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.

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.

The Java 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.

Declaration

public class Socket
   extends Object
      implements Closeable

Constructors

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.

Methods

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.

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

package com.tutorialspoint;

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.Socket;

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();
      }
   }
}

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 −

package com.tutorialspoint;

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;
import java.net.SocketTimeoutException;

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...
Just connected to /127.0.0.1:49462
Hello from /127.0.0.1:49462
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!