JavaFX WeakReference memory leak when drawing charts

Question

I encountered a memory leak I just can't fix in my code. Scenario: The user opens up a new window, in which a chart is drawn (in this case a LineChart). But when the window is closed, java.lang.ref.WeakReference and javafx.beans.property.BooleanPropertyBase$Listener remain in memory. Their number perfectly corresponds to the drawn data points (XYChart.Data). I just can't figure out how to get rid of them. In my code, many of these windows are opened and closed frequently, with 10k-100k data points per chart and memory fills up quite quickly.

I'm sure I made some stupid mistake, but I just can't find it. Help would be greatly appreciated!

Sample Code:

import java.util.ArrayList;
import java.util.List;
import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.chart.LineChart;
import javafx.scene.chart.NumberAxis;
import javafx.scene.chart.XYChart;
import javafx.scene.control.Button;
import javafx.scene.paint.Color;
import javafx.stage.Stage;

public class Test extends Application {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        Application.launch(Test.class, args);
    }

    @Override
    public void start(final Stage primaryStage) {
        primaryStage.setTitle("Hello World");
        Group root = new Group();
        Scene scene = new Scene(root, 300, 250, Color.LIGHTGREEN);
        Button btn = new Button();
        btn.setLayoutX(100);
        btn.setLayoutY(80);
        btn.setText("Create stage");
        btn.setOnAction(new EventHandler<ActionEvent>() {

            public void handle(ActionEvent event) {
                new CreateStage();
                primaryStage.toFront();

            }
        });
        root.getChildren().add(btn);
        primaryStage.setScene(scene);
        primaryStage.show();
    }
}

class CreateStage {

    public CreateStage() {
        Stage stage = new Stage();
            stage.setTitle("Line Chart Sample");
        //Basic Chart attributes
        NumberAxis xAxis = new NumberAxis();
        NumberAxis yAxis = new NumberAxis();
        xAxis.setLabel("RT [minutes]");
        yAxis.setLabel("Intensity");
        LineChart<Number, Number> linechart = new LineChart(xAxis, yAxis);

                XYChart.Series newSeries = new XYChart.Series();

                List<XYChart.Data> list = new ArrayList<>();
                //just fill the chart with data points
                for (int j = 0; j < 10000; j++) {
                    float intensity = j;
                    float currentRT = j;

                    list.add(new XYChart.Data(currentRT, intensity));
                }
                newSeries.getData().addAll(list);

                // add new Series
                linechart.getData().add(newSeries);

        Scene scene  = new Scene(linechart,800,600);       


        stage.setScene(scene);
        stage.show();
    }
}  

Answer 1

I was able to solve the problem. Calling

series.getData().clear();

on every XYChart.Series when closing the window get's rid of the memory leak. Things like javafx.scene.layout.CornerRadii, com.sun.javafx.sg.prism.NGRegion and javafx.scene.layout.Background still remain in memory, but get garbage collected eventually and don't build up.

Here's the fixed code:

import java.util.ArrayList;
import java.util.List;
import javafx.application.Application;
import javafx.collections.ObservableList;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.chart.LineChart;
import javafx.scene.chart.NumberAxis;
import javafx.scene.chart.ScatterChart;
import javafx.scene.chart.XYChart;
import javafx.scene.control.Button;
import javafx.scene.paint.Color;
import javafx.stage.Stage;

public class Test extends Application {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        Application.launch(Test.class, args);
    }

    @Override
    public void start(final Stage primaryStage) {
        primaryStage.setTitle("Hello World");
        Group root = new Group();
        Scene scene = new Scene(root, 300, 250, Color.LIGHTGREEN);
        Button btn = new Button();
        btn.setLayoutX(100);
        btn.setLayoutY(80);
        btn.setText("Create stage");
        btn.setOnAction(new EventHandler<ActionEvent>() {

            public void handle(ActionEvent event) {
                CreateStage();
                primaryStage.toFront();

            }
        });
        root.getChildren().add(btn);
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    public void CreateStage() {

        Stage stage = new Stage();
        stage.setTitle("Line Chart Sample");
        //Basic Chart attributes
        NumberAxis xAxis = new NumberAxis();
        NumberAxis yAxis = new NumberAxis();
        xAxis.setLabel("RT [minutes]");
        yAxis.setLabel("Intensity");

        //linechart.getData().clear();
        LineChart<Number, Number> linechart = new LineChart(xAxis, yAxis);

        XYChart.Series newSeries = new XYChart.Series();

        List<XYChart.Data> list = new ArrayList<>();
        //just fill the chart with data points
        for (int j = 0; j < 10000; j++) {
            float intensity = j;
            float currentRT = j;

            list.add(new XYChart.Data(currentRT, intensity));
        }
        newSeries.getData().addAll(list);

        // add new Series
        linechart.getData().add(newSeries);

        Scene scene = new Scene(linechart, 800, 600);

        stage.setScene(scene);
        stage.show();

        //this fixes it
        stage.setOnCloseRequest(event -> {
            for (XYChart.Series series : linechart.getData()) {
                series.getData().clear();
            }

        });

    }
}

Thanks to @fabian for pointing me in the right direction.

Answer 2

This is most likely not a mistake from your side. I've encountered a similar problem, where some UI data was still left in the system memory after some parts of the UI were closed during runtime.

This is probably not the best solution to the problem, but you can try to reuse your windows.
You can do this by keeping track of all of your open windows and closed windows in separate ArrayLists.

If a user wants to close a window, you set the window to be invisible (instead of completely closing it) and move it into the ArrayList of closed windows. Once a new window is created, you can check your ArrayList of closed windows, if it contains one. If so, you can reuse this window (change the graph of this window and set the window visible) and move it from the ArrayList of closed windows to the ArrayList of open windows.

If you want to open a new window and there is no window inside the ArrayList of open windows, just create a new instance of your window class and add it to your ArrayList of open windows.
This is probably not the best work-around for your problem, but that is how I solved my problem, which is similar to yours.