Aggregator completion thread needs to be daemon

api/src/main/java/com/microsoft/gctoolkit/aggregator/Aggregator.java

@@ -6,6 +6,7 @@
 import com.microsoft.gctoolkit.event.jvm.JVMTermination;
 
 import java.util.Arrays;
+import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 import java.util.function.Consumer;
 
@@ -117,13 +118,22 @@ public void onCompletion(Runnable task) {
         this.completionTask = task;
     }
 
+    private static final ExecutorService executorService =
+        Executors.newSingleThreadExecutor(runnable -> {
+            // Use a daemon thread for executing the completion task; otherwise, the JVM will not exit.
+            Thread thread = new Thread(runnable, "aggregator-completion");
+            thread.setDaemon(true);
+            return thread;
+        }
+    );
+
     /**
      * Call a callback when aggregation is completed.
      */
     private void complete() {
-        if (completionTask != null)
-            Executors.newSingleThreadExecutor().execute(completionTask);
-
+        if (completionTask != null) {
+            executorService.execute(completionTask);
+        }
     }
 
     /**

vertx/src/main/java/com/microsoft/gctoolkit/vertx/DataSourceVerticle.java

@@ -43,11 +43,6 @@ public void start(Promise<Void> promise) {
         }
     }
 
-    @Override
-    public void stop(Promise promise) {
-        promise.complete();
-    }
-
     @Override
     public boolean equals(Object other) {
         // we want Object.equals(other) because it's ok to have more than 1 AggregatorEngine on the bus

vertx/src/main/java/com/microsoft/gctoolkit/vertx/JVMEventVerticle.java

@@ -45,11 +45,6 @@ public void start(Promise<Void> promise) {
         }).completionHandler(result -> {promise.complete();});
     }
 
-    @Override
-    public void stop(Promise promise) {
-        promise.complete();
-    }
-
     @Override
     public boolean equals(Object other) {
         // we want Object.equals(other) because it's ok to have more than 1 AggregatorEngine on the bus