Java rmi tutorial

Java RMI
i
AbouttheTutorial
RMI stands for Remote Method Invocation. It is a mechanism that allows an object
residing in one system (JVM) to access/invoke an object running on another JVM.
RMI is used to build distributed applications; it provides remote communication between
Java programs. It is provided in the package java.rmi.
Audience
This tutorial has been prepared for beginners to make them understand the basics of
Remote Method Invocation in Java.
Prerequisites
For this tutorial, it is assumed that the readers have a prior knowledge of Java
programming language. In some of the programs of this tutorial, we have used JavaFX for
GUI purpose. So, it is recommended that you go through our JavaFX tutorial before
proceeding further. http://www.tutorialspoint.com/javafx/
Copyright&Disclaimer
© Copyright 2017 by Tutorials Point (I) Pvt. Ltd.
All the content and graphics published in this e-book are the property of Tutorials Point (I)
Pvt. Ltd. The user of this e-book is prohibited to reuse, retain, copy, distribute or republish
any contents or a part of contents of this e-book in any manner without written consent
of the publisher.
We strive to update the contents of our website and tutorials as timely and as precisely as
possible, however, the contents may contain inaccuracies or errors. Tutorials Point (I) Pvt.
Ltd. provides no guarantee regarding the accuracy, timeliness or completeness of our
website or its contents including this tutorial. If you discover any errors on our website or
in this tutorial, please notify us at contact@tutorialspoint.com

Java RMI
ii
TableofContents
About the Tutorial .....................................................................................................................................i
Audience....................................................................................................................................................i
Prerequisites..............................................................................................................................................i
Copyright & Disclaimer ..............................................................................................................................i
Table of Contents......................................................................................................................................ii
JAVA RMI ─ INTRODUCTION.................................................................................................1
Architecture of an RMI Application...........................................................................................................1
Working of an RMI Application.................................................................................................................2
Marshalling and Unmarshalling ................................................................................................................2
RMI Registry .............................................................................................................................................3
Goals of RMI .............................................................................................................................................3
JAVA RMI ─ RMI APPLICATION..............................................................................................4
Defining the Remote Interface..................................................................................................................4
Developing the Implementation Class (Remote Object)............................................................................5
Developing the Server Program ................................................................................................................5
Developing the Client Program .................................................................................................................7
Compiling the Application.........................................................................................................................8
Executing the Application .........................................................................................................................9
JAVA RMI ─ GUI APPLICATION............................................................................................11
Server Program.......................................................................................................................................15
Client Program........................................................................................................................................16
Steps to Run the Example .......................................................................................................................17

Java RMI
iii
JAVA RMI ─ DATABASE APPLICATION .................................................................................20
Creating a Student Class..........................................................................................................................20
Server Program.......................................................................................................................................24
Client Program........................................................................................................................................25
Steps to Run the Example .......................................................................................................................26

Java RMI
4
RMI stands for Remote Method Invocation. It is a mechanism that allows an object residing
in one system (JVM) to access/invoke an object running on another JVM.
RMI is used to build distributed applications; it provides remote communication between Java
programs. It is provided in the package java.rmi.
ArchitectureofanRMIApplication
In an RMI application, we write two programs, a server program (resides on the server) and
a client program (resides on the client).
 Inside the server program, a remote object is created and reference of that object is
made available for the client (using the registry).
 The client program requests the remote objects on the server and tries to invoke its
methods.
The following diagram shows the architecture of an RMI application.
JAVA RMI ─ INTRODUCTION

Java RMI
5
Let us now discuss the components of this architecture.
 Transport Layer ─ This layer connects the client and the server. It manages the
existing connection and also sets up new connections.
 Stub ─ A stub is a representation (proxy) of the remote object at client. It resides in
the client system; it acts as a gateway for the client program.
 Skeleton ─ This is the object which resides on the server side. stub communicates
with this skeleton to pass request to the remote object.
 RRL (Remote Reference Layer) ─ It is the layer which manages the references
made by the client to the remote object.
WorkingofanRMIApplication
The following points summarize how an RMI application works:
 When the client makes a call to the remote object, it is received by the stub which
eventually passes this request to the RRL.
 When the client-side RRL receives the request, it invokes a method called invoke() of
the object remoteRef. It passes the request to the RRL on the server side.
 The RRL on the server side passes the request to the Skeleton (proxy on the server)
which finally invokes the required object on the server.
 The result is passed all the way back to the client.
MarshallingandUnmarshalling
Whenever a client invokes a method that accepts parameters on a remote object, the
parameters are bundled into a message before being sent over the network. These
parameters may be of primitive type or objects. In case of primitive type, the parameters are
put together and a header is attached to it. In case the parameters are objects, then they are
serialized. This process is known as marshalling.
At the server side, the packed parameters are unbundled and then the required method is
invoked. This process is known as unmarshalling.

