java tutorial - RMI (Remote Method Invocation) - java programming - learn java - java basics - java for beginners

The RMI (Remote Method Invocation) is an API that provides a mechanism to create distributed application in java. The RMI allows an object to invoke methods on an object running in another JVM

The stub is an object, acts as a gateway for the client side. All the outgoing requests are routed through it. It resides at the client side and represents the remote object. When the caller invokes method on the stub object, it does the following tasks:

The skeleton is an object, acts as a gateway for the server side object. All the incoming requests are routed through it. When the skeleton receives the incoming request, it does the following tasks:

In the Java 2 SDK, an stub protocol was introduced that eliminates the need for skeletons.

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If any application performs these tasks, it can be distributed application.

The RMI application have all these features, so it is called the distributed application.

The is given the 6 steps to write the RMI program.

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import java.rmi.*;  
public interface Adder extends Remote{  
public int add(int x,int y)throws RemoteException;  
}  

Now provide the implementation of the remote interface. For providing the implementation of the Remote interface, we need to

In case, you extend the UnicastRemoteObject class, you must define a constructor that declares RemoteException.

import java.rmi.*;  
import java.rmi.server.*;  
public class AdderRemote extends UnicastRemoteObject implements Adder{  
AdderRemote()throws RemoteException{  
super();  
}  
public int add(int x,int y){return x+y;}  
}  

rmic AdderRemote  

rmiregistry 5000  

Now rmi services need to be hosted in a server process. The Naming class provides methods to get and store the remote object. The Naming class provides 5 methods.

import java.rmi.*;  
import java.rmi.registry.*;  
public class MyServer{  
public static void main(String args[]){  
try{  
Adder stub=new AdderRemote();  
Naming.rebind("rmi://localhost:5000/sonoo",stub);  
}catch(Exception e){System.out.println(e);}  
}  
}  

import java.rmi.*;  
public class MyClient{  
public static void main(String args[]){  
try{  
Adder stub=(Adder)Naming.lookup("rmi://localhost:5000/sonoo");  
System.out.println(stub.add(34,4));  
}catch(Exception e){}  
}  
}  

Example of rmi

For running this rmi example,  
  
1) compile all the java files  
  
javac *.java  
  
2)create stub and skeleton object by rmic tool  
  
rmic AdderRemote  
  
3)start rmi registry in one command prompt  
  
rmiregistry 5000  
  
4)start the server in another command prompt  
  
java MyServer  
  
5)start the client application in another command prompt  
  
java MyClient  

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Let's develop the RMI application by following the steps.

First of all, we need to create the table in the database. Here, we are using Oracle10 database.

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package com.wikitechy;  
public class Customer implements java.io.Serializable{  
    private int acc_no;  
    private String firstname,lastname,email;  
    private float amount;  
//getters and setters  
}  

package com.wikitechy;  
import java.rmi.*;  
import java.util.*;  
interface Bank extends Remote{  
public List<Customer> getCustomers()throws RemoteException;  
}  

package com.wikitechy;  
import java.rmi.*;  
import java.rmi.server.*;  
import java.sql.*;  
import java.util.*;  
class BankImpl extends UnicastRemoteObject implements Bank{  
BankImpl()throws RemoteException{}  
  
public List<Customer> getCustomers(){  
List<Customer> list=new ArrayList<Customer>();  
try{  
Class.forName("oracle.jdbc.driver.OracleDriver");  
Connection con=DriverManager.getConnection("jdbc:oracle:thin:@localhost:1521:xe","system","oracle");  
PreparedStatement ps=con.prepareStatement("select * from customer400");  
ResultSet rs=ps.executeQuery();  
  
while(rs.next()){  
Customer c=new Customer();  
c.setAcc_no(rs.getInt(1));  
c.setFirstname(rs.getString(2));  
c.setLastname(rs.getString(3));  
c.setEmail(rs.getString(4));  
c.setAmount(rs.getFloat(5));  
list.add(c);  
}  
  
con.close();  
}catch(Exception e){System.out.println(e);}  
return list;  
}//end of getCustomers()  
}  

package com.wikitechy;  
import java.rmi.*;  
public class MyServer{  
public static void main(String args[])throws Exception{  
Remote r=new BankImpl();  
Naming.rebind("rmi://localhost:6666/wikitechy",r);  
}}  

File: MyClient.java

package com.wikitechy;  
import java.util.*;  
import java.rmi.*;  
public class MyClient{  
public static void main(String args[])throws Exception{  
Bank b=(Bank)Naming.lookup("rmi://localhost:6666/wikitechy");  
  
List<Customer> list=b.getCustomers();  
for(Customer c:list){  
System.out.println(c.getAcc_no()+" "+c.getFirstname()+" "+c.getLastname()  
+" "+c.getEmail()+" "+c.getAmount());  
}  
  
}}