Motivation

So you want to write plugins in Scala? Great! Or not so great? Scala runtime classes are not on the class/module path by default. Server administrators could add them using the -classpath commandline option, but in reality most bukkit servers run in managed environments by minecraft-specialized server hosts. The standard workaround for this problem is to include the classes in your own plugin and relocate them using a shading plugin in your build process. While this does work, it is not ideal because your plugin will increase in size by a lot. As of Scala 2.12.6, the standard library has a size of 3.5 MB. The reflection library is another 5 MB. Using both libraries in multiple plugins results unnecessarily large plugins sizes, while really those Scala classes should only be loaded once. Spigot 1.16.5 introduced a new library loading api, but it has similar problems. Introducing...

ScalaLoader

ScalaLoader uses a custom PluginLoader that loads the Scala runtime classes for you!

Pros

Write idiomatic Scala!
No need to shade anymore!
Supports different (binary incompatible) Scala versions at the same time! ScalaLoader uses classloader magic to make that work.
Supports custom scala versions by adding/changing URLs in the config file.
Annotation-based detection of the plugin's main class - no need to write a plugin.yml. If you wish to use a plugin.yml still, you can, however I always found it a pain.
A boilerplate-free event api! #getHandlerList(), #getHandlers(), #setCancelled(boolean) and #isCancelled() can be generated at class-load time!
A boilerplate-free ConfigurationSerialization api! #serialize() and #deserialize(Map<String,Object>) can be generated at class-load time! (Scala collection support is a WIP)

Cons

Scala library classes are only accessible to ScalaPlugins (You can still write them in Java though).
ScalaLoaders uses a lot of reflection/injection hacks to make ScalaPlugins accessible to JavaPlugins.

Caveats

ScalaPlugin jars go in the <server_root>/plugins/ScalaLoader/scalaplugins/ directory. I made this choice so that ScalaLoader doesn't try to load JavaPlugins that are loaded already.
By default ScalaLoader downloads the scala libraries from over the network the first time. I made this choice to provide the best possible user experience for server admins. The ScalaLoader jar remains small in size, and there's no manual downloading involved. If you're very security-focused you might want to provide your own jars by changing the URLs to "file://some/location.jar". The scala classes aren't actually loaded until there's a plugin that needs them, so you can run ScalaLoader once without ScalaPlugins to generate the config.
Some helper libraries based on Bukkit will assume that every calling class is in fact loaded from a JavaPlugin (they use JavaPlugin#getProvidingPlugin(Class<?>) internally), which will error when the class is loaded by the ScalaPluginClassLoader. If possible, always use methods that let you pass the Plugin instance explicitly!

Roadmap

There's only ~~seven~~ two features that are missing in my opinion:

~~The first con. I want JavaPlugins te be able to access the Scala library classes, however they will need to tell ScalaLoader somehow which version they want to use.~~ Now implemented in ScalaPluginLoader#openUpToJavaPlugin(ScalaPlugin,JavaPlugin). Currently this does not inject the Scala library classes into the JavaPlugin's classloader, but it's a start.
~~Make the ScalaPluginLoader parallel capable. Right now ScalaPlugins are loaded sequentially.~~
~~Use bukkit's api-version to transform classes so that plugins will be compatible once they are loaded.~~
~~HandlerList- and cancellable-related boilerplate reduction for custom events.~~
API to load third-party libraries (can be specific to certain Scala versions, or not).
~~ConfigurationSerializable-related boilerplate reduction.~~
Link using TASTy if the scalaplugin's jar includes TASTy attributes.

