Add example of using WebSocketServer with systemd socket activation

src/main/example/SocketActivation.java

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+/*
+ * Copyright (c) 2010-2020 Nathan Rajlich
+ *
+ *  Permission is hereby granted, free of charge, to any person
+ *  obtaining a copy of this software and associated documentation
+ *  files (the "Software"), to deal in the Software without
+ *  restriction, including without limitation the rights to use,
+ *  copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  copies of the Software, and to permit persons to whom the
+ *  Software is furnished to do so, subject to the following
+ *  conditions:
+ *
+ *  The above copyright notice and this permission notice shall be
+ *  included in all copies or substantial portions of the Software.
+ *
+ *  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ *  EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
+ *  OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ *  NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+ *  HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+ *  WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ *  FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+ *  OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+import java.io.IOException;
+import java.nio.ByteBuffer;
+import java.nio.channels.ServerSocketChannel;
+import java.util.Collections;
+import java.util.concurrent.atomic.AtomicInteger;
+import org.java_websocket.WebSocket;
+import org.java_websocket.drafts.Draft;
+import org.java_websocket.drafts.Draft_6455;
+import org.java_websocket.handshake.ClientHandshake;
+import org.java_websocket.server.WebSocketServer;
+
+/**
+ * This is a "smart" chat server which will exit when no more clients are left, in order to demonstrate socket activation
+ */
+public class SocketActivation extends WebSocketServer {
+
+  AtomicInteger clients = new AtomicInteger(0);
+
+  public SocketActivation(ServerSocketChannel chan) {
+    super(chan);
+  }
+
+  @Override
+  public void onOpen(WebSocket conn, ClientHandshake handshake) {
+    conn.send("Welcome to the server!"); //This method sends a message to the new client
+    broadcast("new connection: " + handshake.getResourceDescriptor()); //This method sends a message to all clients connected
+    if(clients.get() == 0) {
+      broadcast("You are the first client to join");
+    }
+    System.out.println(conn.getRemoteSocketAddress().getAddress().getHostAddress() + " entered the room!");
+    clients.incrementAndGet();
+  }
+
+  @Override
+  public void onClose(WebSocket conn, int code, String reason, boolean remote) {
+    broadcast(conn + " has left the room!");
+    System.out.println(conn + " has left the room!");
+    if(clients.decrementAndGet() <= 0) {
+      System.out.println("No more clients left, exiting");
+      System.exit(0);
+    }
+  }
+
+  @Override
+  public void onMessage(WebSocket conn, String message) {
+    broadcast(message);
+    System.out.println(conn + ": " + message);
+  }
+
+  @Override
+  public void onMessage(WebSocket conn, ByteBuffer message) {
+    broadcast(message.array());
+    System.out.println(conn + ": " + message);
+  }
+
+
+  public static void main(String[] args) throws InterruptedException, IOException {
+    if(System.inheritedChannel() == null) {
+      System.err.println("System.inheritedChannel() is null, make sure this program is started with file descriptor zero being a listening socket");
+      System.exit(1);
+    }
+    SocketActivation s = new SocketActivation((ServerSocketChannel)System.inheritedChannel());
+    s.start();
+    System.out.println(">>>> SocketActivation started on port: " + s.getPort() + " <<<<");
+  }
+
+  @Override
+  public void onError(WebSocket conn, Exception ex) {
+    ex.printStackTrace();
+  }
+
+  @Override
+  public void onStart() {
+    System.out.println("Server started!");
+  }
+
+}

src/main/example/jws-activation.service

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+[Unit]
+Description=Java-WebSocket systemd activation demo service
+After=network.target jws-activation.socket
+Requires=jws-activation.socket
+
+[Service]
+Type=simple
+# Place the command for running SocketActivation.java in file "$HOME"/jws_activation_command:
+ExecStart=/bin/sh %h/jws_activation_run
+TimeoutStopSec=5
+StandardError=journal
+StandardOutput=journal
+# This is very important - systemd will pass the socket as file descriptor zero, which is what Java expects
+StandardInput=socket
+
+[Install]
+WantedBy=default.target

src/main/example/jws-activation.socket

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+[Unit]
+Description=Java-WebSocket systemd activation demo socket
+PartOf=jws-activation.service
+
+[Socket]
+ListenStream=127.0.0.1:9999
+
+[Install]
+WantedBy=sockets.target