A file server written in java. This was an educational project, and likely has a lot of bugs/issues. I have not followed the specification because I am lazy. Use at your own risk. If you do have any issues though, feel free to reach out to me, I'd e happy to help :)

SSLServer.java

/* Instructions: generate a keystore using:
   keytool -genkey -v -keystore keystore -storepass password -keyalg RSA -keysize 2048
   If you change the keystore file name or the keystore password, make sure to do the same in code as well.
 */
import java.io.*;
import java.net.*;
import java.nio.file.Files;
import javax.net.ssl.KeyManagerFactory;
import javax.net.ssl.SSLContext;
import javax.net.ssl.SSLServerSocket;
import java.io.FileInputStream;
import java.net.ServerSocket;
import java.security.KeyStore;
public class SSLServer {
    final static int port = 8080;
    static String keystore = "keystore";
    static String password = "password";
    public static void main(String[] args) throws Exception {
        SSLContext ctx;
        KeyManagerFactory kmf;
        KeyStore ks;
        char[] passphrase = password.toCharArray();
        ctx = SSLContext.getInstance("TLS");
        kmf = KeyManagerFactory.getInstance("SunX509");
        ks = KeyStore.getInstance("JKS");
        ks.load(new FileInputStream(keystore), passphrase);
        kmf.init(ks, passphrase);
        ctx.init(kmf.getKeyManagers(), null, null);
        ServerSocket serverSocket = ctx.getServerSocketFactory().createServerSocket(port);
        ((SSLServerSocket)serverSocket).setNeedClientAuth(false);
        System.out.println("Ready for connections-");
        while (true) {
            Socket client = serverSocket.accept();
            Thread t = new Thread(new Handler(client));
            t.start();
        }
    }
}

class Handler implements Runnable {
    Socket client;
    String filePath = "/";
    public Handler(Socket c) {
        client = c;
    }
    public void run() {
        try {
            client.setSendBufferSize(4*1024*1024);
            client.setTcpNoDelay(true);
            client.setPerformancePreferences(0, 0, 1);
            client.setSoLinger(true, 30);
            client.setSoTimeout(60*60*1000);
            System.out.println("Client connected from port: " + client.getPort());
            BufferedReader in = new BufferedReader(new InputStreamReader(client.getInputStream()));
            String line;
            line = in.readLine();
            while (line != null) {
                if (line.startsWith("GET ")) {
                    String uri = URLDecoder.decode((line.split(" "))[1], "UTF-8");
                    System.out.println("Client requested: " + uri);
                    File request = new File(filePath + uri);
                    if(request.exists()) {
                        if (request.isDirectory()) {
                            sendDirectory(request);
                        } else {
                            long fromByte = 0;
                            long toByte = 0;
                            while (line != null) {
                                if (line.startsWith("Range: bytes=")) {
                                    String[] range = line.substring(13).split("-");
                                    fromByte = Long.parseLong(range[0]);
                                    if (!line.endsWith("-")) toByte = Long.parseLong(range[1]);
                                    if (line.contains(",")) throw new Exception("RequestNotSupportedException: Client requested a feature that is not supported. Gib pranavgade20 $69 and get this fixed.");
                                }
                                if (line.equals("")) break;
                                line = in.readLine();
                            }
                            sendFile(request, fromByte, toByte);
                        }
                    } else {
                        sendError(404, "File not found.");
                    }
                }

                if (line.equals("")) break;
                line = in.readLine();
            }
            in.close();
            client.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private void sendFile(File file, long fromByte, long toByte) {
        try {
            OutputStream outputStream = client.getOutputStream();
            if (toByte == 0) {
                toByte = Files.size(file.toPath());
            }
            String headers = "HTTP/1.1 200 \r\n";
            if (fromByte > 0) headers = "HTTP/1.1 206 \r\n" +
                    "Content-Range: bytes " + fromByte + "-" + toByte + "/" + file.length() + "\r\n";
            headers += "Content-Type: " + Files.probeContentType(file.toPath()) + "\r\n" +
                    "Accept-Ranges: bytes" + "\r\n" +
                    "Connection: close\r\n" +
                    "Content-Length: " + (toByte - fromByte) + "\r\n" +
                    "\r\n";
            outputStream.write(headers.getBytes());
            BufferedInputStream inputStream = new BufferedInputStream(new FileInputStream(file));
            inputStream.skip(fromByte);
            int bufSize = 16*1024*1024;
            byte[] buf = new byte[(int)(((toByte-fromByte) < bufSize) ?  (toByte-fromByte) : bufSize)];
            long i = Math.floorDiv(toByte - fromByte, bufSize);
            try {
                while (inputStream.read(buf) != -1 && i != 0) {
                    outputStream.write(buf);
                    i--;
                }
                if (i == 0) {
                    byte[] tempBuf = new byte[(int)((toByte - fromByte)%bufSize)];
                    inputStream.read(tempBuf);
                    outputStream.write(tempBuf);
                }
            } catch (Exception e) {
                System.out.println("Client probably paused download. Probably.");
            }

            inputStream.close();
            outputStream.close();
        } catch (Exception exception) {
            exception.printStackTrace();
        }
    }

    private void sendDirectory(File folder) {
        try {
            DataOutputStream dos = new DataOutputStream(client.getOutputStream());
            PrintWriter out = new PrintWriter(dos);
            out.print("HTTP/1.1 200 \r\n"); // Version & status code
            out.print("Content-Type: text/html\r\n"); // The type of data
            out.print("Connection: close\r\n"); // Will close stream
            out.print("\r\n");
            out.print("<!DOCTYPE html>\n" +
                    "<html>\n" +
                    "<head>\n" +
                    "\t<title>File sender!</title>\n" +
                    "</head>\n" +
                    "<body>\n");
            File[] listOfFiles = folder.listFiles();
            if (listOfFiles == null || listOfFiles.length == 0) {
                out.print("<a href=\"/\">No files found.</a>");
            } else {
                for (int i = 0; i < listOfFiles.length; i++) {
                    if (listOfFiles[i].isFile()) {
                        out.println("<a href=\"" + listOfFiles[i].getPath() + "\">" + listOfFiles[i].getName() + "</a>");
                        out.println("<br>");
                    } else if (listOfFiles[i].isDirectory()) {
                        out.println("<a href=\"" + listOfFiles[i].getPath() + "\">" + listOfFiles[i].getName() + '/' + "</a>");
                        out.println("<br>");
                    }
                }
            }

            out.print("</body>\n" +
                    "</html>\n");
            out.print("\r\n");
            out.close();
            dos.close();
        } catch (Exception exception) {
            exception.printStackTrace();
        }
    }

    private void sendError(int errCode, String error) {
        try{
            DataOutputStream dos = new DataOutputStream(client.getOutputStream());
            PrintWriter out = new PrintWriter(dos);
            out.print("HTTP/1.1 " + Integer.toString(errCode) + " \r\n"); // Version & status code
            out.print("Content-Type: text/html\r\n"); // The type of data
            out.print("Connection: close\r\n"); // Will close stream
            out.print("\r\n");
            out.print("<b> " + error + " </b>");
            out.print("\r\n");
            out.close();
            dos.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}