PlaceholderAPI/src/main/java/me/clip/placeholderapi/util/FileUtil.java

package me.clip.placeholderapi.util;

import java.io.File;
import java.io.FilenameFilter;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.ArrayList;
import java.util.List;
import java.util.jar.JarEntry;
import java.util.jar.JarInputStream;

import me.clip.placeholderapi.PlaceholderAPIPlugin;

public class FileUtil {

    public static List<Class<?>> getClasses(String folder, Class<?> type) {
    	List<Class<?>> list = new ArrayList<>();
        try {
            File f = new File(PlaceholderAPIPlugin.getInstance().getDataFolder(), folder);
            if (!f.exists()) {
                if (!f.mkdir()) {
                    PlaceholderAPIPlugin.getInstance().getLogger().severe("Failed to create " + folder + " folder!");
                }
                return list;
            }
            FilenameFilter fileNameFilter = (dir, name) -> {
                int i = name.lastIndexOf('.');
                return i > 0 && name.substring(i).equals(".jar");
            };
            File[] jars = f.listFiles(fileNameFilter);
            if (jars == null) {
                return list;
            }
            for (File file : jars) {
                list = gather(file.toURI().toURL(), list, type);
            }
            return list;
        } catch (Throwable t) {}
        return null;
    }

	private static List<Class<?>> gather(URL jar, List<Class<?>> list, Class<?> clazz) {
    	if (list == null) {
            list = new ArrayList<>();
        }
        try {
            URLClassLoader cl = new URLClassLoader(new URL[]{jar}, clazz.getClassLoader());
            JarInputStream jis = new JarInputStream(jar.openStream());
            while (true) {
                JarEntry j = jis.getNextJarEntry();
                if (j == null) {
                    break;
                }
                String name = j.getName();
                if (name == null || name.isEmpty()) {
                    continue;
                }
                if (name.endsWith(".class")) {
                    name = name.replace("/", ".");
                    String cname = name.substring(0, name.lastIndexOf(".class"));
                    Class<?> c = cl.loadClass(cname);
                    if (clazz.isAssignableFrom(c)) {
                        list.add(c);
                    }
                }
            }
            cl.close();
            jis.close();
        } catch (Throwable t) {
        }
        return list;
    }
}