Implement jar-in-jar loader system (#2899)

This fixes an issue that prevented LuckPerms from loading on Java 16

common/loader-utils/build.gradle

@@ -0,0 +1 @@
+// nothing special to do here (yet)!

common/loader-utils/src/main/java/me/lucko/luckperms/common/loader/JarInJarClassLoader.java

@@ -0,0 +1,140 @@
+/*
+ * This file is part of LuckPerms, licensed under the MIT License.
+ *
+ *  Copyright (c) lucko (Luck) <luck@lucko.me>
+ *  Copyright (c) contributors
+ *
+ *  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.
+ */
+
+package me.lucko.luckperms.common.loader;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.lang.reflect.Constructor;
+import java.net.MalformedURLException;
+import java.net.URL;
+import java.net.URLClassLoader;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.StandardCopyOption;
+
+/**
+ * Classloader that can load a jar from within another jar file.
+ *
+ * <p>The "loader" jar contains the loading code & public API classes,
+ * and is class-loaded by the platform.</p>
+ *
+ * <p>The inner "plugin" jar contains the plugin itself, and is class-loaded
+ * by the loading code & this classloader.</p>
+ */
+public class JarInJarClassLoader extends URLClassLoader {
+    static {
+        ClassLoader.registerAsParallelCapable();
+    }
+
+    /**
+     * Creates a new jar-in-jar class loader.
+     *
+     * @param loaderClassLoader the loader plugin's classloader (setup and created by the platform)
+     * @param jarResourcePath the path to the jar-in-jar resource within the loader jar
+     * @throws LoadingException if something unexpectedly bad happens
+     */
+    public JarInJarClassLoader(ClassLoader loaderClassLoader, String jarResourcePath) throws LoadingException {
+        super(new URL[]{extractJar(loaderClassLoader, jarResourcePath)}, loaderClassLoader);
+    }
+
+    public void addJarToClasspath(URL url) {
+        addURL(url);
+    }
+
+    /**
+     * Creates a new plugin instance.
+     *
+     * @param bootstrapClass the name of the bootstrap plugin class
+     * @param loaderPluginType the type of the loader plugin, the only parameter of the bootstrap
+     *                         plugin constructor
+     * @param loaderPlugin the loader plugin instance
+     * @param <T> the type of the loader plugin
+     * @return the instantiated bootstrap plugin
+     */
+    public <T> LoaderBootstrap instantiatePlugin(String bootstrapClass, Class<T> loaderPluginType, T loaderPlugin) throws LoadingException {
+        Class<? extends LoaderBootstrap> plugin;
+        try {
+            plugin = loadClass(bootstrapClass).asSubclass(LoaderBootstrap.class);
+        } catch (ReflectiveOperationException e) {
+            throw new LoadingException("Unable to load bootstrap class", e);
+        }
+
+        Constructor<? extends LoaderBootstrap> constructor;
+        try {
+            constructor = plugin.getConstructor(loaderPluginType);
+        } catch (ReflectiveOperationException e) {
+            throw new LoadingException("Unable to get bootstrap constructor", e);
+        }
+
+        try {
+            return constructor.newInstance(loaderPlugin);
+        } catch (ReflectiveOperationException e) {
+            throw new LoadingException("Unable to create bootstrap plugin instance", e);
+        }
+    }
+
+    /**
+     * Extracts the "jar-in-jar" from the loader plugin into a temporary file,
+     * then returns a URL that can be used by the {@link JarInJarClassLoader}.
+     *
+     * @param loaderClassLoader the classloader for the "host" loader plugin
+     * @param jarResourcePath the inner jar resource path
+     * @return a URL to the extracted file
+     */
+    private static URL extractJar(ClassLoader loaderClassLoader, String jarResourcePath) throws LoadingException {
+        // get the jar-in-jar resource
+        URL jarInJar = loaderClassLoader.getResource(jarResourcePath);
+        if (jarInJar == null) {
+            throw new LoadingException("Could not locate jar-in-jar");
+        }
+
+        // create a temporary file
+        // on posix systems by default this is only read/writable by the process owner
+        Path path;
+        try {
+            path = Files.createTempFile("luckperms-jarinjar", ".jar.tmp");
+        } catch (IOException e) {
+            throw new LoadingException("Unable to create a temporary file", e);
+        }
+
+        // mark that the file should be deleted on exit
+        path.toFile().deleteOnExit();
+
+        // copy the jar-in-jar to the temporary file path
+        try (InputStream in = jarInJar.openStream()) {
+            Files.copy(in, path, StandardCopyOption.REPLACE_EXISTING);
+        } catch (IOException e) {
+            throw new LoadingException("Unable to copy jar-in-jar to temporary path", e);
+        }
+
+        try {
+            return path.toUri().toURL();
+        } catch (MalformedURLException e) {
+            throw new LoadingException("Unable to get URL from path", e);
+        }
+    }
+
+}

common/loader-utils/src/main/java/me/lucko/luckperms/common/loader/LoaderBootstrap.java

@@ -0,0 +1,39 @@
+/*
+ * This file is part of LuckPerms, licensed under the MIT License.
+ *
+ *  Copyright (c) lucko (Luck) <luck@lucko.me>
+ *  Copyright (c) contributors
+ *
+ *  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.
+ */
+
+package me.lucko.luckperms.common.loader;
+
+/**
+ * Minimal bootstrap plugin, called by the loader plugin.
+ */
+public interface LoaderBootstrap {
+
+    void onLoad();
+
+    void onEnable();
+
+    void onDisable();
+
+}

common/loader-utils/src/main/java/me/lucko/luckperms/common/loader/LoadingException.java

@@ -0,0 +1,41 @@
+/*
+ * This file is part of LuckPerms, licensed under the MIT License.
+ *
+ *  Copyright (c) lucko (Luck) <luck@lucko.me>
+ *  Copyright (c) contributors
+ *
+ *  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.
+ */
+
+package me.lucko.luckperms.common.loader;
+
+/**
+ * Runtime exception used if there is a problem during loading
+ */
+public class LoadingException extends RuntimeException {
+
+    public LoadingException(String message) {
+        super(message);
+    }
+
+    public LoadingException(String message, Throwable cause) {
+        super(message, cause);
+    }
+
+}