An Introduction To RMI With Java - The Concept (Page 2 of 5 )
As I mentioned earlier, RMI is a mechanism through which we can call remote methods as though they were residing on your own machine. This whole process will look transparent to the end user: if I showed you a live demo and didn't tell you it was RMI, then you wouldn't have even realized the method is actually being called from a different machine. Of course, a Java Virtual Machine must be present on both the machines.
Objects which have to be made available to other machines have to be exported to something called a Remote Registry Server so that they can be invoked. So if Machine A wants to call methods of some object on Machine B, then Machine B would have to export that object on its Remote Registry Server. Remote Registry Server is a service that runs on the server and helps client’s search and access objects on the server remotely. Now, if an object has to be capable of being exported then it must implement the Remote Interface present in the RMI package. For example, say that you want an object Xyz on machine A to be available for remote method invocation, then it must implement the Remote interface.
RMI uses something called a stub and a skeleton. The stub is present on the client side, and the skeleton the server side. When you call remote methods, you don't just go directly to other machine and say "hey, here's the method name, here are the parameters, just give me back what has to be returned and I am out of here".
There are a number of events that have to take place beforehand which help in the communication of the data. The stub is like a local object on the client side, which acts like a proxy of the object on the server side. It provides the methods to the client which can be invoked on the server. The Stub then sends the method call to the Skeleton, which is present on the server side. The Skeleton then implements the method on the server side.
The Stub and the Skeleton communicate with each other through something called a Remote Reference Layer. This layer gives the stub and skeleton the capability to send data using the TCP/IP protocol. Let's take a quick look at a simple technique called "Binding".
Whenever a client wants to make a reference to any object on the server, have you thought how he would tell the server what object he wants to create? Well, this is where this concept of "Binding" comes in. On the server end we associate a string variable with an object (we have methods to do this. We will learn more about these when we start coding). The client tells the server what object he wants to create by passing that string to the server, thus letting the server know exactly what object you are talking about. All of these strings and objects are stored in the Remote Registry Server on the server.