Create a simplified FTP (file transport) client and server where the client can send or download text files from the server:

1. ) Client connects to the server and sends a 'file listing' message
2. ) Server sends back the list of the downloadable files
3. ) Client lists the files and asks the user what action they want to take? Upload or download? ('u' or 'd')
4. ) In both cases users must give the full file name with extension
5. ) The client sends the selected file to the server (upload) or downloads the selected file from the server to a specific directory.

1. ) After connecting, it reads the files from the /store subdirectory and sends the file names to the client after receiving the listing message.
2. ) We are waiting for the client's 'u' or 'd' operation
3. ) We get a filename from the client and if the action is 'd' (download), we read the file content and return its contents
4. ) If the operation is 'u' (upload), we open a new file with the specified name and wait for the data to be written to the file.

1. ) The client connects and waits for the list of files coming back and writes it to the console
2. ) We ask for the „u” or „d” key
3. ) Then we'll ask for the file-name as well.
4. ) The client reads the files from the /files folder, or creates the downloaded file here
5. ) If you press „d”, it creates /files/ and writes data from the server
6. ) If you press „u”, /files/ is sent to the server

    import java.io.IOException;
    import java.io.ObjectInputStream;
    import java.io.ObjectOutputStream;
    import java.net.ServerSocket;
    import java.net.Socket;
 
    public class Server {
    	ServerSocket providerSocket;
    	Socket connection = null;
    	ObjectOutputStream out;
    	ObjectInputStream in;
    	String message;
 
    	Server() {
    	}
 
    	void run() {
    		try {
    			// 1. create a socket server listening to port 8080
    			providerSocket = new ServerSocket(8080, 10);
    			// 2. waiting for the connection (here we are waiting until next connection)
    			connection = providerSocket.accept();
    			// 3. create Input and Output streams
    			out = new ObjectOutputStream(connection.getOutputStream());
    			in = new ObjectInputStream(connection.getInputStream());
    			// 4. socket communication
    			do {
    				try {
    					message = (String) in.readObject();
    					System.out.println("client>" + message);
    					if (message.equals("bye")) {
    					    sendMessage("bye");
    					} 
    				} catch (ClassNotFoundException classnot) {
    					System.err.println("Data received in unknown format");
    				}
    			} while (!message.equals("bye"));
    		} catch (IOException ioException) {
    			ioException.printStackTrace();
    		} finally {
    			// 4: close connection
    			try {
    				in.close();
    				out.close();
    				providerSocket.close();
    			} catch (IOException ioException) {
    				ioException.printStackTrace();
    			}
    		}
    	}
 
    	void sendMessage(String msg) {
    		try {
    			out.writeObject(msg);
    			out.flush();
    			System.out.println("server>" + msg);
    		} catch (IOException ioException) {
    			ioException.printStackTrace();
    		}
    	}
 
    	public static void main(String args[]) {
    		Server server = new Server();
    		while (true) {
    			server.run();
    		}
    	}
    }

    import java.io.IOException;
    import java.io.ObjectInputStream;
    import java.io.ObjectOutputStream;
    import java.net.Socket;
    import java.net.UnknownHostException;
 
    public class Client {
    	Socket requestSocket;
    	ObjectOutputStream out;
    	ObjectInputStream in;
    	String message;
 
    	Client() {
    	}
 
    	void run() {
    		try {
    			// 1. try to connect to the socket: localhost:8080
    			requestSocket = new Socket("localhost", 8080);
    			// 2. Input and Output streams
    			out = new ObjectOutputStream(requestSocket.getOutputStream());
    			in = new ObjectInputStream(requestSocket.getInputStream());
    			// 3: communications
    			do {
    				try {
    					sendMessage("Hello server");
    					sendMessage("bye");
    					message = (String) in.readObject();
    				} catch (Exception e) {
    					System.err.println("data received in unknown format");
    				}
    			} while (!message.equals("bye"));
    		} catch (UnknownHostException unknownHost) {
    			System.err.println("You are trying to connect to an unknown host!");
    		} catch (IOException ioException) {
    			ioException.printStackTrace();
    		} finally {
    			// 4: close connection
    			try {
    				in.close();
    				out.close();
    				requestSocket.close();
    			} catch (IOException ioException) {
    				ioException.printStackTrace();
    			}
    		}
    	}
 
    	void sendMessage(String msg) {
    		try {
    			out.writeObject(msg);
    			out.flush();
    			System.out.println("client>" + msg);
    		} catch (IOException ioException) {
    			ioException.printStackTrace();
    		}
    	}
 
    	public static void main(String args[]) {
    		Client client = new Client();
    		client.run();
    	}
    }

The following Agent sends a message and waits for a response on port 8080, also with UDP. In the Eclipse IDE, the text you type on the console can be sent by pressing ctrl+z

Modify the code so that you can transfer a burned-in name and existing text or image file larger than 2 kbytes and verify that it was successfully sent.

  package org.ait;
 
  import java.io.BufferedReader;
  import java.io.InputStreamReader;
  import java.net.DatagramPacket;
  import java.net.DatagramSocket;
  import java.net.InetAddress;
 
  public class UDPClient {
	public static void main(String args[]) throws Exception {
		BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in));
		DatagramSocket clientSocket = new DatagramSocket();
		InetAddress IPAddress = InetAddress.getByName("localhost");
 
		byte[] sendData = new byte[1024];
		byte[] receiveData = new byte[1024];
 
		String sentence = inFromUser.readLine();
		sendData = sentence.getBytes();
 
		DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 8080);
		clientSocket.send(sendPacket);
 
		DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
		clientSocket.receive(receivePacket);
		String modifiedSentence = new String(receivePacket.getData());
 
		System.out.println("converted:" + modifiedSentence);
		clientSocket.close();
	}
	}

2.b) The UDP server waits for the agents messages on port 8080 and converts them to uppercase letters and sends them back to the client UDP socket.

  package org.ait;
 
  import java.net.DatagramPacket;
  import java.net.DatagramSocket;
  import java.net.InetAddress;
 
  public class UDPServer {
	public static void main(String args[]) throws Exception {
 
		DatagramSocket serverSocket = new DatagramSocket(8080);
 
		byte[] bytesReceived = new byte[1024];
		byte[] bytesSent = new byte[1024];
 
 
		DatagramPacket receivePacket = new DatagramPacket(bytesReceived, bytesReceived.length);
		// here we are waiting for the packets
		serverSocket.receive(receivePacket);
 
		String textMessage = new String(receivePacket.getData());
 
		System.out.println("I got: " + textMessage);
 
		InetAddress IPAddress = receivePacket.getAddress();
		int port = receivePacket.getPort();
 
		String upperCaseText = textMessage.toUpperCase();
		bytesSent = upperCaseText.getBytes();
 
		// send back
		DatagramPacket sendPacket = new DatagramPacket(bytesSent, bytesSent.length, IPAddress, port);
		serverSocket.send(sendPacket);
		serverSocket.close();
 
	}
  }