Transcript Document

Remote Method
Invocation
Introduction
Remote Method Invocation (RMI) is a
distributed systems technology that allows
one JVM to invoke object methods that will
be run on another JVM located elsewhere
on a network.
 This technology is very important for the
development of large-scale systems, as it
makes it possible to distribute resources
and processing load across more than one
machine.

RMI is a Java technology that allows one
JVM to communicate with another JVM
and have it execute an object method.
 Using RMI, objects can invoke methods on
other objects located remotely as easily as
if they were on the local host machine.

Java Virtual Machine
Object
main(String[ ] args)
Method request
Java Virtual Machine
Method response
RemoteObject
method1(…)
method2(…)
Each RMI service is defined by an
interface, which describes object methods
that can be executed remotely. This
interface must be shared by all developers
who will write software for that service.
 More than one implementation of the
interface can be created, and developers
do not need to be aware of which
implementation is being used or where it is
located.

How Does RMI Work?

Systems that use RMI for communication
typically are divided into two categories:
 Servers
A server provides an RMI service
 Clients
A client invokes object methods of this service.
RMI servers must register with a lookup
service, to allow clients to find them, or they
can make available a reference to the
service in some other way.
 The Java platform includes an application
called rmiregistry (which runs as a separate
process). This application allows other
applications to register RMI services or
obtain a reference to a named service.



Each service registration is associated with a
name to allow clients to select the appropriate
service.
If a service is moved to another server, the client
need only look up the registry again to find the
new location.
RMI REGISTRY
RMI Server
RMI Server
RMI Server

Once a server has registered, it will then
wait for incoming RMI requests from
clients.
RMI clients will send RMI messages to
invoke an object method remotely.
However, a client must first obtain a
reference for the remote object which is
normally done by looking up a service in the
RMI registry.
 The client requests a particular service
name, and receives a URL to the remote
resource. The URL has the following
format:

rmi://hostname:port/servicename
Once an object reference is obtained, the
client can then interact with the remote
service.
 The networking details of requests are
completely transparent to the application
developer. This is achieved through a
division of the RMI system into two
components i.e. the stub and the skeleton.

The stub object acts as a proxy object,
conveying object requests to the remote
RMI server. It implements a particular RMI
interface, which the client application can
use just like any other object
implementation.
 When the stub object receives a request, it
passes a message to a remote RMI
service, waits for a response, and returns
the response to the calling method.


Hence, the application developer need not
be concerned about where the RMI
resource is located, on which platform it is
running, or how it will fulfill the request.
The RMI client simply invokes a method of
the proxy object which handles all the
implementation details.
RMI client application
stub object
somemethod(…)
request
RMI server
response
skeleton object
somemethod(…)



At the RMI server end, the skeleton object is
responsible for listening for incoming RMI
requests and passing these on to the RMI
service.
Note that the skeleton object does not provide
an implementation of an RMI service. It only
acts as a receiver of requests and passes these
requests on further to the actual implementation
object that implements the RMI interface.
The implementation object executes the
appropriate method and pass the results back to
the stub object in the RMI client.

TCP sockets are used in the
communication that occurs between stub
and skeleton.
LightBulb Example:
Defining an RMI Service Interface
Any system that uses RMI will use a
service interface.
 An RMI service interface defines the
object methods that can be invoked
remotely.
 Stub and skeleton objects, as well as the
RMI service, must implement this
interface.

All RMI service interfaces extend the
java.rmi.Remote interface.
 Only methods defined in a
java.rmi.Remote interface (or its
subclasses) may be executed remotely other methods of an object are hidden
from RMI clients.


Example:
import java.rmi.*;
public interface RMILightBulb extends Remote {
public void on() throws RemoteException;
public void off() throws RemoteException;
public boolean isOn() throws RemoteException;
}

Note that remote methods must be
declared as "throws
java.rmi.RemoteException" as network
errors might occur during communication.
LightBulb Example:
Implementing an RMI Service
Interface
Once a service interface is defined, the
next step is to implement it.
 The implementation not only implements
each method in the RMI interface but can
also define additional methods. However,
only those methods defined in the RMI
interface will be accessible remotely.

Code
LightBulb Example:
Creating Stub and Skeleton
Classes


Stub and skeleton classes are responsible for
dispatching and processing RMI requests.
Developers should not write these classes.
They should be generated using the rmic tool
provided in the JDK.
 Compile
the RMI interface and implementation.
 Run the rmic tool as follows:
rmic RMILightBulbImpl

Two files will be produced:
RMILightBulbImpl_Stub.class
RMILightBulbImpl_Skeleton.class
LightBulb Example:
Creating an RMI Server

The RMI server is responsible for creating
an instance of a service implementation
and then registering it with the RMI
registry.
Code
LightBulb Example:
Creating an RMI Client


An RMI client needs to obtain an object
reference to the remote interface, and doesn't
need to be concerned with how messages are
sent or received or the location of the service.
To find the service initially, a lookup in the RMI
registry is made, and after that, the client can
invoke methods of the service interface just as if
it were a local object.
Code
LightBulb Example:
Running the RMI System

The following steps should be followed:
1.
2.
3.
4.
5.
Copy all necessary files to a directory on the
local file system of all clients and the server.
Check that the current directory is included
in the classpath, or an alternate directory
where the classes are located.
Change to the directory where the files are
located, and run the rmiregistry command.
Run the RMI server: java LightBulbServer
Run the RMI client: java LightBulbClient