Java RMI
6
RMIRegistry
RMIregistry is a namespace on which all server objects are placed. Each time the server
creates an object, it registers this object with the RMIregistry (using bind() or reBind()
methods). These are registered using a unique name known as bind name.
To invoke a remote object, the client needs a reference of that object. At that time, the client
fetches the object from the registry using its bind name (using lookup() method).
The following illustration explains the entire process:
GoalsofRMI
Following are the goals of RMI:
 To minimize the complexity of the application
 To preserve type safety

Java RMI
7
 Distributed garbage collection
 Minimize the difference between working with local and remote objects

Java RMI
8
To write an RMI Java application, you would have to follow the steps given below:
 Define the remote interface
 Develop the implementation class (remote object)
 Develop the server program
 Develop the client program
 Compile the application
 Execute the application
DefiningtheRemoteInterface
A remote interface provides the description of all the methods of a particular remote object.
The client communicates with this remote interface.
To create a remote interface –
 Create an interface that extends the predefined interface Remote which belongs to
the package.
 Declare all the business methods that can be invoked by the client in this interface.
 Since there is a chance of network issues during remote calls, an exception named
RemoteException may occur; throw it.
Following is an example of a remote interface. Here we have defined an interface with the
name Hello and it has a method called printMsg().
import java.rmi.Remote;
import java.rmi.RemoteException;
// Creating Remote interface for our application
public interface Hello extends Remote {
void printMsg() throws RemoteException;
}
JAVA RMI ─ RMI APPLICATION

Java RMI
9
DevelopingtheImplementationClass(RemoteObject)
We need to implement the remote interface created in the earlier step. (We can write an
implementation class separately or we can directly make the server program implement this
interface.)
To develop an implementation class –
 Implement the interface created in the previous step.
 Provide implementation to all the abstract methods of the remote interface.
Following is an implementation class. Here, we have created a class named ImplExample
and implemented the interface Hello created in the previous step and provided body for this
method which prints a message.
// Implementing the remote interface
public class ImplExample implements Hello
{
// Implementing the interface method
public void printMsg() {
System.out.println("This is an example RMI program");
}
}
DevelopingtheServerProgram
An RMI server program should implement the remote interface or extend the implementation
class. Here, we should create a remote object and bind it to the RMIregistry.
To develop a server program –
 Create a class that extends the implementation class implemented in the previous
step. (or implement the remote interface)
 Create a remote object by instantiating the implementation class as shown below.
 Export the remote object using the method exportObject() of the class named
UnicastRemoteObject which belongs to the package java.rmi.server.
 Get the RMI registry using the getRegistry() method of the LocateRegistry class
which belongs to the package java.rmi.registry.
 Bind the remote object created to the registry using the bind() method of the class
named Registry. To this method, pass a string representing the bind name and the
object exported, as parameters.

Java RMI
10
Following is an example of an RMI server program.
import java.rmi.registry.Registry;
import java.rmi.registry.LocateRegistry;
import java.rmi.RemoteException;
import java.rmi.server.UnicastRemoteObject;
public class Server extends ImplExample{
public Server() {}
public static void main(String args[]) {
try {
// Instantiating the implementation class
ImplExample obj = new ImplExample();
// Exporting the object of implementation class
// (here we are exporting the remote object to the stub)
Hello stub = (Hello) UnicastRemoteObject.exportObject(obj, 0);
// Binding the remote object (stub) in the registry
Registry registry = LocateRegistry.getRegistry();
registry.bind("Hello", stub);
System.err.println("Server ready");
} catch (Exception e) {
System.err.println("Server exception: " + e.toString());
e.printStackTrace();
}
}

Java RMI
11
}
DevelopingtheClientProgram
Write a client program in it, fetch the remote object and invoke the required method using
this object.
To develop a client program –
 Create a client class from where you want invoke the remote object.
 Get the RMI registry using the getRegistry() method of the LocateRegistry class
which belongs to the package java.rmi.registry.
 Fetch the object from the registry using the method lookup() of the class Registry
which belongs to the package java.rmi.registry. To this method you need to pass a
string value representing the bind name as a parameter. This will return you the
remote object down cast it.
 The lookup() returns an object of type remote, down cast it to the type Hello.
 Finally invoke the required method using the obtained remote object.
Following is an example of an RMI client program.
import java.rmi.registry.LocateRegistry;
import java.rmi.registry.Registry;
public class Client {
private Client() {}
public static void main(String[] args) {
try {
// Getting the registry
Registry registry = LocateRegistry.getRegistry(null);
// Looking up the registry for the remote object
Hello stub = (Hello) registry.lookup("Hello");

Java RMI
12
// Calling the remote method using the obtained object
stub.printMsg();
// System.out.println("Remote method invoked");
} catch (Exception e) {
System.err.println("Client exception: " + e.toString());
e.printStackTrace();
}
}
}
CompilingtheApplication
To compile the application –
 Compile the Remote interface.
 Compile the implementation class.
 Compile the server program.
 Compile the client program.
Or,
Open the folder where you have stored all the programs and compile all the Java files as
shown below.
Javac *.java

Java RMI
13
ExecutingtheApplication
Step 1: Start the rmi registry using the following command.
start rmiregistry
This will start an rmi registry on a separate window as shown below.

Java RMI
14
Step 2: Run the server class file as shown below.
Java Server
Step 3: Run the client class file as shown below.
java Client

Java RMI
15
Verification: As soon you start the client, you would see the following output in the server.

Java RMI
16
End of ebook preview
If you liked what you saw…
Buy it from our store @ https://store.tutorialspoint.com

Java rmi tutorial

Recommended

More Related Content

What's hot (20)

Similar to Java rmi tutorial (20)

More from HarikaReddy115 (20)

Recently uploaded (20)

Java rmi tutorial