Using Sockets in Java - Server

• Download TCPEchoServer - 793 B

Introduction

In this article I will describe the steps to make a connection in Java using Sockets.Let's start to understand what is a communication created via TCP/IP...A communication link created via TCP/IP sockets is a connection-oriented link.This means that the connection between the server and client remains open throuhout the duration of the dialogue between the two and is only broken (under normal circumstances) when one end of the dialogue formally terminates the exchanges (via an agreed protocol)

Using the code

1.Create a ServerSocket object
ServerSocket mySock = new ServerSocket(1234);
here the server will wait ("listen for") a connection from a client on port 1234

2.Put the server into a waiting state
Socket link = mySock.accept();
here the server waits indefinitely("blocks") for a client to connect with the help of accept method of class ServerSocket, class that will return a Socket object when a connection is made.

Look at this code :

           public class TCPEchoServer
           {    
             private static ServerSocket servSock;
             private static final int PORT = 1234;
        
         public static void main(String[] args)
         {
            System.out.println("Opening port...\n");
            try
                {
                   servSock = new ServerSocket(PORT);      //Step 1.
            }
               catch(IOException ioEx)
            {
                   System.out.println("Unable to attach to port!");
                   System.exit(1);
                }
            do
                {
                   handleClient();
                }
            while (true);
         } 

         private static void handleClient()
         {
            Socket link = null;                        //Step 2.
            try
            {
                   link = servSock.accept();               //Step 2.
        
                   Scanner input = new Scanner(link.getInputStream()); //Step 3.
                   PrintWriter output =  new PrintWriter(link.getOutputStream(),true); //Step 3.

                   int numMessages = 0;
                   String message = input.nextLine();      //Step 4.
                   while (!message.equals("***CLOSE***"))
                   {
                          System.out.println("Message received.");
                          numMessages++;
                              output.println("Message " + numMessages + ": " + message);   //Step 4.
                           message = input.nextLine();
                         }
                   output.println(numMessages + " messages received.");    //Step 4.
              }
              catch(IOException ioEx)
              {
                  ioEx.printStackTrace();
              }
              finally
              {
                  try
                  {
                      System.out.println( "\n* Closing connection... *");
                      link.close();                    //Step 5.
                  }
                        catch(IOException ioEx)
                  {
                      System.out.println("Unable to disconnect!");
                      System.exit(1);
                  }
              }
          }
}

         

History

This example represents the SERVER program, in next article (JavaSocketsClient.aspx) I will describe the CLIENT program, is almost the same.