20/8/2014 Java remote method invocation

https://www.princeton.edu/~achaney/tmve/wiki100k/docs/Java_remote_method_invocation.html 1/2

Java remote method invocation

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

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

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

RMI functionality comes in the package java.rmi, while most of Sun's implementation islocated in the sun.rmi package. Note that with Java versions before Java 5.0 developershad to compile RMI stubs in a separate compilation step using rmic. Version 5.0 of Javaand 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

1 Example2 See also3 References4 External links

Example

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

RmiServer classListens to RMI requests and implements the interface which is used bythe 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);

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20/8/2014 Java remote method invocation

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System.out.println("PeerServer bound in registry"); } catch (Exception e) { System.err.println("RMI server exception:" + e); e.printStackTrace(); } }}

RmiServerIntf classDefines the interface that is used by the client and implemented bythe server.

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

RmiClient classThis is the client which gets the reference to the remote object andinvokes 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 wehave RMI compiller - 'rmic'

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

rmic RmiServer

server.policyThis file is required on the server to allow TCP/IP communication for theremote 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.policyThis 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.policyAlso if you have a troubles with connecting, try this file for server or client.

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