Simple Java HTTPS server

What I eventually used was this:

try {
    // Set up the socket address
    InetSocketAddress address = new InetSocketAddress(InetAddress.getLocalHost(), config.getHttpsPort());

    // Initialise the HTTPS server
    HttpsServer httpsServer = HttpsServer.create(address, 0);
    SSLContext sslContext = SSLContext.getInstance("TLS");

    // Initialise the keystore
    char[] password = "simulator".toCharArray();
    KeyStore ks = KeyStore.getInstance("JKS");
    FileInputStream fis = new FileInputStream("lig.keystore");
    ks.load(fis, password);

    // Set up the key manager factory
    KeyManagerFactory kmf = KeyManagerFactory.getInstance("SunX509");
    kmf.init(ks, password);

    // Set up the trust manager factory
    TrustManagerFactory tmf = TrustManagerFactory.getInstance("SunX509");
    tmf.init(ks);

    // Set up the HTTPS context and parameters
    sslContext.init(kmf.getKeyManagers(), tmf.getTrustManagers(), null);
    httpsServer.setHttpsConfigurator(new HttpsConfigurator(sslContext) {
        public void configure(HttpsParameters params) {
            try {
                // Initialise the SSL context
                SSLContext c = SSLContext.getDefault();
                SSLEngine engine = c.createSSLEngine();
                params.setNeedClientAuth(false);
                params.setCipherSuites(engine.getEnabledCipherSuites());
                params.setProtocols(engine.getEnabledProtocols());

                // Get the default parameters
                SSLParameters defaultSSLParameters = c.getDefaultSSLParameters();
                params.setSSLParameters(defaultSSLParameters);
            } catch (Exception ex) {
                ILogger log = new LoggerFactory().getLogger();
                log.exception(ex);
                log.error("Failed to create HTTPS port");
            }
        }
    });
    LigServer server = new LigServer(httpsServer);
    joinableThreadList.add(server.getJoinableThread());
} catch (Exception exception) {
    log.exception(exception);
    log.error("Failed to create HTTPS server on port " + config.getHttpsPort() + " of localhost");
}

To generate a keystore:

$ keytool -genkeypair -keyalg RSA -alias self_signed -keypass simulator \
  -keystore lig.keystore -storepass simulator

See also here.

Potentially storepass and keypass might be different, in which case the ks.load and kmf.init must use storepass and keypass, respectively.

I updated your answer for a HTTPS server (not socket-based). It might help with CSRF and AJAX calls.

import java.io.*;
import java.net.InetSocketAddress;
import java.lang.*;
import java.net.URL;
import com.sun.net.httpserver.HttpsServer;
import java.security.KeyStore;
import javax.net.ssl.KeyManagerFactory;
import javax.net.ssl.TrustManagerFactory;
import com.sun.net.httpserver.*;
import javax.net.ssl.SSLEngine;
import javax.net.ssl.SSLParameters;

import java.io.InputStreamReader;
import java.io.Reader;
import java.net.URLConnection;

import javax.net.ssl.HostnameVerifier;
import javax.net.ssl.HttpsURLConnection;
import javax.net.ssl.SSLContext;
import javax.net.ssl.SSLSession;
import javax.net.ssl.TrustManager;
import javax.net.ssl.X509TrustManager;
import java.security.cert.X509Certificate;

import java.net.InetAddress;
import com.sun.net.httpserver.HttpExchange;
import com.sun.net.httpserver.HttpHandler;
import com.sun.net.httpserver.HttpServer;
import com.sun.net.httpserver.HttpsExchange;

public class SimpleHTTPSServer {

    public static class MyHandler implements HttpHandler {
        @Override
        public void handle(HttpExchange t) throws IOException {
            String response = "This is the response";
            HttpsExchange httpsExchange = (HttpsExchange) t;
            t.getResponseHeaders().add("Access-Control-Allow-Origin", "*");
            t.sendResponseHeaders(200, response.getBytes().length);
            OutputStream os = t.getResponseBody();
            os.write(response.getBytes());
            os.close();
        }
    }

    /**
     * @param args
     */
    public static void main(String[] args) throws Exception {

        try {
            // setup the socket address
            InetSocketAddress address = new InetSocketAddress(8000);

