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Glossary of Java IDL Terms

attribute (IDL)

That part of an IDL interface that is similar to a public class field or C++ data member. The idltojava compiler maps an OMG IDL attribute to accessor and modifier methods in the Java programming language. For example, an interface ball might include the attribute color. The idltojava compiler would generate a Java programming language method to get the color, and unless the attribute is readonly, a method to set the color. CORBA attributes correspond closely to JavaBeans properties.

client

Any code which invokes an operation on a distributed object. A client might itself be a CORBA object, or it might be a non-object-oriented program, but while invoking a method on a CORBA object, it is said to be acting as client.

client stub

A Java programming language class generated by idltojava and used transparently by the client ORB during object invocation. The remote object reference held by the client points to the client stub. This stub is specific to the IDL interface from which it was generated, and it contains the information needed for the client to invoke a method on the CORBA object that was defined in the IDL interface.

client tier

The portion of a distributed application that requests services from the server tier. Typically, the client tier is characterized by a small local footprint, a graphical user interface, and simplified development and maintenance efforts.

Common Object Request Broker Architecture (CORBA)

An OMG-specified architecture that is the basis for the CORBA object model. The CORBA specification includes an interface definition language (IDL), which is a language-independent way of creating contracts between objects for implementation as distributed applications.
See also: client tier, service tier, data store tier

CORBA object

An entity which (1) is defined by an OMG IDL interface, and (2) for which an object reference is available. Object is also the implicit common base type for object references of IDL interfaces.

data store tier

The portion of a distributed application that manages access to persistent data and its storage mechanisms, such as relational databases.

distributed application

A program designed to run on more than one computer, typically with functionality separated into tiers such as client, service, and data store.

distributed environment

A network of one or more computers that use CORBA objects. Objects are installed on the various machines and can communicate with each other.

Dynamic Invocation Interface (DII)

An API that allows a client to make dynamic invocations on remote CORBA objects. It is used when at compile time a client does not have knowledge about an object it wants to invoke. Once an object is discovered, the client program can obtain a definition of it, issue a parameterized call to it, and receive a reply from it, all without having a type-specific client stub for the remote object.

Dynamic Skeleton Interface (DSI)

An API that provides a way to deliver requests from an ORB to an object implementation when the type of the object implementation is not known at compile time. DSI, which is the server side analog to the client side DII, makes it possible for the application programmer to inspect the parameters of an incoming request to determine a target object and method.

exception (IDL)

An IDL construct that represents an exceptional condition that could be returned in response to an invocation. There are two categories of exceptions: (1) system exceptions, which inherit from org.omg.CORBA.SystemException (which is a java.lang.RuntimeException), and (2) user-defined exceptions, which inherit from org.omg.CORBA.UserException (which is a java.lang.Exception).

factory object

A CORBA object that is used to create new CORBA objects. Factory objects are themselves usually created at server installation time.

idltojava compiler

A tool that takes an interface written in OMG IDL and produces Java programming language interfaces and classes that represent the mapping from the IDL interface to the Java programming language. The resulting files are .java files.

implementation

A concrete class that defines the behavior for all of the operations and attributes of the IDL interface it supports. A servant object is an instance of an implementation. There may be many implementations of a single interface.

initial naming context

The NamingContext object returned by a call to the method orb.resolve_initial_references("NameService"). It is an object reference to the COS Naming Service registered with the ORB and can be used to create other NamingContext objects.
See also: naming context

Interface Definition Language (IDL)

The OMG-standard language for defining the interfaces for all CORBA objects. An IDL interface declares a set of operations, exceptions, and attributes. Each operation has a signature, which defines its name, parameters, result and exceptions. OMG IDL does not include implementations for operations; rather, as its name indicates, it is simply a language for defining interfaces. The complete syntax and semantics for IDL are available in chapter 3 of the OMG specification at the OMG web site.

Interface Repository (IFR)

A service that contains all the registered component interfaces, the methods they support, and the parameters they require. The IFR stores, updates, and manages object interface definitions. Programs may use the IFR APIs to access and update this information. An IFR is not necessary for normal client/server interactions.

Internet InterORB Protocol (IIOP)

The OMG-specified network protocol for communicating between ORBs. Java IDL conforms to IIOP version 1.0.

invocation

The process of performing a method call on a CORBA object, which can be done without knowledge of the object's location on the network. Static invocation, which uses a client stub for the invocation and a server skeleton for the service being invoked, is used when the interface of the object is known at compile time. If the interface is not known at compile time, dynamic invocation must be used.

