If you have a listener on a Service property but don't explicitly hold a reference to it, the Service can be garbage collected, and notifications lost, before the Task completes.

In the code below, there is a listener on Service's State property. If the class MyTask is static, the notifications of RUNNING and SUCCEEDED are never delivered.
If MyTask is made a normal inner class (and thus it holds a reference to the Service), then the RUNNING and SUCCEEDED notifications are delivered.

When there is a listener on the State property, all state transitions should be delivered.

What is happening is that Service attaches the listener using a Weak Reference, so the listener doesn't hold the Service in memory. Under memory pressure the Service gets GC'ed & stops delivering notifications even though the underlying Task succeeds.

This was a really hard bug to track down.

import javafx.application.Application;
import javafx.concurrent.Service;
import javafx.concurrent.Task;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

import java.util.ArrayList;

public class ServiceGCTest extends Application
{
  @Override
  public void start(Stage primaryStage) throws Exception
  {
    startService();

    Label lab = new Label("Hello");
    StackPane root = new StackPane();
    root.getChildren().add(lab);
    primaryStage.setScene(new Scene(root, 200, 250));
    primaryStage.show();

    ArrayList<String> al = new ArrayList<>(); // apply pressure to the GC
    for (int i = 0; i < 100_000; i++)
      al.add("A really long long long really long long long really long long long really long long long String " + System.currentTimeMillis());
    Thread.sleep(1_000);
  }

  private void startService()
  {
    Service s = new MyService();
    s.stateProperty().addListener((observable, oldValue, newValue) -> System.out.println("new value " + newValue));

    s.start();
  }

  public static class MyService extends Service
  {
    @Override
    protected Task createTask()
    {
      return new MyTask();
    }

    static class MyTask extends Task<Void> // if static is removed, the bug goes away!
    {
      @Override
      protected Void call() throws Exception
      {
        return null;
      }
    }
  }
}