Add simple plugin system

src/main/java/emu/nebula/Nebula.java

@@ -15,6 +15,7 @@ import emu.nebula.data.ResourceLoader;
 import emu.nebula.database.DatabaseManager;
 import emu.nebula.game.GameContext;
 import emu.nebula.net.PacketHelper;
+import emu.nebula.plugin.PluginManager;
 import emu.nebula.server.HttpServer;
 import emu.nebula.util.Handbook;
 import emu.nebula.util.JsonUtils;
@@ -38,6 +39,7 @@ public class Nebula {
     
     @Getter private static GameContext gameContext;
     @Getter private static CommandManager commandManager;
+    @Getter private static PluginManager pluginManager;
     
     public static void main(String[] args) {
         // Start Server
@@ -50,6 +52,15 @@ public class Nebula {
         // Load config + commands
         Nebula.loadConfig();
         
+        // Load plugin manager
+        Nebula.pluginManager = new PluginManager();
+
+        try {
+            Nebula.getPluginManager().loadPlugins();
+        } catch (Exception exception) {
+            Nebula.getLogger().error("Unable to load plugins.", exception);
+        }
+        
         // Parse arguments
         for (String arg : args) {
             switch (arg) {
@@ -103,6 +114,9 @@ public class Nebula {
             Nebula.getLogger().error("Unable to start the HTTP server.", exception);
         }
         
+        // Enable plugins
+        Nebula.getPluginManager().enablePlugins();
+        
         // Start console
         Nebula.startConsole();
     }

src/main/java/emu/nebula/plugin/Plugin.java

@@ -0,0 +1,99 @@
+package emu.nebula.plugin;
+
+import lombok.AccessLevel;
+import lombok.Getter;
+import lombok.RequiredArgsConstructor;
+import org.slf4j.Logger;
+
+import java.io.File;
+import java.io.InputStream;
+import java.net.URLClassLoader;
+
+@Getter
+@RequiredArgsConstructor(access = AccessLevel.PROTECTED)
+public abstract class Plugin {
+    private final Identifier identifier;
+    private final URLClassLoader classLoader;
+    private final File dataFolder;
+    private final Logger logger;
+
+    /*
+     * Collection of plugin events which are called by the server.
+     */
+
+    public void onLoad() {}
+    public void onEnable() {}
+    public void onDisable() {}
+
+    /**
+     * Fetches a resource from the plugin's JAR file.
+     *
+     * @param fileName The name of the file to fetch.
+     * @return An {@link InputStream} of the file.
+     */
+    public final InputStream getResource(String fileName) {
+        return this.getClassLoader().getResourceAsStream(fileName);
+    }
+
+    /** Get the plugin's name. */
+    public final String getName() {
+        return this.getIdentifier().name;
+    }
+
+    /** Get the plugin's description. */
+    public final String getDescription() {
+        return this.getIdentifier().description;
+    }
+
+    /** Get the plugin's version. */
+    public final String getVersion() {
+        return this.getIdentifier().version;
+    }
+
+    /** Deserialized plugin config data. */
+    public record Config(
+        String name,
+        String description,
+        String version,
+        String mainClass,
+        Integer api,
+        String[] authors,
+        String[] loadAfter
+    ) {
+        /**
+         * Attempts to validate this config instance.
+         *
+         * @return True if the config is valid, false otherwise.
+         */
+        public boolean validate() {
+            return name != null && description != null && mainClass != null && api != null;
+        }
+    }
+
+    /** Loaded plugin data. */
+    public record Identifier(
+        String name,
+        String description,
+        String version,
+        String[] authors
+    ) {
+        /**
+         * Converts a {@link Config} into a {@link Identifier}.
+         *
+         * @param config The config to convert.
+         * @return An instance of {@link Identifier}.
+         */
+        public static Identifier from(Config config) {
+            if (!config.validate())
+                throw new IllegalArgumentException("Invalid plugin config supplied.");
+            return new Identifier(config.name(), config.description(), config.version(), config.authors());
+        }
+    }
+
+    /** Unloaded plugin data. */
+    public record Data(
+        Plugin instance,
+        URLClassLoader classLoader,
+        String[] dependencies
+    ) {}
+}

