Add a check/warning for outdated Floodgate versions

core/src/main/java/org/geysermc/geyser/GeyserImpl.java

@@ -386,6 +386,8 @@ public class GeyserImpl implements GeyserApi {
 
         if (platformType == PlatformType.STANDALONE) {
             logger.warning(GeyserLocale.getLocaleStringLog("geyser.core.movement_warn"));
+        } else if (config.getRemote().getAuthType() == AuthType.FLOODGATE) {
+            VersionCheckUtils.checkForOutdatedFloodgate(logger);
         }
 
         newsHandler.handleNews(null, NewsItemAction.ON_SERVER_STARTED);

core/src/main/java/org/geysermc/geyser/util/VersionCheckUtils.java

@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2019-2021 GeyserMC. http://geysermc.org
+ *
+ * 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.
+ *
+ * @author GeyserMC
+ * @link https://github.com/GeyserMC/Geyser
+ */
+
+package org.geysermc.geyser.util;
+
+import org.geysermc.geyser.Constants;
+import org.geysermc.geyser.GeyserLogger;
+import org.geysermc.geyser.text.GeyserLocale;
+
+public final class VersionCheckUtils {
+
+    public static void checkForOutdatedFloodgate(GeyserLogger logger) {
+        try {
+            // This class was removed in Floodgate 2.1.0-SNAPSHOT - if it still exists, Floodgate will not work
+            // with this version of Geyser
+            Class.forName("org.geysermc.floodgate.util.TimeSyncerHolder");
+            logger.warning(GeyserLocale.getLocaleStringLog("geyser.bootstrap.floodgate.outdated", Constants.FLOODGATE_DOWNLOAD_LOCATION));
+        } catch (ClassNotFoundException ignored) {
+            // Nothing to worry about; we want this exception
+        }
+    }
+
+    private VersionCheckUtils() {
+    }
+}