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Posted 24 Mar 2008

Using Sockets in Java - Server

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Creating a server program using Java


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 throughout 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.

  3. Set up input and output streams.
    Let's examine two methods: getInputStream and getOuputStream of class Socket. These methods are used to get references to streams associated with the socket returned in step 2.
    You will use these streams later for communication with a client that has just made a connection. If you are not using a GUI application, you can wrap a Scanner object around the InputStream object, object that is returned by method getInputStream, in order to obtain string-oriented input, let's see how:
    Scanner input = new Scanner(link.getInputStream());

    At the same time, we can wrap a PrintWriter object around the OutputStream object returned by method getOutputStream. We supply the PrintWriter constructor with a second argument of true. This argument will cause the output buffer to be flushed for every call of println, let's see how:

    PrintWriter output = new PrintWriter(link.getOutputStream(),true);
  4. Send and receive data.
    After you have set up your Scanner and PrintWriter objects, to send data and to receive is very straightforward. For this, we have to use nextLine method for receiving data and println to send data, let's see how:
    output.println("Awaiting data...");
    String input = input.nextLine();
  5. Close the connection (after completion of the dialogue):

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");
            servSock = new ServerSocket(PORT);      //Step 1.
        catch(IOException ioEx)
            System.out.println("Unable to attach to port!");
        while (true);

    private static void handleClient()
        Socket link = null;                     			//Step 2.
            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.");
                output.println("Message " +
		        numMessages + ": " + message);   		//Step 4.
                message = input.nextLine();
            output.println(numMessages + " messages received."); 	//Step 4.
        catch(IOException ioEx)
                System.out.println( "\n* Closing connection... *");
                link.close();                    //Step 5.
            catch(IOException ioEx)
                System.out.println("Unable to disconnect!");


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


  • 24th March, 2008: Initial post


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Marius Iulian Mihailescu
Instructor / Trainer Lumina - The University of South-East Europe
Romania Romania
No Biography provided

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Comments and Discussions

Questionusage Pin
Member 1086119327-Jun-14 3:22
memberMember 1086119327-Jun-14 3:22 
AnswerRe: usage Pin
Marius Mihailescu28-Jun-14 5:21
memberMarius Mihailescu28-Jun-14 5:21 
GeneralRe: usage Pin
Member 1086119330-Jun-14 2:23
memberMember 1086119330-Jun-14 2:23 
GeneralMy vote of 5 Pin
Member 432084414-Mar-13 12:00
memberMember 432084414-Mar-13 12:00 
GeneralMy vote of 5 Pin
yichangzyh6-Mar-13 14:31
memberyichangzyh6-Mar-13 14:31 
GeneralMy vote of 5 Pin
Member 98464443-Mar-13 6:46
memberMember 98464443-Mar-13 6:46 
QuestionNot really an article... Pin
Dave Kreskowiak3-Mar-13 4:16
mvpDave Kreskowiak3-Mar-13 4:16 
GeneralMy vote of 5 Pin
Member 432084430-Sep-11 3:38
memberMember 432084430-Sep-11 3:38 
General[Message Deleted] Pin
it.ragester2-Apr-09 21:56
memberit.ragester2-Apr-09 21:56 
QuestionWho cares about Java? Pin
Mark Nischalke24-Mar-08 11:00
memberMark Nischalke24-Mar-08 11:00 
AnswerRe: Who cares about Java? Pin
Derek Bartram24-Mar-08 13:16
memberDerek Bartram24-Mar-08 13:16 
GeneralRe: Who cares about Java? Pin
Iulian Marius24-Mar-08 14:10
memberIulian Marius24-Mar-08 14:10 
GeneralRe: Who cares about Java? Pin
Derek Bartram24-Mar-08 22:14
memberDerek Bartram24-Mar-08 22:14 
GeneralRe: Who cares about Java? Pin
Owen Lawrence25-Mar-08 3:17
memberOwen Lawrence25-Mar-08 3:17 
AnswerRe: Who cares about Java? [modified] Pin
Marius Iulian Mihailescu25-Mar-08 4:47
memberMarius Iulian Mihailescu25-Mar-08 4:47 
AnswerRe: Who cares about Java? Pin
Syed M Hussain5-May-08 12:31
memberSyed M Hussain5-May-08 12:31 

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