src/main/java/emu/nebula/plugin/PluginManager.java

@@ -0,0 +1,215 @@
+package emu.nebula.plugin;
+
+import emu.nebula.util.JsonUtils;
+import lombok.Getter;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.File;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.lang.reflect.InvocationTargetException;
+import java.net.URL;
+import java.net.URLClassLoader;
+import java.util.*;
+
+/** 
+ * Manages the server's plugins and the event system.
+ * @author https://github.com/KingRainbow44
+ */
+@Getter
+public final class PluginManager {
+    /*
+     * This should only be changed when a breaking change is made to the plugin API.
+     * A 'breaking change' is something which changes the existing logic of the API.
+     */
+    public static final int API_VERSION = 1;
+
+    /**
+     * The directory where plugins are stored.
+     */
+    public static final File PLUGINS_DIR = new File("plugins");
+
+    /** Map of loaded plugins; Name -> Instance */
+    private final Map<String, Plugin> plugins = new HashMap<>();
+    private final Logger logger = LoggerFactory.getLogger("Plugin Manager");
+
+    private boolean pluginsLoaded = false;
+
+    /**
+     * Loads all plugins from the plugins directory.
+     * This can only be called once.
+     */
+    @SuppressWarnings("resource")
+    public void loadPlugins() throws IOException {
+        if (this.pluginsLoaded) {
+            throw new IllegalStateException("Plugins have already been loaded.");
+        }
+            
+        this.pluginsLoaded = true;
+
+        if (!PLUGINS_DIR.exists() && !PLUGINS_DIR.mkdirs()) {
+            throw new IOException("Failed to create plugins directory.");
+        }
+        
+        // Read files from the directory.
+        var files = PLUGINS_DIR.listFiles();
+        if (files == null) return;
+
+        var loadingExceptions = new ArrayList<Exception>();
+
+        var pluginFiles = Arrays.stream(files)
+            .filter(file -> file.getName().endsWith(".jar"))
+            .toList();
+        var pluginURLs = pluginFiles.stream()
+            .map(file -> {
+                try {
+                    return file.toURI().toURL();
+                } catch (IOException e) {
+                    loadingExceptions.add(e);
+                    return null;
+                }
+            })
+            .filter(Objects::nonNull)
+            .toArray(URL[]::new);
+
+        loadingExceptions.forEach(e -> this.getLogger().warn("Failed to load plugin: " + e.getMessage()));
+
+        // Begin loading plugins.
+        var classLoader = new URLClassLoader(pluginURLs);
+        var dependencies = new ArrayList<Plugin.Data>();
+
+        pluginFiles.forEach(pluginFile -> {
+            try {
+                var pluginUrl = pluginFile.toURI().toURL();
+
+                // Read the plugin's configuration file.
+                var jarReader = new URLClassLoader(new URL[] { pluginUrl });
+                var pluginConfigFile = jarReader.getResourceAsStream("plugin.json");
+                if (pluginConfigFile == null) {
+                    this.getLogger().warn("Plugin {} did not specify a config file.", pluginFile.getName());
+                    return;
+                }
+
+                // Deserialize the plugin's configuration file.
+                var configReader = new InputStreamReader(pluginConfigFile);
+                var pluginConfig = JsonUtils.loadToClass(configReader, Plugin.Config.class);
+                if (pluginConfig == null) {
+                    this.getLogger().warn("Plugin {} has an invalid config file.", pluginFile.getName());
+                    return;
+                }
+                jarReader.close();
+
+                // Validate the plugin's configuration file.
+                if (pluginConfig.api() == null) {
+                    this.getLogger().warn("Plugin {} did not specify an API version.", pluginFile.getName());
+                    return;
+                } else if (pluginConfig.api() != API_VERSION) {
+                    this.getLogger().warn("Plugin {} requires API version {}, but this server is running version {}.", pluginFile.getName(), pluginConfig.api(), API_VERSION);
+                    return;
+                } else if (!pluginConfig.validate()) {
