Warn on outdated Java version

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

@@ -540,6 +540,8 @@ public class GeyserImpl implements GeyserApi {
         if (config.isNotifyOnNewBedrockUpdate()) {
             VersionCheckUtils.checkForGeyserUpdate(this::getLogger);
         }
+
+        VersionCheckUtils.checkForOutdatedJava(logger);
     }
 
     @Override

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

@@ -42,9 +42,12 @@ import javax.annotation.Nonnull;
 import java.util.OptionalInt;
 import java.util.concurrent.CompletableFuture;
 import java.util.function.Supplier;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
 
 public final class VersionCheckUtils {
     private static @Nonnull OptionalInt LATEST_BEDROCK_RELEASE = OptionalInt.empty();
+    private static final int SUPPORTED_JAVA_VERSION = 17;
 
     public static void checkForOutdatedFloodgate(GeyserLogger logger) {
         try {
@@ -57,6 +60,34 @@ public final class VersionCheckUtils {
         }
     }
 
+    public static void checkForOutdatedJava(GeyserLogger logger) {
+        // Taken from Paper
+        String javaVersion = System.getProperty("java.version");
+        Matcher matcher = Pattern.compile("(?:1\\.)?(\\d+)").matcher(javaVersion);
+        if (!matcher.find()) {
+            logger.debug("Could not parse Java version string " + javaVersion);
+            return;
+        }
+
+        String version = matcher.group(1);
+        int majorVersion;
+        try {
+            majorVersion = Integer.parseInt(version);
+        } catch (NumberFormatException e) {
+            logger.debug("Could not format as an int: " + version);
+            return;
+        }
+
+        if (majorVersion < SUPPORTED_JAVA_VERSION) {
+            logger.warning("*********************************************");
+            logger.warning("");
+            logger.warning(GeyserLocale.getLocaleStringLog("geyser.bootstrap.unsupported_java.header"));
+            logger.warning(GeyserLocale.getLocaleStringLog("geyser.bootstrap.unsupported_java.message", SUPPORTED_JAVA_VERSION, javaVersion));
+            logger.warning("");
+            logger.warning("*********************************************");
+        }
+    }
+
     public static void checkForGeyserUpdate(Supplier<GeyserCommandSource> recipient) {
         CompletableFuture.runAsync(() -> {
             try {