA C++ client applications includes the following:

Step 1: Obtaining the OMG IDL File

Generally, the OMG IDL files for the available interfaces and operations are provided to the client programmer by the application designer. This section contains the OMG IDL for the Basic sample application. Listing 2-1 shows the univb.idl file, which defines the following interfaces:

#pragma prefix "beasys.com"

module UniversityB

{

   typedef unsigned long CourseNumber;
   typedef sequence<CourseNumber> CourseNumberList;

   struct CourseSynopsis
     {
        CourseNumber   course_number;
        string         title;
     };

   typedef sequence<CourseSynopsis> CourseSynopsisList;
   interface CourseSynopsisEnumerator
   {
        CourseSynopsisList get_next_n(
            in  unsigned long number_to_get, 
            out unsigned long number_remaining
   };
         void destroy();
   };

   typedef unsigned short Days;
   const Days MONDAY    =  1;
   const Days TUESDAY   =  2;
   const Days WEDNESDAY =  4;
   const Days THURSDAY  =  8;
   const Days FRIDAY    = 16;
  
   struct ClassSchedule
   {
       Days           class_days; // bitmask of days
       unsigned short start_hour; // whole hours in military time
       unsigned short duration;   // minutes
   };

   struct CourseDetails
   {
       CourseNumber   course_number;
       double         cost;
       unsigned short number_of_credits;
       ClassSchedule  class_schedule; 
       unsigned short number_of_seats;
       string         title;
       string         professor;
       string         description;
    };

   typedef sequence<CourseDetails> CourseDetailsList;

   interface Registrar
   {
       CourseSynopsisList
       get_courses_synopsis(
       in string                    search_criteria,
       in unsigned long             number_to_get, // 0 = all
       out unsigned long            number_remaining,
       out CourseSynopsisEnumerator rest
    );

    	CourseDetailsList get_courses_details(in CourseNumberList 
        courses);

   interface RegistrarFactory
   {
        Registrar find_registrar(
    );

  };

};

Step 2: Selecting the Invocation Type

Select the invocation type (static or dynamic) that you will use in the requests in the CORBA client application. You can use both types of invocation in a CORBA client application.

For an overview of static and dynamic invocation, see Static and Dynamic Invocation.

The remainder of this topic assumes that you chose to use static invocation in your CORBA client application. If you chose to use dynamic invocation, see Using the Dynamic Invocation Interface.

Step 3: Compiling the OMG IDL File

When creating CORBA C++ client applications, use the idl command to compile the OMG IDL file and generate the files required for the interface. The following is the syntax of the idl command:

idl idlfilename(s)

The IDL compiler generates a client stub (idlfilename_c.cpp) and a header file (idlfilename_c.h) that describe everything you need to have to use the client stub from the C++ programming language. You need to link these files into your CORBA client application.

In addition, the IDL compiler generates skeletons that contain the signatures of the CORBA object's operations. The generated skeleton information is placed in the idlfilename_s.cpp and idlfilename_s.h files. During development of the CORBA client application, it can be useful to look at the server header files and skeleton file.

For a complete description of the idl command and options, see the BEA Tuxedo Command Reference.

When creating CORBA client applications:

The idltojava command or the IDL compiler generates the following:

Note that each OMG IDL defined exception defines an exception class and its helper and holder classes. The compiled .class files must be in the CLASSPATH of your CORBA client application.

In addition, the idltojava command or the IDL compiler generates skeletons that contain the signatures of the operations of the CORBA object. The generated skeleton information is placed in the _interfaceImplBase file.

Step 4: Writing the CORBA Client Application

To participate in a session with a CORBA server application, a CORBA client application must be able to get an object reference for a CORBA object and invoke operations on the object. To accomplish this, the CORBA client application code must do the following:

The following sections use portions of the client applications in the Basic sample application to illustrate the steps. For information about the Basic sample application, see the Guide to the CORBA University Sample Applications. The Basic sample application is located in the following directory on the BEA Tuxedo software kit:

drive:\tuxdir\samples\corba\university\basic

Initializing the ORB

All CORBA client applications must first initialize the ORB.

