Institute of Information Science - University of Miskolc

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===== Simple Java-RMI Example =====

First of all, let's create an interaction diagram that demonstrates how the Client and RObject communicate with each other through RegisterService:

<mermaid>
sequenceDiagram
    participant C as Client
    participant RS as RegisterService
    participant RO as RObject
    participant RMI as RMI Registry

    RS->>RMI: Register RObject
    RMI-->>RS: RObject Registered
    C->>RMI: Lookup RObjectServer
    RMI-->>C: Return RObject
    C->>RO: primitiveArg(2012)
    RO-->>C: Acknowledge
    C->>RO: argumentByValue(2012)
    RO-->>C: Acknowledge
</mermaid>

This diagram illustrates the steps of the process: RegisterService registers the RObject in the RMI Registry, the Client queries the remote object, then uses its functions.

Now let's see what this system looks like in a structure diagram, which shows the components and their relationships:

<mermaid>
graph TD
    subgraph "RMI Registry"
      RO[RObject Interface]
    end

    subgraph "Server Side"
      ROS[RObjectImpl] -- Implements --> RO
    end

    subgraph "Client Side"
      C[Client] -- Uses --> RO
    end

    RegisterService -- Registers --> RO
    C -- Looks up --> RO
    C -- Calls methods on --> ROS
</mermaid>

The structure diagram clearly depicts the different parts of the system and the relationships between them:

The Server Side section contains the RObjectImpl implementation, which implements the RObject interface. The RMI Registry registers the interface, allowing the Client Side Client to find and use it.

The Client calls the methods of the remote object (ROS), illustrating the two main operations: passing a primitive and passing by value.

**Implementation:**

Install Gradle from here: https://gradle.org/releases/
Add the //bin// directory to the //PATH//.

The complete source code is available here:

  git clone https://github.com/knehez/isi.git
  cd java_rmi

Open two terminals, in one of them:

  gradle runRegisterService

In the other one:

  gradle run

The program in detail:

1.) Define the **RObject** interfaces, both implement the Remote interface

<sxh java>
package org.ait;
import java.rmi.*;

public interface RObject extends Remote {
  // simple argument passing
  void primitiveArg(int num) throws RemoteException;
  
  // pass by value argument
  void argumentByValue(Integer num) throws RemoteException;
}
</sxh>

2.) Implement the code for remote objects

<sxh java>
package org.ait;
import java.rmi.RemoteException;
import java.rmi.server.UnicastRemoteObject;

public class RObjectImpl extends UnicastRemoteObject implements RObject {
      private static final long serialVersionUID = 6350331764929058681L;
      public RObjectImpl() throws RemoteException {
      } 
  
      @Override
      public void primitiveArg(int num) throws RemoteException {
         System.out.println(num);
      }

      @Override
      public void argumentByValue(Integer num) throws RemoteException {
         System.out.println(num);
      }
}
</sxh>

3.) Start the RMI registry. This is a **Java JDK** component, to start it you should be in the created project's **/bin** directory.

4.) Create an instance of the remote object and bind it to the registry

<sxh java>
package org.ait;

import java.rmi.Naming;

public class RegisterService {
	/**
	 * @param args
	 */
	public static void main(String[] args) {
		try {
			java.rmi.registry.LocateRegistry.createRegistry(1099);
			RObject robj = new RObjectImpl();
			Naming.rebind("rmi://localhost:1099/RObjectServer", robj);
			System.out.println("Registered...");
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}
</sxh>

Start the RegisterService, it can only register if the rmiregistry is running.

5.) Use the remote object

Start the following code.

<sxh java>
package org.ait;

import java.rmi.Naming;

public class Client {
	public static void main(String[] args) {
		try {
			// Retrieving the remote object from the registry
			RObject robj = (RObject) Naming.lookup("rmi://localhost:1099/RObjectServer");

			// Simple argument
			robj.primitiveArg(2012);

			// Serialized argument
			robj.argumentByValue(new Integer(2012));

		} catch (Exception e) {
			e.printStackTrace();
			;
		}
	}
}
</sxh>