Basic plugin manager

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

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+package emu.grasscutter.plugin;
+
+import emu.grasscutter.Grasscutter;
+import emu.grasscutter.utils.Utils;
+
+import java.io.File;
+import java.io.InputStreamReader;
+import java.net.URL;
+import java.net.URLClassLoader;
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.stream.Collectors;
+
+/**
+ * Manages the server's plugins & the event system.
+ */
+public final class PluginManager {
+    private final Map<String, Plugin> plugins = new HashMap<>();
+    
+    public PluginManager() {
+        this.loadPlugins(); // Load all plugins from the plugins directory.
+    }
+
+    /**
+     * Loads plugins from the config-specified directory.
+     */
+    private void loadPlugins() {
+        String directory = Grasscutter.getConfig().PLUGINS_FOLDER;
+        File pluginsDir = new File(Utils.toFilePath(directory));
+        if(!pluginsDir.exists() && !pluginsDir.mkdirs()) {
+            Grasscutter.getLogger().error("Failed to create plugins directory: " + pluginsDir.getAbsolutePath());
+            return;
+        }
+        
+        File[] files = pluginsDir.listFiles();
+        if(files == null) {
+            // The directory is empty, there aren't any plugins to load.
+            return;
+        }
+        
+        List<File> plugins = Arrays.stream(files)
+                .filter(file -> file.getName().endsWith(".jar"))
+                .collect(Collectors.toList());
+        
+        plugins.forEach(plugin -> {
+            try {
+                URL url = plugin.toURI().toURL();
+                try (URLClassLoader loader = new URLClassLoader(new URL[]{url})) {
+                    URL configFile = loader.findResource("plugin.json");
+                    InputStreamReader fileReader = new InputStreamReader(configFile.openStream());
+                    
+                    PluginConfig pluginConfig = Grasscutter.getGsonFactory().fromJson(fileReader, PluginConfig.class);
+                    if(!pluginConfig.validate()) {
+                        Utils.logObject(pluginConfig);
+                        Grasscutter.getLogger().warn("Plugin " + plugin.getName() + " has an invalid config file.");
+                        return;
+                    }
+                    
+                    Class<?> pluginClass = loader.loadClass(pluginConfig.mainClass);
+                    Plugin pluginInstance = (Plugin) pluginClass.getDeclaredConstructor(PluginIdentifier.class)
+                            .newInstance(PluginIdentifier.fromPluginConfig(pluginConfig));
+                    this.loadPlugin(pluginInstance);
+                    
+                    fileReader.close(); // Close the file reader.
+                } catch (ClassNotFoundException ignored) {
+                    Grasscutter.getLogger().warn("Plugin " + plugin.getName() + " has an invalid main class.");
+                }
+            } catch (Exception exception) {
+                Grasscutter.getLogger().error("Failed to load plugin: " + plugin.getName(), exception);
+            }
+        });
+    }
+
+    /**
+     * Load the specified plugin.
+     * @param plugin The plugin instance.
+     */
+    private void loadPlugin(Plugin plugin) {
+        Grasscutter.getLogger().info("Loading plugin: " + plugin.getName());
+        
+        // Add the plugin to the list of loaded plugins.
+        this.plugins.put(plugin.getName(), plugin);
+        // Call the plugin's onLoad method.
+        plugin.onLoad();
+    }
+
+    /**
+     * Enables all registered plugins.
+     */
+    public void enablePlugins() {
+        
+    }
+    
+    /**
+     * Disables all registered plugins.
+     */
+    public void disablePlugins() {
+        
+    }
+}