Java - Networking

The term network programming refers to writing programs that execute across multiple devices (computers), in which the devices are all connected to each other using a network.

The java.net package of the J2SE APIs contains a collection of classes and interfaces that provide the low-level communication details, allowing you to write programs that focus on solving the problem at hand.

The java.net package provides support for the two common network protocols −

• TCP − TCP stands for Transmission Control Protocol, which allows for reliable communication between two applications. TCP is typically used over the Internet Protocol, which is referred to as TCP/IP.

• UDP − UDP stands for User Datagram Protocol, a connection-less protocol that allows for packets of data to be transmitted between applications.

This chapter gives a good understanding on the following two subjects −

• Socket Programming − This is the most widely used concept in Networking and it has been explained in very detail.

• URL Processing − This would be covered separately. Click here to learn about URL Processing in Java language.

Socket Programming

Sockets provide the communication mechanism between two computers using TCP. A client program creates a socket on its end of the communication and attempts to connect that socket to a server.

When the connection is made, the server creates a socket object on its end of the communication. The client and the server can now communicate by writing to and reading from the socket.

The java.net.Socket class represents a socket, and the java.net.ServerSocket class provides a mechanism for the server program to listen for clients and establish connections with them.

The following steps occur when establishing a TCP connection between two computers using sockets −

• The server instantiates a ServerSocket object, denoting which port number communication is to occur on.

• The server invokes the accept() method of the ServerSocket class. This method waits until a client connects to the server on the given port.

• After the server is waiting, a client instantiates a Socket object, specifying the server name and the port number to connect to.

• The constructor of the Socket class attempts to connect the client to the specified server and the port number. If communication is established, the client now has a Socket object capable of communicating with the server.

• On the server side, the accept() method returns a reference to a new socket on the server that is connected to the client's socket.

After the connections are established, communication can occur using I/O streams. Each socket has both an OutputStream and an InputStream. The client's OutputStream is connected to the server's InputStream, and the client's InputStream is connected to the server's OutputStream.

TCP is a two-way communication protocol, hence data can be sent across both streams at the same time. Following are the useful classes providing complete set of methods to implement sockets.

ServerSocket Class Methods

The java.net.ServerSocket class is used by server applications to obtain a port and listen for client requests.

The ServerSocket class has four constructors −

If the ServerSocket constructor does not throw an exception, it means that your application has successfully bound to the specified port and is ready for client requests.

Following are some of the common methods of the ServerSocket class −

When the ServerSocket invokes accept(), the method does not return until a client connects. After a client does connect, the ServerSocket creates a new Socket on an unspecified port and returns a reference to this new Socket. A TCP connection now exists between the client and the server, and communication can begin.

Socket Class Methods

The java.net.Socket class represents the socket that both the client and the server use to communicate with each other. The client obtains a Socket object by instantiating one, whereas the server obtains a Socket object from the return value of the accept() method.

The Socket class has five constructors that a client uses to connect to a server −

When the Socket constructor returns, it does not simply instantiate a Socket object but it actually attempts to connect to the specified server and port.

Some methods of interest in the Socket class are listed here. Notice that both the client and the server have a Socket object, so these methods can be invoked by both the client and the server.

InetAddress Class Methods

This class represents an Internet Protocol (IP) address. Here are following usefull methods which you would need while doing socket programming −

Socket Client Example

The following GreetingClient is a client program that connects to a server by using a socket and sends a greeting, and then waits for a response.

Example

// File Name GreetingClient.java
import java.net.*;
import java.io.*;

public class GreetingClient {

   public static void main(String [] args) {
      String serverName = args[0];
      int port = Integer.parseInt(args[1]);
      try {
         System.out.println("Connecting to " + serverName + " on port " + port);
         Socket client = new Socket(serverName, port);
         
         System.out.println("Just connected to " + client.getRemoteSocketAddress());
         OutputStream outToServer = client.getOutputStream();
         DataOutputStream out = new DataOutputStream(outToServer);
         
         out.writeUTF("Hello from " + client.getLocalSocketAddress());
         InputStream inFromServer = client.getInputStream();
         DataInputStream in = new DataInputStream(inFromServer);
         
         System.out.println("Server says " + in.readUTF());
         client.close();
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

Socket Server Example

The following GreetingServer program is an example of a server application that uses the Socket class to listen for clients on a port number specified by a command-line argument −

Example

// File Name GreetingServer.java
import java.net.*;
import java.io.*;

public class GreetingServer extends Thread {
   private ServerSocket serverSocket;
   
   public GreetingServer(int port) throws IOException {
      serverSocket = new ServerSocket(port);
      serverSocket.setSoTimeout(10000);
   }

   public void run() {
      while(true) {
         try {
            System.out.println("Waiting for client on port " + 
               serverSocket.getLocalPort() + "...");
            Socket server = serverSocket.accept();
            
            System.out.println("Just connected to " + server.getRemoteSocketAddress());
            DataInputStream in = new DataInputStream(server.getInputStream());
            
            System.out.println(in.readUTF());
            DataOutputStream out = new DataOutputStream(server.getOutputStream());
            out.writeUTF("Thank you for connecting to " + server.getLocalSocketAddress()
               + "\nGoodbye!");
            server.close();
            
         } catch (SocketTimeoutException s) {
            System.out.println("Socket timed out!");
            break;
         } catch (IOException e) {
            e.printStackTrace();
            break;
         }
      }
   }
   
   public static void main(String [] args) {
      int port = Integer.parseInt(args[0]);
      try {
         Thread t = new GreetingServer(port);
         t.start();
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

Compile the client and the server and then start the server as follows −

$ java GreetingServer 6066
Waiting for client on port 6066...

Check the client program as follows −

Output

$ java GreetingClient localhost 6066
Connecting to localhost on port 6066
Just connected to localhost/127.0.0.1:6066
Server says Thank you for connecting to /127.0.0.1:6066
Goodbye!