visual basic barcode generator The CORBA Object Model in Java

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The CORBA Object Model
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Three elements of the CORBA model for objects which may be distributed over multiple machines connected by a network, are:
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Support for the near-transparent distribution of objects Object references Object adapters (which allow distributed objects to communicate among each other through the use of ORBs)
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The distribution of objects is transparent in the sense that the client using the services provided by another object is nearly unaware of the location of the other object. The other object may be located on a remote machine or it may be located locally. In other words, to a CORBA client, a remote method call looks exactly like a local method call.
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This transparency is made possible by the use of a client stub. Client stubs are discussed later in this section. In the case of distributed objects, two methods are possible for one application component to communicate with the object in another application. One method, which CORBA utilizes, is called passing by reference. In this method, the object being accessed remains in the remote application while an object reference is passed. Operations on the remote object are processed by the remote object itself. The method of passing by reference is illustrated in Figure 5.3. In this illustration, Application A first
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Step 1 Application A Reference Network Step 2 Application A Reference Method Invocation Application B Object Application B Object
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Network Step 3 Application A Reference Network Step 4 Application A Method Return Reference: Network
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Figure 5.3 Passing by reference
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Application B Processing Object
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Application B Object
Distributed Objects and Application Servers
obtains a reference to the remote object in Application B. Then Application A uses the reference to call a remote method on the remote object. Next, Application B processes the method, and finally the remote object sends the response back to Application A. The second method of object communication is called passing by value. In this case, a copy of the object being called is made, which is then sent to the client application by a process called serialization. Next, the client application invokes a method on the copy of the object it has received. In this method of communication, the operation performed cannot change the state of the original remote object. The process of passing by value is illustrated in Figure 5.4. First, Application A invokes a method on a remote object in Application B. Application B makes a copy of
Step 1 Application A Method Invocation Application B
Serialized Object
Object
Network Step 2 Application A Application B
Object s Copy Network Step 3 Application A Processing Object s Copy Network
Figure 5.4 Passing by value
Object
Application A
Object
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the object being called through serialization and sends the serialized copy to Application A. Next, Application A creates a copy of the object locally and invokes a method on this copy of the object. In this way, the original remote object state is not changed by the processing of the request. As mentioned previously, CORBA uses the passing-by-reference method of communication between object exclusively. However, Java RMI allows passing by value for communication between remote objects. Passing by value requires that the object be re-created at the client application. This, in turn, requires that the client be aware of the implementation details for the methods of the remote object. The third element of CORBA s object model is the object adapter. The primary purpose of an object adapter is to interface an object s implementation with its ORB. Three types of object adapters are provided by the CORBA specification. We will be mostly concerned with one type: Basic Object Adapter (BOA). BOA provides CORBA objects with a common set of methods for accessing ORB functions. These functions include object activation, authentication, and persistence. Every ORB implementation, which is CORBA-specification compliant, must provide a BOA.
Roles of the Client and Server in CORBA
As in RPC (or client/server architecture), CORBA maintains the basic notion of client and server. However, the distinction between the two is blurred somewhat, as we discuss in this section, thus moving toward a more peer-to-peer relationship. In CORBA, any application that creates an object and provides other applications with visibility to the object is termed server whereas other applications that use the services provided by the server are called clients. However, an application can act as a client to some applications while acting as a server to other applications. This is illustrated in Figure 5.5, which shows that Application A acts as a client to Application B. At the same time, Application B also acts as a client to Application C. Thus, Application B is simultaneously a CORBA server and a CORBA client. The second situation that blurs the distinction between server and client is when the client passes a reference to an object it owns to
Application A (client)
Application B (client & server)
Application C (server)
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