A CORBA application may consist of one or more CORBA server objects and many clients who connect to these servers.
A CORBA server object makes itself available to the client by registering with the CORBA Naming Service or a CORBA Trader Service. A client locates the desired server object on the network by using this Naming or Trader service. Once the server object is located, the client receives a reference to it. Using this reference, the client can invoke methods on the server object and carry out its desired work. Thus it's the client that usually makes use of server facilities; the server simply returns the results of method invocations to the client. However, in some situations, it may be necessary for the server to invoke a method on the client object. For example, the server may like to notify the client of the occurrence of a certain event on the server or the completion of a processing job requested by the client. This method of invocation on the client is called Callback. CORBA specifications allow Callbacks on clients. This article discusses the implementation of CORBA callbacks using Java.