Provide way to start the client/server as daemons

src/main/java/org/java_websocket/AbstractWebSocket.java

@@ -90,6 +90,13 @@ public abstract class AbstractWebSocket extends WebSocketAdapter {
    */
   private boolean websocketRunning = false;
 
+  /**
+   * Attribute to start internal threads as daemon
+   *
+   * @since 1.5.6
+   */
+  private boolean daemon = false;
+
   /**
    * Attribute to sync on
    */
@@ -182,7 +189,7 @@ protected void startConnectionLostTimer() {
   private void restartConnectionLostTimer() {
     cancelConnectionLostTimer();
     connectionLostCheckerService = Executors
-        .newSingleThreadScheduledExecutor(new NamedThreadFactory("connectionLostChecker"));
+        .newSingleThreadScheduledExecutor(new NamedThreadFactory("connectionLostChecker", daemon));
     Runnable connectionLostChecker = new Runnable() {
 
       /**
@@ -308,4 +315,25 @@ public void setReuseAddr(boolean reuseAddr) {
     this.reuseAddr = reuseAddr;
   }
 
+
+  /**
+   * Getter for daemon
+   *
+   * @return whether internal threads are spawned in daemon mode
+   * @since 1.5.6
+   */
+  public boolean isDaemon() {
+    return daemon;
+  }
+
+  /**
+   * Setter for daemon
+   * <p>
+   * Controls whether or not internal threads are spawned in daemon mode
+   *
+   * @since 1.5.6
+   */
+  public void setDaemon(boolean daemon) {
+    this.daemon = daemon;
+  }
 }

src/main/java/org/java_websocket/client/WebSocketClient.java

@@ -373,6 +373,7 @@ public void connect() {
       throw new IllegalStateException("WebSocketClient objects are not reuseable");
     }
     connectReadThread = new Thread(this);
+    connectReadThread.setDaemon(isDaemon());
     connectReadThread.setName("WebSocketConnectReadThread-" + connectReadThread.getId());
     connectReadThread.start();
   }
@@ -515,6 +516,7 @@ public void run() {
       }
     }
     writeThread = new Thread(new WebsocketWriteThread(this));
+    writeThread.setDaemon(isDaemon());
     writeThread.start();
 
     byte[] rawbuffer = new byte[WebSocketImpl.RCVBUF];

src/main/java/org/java_websocket/server/WebSocketServer.java

@@ -245,7 +245,9 @@ public void start() {
     if (selectorthread != null) {
       throw new IllegalStateException(getClass().getName() + " can only be started once.");
     }
-    new Thread(this).start();
+    Thread t = new Thread(this);
+    t.setDaemon(isDaemon());
+    t.start();
   }
 
   public void stop(int timeout) throws InterruptedException {
@@ -326,6 +328,20 @@ public int getPort() {
     return port;
   }
 
+  @Override
+  public void setDaemon(boolean daemon) {
+    // pass it to the AbstractWebSocket too, to use it on the connectionLostChecker thread factory
+    super.setDaemon(daemon);
+    // we need to apply this to the decoders as well since they were created during the constructor
+    for (WebSocketWorker w : decoders) {
+      if (w.isAlive()) {
+        throw new IllegalStateException("Cannot call setDaemon after server is already started!");
+      } else {
+        w.setDaemon(daemon);
+      }
+    }
+  }
+
   /**
    * Get the list of active drafts
    *

src/main/java/org/java_websocket/util/NamedThreadFactory.java

@@ -34,14 +34,22 @@ public class NamedThreadFactory implements ThreadFactory {
   private final ThreadFactory defaultThreadFactory = Executors.defaultThreadFactory();
   private final AtomicInteger threadNumber = new AtomicInteger(1);
   private final String threadPrefix;
+  private final boolean daemon;
 
   public NamedThreadFactory(String threadPrefix) {
     this.threadPrefix = threadPrefix;
+    this.daemon = false;
+  }
+
+  public NamedThreadFactory(String threadPrefix, boolean daemon) {
+    this.threadPrefix = threadPrefix;
+    this.daemon = daemon;
   }
 
   @Override
   public Thread newThread(Runnable runnable) {
     Thread thread = defaultThreadFactory.newThread(runnable);
+    thread.setDaemon(daemon);
     thread.setName(threadPrefix + "-" + threadNumber);
     return thread;
   }

src/test/java/org/java_websocket/server/DaemonThreadTest.java

@@ -0,0 +1,75 @@
+package org.java_websocket.server;
+
+import java.io.IOException;
+import java.net.*;
+import java.util.Set;
+import java.util.concurrent.CountDownLatch;
+import org.java_websocket.WebSocket;
+import org.java_websocket.handshake.*;
+import org.java_websocket.client.*;
+import org.java_websocket.server.WebSocketServer;
+import org.java_websocket.util.SocketUtil;
+import org.junit.Test;
+import static org.junit.Assert.assertTrue;
+
+public class DaemonThreadTest {
+
+  @Test(timeout = 1000)
+  public void test_AllCreatedThreadsAreDaemon() throws Throwable {
+
+    Set<Thread> threadSet1 = Thread.getAllStackTraces().keySet();
+    final CountDownLatch ready = new CountDownLatch(1);
+
+    WebSocketServer server = new WebSocketServer(new InetSocketAddress(SocketUtil.getAvailablePort())) {
+      @Override
+      public void onOpen(WebSocket conn, ClientHandshake handshake) {}
+      @Override
+      public void onClose(WebSocket conn, int code, String reason, boolean remote) {}
+      @Override
+      public void onMessage(WebSocket conn, String message) {}
+      @Override
+      public void onError(WebSocket conn, Exception ex) {}
+      @Override
+      public void onStart() {}
+    };
+    server.setDaemon(true);
+    server.setDaemon(false);
+    server.setDaemon(true);
+    server.start();
+
+    WebSocketClient client = new WebSocketClient(URI.create("ws://localhost:" + server.getPort())) {
+      @Override
+      public void onOpen(ServerHandshake handshake) {
+        ready.countDown();
+      }
+      @Override
+      public void onClose(int code, String reason, boolean remote) {}
+      @Override
+      public void onMessage(String message) {}
+      @Override
+      public void onError(Exception ex) {}
+    };
+    client.setDaemon(false);
+    client.setDaemon(true);
+    client.connect();
+
+    ready.await();
+    Set<Thread> threadSet2 = Thread.getAllStackTraces().keySet();
+    threadSet2.removeAll(threadSet1);
+
+    assertTrue("new threads created (no new threads indicates issue in test)", !threadSet2.isEmpty());
+
+    for (Thread t : threadSet2)
+      assertTrue(t.getName(), t.isDaemon());
+
+    boolean exception = false;
+    try {
+      server.setDaemon(false);
+    } catch(IllegalStateException e) {
+      exception = true;
+    }
+    assertTrue("exception was thrown when calling setDaemon on a running server", exception);
+
+    server.stop();
+  }
+}