Java remote method invocation

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The Java Remote Method Invocation Application Programming Interface (API), or Java RMI, is a Java application programming interface that performs the object-oriented equivalent of remote procedure calls (RPC).

Usage of the term RMI may denote solely the programming interface or may signify both the API and JRMP, whereas the term RMI-IIOP (read: RMI over IIOP) denotes the RMI interface delegating most of the functionality to the supporting CORBA implementation.

The programmers of the original RMI API generalized the code somewhat to support different implementations, such as a HTTP transport. Additionally, the ability to pass arguments "by value" was added to CORBA in order to support the RMI interface. Still, the RMI-IIOP and JRMP implementations do not have fully identical interfaces.

RMI functionality comes in the package java.rmi, while most of Sun's implementation is located in the sun.rmi package. Note that with Java versions before Java 5.0 developers had to compile RMI stubs in a separate compilation step using rmic. Version 5.0 of Java and beyond no longer require this step.

Jini offers a more advanced version of RMI in Java. It functions similarly but provides more advanced searching capabilities and mechanisms for distributed object applications.[1]

Contents

Example

The following classes implement a simple client-server program using RMI that displays a message.

RmiServer class—Listens to RMI requests and implements the interface which is used by the client to invoke remote methods.

import java.rmi.Naming;
import java.rmi.RemoteException;
import java.rmi.RMISecurityManager;
import java.rmi.server.UnicastRemoteObject;
import java.rmi.registry.*; 
 
public class RmiServer extends UnicastRemoteObject 
    implements RmiServerIntf {
    public static final String MESSAGE = "Hello world";
 
    public RmiServer() throws RemoteException {
    }
 
    public String getMessage() {
        return MESSAGE;
    }
 
 
    public static void main(String args[]) {
        System.out.println("RMI server started");
 
        // Create and install a security manager
        if (System.getSecurityManager() == null) {
            System.setSecurityManager(new RMISecurityManager());
            System.out.println("Security manager installed.");
        } else {
            System.out.println("Security manager already exists.");
        }
 
        try { //special exception handler for registry creation
            LocateRegistry.createRegistry(1099); 
            System.out.println("java RMI registry created.");
        } catch (RemoteException e) {
            //do nothing, error means registry already exists
            System.out.println("java RMI registry already exists.");
        }
 
        try {
            //Instantiate RmiServer
            RmiServer obj = new RmiServer();
 
            // Bind this object instance to the name "RmiServer"
            Naming.rebind("//localhost/RmiServer", obj);
 
            System.out.println("PeerServer bound in registry");
        } catch (Exception e) {
            System.err.println("RMI server exception:" + e);
            e.printStackTrace();
        }
    }
}

RmiServerIntf class—Defines the interface that is used by the client and implemented by the server.

import java.rmi.Remote;
import java.rmi.RemoteException;
 
public interface RmiServerIntf extends Remote {
    public String getMessage() throws RemoteException;
}

RmiClient class—This is the client which gets the reference to the remote object and invokes its method to get a message.

import java.rmi.Naming;
import java.rmi.RemoteException;
import java.rmi.RMISecurityManager;
 
public class RmiClient { 
    // "obj" is the reference of the remote object
    RmiServerIntf obj = null; 
 
    public String getMessage() { 
        try { 
            obj = (RmiServerIntf)Naming.lookup("//localhost/RmiServer");
            return obj.getMessage(); 
        } catch (Exception e) { 
            System.err.println("RmiClient exception: " + e); 
            e.printStackTrace(); 
 
            return e.getMessage();
        } 
    } 
 
    public static void main(String args[]) {
        // Create and install a security manager
        if (System.getSecurityManager() == null) {
            System.setSecurityManager(new RMISecurityManager());
        }
 
        RmiClient cli = new RmiClient();
 
        System.out.println(cli.getMessage());
    }
}

Before running this subj, we need to make 'Stub' file of interface we used. For this task we have RMI compiller - 'rmic'

  • Note: we make stub file from *.class with implementation remote interface, not '*.java'*
rmic RmiServer

server.policy—This file is required on the server to allow TCP/IP communication for the remote registry and for the RMI server.

grant {
    permission java.net.SocketPermission "127.0.0.1:*", "connect,resolve";
    permission java.net.SocketPermission "127.0.0.1:*", "accept";
};

The server.policy file should be used using the D switch of Java RTE, e.g.:

java.exe -Djava.security.policy=server.policy RmiServer

client.policy—This file is required on the client to connect to RMI Server using TCP/IP.

grant {
    permission java.net.SocketPermission "127.0.0.1:*", "connect,resolve";
};

no.policy—Also if you have a troubles with connecting, try this file for server or client.

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