+                    this.getLogger().warn("Plugin {} has an invalid config file.", pluginFile.getName());
+                    return;
+                }
+
+                // Instantiate the plugin.
+                var pluginClass = classLoader.loadClass(pluginConfig.mainClass());
+                var pluginInstance = (Plugin) pluginClass.getDeclaredConstructor(
+                    Plugin.Identifier.class,
+                    URLClassLoader.class,
+                    File.class,
+                    Logger.class
+                ).newInstance(
+                    Plugin.Identifier.from(pluginConfig),
+                    classLoader,
+                    new File(PLUGINS_DIR, pluginConfig.name()),
+                    LoggerFactory.getLogger(pluginConfig.name())
+                );
+
+                // Check for plugin dependencies.
+                var loadAfter = pluginConfig.loadAfter();
+                if (loadAfter != null && loadAfter.length > 0) {
+                    dependencies.add(new Plugin.Data(
+                        pluginInstance,
+                        classLoader,
+                        loadAfter
+                    ));
+                } else try {
+                    pluginInstance.onLoad();
+                    this.plugins.put(pluginInstance.getName(), pluginInstance);
+                } catch (Throwable exception) {
+                    this.getLogger().warn("Failed to load plugin {}.", pluginFile.getName());
+                }
+            } catch (IOException | ClassNotFoundException e) {
+                this.getLogger().warn("Failed to load plugin {}.", pluginFile.getName());
+            } catch (InvocationTargetException | InstantiationException | IllegalAccessException | NoSuchMethodException e) {
+                throw new RuntimeException(e);
+            }
+        });
+
+        // Load all plugins with dependencies.
+        var depth = 0;
+        final var maxDepth = 30;
+        while (!dependencies.isEmpty()) {
+            // Check if the depth is too high.
+            if (depth >= maxDepth) {
+                this.getLogger().warn("Failed to load plugins due to circular dependencies.");
+                break;
+            }
+
+            try {
+                // Get the next plugin to load.
+                var pluginData = dependencies.get(0);
+
+                // Check if the plugin's dependencies are loaded.
+                if (!this.plugins.keySet().containsAll(List.of(pluginData.dependencies()))) {
+                    depth++; // Increase depth counter.
+                    continue; // Continue to next plugin.
+                }
+
+                // Remove the plugin from the list of dependencies.
+                dependencies.remove(pluginData);
+
+                // Load the plugin.
+                pluginData.instance().onLoad();
+                this.plugins.put(pluginData.instance().getName(), pluginData.instance());
+            } catch (Throwable exception) {
+                this.getLogger().warn("Failed to load plugin {}.", exception.getMessage());
+                depth++;
+            }
+        }
+    }
+
+    /**
+     * Enables all plugins.
+     */
+    public void enablePlugins() {
+        this.getPlugins().forEach((name, plugin) -> {
+            try {
+                this.getLogger().info("Enabling plugin {}.", name);
+                plugin.onEnable();
+                return;
+            } catch (NoSuchMethodError | NoSuchFieldError ignored) {
+                this.getLogger().warn("Plugin {} is not compatible with this server version.", name);
+            } catch (Throwable exception) {
+                this.getLogger().warn("Failed to enable plugin {}.", name);
+            }
+
+            plugin.onDisable();
+        });
+    }
+
+    /**
+     * Disables all plugins.
+     */
+    public void disablePlugins() {
+        this.getPlugins().forEach((name, plugin) -> {
+            try {
+                this.getLogger().info("Disabling plugin {}.", name);
+                plugin.onDisable();
+            } catch (Throwable exception) {
+                this.getLogger().warn("Failed to disable plugin {}.", name);
+            }
+        });
+    }
+}