Enable Multi-Release plugin jar support

Fixes #4841

Spigot-API-Patches/0241-Enable-multi-release-plugin-jars.patch

@@ -0,0 +1,30 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Kyle Wood <demonwav@gmail.com>
+Date: Fri, 4 Dec 2020 15:53:19 -0800
+Subject: [PATCH] Enable multi-release plugin jars
+
+
+diff --git a/src/main/java/org/bukkit/plugin/java/PluginClassLoader.java b/src/main/java/org/bukkit/plugin/java/PluginClassLoader.java
+index 62f7a6817da079513f471e36cd79739d36a36d86..7760be3e34fa20825faf145d9fb5b2855c1a4602 100644
+--- a/src/main/java/org/bukkit/plugin/java/PluginClassLoader.java
++++ b/src/main/java/org/bukkit/plugin/java/PluginClassLoader.java
+@@ -56,7 +56,18 @@ public final class PluginClassLoader extends URLClassLoader { // Spigot
+         this.description = description;
+         this.dataFolder = dataFolder;
+         this.file = file;
+-        this.jar = new JarFile(file);
++        // Paper - enable multi-release jars for Java 9+
++        JarFile jarFile;
++        try {
++            final java.lang.reflect.Method runtimeVersionMethod = JarFile.class.getMethod("runtimeVersion");
++            final Object runtimeVersion = runtimeVersionMethod.invoke(null);
++            @SuppressWarnings("JavaReflectionMemberAccess") final java.lang.reflect.Constructor<JarFile> constructor = JarFile.class.getConstructor(File.class, boolean.class, int.class, runtimeVersion.getClass());
++            jarFile = constructor.newInstance(file, true, java.util.zip.ZipFile.OPEN_READ, runtimeVersion);
++        } catch (Exception ignored) {
++            jarFile = new JarFile(file);
++        }
++        this.jar = jarFile;
++        // Paper end
+         this.manifest = jar.getManifest();
+         this.url = file.toURI().toURL();
+ 

Spigot-Server-Patches/0611-Add-warning-for-servers-not-running-on-Java-11.patch

@@ -6,64 +6,57 @@ Subject: [PATCH] Add warning for servers not running on Java 11
 
 diff --git a/src/main/java/com/destroystokyo/paper/util/PaperJvmChecker.java b/src/main/java/com/destroystokyo/paper/util/PaperJvmChecker.java
 new file mode 100644
-index 0000000000000000000000000000000000000000..023123d52fbb458e6ee2d38e42c9651a5c7be728
+index 0000000000000000000000000000000000000000..afe07487b1e7337293f7147619a6f10fcdef9d0e
 --- /dev/null
 +++ b/src/main/java/com/destroystokyo/paper/util/PaperJvmChecker.java
-@@ -0,0 +1,56 @@
+@@ -0,0 +1,49 @@
 +package com.destroystokyo.paper.util;
 +
 +import org.apache.logging.log4j.LogManager;
 +import org.apache.logging.log4j.Logger;
 +
-+public final class PaperJvmChecker {
++public class PaperJvmChecker {
 +
-+    private PaperJvmChecker() {}
++    private static int getJvmVersion() {
-+
++        String javaVersion = System.getProperty("java.version");
-+    public static void checkJvm() {
++        int dotIndex = javaVersion.indexOf('.');
-+        final Logger logger = LogManager.getLogger();
-+
-+        final String javaVersion = System.getProperty("java.version");
-+        final int dotIndex = javaVersion.indexOf('.');
 +
 +        if (javaVersion.startsWith("1.")) {
-+            // definitely old
++            // For Java 8 and below, trim off the 1. prefix
-+            PaperJvmChecker.printWarning(javaVersion, logger);
++            javaVersion = javaVersion.substring(2);
-+            return;
++            dotIndex = javaVersion.indexOf('.');
 +        }
 +
 +        final int endIndex = dotIndex == -1 ? javaVersion.length() : dotIndex;
 +        final String version = javaVersion.substring(0, endIndex);
 +
-+        final int javaVersionNum;
 +        try {
-+            javaVersionNum = Integer.parseInt(version);
++            return Integer.parseInt(version);
 +        } catch (final NumberFormatException e) {
-+            logger.warn("Failed to determine Java version; Could not parse {}", version, e);
++            LogManager.getLogger().warn("Failed to determine Java version; Could not parse {}", version, e);
-+            PaperJvmChecker.printWarning(javaVersion, logger);
++            return -1;
-+            return;
-+        }
-+
-+        if (javaVersionNum < 11) {
-+            PaperJvmChecker.printWarning(javaVersion, logger);
 +        }
 +    }
 +
-+    private static void printWarning(final String version, final Logger logger) {
++    public static void checkJvm() {
-+        logger.warn("************************************************************");
++        if (getJvmVersion() < 11) {
-+        logger.warn("* WARNING - YOU ARE RUNNING AN OUTDATED VERSION OF JAVA.");
++            final Logger logger = LogManager.getLogger();
-+        logger.warn("* PAPER WILL STOP BEING COMPATIBLE WITH THIS VERSION OF");
++            logger.warn("************************************************************");
-+        logger.warn("* JAVA WHEN MINECRAFT 1.17 IS RELEASED.");
++            logger.warn("* WARNING - YOU ARE RUNNING AN OUTDATED VERSION OF JAVA.");
-+        logger.warn("*");
++            logger.warn("* PAPER WILL STOP BEING COMPATIBLE WITH THIS VERSION OF");
-+        logger.warn("* Please update the version of Java you use to run Paper");
++            logger.warn("* JAVA WHEN MINECRAFT 1.17 IS RELEASED.");
-+        logger.warn("* to at least Java 11. When Paper for Minecraft 1.17 is");
++            logger.warn("*");
-+        logger.warn("* released support for versions of Java before 11 will");
++            logger.warn("* Please update the version of Java you use to run Paper");
-+        logger.warn("* be dropped.");
++            logger.warn("* to at least Java 11. When Paper for Minecraft 1.17 is");
-+        logger.warn("*");
++            logger.warn("* released support for versions of Java before 11 will");
-+        logger.warn("* Current Java version: " + version);
++            logger.warn("* be dropped.");
-+        logger.warn("*");
++            logger.warn("*");
-+        logger.warn("* Check this forum post for more information: ");
++            logger.warn("* Current Java version: {}", System.getProperty("java.version"));
-+        logger.warn("*   https://papermc.io/java11");
++            logger.warn("*");
-+        logger.warn("************************************************************");
++            logger.warn("* Check this forum post for more information: ");
++            logger.warn("*   https://papermc.io/java11");
++            logger.warn("************************************************************");
++        }
 +    }
 +}
 diff --git a/src/main/java/net/minecraft/server/MinecraftServer.java b/src/main/java/net/minecraft/server/MinecraftServer.java