Java IDL

The classes, libraries, and tools that make it possible to use CORBA objects from the Java programming language. The main components of Java IDL are an ORB, a naming service, and the idltojava compiler. The ORB and naming service are part of JDK1.2; the idltojava compiler can be downloaded from the Java Developer Connection (JDC) web site.

name binding

The association of a name with an object reference. Name bindings are stored in a naming context.

namespace

A collection of naming contexts that are grouped together.

naming context

A CORBA object that supports the NamingContext interface and functions as a sort of directory which contains (points to) other naming contexts and/or simple names. Similar to a directory structure, where the last item is a file and preceding items are directories, in a naming context, the last item is an object reference name, and the preceding items are naming contexts.

naming service

A CORBA service that allows CORBA objects to be named by means of binding a name to an object reference. The name binding may be stored in the naming service, and a client may supply the name to obtain the desired object reference.

object

A computational grouping of operations and data into a modular unit. An object is defined by the interface it presents to others, its behavior when operations on its interface are invoked, and its state.

object implementation

See implementation.

Object Management Group (OMG)

An international organization with over 700 members that establishes industry guidelines and object management specifications in order to provide a common framework for object-oriented application development. Its members include platform vendors, object-oriented database vendors, software tool developers, corporate developers, and software application vendors. The OMG Common Object Request Broker Architecture specifies the CORBA object model. See www.omg.org for more information.

object reference

A construct containing the information needed to specify an object within an ORB. An object reference is used in method invocations to locate a CORBA object. Object references are the CORBA object equivalent to programming language-specific object pointers. They may be obtained from a factory object or from the Naming Service. An object reference, which is opaque (its internal structure is irrelevant to application developers), identifies the same CORBA object each time it is used. It is possible, however, for multiple object references to refer to the same CORBA object.

Object Request Broker (ORB)

The libraries, processes, and other infrastructure in a distributed environment that enable CORBA objects to communicate with each other. The ORB connects objects requesting services to the objects providing them.

operation (IDL)

The construct in an IDL interface that maps to a Java programming language method. For example, an interface ball might support the operation bounce. Operations may take parameters, return a result, or raise exceptions. IDL operations can be oneway, in which case they cannot return results (return values or out arguments) or raise exceptions.

parameter (IDL)

One or more objects the client passes to an IDL operation when it invokes the operation. Parameters may be declared as "in" (passed from client to server), "out" (passed from server to client), or "inout" (passed from client to server and then back from server to client).

PIDL (Pseudo-IDL)

The interface definition language for describing a CORBA pseudo-object. Each language mapping, including the mapping from IDL to the Java programming language, describes how pseudo objects are mapped to language-specific constructs. PIDL mappings may or may not follow the rules that apply to mapping regular CORBA objects.

pragma

A directive to the idltojava compiler to perform certain operations while compiling an IDL file. For example, the pragma "javaPackage" directs the idltojava compiler to put the Java programming language interfaces and classes it generates from the IDL interface into the Java programming language package specified.

pseudo-object

An object similar to a CORBA object in that it is described in IDL, but unlike a CORBA object, it cannot be passed around using its object reference, nor can it be narrowed or stringified. Examples of pseudo-objects include the Interface Repository and DII which, although implemented as libraries, are more clearly described in OMG specifications as pseudo-objects with IDL interfaces. The IDL for pseudo-objects is called "PIDL" to indicate that a pseudo-object is being defined.

servant object

An instance of an object implementation for an IDL interface. The servant object is registered with the ORB so that the ORB knows where to send invocations. It is the servant that performs the services requested when a CORBA object's method is invoked.

server

A program that contains the implementations of one or more IDL interfaces. For example, a desktop publishing server might implement a Document object type, a ParagraphTag object type, and other related object types. The server is required to register each implementation (servant object) with the ORB so that the ORB knows about the servant. Servers are sometimes referred to as object servers.

server skeleton

A public abstract class generated by the idltojava compiler that provides the ORB with information it needs in dispatching method invocations to the servant object(s). A server skeleton, like a client stub, is specific to the IDL interface from which it is generated. A server skeleton is the server side analog to a client stub, and these two classes are used by ORBs in static invocation.

service tier

The portion of a distributed application that contains the business logic and performs most of the computation. The service tier is typically located on a shared machine for optimum resource use.

static invocation

See invocation.

stringified object reference

An object reference that has been converted to a string so that it may be stored on disk in a text file (or stored in some other manner). Such strings should be treated as opaque because they are ORB-implementation independent. Standard object_to_string and string_to_object methods on org.omg.CORBA.Object make stringified references available to all CORBA Objects.

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