Java remote method invocation

A typical implementation model of Java-RMI using stub and skeleton objects. Java 2 SDK, Standard Edition, v1.2 removed the need for a skeleton.

The Java Remote Method Invocation Application Programming Interface (API), or Java RMI, is a Java API that performs the object-oriented equivalent of remote procedure calls (RPC), with support for direct transfer of serialized Java objects and distributed garbage collection.

1. The original implementation depends on Java Virtual Machine (JVM) class representation mechanisms and it thus only supports making calls from one JVM to another. The protocol underlying this Java-only implementation is known as Java Remote Method Protocol (JRMP).
2. In order to support code running in a non-JVM context, a CORBA version was later developed.

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.

Generalized code

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 version

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.

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 interface — 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 (a proxy) to the remote object living on the server and invokes its method to get a message. If the server object implemented java.io.Serializable instead of java.rmi.Remote, it would be serialized and passed to the client as a value.

import java.rmi.Naming;
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 example, we need to make a 'stub' file of the interface we used. For this task we have the RMI compiller - 'rmic'

• Note: we make a stub file from the '*.class' file with the implementation of the remote interface, not from the '*.java' file.

rmic RmiServer

Note that since version 5.0 of J2SE support for dynamically generated stub files has been added, and rmic is only provided for backwards compatibility with earlier run times.

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 -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 any troubles with connecting, try this file for server or client.

grant {
    permission java.security.AllPermission;
};

External links

• [1] - Remote Method Invocation Home
• The Java RMI tutorial - a good starting point to learn RMI. Also check the Hello World in RMI
• the Java RMI online training - Very good for training JavaRMI and as reference
• The RMI page in the JDK docs
• java.rmi (Sun's Java API Reference for the RMI package)
• Wollrath, Ann; Riggs, Roger; Waldo, Jim. A Distributed Object Model for the Java System (PDF). Retrieved 2009-02-11. 
• Programming WebLogic RMI - an introduction to RMI in Oracle Weblogic.
• General Remote Method Invocation
• RMI - Servlet Integration