Example Plugin

package xyz.janboerman.scalaloader.example.scala

import org.bukkit.ChatColor
import org.bukkit.command.{CommandSender, Command}
import org.bukkit.event.{EventHandler, Listener}
import org.bukkit.event.player.PlayerJoinEvent
import org.bukkit.permissions.PermissionDefault
import xyz.janboerman.scalaloader.plugin.ScalaPluginDescription.{Command => SPCommand, Permission => SPPermission}
import xyz.janboerman.scalaloader.plugin.{ScalaPlugin, ScalaPluginDescription}
import xyz.janboerman.scalaloader.plugin.description.{Scala, ScalaVersion, Api, ApiVersion}

@Scala(version = ScalaVersion.v2_13_7)
@Api(ApiVersion.v1_18)
object ExamplePlugin
    extends ScalaPlugin(new ScalaPluginDescription("ScalaExample", "1.0")
        .commands(new SPCommand("foo")
            .permission("scalaexample.foo"))
        .permissions(new SPPermission("scalaexample.foo")
            .permissionDefault(PermissionDefault.TRUE)))
    with Listener {

    getLogger().info("ScalaExample - I am constructed!")

    override def onLoad(): Unit = {
        getLogger.info("ScalaExample - I am loaded!")
    }

    override def onEnable(): Unit = {
        getLogger.info("ScalaExample - I am enabled!")
        getServer.getPluginManager.registerEvents(this, this)
    }

    override def onDisable(): Unit = {
        getLogger.info("ScalaExample - I am disabled!")
    }

    @EventHandler
    def onJoin(event: PlayerJoinEvent): Unit = {
        event.setJoinMessage(s"${ChatColor.GREEN} Howdy ${event.getPlayer.getName}!")
    }

    override def onCommand(sender: CommandSender, command: Command, label: String, args: Array[String]): Boolean = {
        sender.sendMessage("Executed foo command!")
        true
    }

    def getInt() = 42

}

Depending on a ScalaPlugin from a JavaPlugin

plugin.yml:

name: DummyPlugin
version: 1.0
main: xyz.janboerman.dummy.dummyplugin.DummyPlugin
depend: [ScalaLoader]
softdepend: [ScalaExample] #A hard dependency will not work! Your plugin will not load!

Java code:

package xyz.janboerman.dummy.dummyplugin;

import org.bukkit.plugin.java.JavaPlugin;
import xyz.janboerman.scalaloader.plugin.ScalaPluginLoader;
import xyz.janboerman.scalaloader.example.scala.ExamplePlugin$;

public final class DummyPlugin extends JavaPlugin {

    @Override
    public void onEnable() {
        //get the plugin instance
        ExamplePlugin$ plugin = (ExamplePlugin$) getServer().getPluginManager().getPlugin("ScalaExample");
        
        //make sure all classes from the scala plugin can be accessed
        ScalaPluginLoader.getInstance().openUpToJavaPlugin(plugin, this);
        
        //do whatever you want afterwards!
        getLogger().info("We got " + plugin.getInt() + " from Scala!");
    }

}

Compiling

It's a maven project, so just run mvn package and you're good to go. The jar file will be built at ./ScalaLoader/target/ScalaLoader-<version>.jar Note that while ScalaLoader can run on Java 8, it requires JDK-18 to compile.

Pre-built plugin jar file?

Available on SpigotMC

Dependency Information

SBT

resolvers += "codemc-snapshots" at "https://repo.codemc.io/repository/maven-snapshots/"
libraryDependencies += "xyz.janboerman" % "ScalaLoader" % "0.17.16-SNAPSHOT" % "provided"

Maven

<repository>
    <id>codemc-snapshots</id>
    <url>https://repo.codemc.io/repository/maven-snapshots/</url>
</repository>

<dependency>
    <groupId>xyz.janboerman</groupId>
    <artifactId>ScalaLoader</artifactId>
    <version>0.17.16-SNAPSHOT</version>
    <scope>provided</scope>
</dependency>

License

LGPL, because I want forks of this thing to be open for auditing. If you however which to include parts this code base in your own open source project but not adopt the (L)GPL license, please contact me and I will likely permit you to use this under a different license. Sending me a private message on the SpigotMC forums or an issue on this repository will do.

EpiCanard / ScalaPluginLoader