Editing Java remote method invocation

{{Short description|Programming interface}}
[[File:RMI-Stubs-Skeletons.svg|thumb|right|400px|A typical implementation model of Java-RMI using [[Class stub|stub]] and [[Distributed object communication#Skeleton|skeleton]] objects. Java 2 SDK, Standard Edition, v1.2 removed the need for a skeleton.]]

The '''Java Remote Method Invocation''' ('''Java RMI''') is a [[Java (programming language)|Java]] [[Application programming interface|API]] that performs [[remote method invocation]], the object-oriented equivalent of [[remote procedure call]]s (RPC), with support for direct transfer of [[Serialization#Java|serialized]] Java classes and [[Distributed Garbage Collection|distributed garbage-collection]].

The original implementation depends on [[Java virtual machine|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). In order to support code running in a non-JVM context, programmers later developed a [[Common Object Request Broker Architecture|CORBA]] version.

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

The basic idea of Java RMI, the distributed garbage-collection (DGC) protocol, and much of the architecture underlying the original Sun implementation, come from the "network objects" feature of [[Modula-3]].

==Generalized code==
The programmers of the original RMI API generalized the code somewhat to support different implementations, such as a [[Hypertext Transfer Protocol|HTTP]] transport. Additionally, the ability to pass arguments "[[Call by value|by value]]" was added to CORBA in order to be compatible with the RMI interface. Still, the RMI-IIOP and JRMP implementations do not have fully identical interfaces.

RMI functionality comes in the package {{Javadoc:SE|package=java.rmi|java/rmi|module=java.rmi}}, while most of Sun's implementation is located in the <code>sun.rmi</code> package. Note that with Java versions before Java 5.0 developers had to compile RMI stubs in a separate compilation step using <code>'''rmic'''</code>. 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 security, object discovery capabilities, and other mechanisms for distributed object applications.<ref name="From P2P to Web Services and Grids 2005">{{cite book |first=Ian J |last=Taylor |title=From P2P to Web Services and Grids : Peers in a Client/Server World |series=Computer Communications and Networks |publisher=Springer-Verlag |location=London |year=2005 |isbn=1852338695 |doi=10.1007/b138333 |url=https://archive.org/details/fromp2ptowebserv0000tayl |oclc=827073874 |url-access=registration }}{{page needed|date=September 2017}}</ref>


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

; <code>RmiServerIntf</code> interface : defines the interface that is used by the client and implemented by the server.

<syntaxhighlight lang=java>
import java.rmi.Remote;
import java.rmi.RemoteException;

public interface RmiServerIntf extends Remote {
    String getMessage() throws RemoteException;
}
</syntaxhighlight>

; <code>RmiServer</code> class : listens to RMI requests and implements the interface which is used by the client to invoke remote methods.

<syntaxhighlight lang=java>
import java.rmi.Naming;
import java.rmi.RemoteException;
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 {
        super(0); // required to avoid the 'rmic' step, see below
    }

    public String getMessage() {
        return MESSAGE;
    }

    public static void main(String args[]) throws Exception {
        System.out.println("RMI server started");

        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.");
        }
           
        //Instantiate RmiServer
        RmiServer server = new RmiServer();

        // Bind this object instance to the name "RmiServer"
        Naming.rebind("//localhost/RmiServer", server);
        System.out.println("PeerServer bound in registry");
    }
}
</syntaxhighlight>

; <code>RmiClient</code> 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.<ref>{{cite web |last1=Wilson |first1=M. Jeff |date=2000-11-10 |df=mdy |url=https://www.infoworld.com/article/2076234/get-smart-with-proxies-and-rmi.html |title=Get smart with proxies and RMI |work=[[JavaWorld]] |access-date=2020-07-18}}</ref>

<syntaxhighlight lang=java>
import java.rmi.Naming;

public class RmiClient {
    public static void main(String args[]) throws Exception {
        RmiServerIntf server = (RmiServerIntf)Naming.lookup("//localhost/RmiServer");
        System.out.println(server.getMessage());
    }
}
</syntaxhighlight>

Before running this example, we need to make a 'stub' file for the interface we used. For this task we have the RMI compiler - '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 runtimes,<ref>{{cite web|title=Java RMI Release Notes|url=http://docs.oracle.com/javase/1.5.0/docs/guide/rmi/relnotes.html|publisher=Oracle|access-date=9 May 2012}}</ref> or for programs that don't provide an explicit port number (or zero) when exporting remote objects, which is required for generated stubs to be possible, as described in the Javadoc for UnicastRemoteObject. See the comment in the constructor above.

==References==
{{reflist}}

==External links==
*{{cite web |title=Remote Method Invocation Home |url=http://www.oracle.com/technetwork/java/javase/tech/index-jsp-136424.html |website= Oracle Technology Network for Java Developers |publisher=[[Oracle Corporation]] |location=Redwood Shores, CA, USA |access-date=2014-07-14}}
* [http://download.oracle.com/javase/tutorial/rmi/index.html The Java RMI tutorial] - a good starting point to learn RMI. Also check the [http://java.sun.com/j2se/1.5.0/docs/guide/rmi/hello/hello-world.html Hello World in RMI]
* [http://java.sun.com/developer/onlineTraining/rmi/RMI.html the Java RMI online training] - Very good for training JavaRMI and as reference
* [http://docs.oracle.com/javase/7/docs/technotes/guides/rmi/index.html The RMI page in the JDK docs]
* {{Javadoc:SE|package=java.rmi|java/rmi}} (Sun's Java API Reference for the RMI package)
* {{cite web | author1= Ann Wollrath | author2= Roger Riggs | author3 = Jim Waldo |author3-link=Jim Waldo
  | title= A Distributed Object Model for the Java System 
  | url=http://pdos.csail.mit.edu/6.824/papers/waldo-rmi.pdf |archive-url=https://ghostarchive.org/archive/20221010/http://pdos.csail.mit.edu/6.824/papers/waldo-rmi.pdf |archive-date=2022-10-10 |url-status=live | access-date= 2009-02-11}}
* [http://docs.oracle.com/cd/E12840_01/wls/docs103/rmi/rmi_intro.html Programming WebLogic RMI] - an introduction to RMI in Oracle Weblogic.
* [http://notes.corewebprogramming.com/student/RMI.pdf General Remote Method Invocation]

[[Category:JDK components|RMI]]
[[Category:Remote procedure call]]
[[Category:Articles with example Java code]]