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{{Short description|Java application-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]] [[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]] (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 [[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 - and static stubs have been deprecated since Java 8.

==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. This extends the {{Javadoc:SE|module=java.rmi|package=java.rmi|java/rmi|Remote}} interface, which serves to identify an implementing class as one with remotely-invokable methods.

<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 (computer networking)|port number]] (or zero) when exporting remote objects, which is required for generated stubs to be possible, as described in the [[Javadoc]] for {{Javadoc:SE|module=java.rmi|java/rmi/server|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}}
* [https://docs.oracle.com/javase/tutorial/rmi/index.html The Java RMI tutorial] - a good starting point to learn RMI. Also check the [https://docs.oracle.com/javase/1.5.0/docs/guide/rmi/hello/hello-world.html Hello World in RMI (Java 5)]
* {{Webarchive|url=https://web.archive.org/web/20120812071936/http://java.sun.com/developer/onlineTraining/rmi/RMI.html|date=12-08-2012|title=Java RMI online training (Java 7)|nolink=y}} - Very good for training JavaRMI and as reference
* [https://docs.oracle.com/javase/8/docs/technotes/guides/rmi/index.html The RMI page in the JDK 8 docs]
* {{Javadoc:SE|package=java.rmi|module=java.rmi|java/rmi}} (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]]

Java remote method invocation - Revision history

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