Java remote method invocation: Difference between revisions

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{{Short description|Java application-programming interface}}

{{one source|date=February 2012}}

[[ImageFile:RMI-Stubs-Skeletons.svg|thumb|right|400px|A typical implementation model of Java-RMI using [[Class stub|stub]] and [[ClassDistributed object skeletoncommunication#Skeleton|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 (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 objectsclasses 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 [[JRMPCommon Object Request Broker Architecture|CORBA]]) version.

# In order to support code running in a non-JVM context, 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, [[JRMPIIOP]], 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 supportbe 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 {

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.RMISecurityManager;

import java.rmi.server.UnicastRemoteObject;

import java.rmi.registry.*;

public class RmiServer extends UnicastRemoteObject implements RmiServerIntf {

public static final String MESSAGE = "Hello World";

implements RmiServerIntf {

public static final String MESSAGE = "Hello world";

public RmiServer() throws RemoteException {

super(0); // required to avoid the 'rmic' step, see below

}

Line 41 ⟶ 51:

}

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

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) {

Line 59 ⟶ 61:

System.out.println("java RMI registry already exists.");

}

try//Instantiate {RmiServer

RmiServer server = //Instantiatenew 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");

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

} catch (Exception e) {

System.err.println("RMI server exception:" + e);

e.printStackTrace();

}

}

}

</syntaxhighlight>

</source>

; <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>

'''<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;

import java.rmi.RMISecurityManager;

public class RmiClient {

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

// "obj" is the reference of the remote object

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

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

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());

}

}

</syntaxhighlight>

</source>

Before running this example, we need to make a 'stub' file offor the interface we used. For this task we have the RMI compillercompiler - '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 run times.<ref>{{cite web|title=Jave RMI Release Notes|url=http://docs.oracle.com/javase/1.5.0/docs/guide/rmi/relnotes.html|publisher=Oracle|accessdate=9 May 2012}}</ref>

'''<code>server.policy</code>''' &mdash; this file is required on the server to allow TCP/IP communication for the remote registry and for the RMI server.

<source lang=java>

grant {

permission java.net.SocketPermission "127.0.0.1:*", "connect,resolve";

permission java.net.SocketPermission "127.0.0.1:*", "accept";

};

</source>

The server.policy file should be used using the D switch of Java RTE, e.g.:

<source lang=dos>

java -Djava.security.policy=server.policy RmiServer

</source>

'''<code>client.policy</code>''' &mdash; this file is required on the client to connect to RMI Server using TCP/IP.

<source lang=java>

grant {

permission java.net.SocketPermission "127.0.0.1:*", "connect,resolve";

};

</source>

rmic RmiServer

'''<code>no.policy</code>''' &mdash; also if you have any troubles with connecting, try this file for server or client.

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.

<source lang=java>

grant {

permission java.security.AllPermission;

};

</source>

==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 RemoteShores, MethodCA, InvocationUSA Home|access-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 | authorauthor1= Wollrath, Ann Wollrath | coauthorsauthor2= Roger Riggs, Roger;| author3 = Jim Waldo, |author3-link=Jim Waldo

| title= A Distributed Object Model for the Java System | version= | pages= | publisher= | date= | doi= | url=http://pdos.csail.mit.edu/6.824/papers/waldo-rmi.pdf | format= PDF

| 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}}

| accessdate= 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]

* [http://robb19y.altervista.org/RMIHttpServlet RMI - Servlet Integration]

[[Category:JavaJDK platformcomponents|RMI]]

[[Category:Remote procedure call]]

[[Category:Articles with example Java code]]