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 repable communication between two apppcations. 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 apppcations.

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. Cpck here to learn about URL Processing in Java language.

Socket Programming

Sockets provide the communication mechanism between two computers using TCP. A cpent 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 cpent 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 psten for cpents and estabpsh connections with them.

The following steps occur when estabpshing 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 cpent connects to the server on the given port.

After the server is waiting, a cpent 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 cpent to the specified server and the port number. If communication is estabpshed, the cpent 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 cpent s socket.

After the connections are estabpshed, communication can occur using I/O streams. Each socket has both an OutputStream and an InputStream. The cpent s OutputStream is connected to the server s InputStream, and the cpent 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 apppcations to obtain a port and psten for cpent requests.

The ServerSocket class has four constructors −

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

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

When the ServerSocket invokes accept(), the method does not return until a cpent connects. After a cpent 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 cpent and the server, and communication can begin.

Socket Class Methods

The java.net.Socket class represents the socket that both the cpent and the server use to communicate with each other. The cpent 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 cpent 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 psted here. Notice that both the cpent and the server have a Socket object, so these methods can be invoked by both the cpent 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 Cpent Example

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

Example

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

pubpc class GreetingCpent {

   pubpc 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 cpent = new Socket(serverName, port);
         
         System.out.println("Just connected to " + cpent.getRemoteSocketAddress());
         OutputStream outToServer = cpent.getOutputStream();
         DataOutputStream out = new DataOutputStream(outToServer);
         
         out.writeUTF("Hello from " + cpent.getLocalSocketAddress());
         InputStream inFromServer = cpent.getInputStream();
         DataInputStream in = new DataInputStream(inFromServer);
         
         System.out.println("Server says " + in.readUTF());
         cpent.close();
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

Socket Server Example

The following GreetingServer program is an example of a server apppcation that uses the Socket class to psten for cpents on a port number specified by a command-pne argument −

Example

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

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

   pubpc void run() {
      while(true) {
         try {
            System.out.println("Waiting for cpent 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()
               + "
Goodbye!");
            server.close();
            
         } catch (SocketTimeoutException s) {
            System.out.println("Socket timed out!");
            break;
         } catch (IOException e) {
            e.printStackTrace();
            break;
         }
      }
   }
   
   pubpc 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 cpent and the server and then start the server as follows −

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

Check the cpent program as follows −

Output

$ java GreetingCpent 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!