Add note about outdated java version

worldedit-bukkit/src/main/java/com/boydti/fawe/bukkit/util/JavaVersionCheck.java

@@ -0,0 +1,53 @@
+package com.boydti.fawe.bukkit.util;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+public class JavaVersionCheck {
+
+    public static final Logger logger = LoggerFactory.getLogger(JavaVersionCheck.class);
+
+    private static int checkJavaVersion() {
+        String javaVersion = System.getProperty("java.version");
+        final Matcher matcher = Pattern.compile("(?:1\\.)?(\\d+)").matcher(javaVersion);
+        if (!matcher.find()) {
+            logger.warn("Failed to determine Java version; Could not parse: {}", javaVersion);
+            return -1;
+        }
+
+        final String version = matcher.group(1);
+        try {
+            return Integer.parseInt(version);
+        } catch (final NumberFormatException e) {
+            logger.warn("Failed to determine Java version; Could not parse {} from {}", version, javaVersion, e);
+            return -1;
+        }
+    }
+
+    public static void checkJvm() {
+        if (checkJavaVersion() < 11) {
+            logger.warn("************************************************************");
+            logger.warn("* WARNING - YOU ARE RUNNING AN OUTDATED VERSION OF JAVA.");
+            logger.warn("* FASTASYNCWORLDEDIT WILL STOP BEING COMPATIBLE WITH THIS VERSION OF");
+            logger.warn("* JAVA WHEN MINECRAFT 1.17 IS RELEASED.");
+            logger.warn("*");
+            logger.warn("* Please update the version of Java to 11. When Minecraft 1.17");
+            logger.warn("* is released, support for versions of Java prior to 11 will");
+            logger.warn("* be dropped.");
+            logger.warn("*");
+            logger.warn("* Current Java version: {}", System.getProperty("java.version"));
+            logger.warn("************************************************************");
+        }
+        if (checkJavaVersion() >= 15) {
+            logger.warn("************************************************************");
+            logger.warn("* FastAsyncWorldEdit uses Nashorn for the craftscript engine.");
+            logger.warn("* Within Java 15, Nashorn has been removed from Java.");
+            logger.warn("* Until we add a suitable workaround, you should stick to Java 11");
+            logger.warn("* to use all features of FastAsyncWorldEdit.");
+            logger.warn("************************************************************");
+        }
+    }
+}

worldedit-bukkit/src/main/java/com/sk89q/worldedit/bukkit/WorldEditPlugin.java

@@ -84,6 +84,7 @@ import java.util.Locale;
 import java.util.Optional;
 import java.util.logging.Level;
 
+import static com.boydti.fawe.bukkit.util.JavaVersionCheck.checkJvm;
 import static com.google.common.base.Preconditions.checkNotNull;
 import static com.sk89q.worldedit.internal.anvil.ChunkDeleter.DELCHUNKS_FILE_NAME;
 
@@ -186,6 +187,9 @@ public class WorldEditPlugin extends JavaPlugin { //implements TabCompleter
 
         // Setup metrics
         new Metrics(this, BSTATS_PLUGIN_ID);
+
+        // Check whether the server runs on 11 or greater
+        checkJvm();
     }
 
     private void setupPreWorldData() {