Developing Clients

This document was not updated for this release.

When updated, topics in this section will include:

ORBs and Invocations

For a client to invoke a CORBA object operation, both the client and the server (the object implementation) must use a CORBA software component called an ORB (object request broker). ORBs are the common denominators that bridge the differences in location, platform, and programming language that can separate a client and a server. ORBs can contact each other across the network, can create and interpret object references (CORBA object handles), and can marshal parameters into and out of the format used by IIOP. In addition to enabling client/server communication, ORBs provide other services, but they are not described here.

The two ways to invoke an operation on a CORBA object are:

Static Invocation

To make a static invocation on a CORBA object, a Java client needs an object reference to the servant that performs the operation. The object reference has two important functions:

Clients don't create object references but obtain them, typically from other objects such as factories or naming contexts.

OMG IDL is the language in which CORBA object interfaces are defined. For each OMG IDL module, the idlj compiler generates a Java package. For each interface Foo defined in an OMG IDL module, the generated package contains the following items of interest to the client programmer:

Dynamic Invocation

CORBA dynamic invocation uses an object called a request to hold everything pertinent to an invocation: the object reference, the name of the operation, its parameters, and space for the result. The client builds a request object describing an operation, then calls the request's invoke method, which dispatches the request just as a stub would. When the invoke method returns, the result is available in the request object.

The key to dynamic invocation is the ability of requests to hold self-describing data. This facility enables a request object to represent any invocation of any operation, regardless of its parameters. Each self-describing data element has a special type known in OMG IDL as Any. An Any consists of a typecode (whose values are defined by OMG IDL) and a value; the typecode specifies the type of the value.

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