            // initialise the HTTPS server
            HttpsServer httpsServer = HttpsServer.create(address, 0);
            SSLContext sslContext = SSLContext.getInstance("TLS");

            // initialise the keystore
            char[] password = "password".toCharArray();
            KeyStore ks = KeyStore.getInstance("JKS");
            FileInputStream fis = new FileInputStream("testkey.jks");
            ks.load(fis, password);

            // setup the key manager factory
            KeyManagerFactory kmf = KeyManagerFactory.getInstance("SunX509");
            kmf.init(ks, password);

            // setup the trust manager factory
            TrustManagerFactory tmf = TrustManagerFactory.getInstance("SunX509");
            tmf.init(ks);

            // setup the HTTPS context and parameters
            sslContext.init(kmf.getKeyManagers(), tmf.getTrustManagers(), null);
            httpsServer.setHttpsConfigurator(new HttpsConfigurator(sslContext) {
                public void configure(HttpsParameters params) {
                    try {
                        // initialise the SSL context
                        SSLContext context = getSSLContext();
                        SSLEngine engine = context.createSSLEngine();
                        params.setNeedClientAuth(false);
                        params.setCipherSuites(engine.getEnabledCipherSuites());
                        params.setProtocols(engine.getEnabledProtocols());

                        // Set the SSL parameters
                        SSLParameters sslParameters = context.getSupportedSSLParameters();
                        params.setSSLParameters(sslParameters);

                    } catch (Exception ex) {
                        System.out.println("Failed to create HTTPS port");
                    }
                }
            });
            httpsServer.createContext("/test", new MyHandler());
            httpsServer.setExecutor(null); // creates a default executor
            httpsServer.start();

        } catch (Exception exception) {
            System.out.println("Failed to create HTTPS server on port " + 8000 + " of localhost");
            exception.printStackTrace();

        }
    }

}

To create a self-signed certificate:

keytool -genkeypair -keyalg RSA -alias selfsigned -keystore testkey.jks -storepass password -validity 360 -keysize 2048

With ServerSocket

You can use the class that HttpsServer is built around to be even more light-weight: ServerSocket.

Single-threaded

The following program is a very simple, single-threaded server listening on port 8443. Messages are encrypted with TLS using the keys in ./keystore.jks:

public static void main(String... args) {
    var address = new InetSocketAddress("0.0.0.0", 8443);

    startSingleThreaded(address);
}

public static void startSingleThreaded(InetSocketAddress address) {

    System.out.println("Start single-threaded server at " + address);

    try (var serverSocket = getServerSocket(address)) {

        var encoding = StandardCharsets.UTF_8;

        // This infinite loop is not CPU-intensive since method "accept" blocks
        // until a client has made a connection to the socket
        while (true) {
            try (var socket = serverSocket.accept();
                 // Use the socket to read the client's request
                 var reader = new BufferedReader(new InputStreamReader(
                         socket.getInputStream(), encoding.name()));
                 // Writing to the output stream and then closing it sends
                 // data to the client
                 var writer = new BufferedWriter(new OutputStreamWriter(
                         socket.getOutputStream(), encoding.name()))
            ) {
                getHeaderLines(reader).forEach(System.out::println);

                writer.write(getResponse(encoding));
                writer.flush();

            } catch (IOException e) {
                System.err.println("Exception while handling connection");
                e.printStackTrace();
            }
        }
    } catch (Exception e) {
        System.err.println("Could not create socket at " + address);
        e.printStackTrace();
    }
}

private static ServerSocket getServerSocket(InetSocketAddress address)
        throws Exception {

    // Backlog is the maximum number of pending connections on the socket,
    // 0 means that an implementation-specific default is used
    int backlog = 0;

    var keyStorePath = Path.of("./keystore.jks");
    char[] keyStorePassword = "pass_for_self_signed_cert".toCharArray();

    // Bind the socket to the given port and address
    var serverSocket = getSslContext(keyStorePath, keyStorePassword)
            .getServerSocketFactory()
            .createServerSocket(address.getPort(), backlog, address.getAddress());

