Sample code with groovy class loader

App.java

/*
 * This Java source file was generated by the Gradle 'init' task.
 */
import groovy.lang.GroovyClassLoader;
import groovy.cli.commons.CliBuilder;
import groovy.cli.Option;

import java.io.File;
import java.io.IOException;
import java.nio.file.*;
import java.util.function.IntSupplier;

import static java.nio.file.StandardWatchEventKinds.*;

public class App {
    interface OptionInterface {
        @Option(longName="root", description="Path to the root directory to load plugins") String root();
        @Option(longName="plugin", description="Name of the plugin to load") String plugin();
    }

    public static void main(String[] args) throws IOException {
        CliBuilder builder = new CliBuilder();
        builder.setUsage("groovy-plugin --root [path]");

        OptionInterface options = builder.parseFromSpec(OptionInterface.class, args);
        Path root = Paths.get(options.root());

        GroovyClassLoader gcl= new GroovyClassLoader();
        gcl.addClasspath(options.root());

        WatchService watcher = FileSystems.getDefault().newWatchService();
        Files.walk(root)
                .filter(p -> p.toFile().isDirectory())
                .forEach(p -> {
                    try {
                        System.out.println("Registering '" + p.toAbsolutePath().toString() + "'");
                        p.register(watcher, ENTRY_CREATE, ENTRY_MODIFY);
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                });

        while(true) {
            try {
                WatchKey result = watcher.take();
                for(WatchEvent<?> e: result.pollEvents()) {
                    Path resultRoot = (Path) result.watchable();
                    Path relative = Paths.get(options.root()).relativize(resultRoot);

                    String filename = relative.resolve(((WatchEvent<Path>) e).context()).toString();

                    if (filename.endsWith(".groovy")) {
                        String classname = filename.substring(0, filename.lastIndexOf('.'))
                                .replace(File.separator, ".");
                        if (classname.contains(options.plugin())) {
                            Class<?> clazz = gcl.loadClass(classname, true, false);

                            Object newObj = clazz.newInstance();

                            System.out.printf("%s()=%d%n",
                                    filename,
                                    ((IntSupplier) newObj).getAsInt()
                            );
                        }
                    }
                }

                result.reset();
            } catch (InterruptedException | InstantiationException | IllegalAccessException | ClassNotFoundException e) {
                e.printStackTrace();
            }
        }
    }
}

AppScript.java

import groovy.cli.Option;
import groovy.cli.commons.CliBuilder;
import groovy.lang.Binding;
import groovy.util.GroovyScriptEngine;
import groovy.util.ResourceException;
import groovy.util.ScriptException;

import java.io.IOException;
import java.net.URL;
import java.nio.file.*;

import static java.nio.file.StandardWatchEventKinds.ENTRY_CREATE;
import static java.nio.file.StandardWatchEventKinds.ENTRY_MODIFY;

public class AppScript {
    interface OptionInterface {
        @Option(longName="root", description="Path to the root directory to load plugins") String root();
        @Option(longName="script", description="Groovy script to execute") String script();
    }

    public static void main(String[] args) throws IOException {
        CliBuilder builder = new CliBuilder();
        builder.setUsage("groovy-plugin --root [path] --script [name]");

        OptionInterface options = builder.parseFromSpec(OptionInterface.class, args);
        Path root = Paths.get(options.root());

        WatchService watcher = FileSystems.getDefault().newWatchService();
        Files.walk(root)
                .filter(p -> p.toFile().isDirectory())
                .forEach(p -> {
                    try {
                        System.out.println("Registering '" + p.toAbsolutePath().toString() + "'");
                        p.register(watcher, ENTRY_CREATE, ENTRY_MODIFY);
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                });

        Binding binding = new Binding();
        binding.setVariable("n", 10);
        GroovyScriptEngine engine = new GroovyScriptEngine(new URL[] {
                root.toUri().toURL()
        });
        while(true) {
            try {
                WatchKey result = watcher.take();
                for(WatchEvent<?> e: result.pollEvents()) {
                    Path resultRoot = (Path) result.watchable();
                    Path relative = Paths.get(options.root()).relativize(resultRoot);

                    String filename = relative.resolve(((WatchEvent<Path>) e).context()).toString();

                    if (filename.equals(options.script())) {
                        engine.run(filename, binding);
                    }
                }

                result.reset();
            } catch (InterruptedException | ResourceException | ScriptException e) {
                e.printStackTrace();
            }
        }
    }
}