Client Implementation

The client in this example remotely invokes the getMessage() method in order to get the string "Hello World". Here is the code:

Listing 3.3 (HelloClient.java)

import java.rmi.*;
import java.net.MalformedURLException;

//Client implementation
public class HelloClient {
	
  //main method
  public static void main(String []helloClient) {
		
     try{
             //get a reference to the stub
             HelloInterface server = 
			(HelloInterface)Naming.lookup("rmi://localhost:1099/Server");
			
             //Call the remote method
             String msg = server.getMessage();
             System.out.println(msg);
         }catch(RemoteException ex) {
	   System.out.println("Error " +ex.getMessage());
         }catch(MalformedURLException ex) {
	   System.out.println("Error " +ex.getMessage());
         } 
         catch(NotBoundException ex) {
	   System.out.println("Error " +ex.getMessage());
         } 
    }
}

Explanation of Code

Import the necessary packages.

import java.rmi.*;
import java.net.MalformedURLException;

The client does not need to extend UnicastRemoteObject or implement any remote interface unless it is also exporting some objects. Here is the HelloClient class declaration:

public class HelloClient {

This client will run as a stand alone application so it must declare a main() method.

public static void main(String []helloClient) {
         try{
		
            //get a reference to the stub
            HelloInterface server = 
			(HelloInterface)Naming.lookup("rmi://localhost:1099/Server");
			
            //Call the remote method
            String msg = server.getMessage();
            System.out.println(msg);
         }catch(RemoteException ex) {
			System.out.println("Error " +ex.getMessage());
         }catch(MalformedURLException ex) {
			System.out.println("Error " +ex.getMessage());
        } catch(NotBoundException ex) {
			System.out.println("Error " +ex.getMessage());
        } 
  }

First, the client gets a reference to the remote object implementation from the server(s) registry. The Naming.lookup() method takes a URL-formatted string. It returns a stub, for the remote object associated with the specified name. The lookup() method may throw a number of exceptions:

• NotBoundException - if name is not currently bound
• RemoteException - if registry could not be contacted
• MalformedURLException - if the name is not an appropriately formatted URL

 HelloInterface server = (HelloInterface)Naming.lookup("Server");

After execution of the above statement, the variable "server" will contain a reference to an instance of the stub class that actually handles the client side RMI. Now the client can invoke methods on the remote object.

String msg = server.getMessage();

Finally, the program prints "HelloWorld" returned by the server.

System.out.println(msg);

The code for this example is complete.

Figure 3.3