Java programming course: 20.4 Networking

• Previous lesson: 20.3 JavaBeans
• Course contents 

Networking

Two of the main ways of programming networking applications in Java is through sockets and Remote Method Invocation (RMI):  

Sockets

Java supports connection to other hosts through the use of sockets. You can use the Socket class (in package java.net) to connect to a remote host by specifying an IP address and port number:

Socket socket = new Socket("catalog.example.com", 4321); 

You can then obtain an InputStream and OutputStream object from the socket to allow you to read and write (subject to any permissions) to the host:

InputStream is = socket.getInputStream();
OutputStream os = socket.getOutputStream();
 

In the above case you are the client and the host you are connecting to is the server. If you want to create a server application, you can use the ServerSocket class:

ServerSocket serverSocket = new ServerSocket(80); // port 80 = http 

You then need to wait for clients to connect to your server through the accept() method, which returns a Socket object:

Socket clientSocket = serverSocket.accept(); 

A typical way of coding server programs is to listen for client connections (via accept()) inside a while loop which continues until the server application is stopped. Each time a client connects, a new Thread would be created for that client so that multiple clients can connect simultaneously.

Remote Method Invocation (RMI)

Remote Method Invocation provides a means for you to create both a Java server and a Java client program that can communicate with each other without the need to create sockets. Your client object can invoke the methods of the server object just as if they were on the same physical machine, where the RMI mechanism takes care of the network connections for you.

On the server side, you need to define an interface that contains the methods which should be able to be invoked by a remote client. The interface must extend the Remote interface (in the java.rmi package), and because network connections are involved which could fail, your method must specify that they throw RemoteException. Here is a simple interface to convert a String to uppercase[1]:

package somepackage;

import java.rmi.*;

public interface WordUtilities extends Remote {

    public void makeUpperCase(String s) throws RemoteException;

}
 

Also on the server, you need to create a class which implements your interface and extends UnicastRemoteObject:

package somepackage;

import java.rmi.*;

public class WordUtilitiesImpl extends UnicastRemoteObject
				     implements WordUtilities {

    public void makeUpperCase(String s) throws RemoteException {
        return s.toUpperCase();
    }

}
 

Your server program needs to start the RMI registry, an instance of the WordUtilites object, and bind it to the registry:

try {
    LocateRegistry.createRegistry(1099); // 1099 is default RMI port
    WordUtilities wu = new  WordUtilitiesImpl();
    Naming.rebind("rmi://example.com/wordutilapp", wu);
    System.out.println("Server has started");

} catch (Exception ex) {
    ex.printStackTrace();
}
 

At the client end your application needs to connect to the server host before it can invoke the methods remotely:

try {
    WordUtilities wu =
      (WordUtilities) Naming.lookup("rmi://example.com/wordutilapp");
    String sUpper = wu.makeUpperCase("make this upper-case");
    System.out.println(sUpper);
} catch (Exception ex) {
    ex.printStackTrace();
}