Remote Method Invocation in Java

Remote Method Invocation (RMI) is an API which allows an object to invoke a method on an object that exists in another address space, which could be on the same machine or on a remote machine. Through RMI, object running in a JVM present on a computer (Client side) can invoke methods on an object present in another JVM (Server side). RMI creates a public remote server object that enables client and server side communications through simple method calls on the server object.

Working of RMI

The communication between client and server is handled by using two intermediate objects: Stub object (on client side) and Skeleton object (on server side).

Stub Object

The stub object on the client machine builds an information block and sends this information to the server. The block consists of

• An identifier of the remote object to be used
• Method name which is to be invoked
• Parameters to the remote JVM

Skeleton Object

The skeleton object passes the request from the stub object to the remote object. It performs following tasks

• It calls the desired method on the real object present on the server.
• It forwards the parameters received from the stub object to the method.

Steps to implement Interface

1. Defining a remote interface
2. Implementing the remote interface
3. Creating Stub and Skeleton objects from the implementation class using rmic (rmi complier)
4. Start the rmiregistry
5. Create and execute the server application program
6. Create and execute the client application program.

Step 1: Defining the remote interface
The first thing to do is to create an interface which will provide the description of the methods that can be invoked by remote clients. This interface should extend the Remote interface and the method prototype within the interface should throw the RemoteException.

// Creating a Search interface import java.rmi.*; public interface Search extends Remote {     // Declaring the method prototype     public String query(String search) throws RemoteException; }Step 2: Implementing the remote interface

The next step is to implement the remote interface. To implement the remote interface, the class should extend to UnicastRemoteObject class of java.rmi package. Also, a default constructor needs to be created to throw the java.rmi.RemoteException from its parent constructor in class.

// Java program to implement the Search interface import java.rmi.*; import java.rmi.server.*; public class SearchQuery extends UnicastRemoteObject                          implements Search {     // Default constructor to throw RemoteException     // from its parent constructor     SearchQuery() throws RemoteException     {         super();     }     // Implementation of the query interface     public String query(String search)                        throws RemoteException     {         String result;         if (search.equals("Reflection in Java"))             result = "Found";         else             result = "Not Found";         return result;     } }

Step 3:  Creating Stub and Skeleton objects from the implementation class using rmic
The rmic tool is used to invoke the rmi compiler that creates the Stub and Skeleton objects. Its prototype is
rmic classname. For above program the following command need to be executed at the command prompt
rmic SearchQuery

STEP 4: Start the rmiregistry
Start the registry service by issuing the following command at the command prompt start rmiregistry

STEP 5: Create and execute the server application program
The next step is to create the server application program and execute it on a separate command prompt.

• The server program uses createRegistry method of LocateRegistry class to create rmiregistry within the server JVM with the port number passed as argument.
• The rebind method of Naming class is used to bind the remote object to the new name.

//program for server application import java.rmi.*; import java.rmi.registry.*; public class SearchServer {     public static void main(String args[])     {         try         {             // Create an object of the interface             // implementation class             Search obj = new SearchQuery();             // rmiregistry within the server JVM with             // port number 1900             LocateRegistry.createRegistry(1900);             // Binds the remote object by the name             // <a href="#">geeksforgeeks</a>             Naming.rebind("rmi://localhost:1900"+                           "/<a href="#">geeksforgeeks</a>",obj);         }         catch(Exception ae)         {             System.out.println(ae);         }     } }

Step 6: Create and execute the client application program
The last step is to create the client application program and execute it on a separate command prompt .The lookup method of Naming class is used to get the reference of the Stub object.

//program for client application import java.rmi.*; public class ClientRequest {     public static void main(String args[])     {         String answer,value="Reflection in Java";         try         {             // lookup method to find reference of remote object             Search access =                 (Search)Naming.lookup("rmi://localhost:1900"+                                       "/<a href="#">geeksforgeeks</a>");             answer = access.query(value);             System.out.println("Article on " + value +                             " " + answer+" at <a href="#">GeeksforGeeks</a>");         }         catch(Exception ae)         {             System.out.println(ae);         }     } }