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.
