Socket - Learn Java by Examples

In this example you’ll see how to create a simple port scanner program to check the open ports for the specified host name.

package org.kodejava.net;

import java.io.IOException;
import java.net.InetAddress;
import java.net.Socket;

public class PortScanner {
    public static void main(String[] args) throws Exception {
        String host = "localhost";
        InetAddress inetAddress = InetAddress.getByName(host);

        String hostName = inetAddress.getHostName();
        for (int port = 0; port <= 65535; port++) {
            try {
                Socket socket = new Socket(hostName, port);
                String text = hostName + " is listening on port " + port;
                System.out.println(text);
                socket.close();
            } catch (IOException e) {
                // empty catch block
            }
        }
    }
}

The result of the code snippet above are:

localhost is listening on port 21
localhost is listening on port 80
localhost is listening on port 135
localhost is listening on port 445
localhost is listening on port 3000
...
...
localhost is listening on port 63342
localhost is listening on port 63467
localhost is listening on port 64891
localhost is listening on port 64921
localhost is listening on port 65001

In this example you’ll see how to create a client-server socket communication. The example below consist of two main classes, the ServerSocketExample and the ClientSocketExample. The server application listen to port 7777 at the localhost. When we send a message from the client application the server receive the message and send a reply to the client application.

The communication in this example using the TCP socket, it means that there is a fixed connection line between the client application and the server application.

package org.kodejava.net;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.lang.ClassNotFoundException;
import java.lang.Runnable;
import java.lang.Thread;
import java.net.ServerSocket;
import java.net.Socket;

public class ServerSocketExample {
    private static final int PORT = 7777;
    private ServerSocket server;

    private ServerSocketExample() {
        try {
            server = new ServerSocket(PORT);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        ServerSocketExample example = new ServerSocketExample();
        example.handleConnection();
    }

    private void handleConnection() {
        System.out.println("Waiting for client message...");

        // The server do a loop here to accept all connection initiated by the
        // client application.
        while (true) {
            try {
                Socket socket = server.accept();
                new ConnectionHandler(socket);
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

class ConnectionHandler implements Runnable {
    private final Socket socket;

    ConnectionHandler(Socket socket) {
        this.socket = socket;

        Thread t = new Thread(this);
        t.start();
    }

    public void run() {
        try {
            // Read a message sent by client application
            ObjectInputStream ois = new ObjectInputStream(socket.getInputStream());
            String message = (String) ois.readObject();
            System.out.println("Message Received: " + message);

            // Send a response information to the client application
            ObjectOutputStream oos = new ObjectOutputStream(socket.getOutputStream());
            oos.writeObject("Hi...");

            ois.close();
            oos.close();
            socket.close();

            System.out.println("Waiting for client message...");
        } catch (IOException | ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
}

package org.kodejava.net;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.lang.ClassNotFoundException;
import java.net.InetAddress;
import java.net.Socket;

public class ClientSocketExample {
    public static void main(String[] args) {
        try {
            // Create a connection to the server socket on the server application
            InetAddress host = InetAddress.getLocalHost();
            Socket socket = new Socket(host.getHostName(), 7777);

            // Send a message to the client application
            ObjectOutputStream oos = new ObjectOutputStream(socket.getOutputStream());
            oos.writeObject("Hello There...");

            // Read and display the response message sent by server application
            ObjectInputStream ois = new ObjectInputStream(socket.getInputStream());
            String message = (String) ois.readObject();
            System.out.println("Message: " + message);

            ois.close();
            oos.close();
        } catch (IOException | ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
}

To test the application you need to start the server application. Each time you run the client application it will send a message “Hello There…” and in turns the server reply with a message “Hi…”.

Tag Archives: Socket

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