ConcurrentModificationException when using broadcast

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

@@ -924,13 +924,17 @@ private void doBroadcast(Object data, Collection<WebSocket> clients) {
 			return;
 		}
 		Map<Draft, List<Framedata>> draftFrames = new HashMap<Draft, List<Framedata>>();
-		for( WebSocket client : clients ) {
-			if( client != null ) {
+		List<WebSocket> clientCopy;
+		synchronized (clients) {
+			clientCopy = new ArrayList<WebSocket>(clients);
+		}
+		for (WebSocket client : clientCopy) {
+			if (client != null) {
 				Draft draft = client.getDraft();
 				fillFrames(draft, draftFrames, sData, bData);
 				try {
-					client.sendFrame( draftFrames.get( draft ) );
-				} catch ( WebsocketNotConnectedException e ) {
+					client.sendFrame(draftFrames.get(draft));
+				} catch (WebsocketNotConnectedException e) {
 					//Ignore this exception in this case
 				}
 			}

src/test/java/org/java_websocket/issues/Issue879Test.java

@@ -0,0 +1,170 @@
+/*
+ * Copyright (c) 2010-2019 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.
+ *
+ */
+
+package org.java_websocket.issues;
+
+import org.java_websocket.WebSocket;
+import org.java_websocket.client.WebSocketClient;
+import org.java_websocket.handshake.ClientHandshake;
+import org.java_websocket.handshake.ServerHandshake;
+import org.java_websocket.server.WebSocketServer;
+import org.java_websocket.util.SocketUtil;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.Parameterized;
+
+import java.io.IOException;
+import java.net.BindException;
+import java.net.InetSocketAddress;
+import java.net.URI;
+import java.net.URISyntaxException;
+import java.nio.ByteBuffer;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.ConcurrentModificationException;
+import java.util.List;
+import java.util.concurrent.CountDownLatch;
+
+import static org.junit.Assert.assertFalse;
+
+@RunWith(Parameterized.class)
+public class Issue879Test {
+
+    private static final int NUMBER_OF_TESTS = 20;
+
+    @Parameterized.Parameter
+    public int numberOfConnections;
+
+
+    @Test(timeout= 10000)
+    public void QuickStopTest() throws IOException, InterruptedException, URISyntaxException {
+        final boolean[] wasBindException = {false};
+        final boolean[] wasConcurrentException = new boolean[1];
+        final CountDownLatch countDownLatch = new CountDownLatch(1);
+
+        class SimpleServer extends WebSocketServer {
+            public SimpleServer(InetSocketAddress address) {
+                super(address);
+            }
+
+            @Override
+            public void onOpen(WebSocket conn, ClientHandshake handshake) {
+                broadcast("new connection: " + handshake.getResourceDescriptor()); //This method sends a message to all clients connected
+            }
+
+            @Override
+            public void onClose(WebSocket conn, int code, String reason, boolean remote) {
+            }
+
+            @Override
+            public void onMessage(WebSocket conn, String message) {
+            }
+
+            @Override
+            public void onMessage(WebSocket conn, ByteBuffer message) {
+            }
+
+            @Override
+            public void onError(WebSocket conn, Exception ex) {
+                if (ex instanceof BindException) {
+                    wasBindException[0] = true;
+                }
+                if (ex instanceof ConcurrentModificationException) {
+                    wasConcurrentException[0] = true;
+                }
+            }
+
+            @Override
+            public void onStart() {
+                countDownLatch.countDown();
+            }
+        }
+        int port = SocketUtil.getAvailablePort();
+        SimpleServer serverA = new SimpleServer(new InetSocketAddress( port));
+        SimpleServer serverB = new SimpleServer(new InetSocketAddress( port));
+        serverA.start();
+        countDownLatch.await();
+        List<WebSocketClient> clients = startNewConnections(numberOfConnections, port);
+        Thread.sleep(100);
+        int numberOfConnected = 0;
+        for (WebSocketClient client : clients) {
+            if (client.isOpen())
+                numberOfConnected++;
+        }
+        // Number will differ since we use connect instead of connectBlocking
+        // System.out.println(numberOfConnected + " " + numberOfConnections);
+
+        serverA.stop();
+        serverB.start();
+        clients.clear();
+        assertFalse("There was a BindException", wasBindException[0]);
+        assertFalse("There was a ConcurrentModificationException", wasConcurrentException[0]);
+    }
+
+    @Parameterized.Parameters
+    public static Collection<Integer[]> data() {
+        List<Integer[]> ret = new ArrayList<Integer[]>(NUMBER_OF_TESTS);
+        for (int i = 0; i < NUMBER_OF_TESTS; i++) ret.add(new Integer[]{25+ i*25});
+        return ret;
+    }
+
+    private List<WebSocketClient> startNewConnections(int numberOfConnections, int port) throws URISyntaxException, InterruptedException {
+        List<WebSocketClient> clients = new ArrayList<WebSocketClient>(numberOfConnections);
+        for (int i = 0; i < numberOfConnections; i++) {
+            WebSocketClient client = new SimpleClient(new URI("ws://localhost:" + port));
+            client.connect();
+            Thread.sleep(1);
+            clients.add(client);
+        }
+        return clients;
+    }
+    class SimpleClient extends WebSocketClient {
+
+        public SimpleClient(URI serverUri) {
+            super(serverUri);
+        }
+
+        @Override
+        public void onOpen(ServerHandshake handshakedata) {
+
+        }
+
+        @Override
+        public void onMessage(String message) {
+
+        }
+
+        @Override
+        public void onClose(int code, String reason, boolean remote) {
+
+        }
+
+        @Override
+        public void onError(Exception ex) {
+
+        }
+    }
+}