Add java version checker

Bukkit/src/main/java/com/plotsquared/bukkit/BukkitPlatform.java

@@ -156,6 +156,7 @@ import java.util.concurrent.Executors;
 import java.util.concurrent.LinkedBlockingQueue;
 import java.util.concurrent.TimeUnit;
 
+import static com.plotsquared.bukkit.util.JavaVersionCheck.checkJavaVersion;
 import static com.plotsquared.core.util.PremiumVerification.getDownloadID;
 import static com.plotsquared.core.util.PremiumVerification.getResourceID;
 import static com.plotsquared.core.util.PremiumVerification.getUserID;
@@ -242,6 +243,9 @@ public final class BukkitPlatform extends JavaPlugin implements Listener, PlotPl
             return;
         }
 
+        // Check whether the server runs on 11 or greater
+        checkJavaVersion();
+
         // We create the injector after PlotSquared has been initialized, so that we have access
         // to generated instances and settings
         this.injector = Guice

Bukkit/src/main/java/com/plotsquared/bukkit/util/JavaVersionCheck.java

@@ -0,0 +1,74 @@
+/*
+ *       _____  _       _    _____                                _
+ *      |  __ \| |     | |  / ____|                              | |
+ *      | |__) | | ___ | |_| (___   __ _ _   _  __ _ _ __ ___  __| |
+ *      |  ___/| |/ _ \| __|\___ \ / _` | | | |/ _` | '__/ _ \/ _` |
+ *      | |    | | (_) | |_ ____) | (_| | |_| | (_| | | |  __/ (_| |
+ *      |_|    |_|\___/ \__|_____/ \__, |\__,_|\__,_|_|  \___|\__,_|
+ *                                    | |
+ *                                    |_|
+ *            PlotSquared plot management system for Minecraft
+ *                  Copyright (C) 2020 IntellectualSites
+ *
+ *     This program is free software: you can redistribute it and/or modify
+ *     it under the terms of the GNU General Public License as published by
+ *     the Free Software Foundation, either version 3 of the License, or
+ *     (at your option) any later version.
+ *
+ *     This program is distributed in the hope that it will be useful,
+ *     but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *     GNU General Public License for more details.
+ *
+ *     You should have received a copy of the GNU General Public License
+ *     along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package com.plotsquared.bukkit.util;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class JavaVersionCheck {
+
+    private static final Logger logger = LoggerFactory.getLogger("P2/" + JavaVersionCheck.class.getSimpleName());
+
+    public JavaVersionCheck() {}
+
+    public static void checkJavaVersion() {
+        final String javaVersion = System.getProperty("java.version");
+        final int dotIndex = javaVersion.indexOf('.');
+
+        if (javaVersion.startsWith("1.")) {
+            JavaVersionCheck.notify(javaVersion, logger);
+            return;
+        }
+
+        final int endIndex = dotIndex == -1 ? javaVersion.length() : dotIndex;
+        final String version = javaVersion.substring(0, endIndex);
+
+        final int javaVersionNumber;
+        try {
+            javaVersionNumber = Integer.parseInt(version);
+        } catch (final NumberFormatException e) {
+            logger.error("Failed to determine Java version. Could not parse {}", version, e);
+            JavaVersionCheck.notify(javaVersion, logger);
+            return;
+        }
+
+        if (javaVersionNumber < 11) {
+            JavaVersionCheck.notify(javaVersion, logger);
+        }
+    }
+    
+    public static void notify(final String version, final Logger logger) {
+        logger.error("************************************************************");
+        logger.error("* WARNING - YOU ARE RUNNING AN OUTDATED VERSION OF JAVA.");
+        logger.error("* PLOTSQUARED WILL STOP BEING COMPATIBLE WITH THIS VERSION OF");
+        logger.error("* JAVA WHEN MINECRAFT 1.17 IS RELEASED.");
+        logger.error("*");
+        logger.error("* Please update the version of Java to 11.");
+        logger.error("*");
+        logger.error("* Current Java version: " + version);
+        logger.error("************************************************************");
+    }
+}