Java remote method invocation: Difference between revisions

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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 [[ClassDistributed skeletonobject 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]] (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 was later developed.

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 - 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 searchingsecurity, object discovery capabilities, and other mechanisms for [[distributed object]] applications.<ref name="From P2P to Web Services and Grids 2005">Taylor,{{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>

</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.

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

<sourcesyntaxhighlight lang=java>

import java.rmi.Remote;

import java.rmi.RemoteException;

public interface RmiServerIntf extends Remote {

public String getMessage() throws RemoteException;

}

</syntaxhighlight>

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

<sourcesyntaxhighlight 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

}

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}

public static void main(String args[] 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) {

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//Instantiate RmiServer

RmiServer objserver = new RmiServer();

RmiServer obj = new RmiServer();

// Bind this object instance to the name "RmiServer"

Naming.rebind("//localhost/RmiServer", objserver);

System.out.println("PeerServer bound in registry");

}

}

</syntaxhighlight>

</source>

'''; <code>RmiClient </code> class''' &mdash;: 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>[http{{cite web |last1=Wilson |first1=M. Jeff |date=2000-11-10 |df=mdy |url=https://www.javaworldinfoworld.com/javaworldarticle/jw2076234/get-11smart-2000/jwwith-1110proxies-smartproxyand-rmi.html |title=Get smart with proxies and RMI - |work=[[JavaWorld]] |access-date=2020-07-18}}</ref>

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

<sourcesyntaxhighlight lang=java>

import java.rmi.Remote;

import java.rmi.RemoteException;

public interface RmiServerIntf extends Remote {

public String getMessage() throws RemoteException;

}

</source>

'''<code>RmiClient </code> class''' &mdash; 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>[http://www.javaworld.com/javaworld/jw-11-2000/jw-1110-smartproxy.html Get smart with proxies and RMI - JavaWorld]</ref>

<source lang=java>

import java.rmi.Naming;

public class RmiClient {

public static void main(String args[] args) throws Exception {

RmiServerIntf objserver = (RmiServerIntf)Naming.lookup("//localhost/RmiServer");

System.out.println(objserver.getMessage());

}

}

</syntaxhighlight>

</source>

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.

<source lang=dos>

rmic RmiServer

</source>

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|accessdateaccess-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==

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==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 |accessdateaccess-date=2014-07-14}}

* [httphttps://downloaddocs.oracle.com/javase/tutorial/rmi/index.html The Java RMI tutorial] - a good starting point to learn RMI. Also check the [httphttps://javadocs.sunoracle.com/j2sejavase/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 the |date=12-08-2012|title=Java RMI online training] (Java 7)|nolink=y}} - Very good for training JavaRMI and as reference

* [httphttps://docs.oracle.com/javase/78/docs/technotes/guides/rmi/index.html The RMI page in the JDK 8 docs]

* {{Javadoc:SE|package=java.rmi|module=java.rmi|java/rmi}} (Sun's Java API Reference for the RMI package)

* {{cite paperweb | author1= Wollrath, Ann Wollrath | author2= Riggs, Roger Riggs | author3 = Jim Waldo, |author3-link=Jim Waldo

| title= A Distributed Object Model for the Java System | version= | pages= | publisher= | doi=

| 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 format|archive-date=2022-10-10 PDF|url-status=live | accessdateaccess-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:JavaJDK platformcomponents|RMI]]

[[Category:Remote procedure call]]

[[Category:Articles with example Java code]]