initial working version

Currenttly only features systemd ready-notification on startup.

build.gradle

+plugins {
+	id 'com.github.johnrengelman.shadow' version '2.0.4'
+	id 'java'
+}
+
+repositories {
+	mavenCentral()
+	maven { url = 'https://hub.spigotmc.org/nexus/content/repositories/snapshots/' }
+	maven { url = 'https://oss.sonatype.org/content/repositories/snapshots' }
+}
+
+dependencies {
+	compileOnly 'org.spigotmc:spigot-api:1.12.2-R0.1-SNAPSHOT'
+	compile 'info.faljse:SDNotify:1.1'
+}

settings.gradle

+rootProject.name = 'SpigotSystemd'

src/main/java/jetzt/nicht/minecraft/spigotSystemd/PluginListener.java

+package jetzt.nicht.minecraft.spigotSystemd;
+
+import org.bukkit.event.EventHandler;
+import org.bukkit.event.EventPriority;
+import org.bukkit.event.Listener;
+import org.bukkit.event.server.PluginEnableEvent;
+
+import jetzt.nicht.minecraft.spigotSystemd.SpigotSystemdPlugin;
+
+public class PluginListener implements Listener {
+	SpigotSystemdPlugin mainPlugin;
+
+	public PluginListener(SpigotSystemdPlugin mainPlugin) {
+		this.mainPlugin = mainPlugin;
+	}
+
+	@EventHandler(priority = EventPriority.MONITOR)
+	public void onPluginEnabled(PluginEnableEvent event) {
+		System.out.println("Some plugin has been enabled!");
+		this.mainPlugin.onPluginEnabled();
+	}
+}

src/main/java/jetzt/nicht/minecraft/spigotSystemd/SpigotSystemdPlugin.java

+package jetzt.nicht.minecraft.spigotSystemd;
+
+import org.bukkit.event.HandlerList;
+import org.bukkit.plugin.Plugin;
+import org.bukkit.plugin.java.JavaPlugin;
+import info.faljse.SDNotify.SDNotify;
+
+import jetzt.nicht.minecraft.spigotSystemd.PluginListener;
+import jetzt.nicht.minecraft.spigotSystemd.StartupListener;
+
+public class SpigotSystemdPlugin extends JavaPlugin {
+	PluginListener pluginListener;
+	StartupListener startupListener;
+
+	@Override
+	public void onEnable() {
+		// we have been loaded, so the server has just been started
+		// XXX: Check some other state, to become reload-safe!
+		// Check systemd if our service is started!
+
+		// Register our listener that waits for the server to be ready.
+		// We pass in a reference to ourselves so the listener can call back.
+		this.startupListener = new StartupListener(this);
+		System.out.println("Registering StartupListener...");
+		getServer().getPluginManager()
+			.registerEvents(this.startupListener, this);
+	}
+
+	@Override
+	public void onDisable() {
+	}
+
+	public void onServerMostlyReady() {
+		System.out.println("The server is now \"mostly ready\"!");
+
+		// Unregister the StartupListener, as the server is now
+		// deemed "mostly ready"
+		System.out.println("Unregistering StartupListener...");
+		HandlerList.unregisterAll(this.startupListener);
+
+		// Register our listener that waits for plugins to be enabled. See above.
+		this.pluginListener = new PluginListener(this);
+		System.out.println("Registering PluginListener...");
+		getServer().getPluginManager()
+			.registerEvents(this.pluginListener, this);
+
+		// Trigger one execution of onPluginEnabled, as all plugins might have
+		// been enabled already. This is a bit ugly but technically correct,
+		// because at least we ourselves have been enabled.
+		this.onPluginEnabled();
+	}
+
+	public void onPluginEnabled() {
+		if (this.checkPluginsEnabled()) {
+			System.out.println("All plugins have been enabled!");
+			System.out.println("Unregistering PluginListener...");
+			HandlerList.unregisterAll(this.pluginListener);
+
+			System.out.println("Signalling readyness to systemd...");
+			SDNotify.sendNotify();
+		} else {
+			System.out.println("There are still plugins to be enabled, continuing...");
+		}
+	}
+
+	public boolean checkPluginsEnabled() {
+		System.out.println("Checking whether all plugins have been enabled...");
+		Plugin[] loadedPlugins = getServer().getPluginManager().getPlugins();
+		boolean allPluginsEnabled = true;
+		for (Plugin plugin : loadedPlugins) {
+			System.out.println(
+					String.format("%s: %b", plugin, plugin.isEnabled())
+					);
+			if (plugin.isEnabled() == false) {
+				allPluginsEnabled = false;
+				break;
+			}
+		}
+		return allPluginsEnabled;
+	}
+}

src/main/java/jetzt/nicht/minecraft/spigotSystemd/StartupListener.java

+package jetzt.nicht.minecraft.spigotSystemd;
+
+import org.bukkit.event.EventHandler;
+import org.bukkit.event.EventPriority;
+import org.bukkit.event.Listener;
+import org.bukkit.event.world.WorldInitEvent;
+
+import jetzt.nicht.minecraft.spigotSystemd.SpigotSystemdPlugin;
+
+public class StartupListener implements Listener {
+	SpigotSystemdPlugin mainPlugin;
+
+	public StartupListener(SpigotSystemdPlugin mainPlugin) {
+		this.mainPlugin = mainPlugin;
+	}
+
+	@EventHandler(priority = EventPriority.MONITOR)
+	public void onWorldInitialized(WorldInitEvent event) {
+		System.out.println("Some World has been initialized!");
+		this.mainPlugin.onServerMostlyReady();
+	}
+}

src/main/resources/plugin.yml

+---
+name: systemd
+description: Integrates Spigot with systemd when running as a systemd service.
+author: Gigadoc2
+version: 0.1.0
+
+main: jetzt.nicht.minecraft.spigotSystemd.SpigotSystemdPlugin
+load: STARTUP