IBM User Guide for Java V7 on Windows > IBM SDK for Java > The ORB > How the ORB works > The client side

Remote method invocation

The client holds a reference to the remote object that is an instance of the stub class. The next step is to call the method on that reference. The stub implements the Sample interface and therefore contains the message() method that the client has called.

First, the stub code determines whether the implementation of the remote object is located on the same ORB instance. If so, the object can be accessed without using the Internet.

If the implementation of the remote object is located on the same ORB instance, the performance improvement can be significant because a direct call to the object implementation is done. If no local servant can be found, the stub first asks the ORB to create a request by calling the _request() method, specifying the name of the method to call and whether a reply is expected or not.

The CORBA specification imposes an extra layer of indirection between the ORB code and the stub. This layer is commonly known as delegation. CORBA imposes the layer using an interface named Delegate. This interface specifies a portable API for ORB-vendor-specific implementation of the org.omg.CORBA.Object methods. Each stub contains a delegate object, to which all org.omg.CORBA.Object method invocations are forwarded. Using the delegate object means that a stub generated by the ORB from one vendor is able to work with the delegate from the ORB of another vendor.

When creating a request, the ORB first checks whether the enableLocateRequest property is set to true, in which case, a LocateRequest is created. The steps of creating this request are like the full Request case.

The ORB obtains the IOR of the remote object (the one that was retrieved by a naming service, for example) and passes the information that is contained in the IOR (Profile object) to the transport layer.

The transport layer uses the information that is in the IOR (IP address, port number, and object key) to create a connection if it does not exist. The ORB TCP/IP transport has an implementation of a table of cached connections for improving performances, because the creation of a new connection is a time-consuming process. The connection is not an open communication channel to the server host. It is only an object that has the potential to create and deliver a TCP/IP message to a location on the Internet. Typically, that involves the creation of a Java™ socket and a reader thread that is ready to intercept the server reply. The ORB.connect() method is called as part of this process.

When the ORB has the connection, it proceeds to create the Request message. The message contains the header and the body of the request. The CORBA 2.3 specification specifies the exact format. The header contains these items:

• Local IP address
• Local port
• Remote IP address
• Remote port
• Message size
• Version of the CORBA stream format
• Byte sequence convention
• Request types
• IDs

See ORB problem determination for a detailed description and example.

The body of the request contains several service contexts and the name and parameters of the method invocation. Parameters are typically serialized.

A service context is some extra information that the ORB includes in the request or reply, to add several other functions. CORBA defines a few service contexts, such as the codebase and the codeset service contexts. The first is used for the callback feature which is described in the CORBA specification. The second context is used to specify the encoding of strings.

In the next step, the stub calls _invoke(). The effect is to run the delegate invoke() method. The ORB in this chain of events calls the send() method on the connection that writes the request to the socket buffer and then flushes it away. The delegate invoke() method waits for a reply to arrive. The reader thread that was spun during the connection creation gets the reply message, processes it, and returns the correct object.

Parent: The client side

Previous topic: Obtaining the remote object

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