better ;)

PluginManager.java

package org.heater.core.plugin;

import com.google.inject.AbstractModule;
import com.google.inject.Inject;
import com.google.inject.multibindings.Multibinder;
import org.heater.api.HeaterActivator;

import java.io.File;
import java.io.FileFilter;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.*;

public class PluginManager {
    private final List<HeaterActivator> instances;
    private AbstractModule module;

    @Inject Set<HeaterActivator> plugins;

    public PluginManager() {
        this.instances = new ArrayList<>();
    }

    public PluginManager initialize() {
        File[] files = new File("plugins").listFiles(file -> file.getPath().toLowerCase().endsWith(".jar"));

        URL[] urls = new URL[files.length];

        for (int i = 0; i < files.length; i++) {
            try {
                urls[i] = files[i].toURI().toURL();
            } catch (MalformedURLException e) {
                e.printStackTrace();
            }
        }

        ServiceLoader<HeaterActivator> loader = ServiceLoader.load(HeaterActivator.class, new URLClassLoader(urls));
        for(HeaterActivator instance: loader)
            this.instances.add(instance);
        
        return this;
    }

    public void terminate() {
        if(plugins == null) return;

        for(HeaterActivator plugin: plugins)
            plugin.stop();
    }

    public void installPlugins() {
        if(plugins == null) return;

        for(HeaterActivator plugin: plugins)
            plugin.start();
    }

    private AbstractModule buildModule() {
        return new AbstractModule() {
            @Override
            protected void configure() {
                bind(PluginManager.class).toInstance(PluginManager.this);

                Multibinder<HeaterActivator> binder = Multibinder.newSetBinder(binder(), HeaterActivator.class);
                for(HeaterActivator instance: instances) {
                    binder.addBinding().toInstance(instance);
                    install(instance.pluginModule());
                }
            }
        };
    }

    public AbstractModule getModule() {
        return module != null ? module : (module = buildModule());
    }
}