Java IDL Programming Concepts

This topic includes the following sections:

Exceptions

CORBA has two types of exceptions: standard system exceptions, which are fully specified by the OMG, and user exceptions, which are defined by the individual application programmer. CORBA exceptions differ slightly from Java exception objects, but those differences are largely handled in the mapping from IDL-to-Java.

Topics in this section include:

Differences Between CORBA and Java Exceptions

To specify an exception in IDL, the interface designer uses the raises keyword. This is similar to the throws specification in Java. When you use the exception keyword in IDL, you create a user-defined exception. The standard system exceptions need not (and cannot) be specified this way.

System Exceptions

CORBA defines a set of standard system exceptions, which are generally raised by the ORB libraries to signal systemic error conditions including:

All IDL operations can throw system exceptions when invoked. The interface designer need not specify anything to enable operations in the interface to throw system exceptions; the capability is automatic.

This makes sense because no matter how trivial an operation's implementation is, the potential of an operation invocation coming from a client that is in another process, and perhaps (likely) on another machine, means that a whole range of errors is possible.

Therefore, a CORBA Java client should always catch CORBA system exceptions. Moreover, developers cannot rely on the idltojava compiler to notify them of a system exception they should catch, because CORBA system exceptions are descendants of java.lang.RuntimeException.

System Exception Structure

All CORBA system exceptions have the same structure:

exception <SystemExceptionName> { // descriptive of error
    unsigned long minor;          // more detail about error
    CompletionStatus completed;   // yes, no, maybe
}

System exceptions are subtypes of java.lang.RuntimeException through org.omg.CORBA.SystemException:

java.lang.Exception
 |
 +--java.lang.RuntimeException
     |
     +--org.omg.CORBA.SystemException
         |
         +--BAD_PARAM
         |
         +--//etc. 

Completion Status

All CORBA system exceptions have a completion status field which indicates the status of the operation that threw the exception. The completion codes are:

User Exceptions

CORBA user exceptions are subtypes of java.lang.Exception through org.omg.CORBA.UserException:

java.lang.Exception
 |
 +--org.omg.CORBA.UserException
      |
      +-- Stocks.BadSymbol
      |
      +--//etc. 

Each user-defined exception specified in IDL results in a generated Java exception class. These exceptions are entirely defined and implemented by the programmer.

Minor Code Meanings

Table 4-1 ORB Minor Codes and Their Meanings

Table 4-2 Name Server Minor Codes and Their Meanings

Initializations

Before a CORBA Java client or a CORBA Java joint client/server can use CORBA objects, it must initialize itself by:

Creating an ORB Object

Before it can create or invoke a CORBA object, a CORBA Java client or a CORBA Java joint client/server must first create an ORB object. By creating an ORB object, the client or joint client/server introduces itself to the ORB and obtains access to important operations that are defined on the ORB object.

Clients and joint client/servers create ORB instances slightly differently, because their parameters, which must be passed in the ORB.init() call, are arranged differently.

Creating an ORB for an Application

The following code fragment shows how a CORBA Java client might create an ORB:

    import org.omg.CORBA.ORB;
 
    public static void main(String args[])
    {
      try{
        ORB orb = ORB.init(args, null); 
    // code continues

Creating an ORB for an Applet

A Java applet creates an ORB like this:

    import org.omg.CORBA.ORB;
 
    public void init() {
      try {
        ORB orb = ORB.init(this, null);
    // code continues

Some Web browsers have a built-in ORB. This can cause problems if that ORB is not entirely compliant. In this case, special steps must be taken to initialize the Java IDL ORB specifically. For example, because of missing classes in the installed ORB in Netscape Communicator 4.01, an applet displayed in that browser must contain code similar to the following in its init() method:

    import java.util.Properties;
    import org.omg.CORBA.*;

    public class MyApplet extends java.applet.Applet {
      public void init()
      {
        // Instantiate the Java IDL ORB, passing in this applet 
        // so that the ORB can retrieve the applet properties.
        Properties p = new Properties();
        p.put("org.omg.CORBA.ORBClass", "com.sun.CORBA.iiop.ORB");
        p.put("org.omg.CORBA.ORBSingletonClass","com.sun.CORBA.idl.ORBSingleton");
        System.setProperties(p);
        ORB orb = ORB.init(args, p);
        ...
      }
    }

Arguments to ORB.init()

For both applications and applets, the arguments for the initialization method are:

The init() operation uses these parameters, as well as the system properties, to obtain information it needs to configure the ORB. It searches for ORB configuration properties in the following places and order:

The first value found for a particular property is the value used by the init() operation. If a configuration property cannot be found in any of these places, the init() operation assumes an implementation-specific value for it. For maximum portability among ORB implementations, applets and applications should explicitly specify configuration property values that affect their operation, rather than relying on the assumptions of the ORB in which they are running.

System Properties

The BEA Tuxedo product uses the Sun Microsystem, Inc. Java virtual machine, which adds -D command-line arguments to it. Other Java virtual machines may or may not do the same.

Currently, the following configuration properties are defined for all ORB implementations:

Applet parameters should specify the full property names. The conventions for applications differ from applets so as not to expose language-specific details in command-line invocations.

Obtaining Initial Object References

To invoke a CORBA object, an applet or application must have a reference for it. There are three ways to get a reference for a CORBA object:

Stringified Object References

The first technique, converting a stringified reference to an actual object reference, is ORB-implementation independent. Regardless of which ORB an applet or application runs on, it can convert a stringified object reference. However, it is up to the applet or application developer to:

Getting References from the ORB

If you do not use a stringified reference to get an initial CORBA object, you use the ORB itself to produce an initial object reference.

The Bootstrap object defines an operation called resolve_initial_references() that is intended for bootstrapping object references into a newly started application or applet. The operation takes a string argument that names one of a few recognized objects; it returns a CORBA Object, which must be narrowed to the type the applet or application knows it to be.

Using the Bootstrap object, you can obtain the following object references (SecurityCurrent, TransactionCurrent, FactoryFinder, NotificationService, Tobj_SimpleEventsService, NameService, and InterfaceRepository). The object of concern to us here is the FactoryFinder object.

The FactoryFinder interface provides clients with one object reference that serves as the single point of entry into the BEA Tuxedo domain. The FactoryFinder object is used to obtain a specific factory object, which in turn can create the needed objects.

The FactoryFinder Interface

The FactoryFinder interface provides clients with one object reference that serves as the single point of entry into the BEA Tuxedo domain. Multiple FactoryFinders provide increased availability and reliability. Mapping across multiple domains is supported.

The FactoryFinder interface and the NameManager are a mechanism for registering, storing, and finding objects. In the BEA Tuxedo environment, you can: