Extensions will be unloaded when server is stopped

src/main/java/net/minestom/server/MinecraftServer.java

@@ -784,6 +784,7 @@ public final class MinecraftServer {
     public static void stopCleanly() {
         stopping = true;
         LOGGER.info("Stopping Minestom server.");
+        extensionManager.unloadAllExtensions();
         updateManager.stop();
         schedulerManager.shutdown();
         connectionManager.shutdown();

src/main/java/net/minestom/server/extensions/Extension.java

@@ -24,7 +24,7 @@ public abstract class Extension {
 
     /**
      * Observers that will be notified of events related to this extension.
-     * Kept as WeakReference because entities can be observer, but could become candidate to be garbage-collected while
+     * Kept as WeakReference because entities can be observers, but could become candidate to be garbage-collected while
      * this extension holds a reference to it. A WeakReference makes sure this extension does not prevent the memory
      * from being cleaned up.
      */

src/main/java/net/minestom/server/extensions/ExtensionManager.java

@@ -710,4 +710,17 @@ public class ExtensionManager {
         }
         LOGGER.info("Early load of code modifiers from extensions done!");
     }
+
+    /**
+     * Unloads all extensions
+     */
+    public void unloadAllExtensions() {
+        // copy names, as the extensions map will be modified via the calls to unload
+        Set<String> extensionNames = new HashSet<>(extensions.keySet());
+        for(String ext : extensionNames) {
+            if(extensions.containsKey(ext)) { // is still loaded? Because extensions can depend on one another, it might have already been unloaded
+                unloadExtension(ext);
+            }
+        }
+    }
 }

src/test/java/improveextensions/MixinIntoMinestomCoreLauncher.java

@@ -8,6 +8,9 @@ import net.minestom.server.Bootstrap;
 // -Dminestom.extension.indevfolder.classes=build/classes/java/test/ -Dminestom.extension.indevfolder.resources=build/resources/test/improveextensions
 // To test disabling early Mixin injections:
 // -Dminestom.extension.disable_early_load=true -Dminestom.extension.indevfolder.classes=build/classes/java/test/ -Dminestom.extension.indevfolder.resources=build/resources/test/improveextensions/disableearlyload
+
+// To test extension termination when the server quits:
+// -Dminestom.extension.indevfolder.classes=build/classes/java/test/ -Dminestom.extension.indevfolder.resources=build/resources/test/improveextensions/unloadonstop
 public class MixinIntoMinestomCoreLauncher {
      public static void main(String[] args) {
             Bootstrap.bootstrap("demo.MainDemo", args);

src/test/java/improveextensions/unloadextensiononstop/UnloadExtensionOnStop.java

@@ -0,0 +1,37 @@
+package improveextensions.unloadextensiononstop;
+
+import net.minestom.server.MinecraftServer;
+import net.minestom.server.entity.type.monster.EntityZombie;
+import net.minestom.server.event.EventCallback;
+import net.minestom.server.event.GlobalEventHandler;
+import net.minestom.server.event.entity.EntityTickEvent;
+import net.minestom.server.event.instance.InstanceTickEvent;
+import net.minestom.server.extensions.Extension;
+import net.minestom.server.extras.selfmodification.MinestomRootClassLoader;
+import net.minestom.server.instance.Instance;
+import net.minestom.server.utils.Position;
+import net.minestom.server.utils.time.TimeUnit;
+import org.junit.jupiter.api.Assertions;
+import org.opentest4j.AssertionFailedError;
+
+import java.util.concurrent.atomic.AtomicBoolean;
+
+public class UnloadExtensionOnStop extends Extension {
+
+    private boolean terminated = false;
+
+    @Override
+    public void initialize() {
+        MinecraftServer.getSchedulerManager().buildShutdownTask(() -> {
+            Assertions.assertTrue(terminated, "Extension should have been terminated on shutdown.");
+            System.out.println("All tests passed.");
+        }).makeTransient().schedule();
+
+        MinecraftServer.getSchedulerManager().buildTask(MinecraftServer::stopCleanly).makeTransient().delay(1L, TimeUnit.SECOND).schedule();
+    }
+
+    @Override
+    public void terminate() {
+        terminated = true;
+    }
+}

src/test/resources/improveextensions/unloadonstop/extension.json

@@ -0,0 +1,4 @@
+{
+  "entrypoint": "improveextensions.unloadextensiononstop.UnloadExtensionOnStop",
+  "name": "UnloadExtensionOnStop"
+}