Remote Method Invocation in Java

Remote Method Invocation (RMI) in Java is an API that enables an object in one JVM to invoke methods on an object located in another JVM, either on the same machine or a remote system. It supports building distributed applications by allowing seamless client-server communication through method calls.

• Uses a client-server architecture where the client invokes methods on remote objects.
• Relies on the java.rmi package and requires remote interfaces extending Remote.
• Communication is managed internally by the JVM, simplifying remote interaction.

Stub (Client-side Proxy): It acts as a proxy for the remote object and forwards method calls from the client to the server.

The block consists of

• An identifier of the remote object to be used
• Method name which is to be invoked
• Parameters to the remote JVM

Working of RMI

Communication between client and server is handled using a Stub (client-side proxy), while server-side request handling is managed internally by the RMI runtime.

Note: In earlier RMI versions, Skeleton was used on the server side, but it is now deprecated and no longer required.

👁 Image

The steps to implement RMI are as follows

The following steps demonstrate how to build and run a basic RMI application in Java.

Step 1: Defining the remote interface

The first thing to do is to create an interface that will provide the description of the methods that can be invoked by remote clients. This interface should extend the Remote interface and the method prototype within the interface should throw the RemoteException.

Example:

Step 2: Implementing the remote interface

The next step is to implement the remote interface. To implement the remote interface, the class should extend to UnicastRemoteObject class of java.rmi package. Also, a default constructor needs to be created to throw the java.rmi.RemoteException from its parent constructor in class.

Step 3: No need to generate Stub/Skeleton manually

In modern Java, stub classes are generated dynamically by the JVM, so the rmic tool is not required.

Step 4: Start the rmiregistry

Start the registry service by issuing the following command at the command prompt start rmiregistry

Step 5: Create and execute the server application program 

The next step is to create the server application program and execute it on a separate command prompt.

• The server program uses createRegistry method of LocateRegistry class to create rmiregistry within the server JVM with the port number passed as an argument.
• The rebind method of Naming class is used to bind the remote object to the new name.

Step 6: Create and execute the client application program

The last step is to create the client application program and execute it on a separate command prompt . The lookup method of the Naming class is used to get the reference of the Stub object.

Note: The above client and server program is executed on the same machine so localhost is used. In order to access the remote object from another machine, localhost is to be replaced with the IP address where the remote object is present.

save the files respectively as per class name as

Search.java , SearchQuery.java , SearchServer.java & ClientRequest.java

Important Observations:

• RMI is a pure java solution to Remote Procedure Calls (RPC) and is used to create the distributed applications in java.
• Stub objects are used on the client side, while server-side communication is handled internally by the RMI runtime.