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In this section, you get some additional detail on CORBA, IDL, and the Java IDL ORB promised earlier.Any relationship between distributed objects has two sides: the client and the server. The server provides a remote interface, and the client calls a remote interface. These relationships are common to most distributed object standards, including RMI and CORBA. Note that in this context, the terms client and server define object-level rather than application-level interaction--any application could be a server for some objects and a client of others. In fact, a single object could be the client of an interface provided by a remote object and at the same time implement an interface to be called remotely by other objects.
This figure shows how a one-method distributed object is shared between a CORBA client and server to implement the classic "Hello World" application.
A one-method distributed object shared between a CORBA client and server. On the client side, the application includes a reference for the remote object. The object reference has a stub method, which is a stand-in for the method being called remotely. The stub is actually wired into the ORB, so that calling it invokes the ORB's connection capabilities, which forwards the invocation to the server.
On the server side, the ORB uses skeleton code to translate the remote invocation into a method call on the local object. The skeleton translates the call and any parameters to their implementation-specific format and calls the method being invoked. When the method returns, the skeleton code transforms results or errors, and sends them back to the client via the ORBs.
Between the ORBs, communication proceeds by means of a shared protocol, IIOP--the Internet Inter-ORB Protocol. IIOP, which is based on the standard TCP/IP internet protocol, defines how CORBA-compliant ORBs pass information back and forth. Like CORBA and IDL, the IIOP standard is defined by OMG, the Object Management Group.
In addition to these simple distributed object capabilities, CORBA-compliant ORBs can provide a number of optional services defined by the OMG. These include services for looking up objects by name, maintaining persistent objects, supporting transaction processing, enabling messaging, and many other abilities useful in today's distributed, multi-tiered computing environments. Several Java ORBs from third-party vendors support some or all of these additional capabilities. The ORB provided with Java IDL supports one optional service, the ability to locate objects by name.
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