Implement a PluginLoader, which can load plugin jars.

src/org/nwapw/abacus/plugin/PluginLoader.java

@@ -0,0 +1,106 @@
+package org.nwapw.abacus.plugin;
+
+import java.io.File;
+import java.io.IOException;
+import java.net.URL;
+import java.net.URLClassLoader;
+import java.util.ArrayList;
+import java.util.Properties;
+import java.util.jar.JarFile;
+
+/**
+ * A plugin loader, used to scan a directory for
+ * plugins and load them into classes that can then be
+ * used by the plugin manager.
+ */
+public class PluginLoader {
+
+    /**
+     * An internal class that represents a Jar file that
+     * has been founded, but not loaded.
+     */
+    private static final class PluginEntry {
+        String mainClass;
+        File jarPath;
+    }
+
+    /**
+     * The path which to search for plugins.
+     */
+    private File path;
+    /**
+     * The array of loaded plugin main classes.
+     */
+    private ArrayList<Class<?>> foundMainClasses;
+
+    /**
+     * Creates a new plugin loader at the given path.
+     * @param path the path which to search for plugins.
+     */
+    public PluginLoader(File path) {
+        this.path = path;
+    }
+
+    /**
+     * Loads all the plugin classes that have been found.
+     * @return the list of loaded classes.
+     * @throws IOException thrown when loading classes from URL fails.
+     * @throws ClassNotFoundException thrown when the class specified in plugin.properties is missing.
+     */
+    private ArrayList<Class<?>> loadPluginClasses() throws IOException, ClassNotFoundException {
+        ArrayList<Class<?>> foundMainClasses = new ArrayList<>();
+        for(PluginEntry entry : findPlugins()){
+            if(entry.mainClass == null) continue;
+            ClassLoader loader = URLClassLoader.newInstance(
+                    new URL[]{ entry.jarPath.toURI().toURL() },
+                    getClass().getClassLoader());
+            Class<?> loadedClass = loader.loadClass(entry.mainClass);
+            if(!Plugin.class.isAssignableFrom(loadedClass)) continue;
+            foundMainClasses.add(loadedClass);
+        }
+        return foundMainClasses;
+    }
+
+    /**
+     * Find all jars that have a plugin.properties file in the plugin folder.
+     * @return the list of all plugin entries, with their main class names and the jars files.
+     * @throws IOException thrown if reading the jar file fails
+     */
+    private ArrayList<PluginEntry> findPlugins() throws IOException {
+        ArrayList<PluginEntry> pluginEntries = new ArrayList<>();
+        File[] childFiles = path.listFiles();
+
+        if(childFiles == null) return pluginEntries;
+        for(File file : childFiles){
+            if(!file.isFile() || !file.getName().endsWith(".jar")) continue;
+            JarFile jarFile = new JarFile(file);
+            if(jarFile.getEntry("plugin.properties") == null) continue;
+            Properties properties = new Properties();
+            properties.load(jarFile.getInputStream(jarFile.getEntry("plugin.properties")));
+
+            PluginEntry entry = new PluginEntry();
+            entry.mainClass = properties.getProperty("mainClass");
+            entry.jarPath = file;
+            pluginEntries.add(entry);
+        }
+        return pluginEntries;
+    }
+
+    /**
+     * Loads all valid plugins and keeps track of them.
+     * @throws IOException thrown if loading from jar files fails.
+     * @throws ClassNotFoundException thrown if class specified in plugin.properties doesn't exist.
+     */
+    public void loadValidPlugins() throws IOException, ClassNotFoundException {
+        foundMainClasses = loadPluginClasses();
+    }
+
+    /**
+     * Gets the list of all the plugins that have last been loaded.
+     * @return the list of loaded class files.
+     */
+    public ArrayList<Class<?>> getFoundMainClasses() {
+        return foundMainClasses;
+    }
+
+}