    // We don't need the password anymore → Overwrite it
    Arrays.fill(keyStorePassword, '0');

    return serverSocket;
}

private static SSLContext getSslContext(Path keyStorePath, char[] keyStorePass)
        throws Exception {

    var keyStore = KeyStore.getInstance("JKS");
    keyStore.load(new FileInputStream(keyStorePath.toFile()), keyStorePass);

    var keyManagerFactory = KeyManagerFactory.getInstance("SunX509");
    keyManagerFactory.init(keyStore, keyStorePass);

    var sslContext = SSLContext.getInstance("TLS");
    // Null means using default implementations for TrustManager and SecureRandom
    sslContext.init(keyManagerFactory.getKeyManagers(), null, null);
    return sslContext;
}

private static String getResponse(Charset encoding) {
    var body = "The server says hi 👋\r\n";
    var contentLength = body.getBytes(encoding).length;

    return "HTTP/1.1 200 OK\r\n" +
            String.format("Content-Length: %d\r\n", contentLength) +
            String.format("Content-Type: text/plain; charset=%s\r\n",
                    encoding.displayName()) +
            // An empty line marks the end of the response's header
            "\r\n" +
            body;
}

private static List<String> getHeaderLines(BufferedReader reader)
        throws IOException {
    var lines = new ArrayList<String>();
    var line = reader.readLine();
    // An empty line marks the end of the request's header
    while (!line.isEmpty()) {
        lines.add(line);
        line = reader.readLine();
    }
    return lines;
}

Here's a project using this socket-based approach.

Multi-threaded

To use more than one thread for the server, you can employ a thread pool:

public static void startMultiThreaded(InetSocketAddress address) {

    try (var serverSocket = getServerSocket(address)) {

        System.out.println("Started multi-threaded server at " + address);

        // A cached thread pool with a limited number of threads
        var threadPool = newCachedThreadPool(8);

        var encoding = StandardCharsets.UTF_8;

        // This infinite loop is not CPU-intensive since method "accept" blocks
        // until a client has made a connection to the socket
        while (true) {
            try {
                var socket = serverSocket.accept();
                // Create a response to the request on a separate thread to
                // handle multiple requests simultaneously
                threadPool.submit(() -> {

                    try ( // Use the socket to read the client's request
                          var reader = new BufferedReader(new InputStreamReader(
                                  socket.getInputStream(), encoding.name()));
                          // Writing to the output stream and then closing it
                          // sends data to the client
                          var writer = new BufferedWriter(new OutputStreamWriter(
                                  socket.getOutputStream(), encoding.name()))
                    ) {
                        getHeaderLines(reader).forEach(System.out::println);
                        writer.write(getResponse(encoding));
                        writer.flush();
                        // We're done with the connection → Close the socket
                        socket.close();

                    } catch (Exception e) {
                        System.err.println("Exception while creating response");
                        e.printStackTrace();
                    }
                });
            } catch (IOException e) {
                System.err.println("Exception while handling connection");
                e.printStackTrace();
            }
        }
    } catch (Exception e) {
        System.err.println("Could not create socket at " + address);
        e.printStackTrace();
    }
}

private static ExecutorService newCachedThreadPool(int maximumNumberOfThreads) {
    return new ThreadPoolExecutor(0, maximumNumberOfThreads,
            60L, TimeUnit.SECONDS,
            new SynchronousQueue<>());
}

Create a certificate

Use the keytool to create a self-signed certificate (you can get a proper certificate from Let's Encrypt for free):

keytool -genkeypair -keyalg RSA -alias selfsigned -keystore keystore.jks \
        -storepass pass_for_self_signed_cert \
        -dname "CN=localhost, OU=Developers, O=Bull Bytes, L=Linz, C=AT"

Contact the server

After starting the server, connect to it with curl:

curl -k https://localhost:8443

This will fetch a message from the server:

The server says hi 👋

Inspect which protocol and cipher suite were established by curl and your server with

curl -kv https://localhost:8443

Using JDK 13 and curl 7.66.0, this produced

SSL connection using TLSv1.3 / TLS_AES_256_GCM_SHA384

Refer to Java Network Programming by Elliotte Rusty Harold for more on the topic.