sockets and selectors

import java.io.*;
import java.net.*;
 
void main() throws Exception {
    try (var server = new ServerSocket(0)) {             // port 0: any free port
        int port = server.getLocalPort();
 
        Thread.ofVirtual().start(() -> {                 // the server side
            try (Socket s = server.accept();
                 var in = new BufferedReader(new InputStreamReader(s.getInputStream()));
                 var out = new PrintWriter(s.getOutputStream(), true)) {
                String line;
                while ((line = in.readLine()) != null) out.println(line.toUpperCase());
            } catch (IOException e) { throw new UncheckedIOException(e); }
        });
 
        try (var s = new Socket("localhost", port);       // the client side
             var in = new BufferedReader(new InputStreamReader(s.getInputStream()));
             var out = new PrintWriter(s.getOutputStream(), true)) {
            for (String msg : new String[] { "hello", "tcp" }) {
                out.println(msg);
                IO.println(msg + " → " + in.readLine());
            }
        }
    }
}

hello → HELLO
tcp → TCP

this one needs a real JVM (virtual threads), so it doesn't run in the browser. copy it into java Main.java to try it.

import java.net.*;
 
void main() throws Exception {
    try (var server = new ServerSocket(0);
         var client = new Socket("localhost", server.getLocalPort());
         var conn = server.accept()) {
 
        client.getOutputStream().write("hello".getBytes());   // two separate writes
        client.getOutputStream().write("world".getBytes());
        Thread.sleep(100);                                     // let both arrive
 
        byte[] buf = new byte[100];
        int n = conn.getInputStream().read(buf);               // one read
        IO.println(n + " bytes in one read: " + new String(buf, 0, n));
    }
}

10 bytes in one read: helloworld

this one needs a real JVM (files, the network or a database), so it doesn't run in the browser. copy it into java Main.java to try it.

try (var server = new ServerSocket(8080)) {
    while (true) {
        Socket s = server.accept();
        Thread.ofPlatform().start(() -> handle(s));   // one thread per client
    }
}

import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.*;
 
void main() throws Exception {
    Selector selector = Selector.open();
    ServerSocketChannel server = ServerSocketChannel.open();
    server.bind(new InetSocketAddress("localhost", 0));
    server.configureBlocking(false);
    server.register(selector, SelectionKey.OP_ACCEPT);
 
    Thread.ofPlatform().daemon().start(() -> {           // the whole server: one thread
        try {
            while (true) {
                selector.select();                       // sleep until some channel is ready
                var it = selector.selectedKeys().iterator();
                while (it.hasNext()) {
                    SelectionKey key = it.next();
                    it.remove();
                    if (key.isAcceptable()) {
                        SocketChannel c = server.accept();
                        c.configureBlocking(false);
                        c.register(selector, SelectionKey.OP_READ, ByteBuffer.allocate(256));
                    } else if (key.isReadable()) {
                        var c = (SocketChannel) key.channel();
                        var buf = (ByteBuffer) key.attachment();
                        if (c.read(buf) == -1) { c.close(); continue; }
                        buf.flip();
                        c.write(buf);                    // echo it back
                        buf.clear();
                    }
                }
            }
        } catch (Exception e) { throw new RuntimeException(e); }
    });
 
    var clients = new SocketChannel[3];
    for (int i = 0; i < 3; i++) clients[i] = SocketChannel.open(server.getLocalAddress());
 
    for (int i = 2; i >= 0; i--) {                       // any order: nobody is blocked on a client
        clients[i].write(ByteBuffer.wrap(("client " + i).getBytes()));
        var reply = ByteBuffer.allocate(256);
        clients[i].read(reply);
        IO.println("echo: " + new String(reply.array(), 0, reply.position()));
    }
    for (var c : clients) c.close();
}

echo: client 2
echo: client 1
echo: client 0

this one needs a real JVM (catching a runtime exception), so it doesn't run in the browser. copy it into java Main.java to try it.

import java.net.*;
import java.nio.charset.StandardCharsets;
 
void main() throws Exception {
    try (var receiver = new DatagramSocket(0, InetAddress.getLoopbackAddress());
         var sender = new DatagramSocket()) {
        byte[] msg = "ping".getBytes(StandardCharsets.UTF_8);
        sender.send(new DatagramPacket(msg, msg.length, receiver.getLocalSocketAddress()));
 
        var buf = new byte[1500];
        var packet = new DatagramPacket(buf, buf.length);
        receiver.setSoTimeout(2000);                  // never wait forever on UDP
        receiver.receive(packet);
        IO.println(new String(buf, 0, packet.getLength(), StandardCharsets.UTF_8)
            + " (" + packet.getLength() + " bytes, one datagram)");
    }
}

ping (4 bytes, one datagram)

this one needs a real JVM (files, the network or a database), so it doesn't run in the browser. copy it into java Main.java to try it.