Java Reference
In-Depth Information
CORBA
Packages
JavaIDL:
org.omg.CORBA, org.omg.CORBA_2_3, org.omg.CORBA_2_3. portable,
org.omg.CORBA.DynAnyPackage, org.omg.CORBAORBPackage, org.omg.CORBA.portable,
org.omg.CORBATypeCodePackage
CosNaming:
org.omg.CosNaming, org.omg.CosNaming. NamingContextPackage, org.omg.SendingContext
RMI-IIOP:
javax.rmi.CORBA, org.omg.stub.java.rmi
Use:
J2SE (JDK 1.2)
Overview
The Common Object Request Broker Architecture, or CORBA, is a distributed object communication
architecture. In simplest terms, it's a way for an application to request services from another application by calling
remote methods.
CORBA pretty much provides the ultimate in interoperability. Many programming languages support the
CORBA standard, and the architecture is defined so that a client and server written in different
CORBA-compliant languages can interact without knowing or caring about remote implementation details.
Central to the architecture is the Object Request Broker, or ORB. The ORB functions as the router for all
distributed communication. Every client and server in a system depends on an ORB for messaging.
To interact with the ORB, CORBA participants map their code to IDL, the Interface Definition Language. IDL is
a platform-neutral language used to define calling interfaces for CORBA participants. This is key to the magic
behind client-server communication. IDL provides a description of the contract between a client and server. In
this way, a client knows what methods it can invoke, but does
not
know what language the methods will be
written in.
The CORBA framework uses the Internet Inter-ORB Protocol (IIOP) to manage its distributed communication. This lower-level
protocol was introduced in recent revisions of the CORBA specification to enable distributed communications across the most pervasive
of networks—the Internet.
Java and CORBA
The Java API was created to allow Java programs to interoperate with the CORBA model at several levels. Three
CORBA-related technologies are represented in Java APIs: JavaIDL, CosNaming, and RMI-IIOP.
JavaIDL - JavaIDL represents direct CORBA capabilities in the Java programming language. The API, with its
associated compiler tool, provides a way to map between Java code and the Interface Definition Language.
Practically speaking, this means that Java code can use the API to function as a CORBA client or server module.
In broader terms, the API allows a Java program to interact with an ORB, effectively allowing Java programs to
leverage the power of CORBA.
CosNaming - The CosNaming service provides a way for Java programs to use a CORBA naming service. CORBA's naming service
pretty much does what you'd expect it to do—it keeps track of an object with a string representing the object's name.
A Java server using CosNaming will register its remote objects with the service; a client will look them up and
use them. This is sort of like the services provided by the Java Naming and Directory Interface—well, actually,
it's identical. CosNaming was introduced before JNDI to accommodate Java CORBA applications. JNDI has
subsequently integrated CosNaming as one of its possible services. This means that you can write CORBA
programs that use the original API to implement Cos-Naming, or you can use JNDI to access the CosNaming
services.
RMI-IIOP - RMI-IIOP was a recent addition to the batch of CORBA technologies. It allows RMI programs to
communicate using CORBA's underlying protocol. Originally, RMI used a protocol called the Java Remote
Method Protocol (JRMP) to provide transport capabilities. This protocol was developed by Java and for Java;
there are no other technologies that use or support it. IIOP, on the other hand, which is CORBA's communication
protocol, is very widely supported. It makes sense, given Java's goals of universality, to use a protocol like IIOP
for interapplication communication.