Use the following code to initialize the ORB from a CORBA C++ client application:

C++

CORBA::ORB_var orb=CORBA::ORB_init(argc, argv, ORBid);

Typically, no ORBid is specified and the default ORBid specified during installation is used. However, when a CORBA client application is running on a machine that also has CORBA server applications running and the CORBA client application wants to access server applications in another BEA Tuxedo domain, you need to override the default ORBid. This can be done by hard coding the ORBid as BEA_IIOP or by passing the ORBid in the command line as _ORBid BEA_IIOP.

Establishing Communication with the BEA Tuxedo Domain

The CORBA client application creates a Bootstrap object. A list of IIOP Listener/Handlers can be supplied either as a parameter, via the TOBJADDR Java property or applet property. A single IIOP Listener/Handler is specified as follows:

//host:port

When the IIOP Listerner/Handler is provided via TOBJADDR, the second argument of the constructor can be null.

The host and port combination for the IIOP Listener/Handler is defined in the UBBCONFIG file. The host and port combination that is specified for the Bootstrap object must exactly match the ISL parameter in the BEA Tuxedo domain's UBBCONFIG file. The format of the host and port combination, as well as the capitalization, must match. If the addresses do not match, the call to the Bootstrap object will fail and the following message appears in the log file:

Error: Unofficial connection from client at <tcp/ip address>/<portnumber>

For example, if the network address is specified as //TRIXIE::3500 in the ISL parameter in the UBBCONFIG file, specifying either //192.12.4.6.:3500 or //trixie:3500 in the Bootstrap object will cause the connection attempt to fail.

On UNIX systems, use the uname -n command on the host system to determine the capitalization used. On Window 2000, use the Network Control Panel to determine the capitalization.

The following C++ and Java examples show how to use the Bootstrap object:

C++

       Tobj_Bootstrap* bootstrap = new Tobj_Bootstrap(orb, "//host:port");

Java Applet

       Tobj_Bootstrap bootstrap = new Tobj_Bootstrap(orb, "//host:port", this); 

where this is the name of the Java applet

A BEA Tuxedo domain can have multiple IIOP Listener/Handlers. If you are accessing a BEA Tuxedo domain with multiple IIOP Listener/Handlers, you supply a list of Host:Port combinations to the Bootstrap object. If the second parameter of the Bootstrap command is an empty string, the Bootstrap object walks through the list until it connects to a BEA Tuxedo domain. The list of IIOP Listener/Handlers can also be specified in TOBJADDR.

If you want to access multiple BEA Tuxedo domains, you must create a Bootstrap object for each BEA Tuxedo domain you want to access.

Resolving Initial References to the FactoryFinder Object

The CORBA client application must obtain initial references to the environmental objects that provide services for the CORBA application. The Bootstrap object's resolve_initial_references operation can be called to obtain references to the FactoryFinder, InterfaceRepository, SecurityCurrent, TransactionCurrent, NotificationService, Tobj_SimpleEventsService, and NameService environmental objects. The argument passed to the operation is a string containing the name of the desired object reference. You need to get initial references only for the environmental objects you plan to use in your CORBA client application.

The following C++ and Java examples show how to use the Bootstrap object to resolve initial references to the FactoryFinder object:

C++

//Resolve Factory Finder
CORBA::Object_var var_factory_finder_oref = bootstrap.resolve_initial_references
       ("FactoryFinder");
Tobj::FactoryFinder_var var_factory_finder_ref = Tobj::FactoryFinder::_narrow
      (factory_finder_oref.in());

Java

//Resolve Factory Finder
org.omg.CORBA.Object off = bootstrap.resolve_initial_references
      ("FactoryFinder");
FactoryFinder ff=FactoryFinderHelper.narrow(off);

Using the FactoryFinder Object to Get a Factory

CORBA client applications get object references to CORBA objects from factories. A factory is any CORBA object that returns an object reference to another CORBA object and registers itself as a factory. The CORBA client application invokes an operation on a factory to obtain an object reference to a CORBA object of a specific type. To use factories, the CORBA client application must be able to locate the factory it needs. The FactoryFinder object serves this purpose. For information about the function of the FactoryFinder object, see CORBA Client Application Development Concepts.

The FactoryFinder object has the following methods:

The following C++ and Java examples show how to use the FactoryFinder find_one_factory_by_id method to get a factory for the Registrar object used in the CORBA client application for the Basic sample applications:

C++

CORBA::Object_var var_registrar_factory_oref = var_factory_finder_ref->
     find_one_factory_by_id(UniversityB::_tc_RegistrarFactory->id()
);
UniversityB::RegistrarFactory_var var_RegistrarFactory_ref =
     UniversityB::RegistrarFactory::_narrow(
          var_RegistrarFactory_oref.in()
     );

Java

org.omg.CORBA.Object of = FactoryFinder.find_one_factory_by_id
    (UniversityB.RegistrarFactoryHelper.id());
UniversityB.RegistrarFactory F = UniversityB.RegistrarFactoryHelper.narrow(of);

Using a Factory to Get a CORBA Object

CORBA client applications call the factory to get an object reference to a CORBA object. The CORBA client applications then invoke operations on the CORBA object by passing it a pointer to the factory and any arguments that the operation requires.

The following C++ and Java examples illustrate getting the factory for the Registrar object and then invoking the get_courses_details() method on the Registrar object:

C++

UniversityB::Registrar_var var_Registrar = var_RegistrarFactory->
             find_Registrar();
UniversityB::CourseDetailsList_var course_details_list = Registrar_oref->
             get_course_details(CourseNumberList);

Java

UniversityB.Registrar gRegistrarObjRef = F.find_registrar();
gRegistrarObjRef.get_course_details(selected_course_numbers);

Step 5: Building the CORBA Client Application

The final step in the development of the CORBA client application is to produce the executable for the client application. To do this, you need to compile the code and link against the client stub.

When creating CORBA C++ client applications, use the buildobjclient command to construct a CORBA client application executable. The command combines the client stubs for interfaces that use static invocation, and the associated header files with the standard BEA Tuxedo libraries to form a client executable. For the syntax of the buildobjclient command, see the BEA Tuxedo Command Reference.

Server Applications Acting as Client Applications

To process a request from a CORBA client application, the CORBA server application may need to request processing from another server application. In this situation, the CORBA server application is acting as a CORBA client application.

To act as a CORBA client application, the CORBA server application must obtain a Bootstrap object for the current BEA Tuxedo domain. The Bootstrap object for the CORBA server application is already available via TP::Bootstrap (for CORBA C++ client applications). The CORBA server application then uses the FactoryFinder object to locate a factory for the CORBA object that can satisfy the request from the CORBA client application.

Using Java2 Applets

The CORBA environment in the BEA Tuxedo product supports Java2 applets as clients. To run Java2 applets, you need to install the Java Plug-In product from Sun Microsystems, Inc. The Java Plug-in runs Java applets in an HTML page using Sun's Java Virtual Machine (JVM).

Before downloading the Java Plug-in kit from the Sun Web site, verify whether or not the Java Plug-In is already installed on your machine.

Netscape Navigator

In Netscape Navigator, choose the About Plug-Ins option from the Help menu in the browser window. The following will appear if the Java Plug-In is installed:

application/x-java-applet;version 1.2

Internet Explorer

From the Start menu in Windows, select the Programs option. If the Java Plug-In is installed, a Java Plug-In Control Panel option will appear.

If the Java Plug-In is not installed, you need to download and install the JDK1.2 plug-in (jre12-win32.exe) and the HTML converter tool (htmlconv12.zip). You can obtain both these products from java.sun.com/products/plugin.

You also need to read the Java Plug-In HTML Specification located at java.sun.com/products/plugin/1.2/docs. This specification explains the changes Web page authors need to make to their existing HTML code to have existing JDK 1.2 applets run using the Java Plug-In rather that the brower's default Java run-time environment.

Write your Java applet. Use the following command to intialize the ORB from the Java applet:

org.omg.CORBA.ORB orb = org.omg.CORBA.ORB.init (this,null);

To automatically launch the Java Plug-In when Internet Explorer or Netscape Navigator browses the HTML page for your applet, use the OBJECT tag and the EMBED tag in the HTML specification. If you use the HTML Converter tool to convert your applet to HTML, these tags are automatically inserted. For more information about using the OBJECT and EMBED tags, see java.sun.com/products/plugin/1.2/docs/tags.html.