Java remote method invocation

A typical implementation model of Java-RMI usingstubandskeletonobjects. Java 2 SDK, Standard Edition, v1.2 removed the need for a skeleton.

TheJava Remote Method Invocation(Java RMI) is aJavaAPIthat performsremote method invocation,the object-oriented equivalent ofremote procedure calls(RPC), with support for direct transfer ofserializedJava classes anddistributed garbage-collection.

The original implementation depends onJava 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 aCORBAversion.

Usage of the termRMImay denote solely the programming interface or may signify both the API and JRMP,IIOP,or another implementation, whereas the termRMI-IIOP(read: RMI overIIOP) specifically denotes the RMI interface delegating most of the functionality to the supportingCORBAimplementation.

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 ofModula-3.

Generalized code

The programmers of the original RMI API generalized the code somewhat to support different implementations, such as aHTTPtransport. Additionally, the ability to pass arguments "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 packagejava.rmi,while most of Sun's implementation is located in thesun.rmipackage. Note that with Java versions before Java 5.0 developers had to compile RMI stubs in a separate compilation step usingrmic.Version 5.0 of Java and beyond no longer require this step.

Jini version

Jinioffers 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.

Example

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

RmiServerIntfinterface

defines the interface that is used by the client and implemented by the server.

importjava.rmi.Remote;
importjava.rmi.RemoteException;

publicinterfaceRmiServerIntfextendsRemote{
StringgetMessage()throwsRemoteException;
}

RmiServerclass

listens to RMI requests and implements the interface which is used by the client to invoke remote methods.

importjava.rmi.Naming;
importjava.rmi.RemoteException;
importjava.rmi.server.UnicastRemoteObject;
importjava.rmi.registry.*;

publicclassRmiServerextendsUnicastRemoteObjectimplementsRmiServerIntf{
publicstaticfinalStringMESSAGE="Hello World";

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

publicStringgetMessage(){
returnMESSAGE;
}

publicstaticvoidmain(Stringargs[])throwsException{
System.out.println("RMI server started");

try{//special exception handler for registry creation
LocateRegistry.createRegistry(1099);
System.out.println("java RMI registry created.");
}catch(RemoteExceptione){
//do nothing, error means registry already exists
System.out.println("java RMI registry already exists.");
}

//Instantiate RmiServer
RmiServerserver=newRmiServer();

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

RmiClientclass

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.

importjava.rmi.Naming;

publicclassRmiClient{
publicstaticvoidmain(Stringargs[])throwsException{
RmiServerIntfserver=(RmiServerIntf)Naming.lookup("//localhost/RmiServer");
System.out.println(server.getMessage());
}
}

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,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.

External links

• "Remote Method Invocation Home".Oracle Technology Network for Java Developers.Redwood Shores, CA, USA:Oracle Corporation.Retrieved2014-07-14.
• The Java RMI tutorial- a good starting point to learn RMI. Also check theHello World in RMI
• the Java RMI online training- Very good for training JavaRMI and as reference
• The RMI page in the JDK docs
• java.rmi(Sun's Java API Reference for the RMI package)
• Ann Wollrath; Roger Riggs;Jim Waldo."A Distributed Object Model for the Java System"(PDF).Archived(PDF)from the original on 2022-10-10.Retrieved2009-02-11.
• Programming WebLogic RMI- an introduction to RMI in Oracle Weblogic.
• General Remote Method Invocation