Working with Sockets

What is a Socket?

A socket is another word for a connection. Sockets are used to link different computers together, through a network. This allows for the flow of information; being sent and received on both ends of the connection.

Hello World!

Lets get started on a simple client-server program. This program will have a server, which sends a string and a client that receives and outputs the string to the console.

Lets start off with creating a server class.

import java.io.*;
import java.net.*;

public class Server {

    public static Socket clientSocket = null;
    public static ServerSocket serverSocket = null;

    private static ObjectOutputStream output;

    public static void main(String[] args) {

    }
}

Here we need 2 sockets :

ServerSocket - Waits and accepts requests from the network

Socket - The accepted connection from the client

The ObjectOutStream will be used to send information to the client.

Next lets create the methods used in each stage of the server loop.

import java.io.*;
import java.net.*;

public class Server {
    public static Socket clientSocket = null;
    public static ServerSocket serverSocket = null;

    private static ObjectOutputStream output;

    public static void setup() throws IOException {
        serverSocket = new ServerSocket(1234);
        System.out.println("SERVER: Ready to roll!");
    }

    public static void waitForClient() throws IOException {
        System.out.println("SERVER: Waiting for client to accept connection...");
        clientSocket = serverSocket.accept();
        output = new ObjectOutputStream(clientSocket.getOutputStream());
        System.out.println("SERVER: A client has accepted");
    }

    public static void sendString(String string) throws IOException {
        System.out.println("SERVER: Sending...");
        output.writeObject(string);
        output.flush();
    }

    public static void end() throws IOException {
        System.out.println("SERVER: Time to close up shop :) ");
        serverSocket.close();
        clientSocket.close();
        output.close();
    }

    public static void main(String[] args) {

    }
}

1. setup() - This method is called initially and is used to setup the ServerSocket
2. waitForClient() - This method waits for a client connection through the ServerSocket and sets the clientSocket to that connection
3. sendString() - This method sends a given string through the clientSocket
4. end() - This method closes up all the sockets and streams

Now that basic functionality has been created, lets put everything in order in the main() method

import java.io.*;
import java.net.*;

public class Server {
    public static Socket clientSocket = null;
    public static ServerSocket serverSocket = null;

    private static ObjectOutputStream output;

    public static void setup() throws IOException {
        serverSocket = new ServerSocket(1234);
        System.out.println("SERVER: Ready to roll!");
    }

    public static void waitForClient() throws IOException {
        System.out.println("SERVER: Waiting for client to accept connection...");
        clientSocket = serverSocket.accept();
        output = new ObjectOutputStream(clientSocket.getOutputStream());
        System.out.println("SERVER: A client has accepted");
    }

    public static void sendString(String string) throws IOException {
        System.out.println("SERVER: Sending...");
        output.writeObject(string);
        output.flush();
    }

    public static void end() throws IOException {
        System.out.println("SERVER: Time to close up shop :) ");
        serverSocket.close();
        clientSocket.close();
        output.close();
    }

    public static void main(String[] args) {
        try {
            setup();
            waitForClient();
            for(int i = 0; i < 100; i++) {
            sendString("Hello World! " + String.valueOf(i));
            }    
            end();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

The main method is what gets called when a Server object is instantiated. The order of the methods is preserved and "Hello World!" is sent through the socket 100 times.

Now lets work on the client class.

import java.io.*;
import java.net.*;

public class Client {
    public static Socket serverSocket = null;

    private static ObjectInputStream input;

    public Client(String ip, int port) {
        try {
            serverSocket = new Socket(ip, port);
        } catch (UnknownHostException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {

    }
}

The client class is relatively similar to the server class, except instead of a ServerSocket it has a socket and instead of an ObjectOutputStream it contains an ObjectInputStream. The constructor of the Client class takes a given ipaddress_ (of the server the client connects to) _and a port number that is used to connect to the server.

Next lets create the methods used in the main() method.

import java.io.*;
import java.net.*;

public class Client {
    public static Socket serverSocket = null;

    private static ObjectInputStream input;

    public Client(String ip, int port) {
        try {
            serverSocket = new Socket(ip, port);
        } catch (UnknownHostException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    public static void setup() throws IOException {
        input = new ObjectInputStream(serverSocket.getInputStream());
        System.out.println("CLIENT: All setup!");
    }


    public static String receive() throws IOException, ClassNotFoundException {
        return (String) input.readObject();
    }

    public static void end() throws IOException {
        System.out.println("CLIENT: Time to go fishing :)");
        serverSocket.close();
        input.close();
    }

    public static void main(String[] args) {

    }
}

1. setup() - creates a new InputObjectStream from the server connection
2. receive() - returns a received string from the server connection
3. end() - closes off the server connection and the input stream

Now lets put everything together.

import java.io.*;
import java.net.*;

public class Client {
    public static Socket serverSocket = null;

    private static ObjectInputStream input;

    public Client(String ip, int port) {
        try {
            serverSocket = new Socket(ip, port);
        } catch (UnknownHostException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    public static void setup() throws IOException {
        input = new ObjectInputStream(serverSocket.getInputStream());
        System.out.println("CLIENT: All setup!");
    }


    public static String receive() throws IOException, ClassNotFoundException {
        return (String) input.readObject();
    }

    public static void end() throws IOException {
        System.out.println("CLIENT: Time to go fishing :)");
        serverSocket.close();
        input.close();
    }

    public static void main(String[] args) {
        Client client = new Client("localhost", 1234);
        try {
            client.setup();
            for(int i = 0; i < 100; i++) {
            System.out.println("CLIENT: RECIEVED MESSAGE: " + client.receive());
            }    
            client.end();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

The main method creates a new client - for our testing purposes the ipaddress will be "localhost" and the port will be 1234, so that we can test both programs on 1 computer. If you have multiple computers, change the ipaddress to the address of the host computer. The client will receive 100 messages and output them into the console.