Need clarification on changing data in javafx application thread

Question

I've been migrating a project of mine to JavaFX and started running into thread issues. I'll attach a short example. After much searching I managed to sort out the problem. I can't change the tableView data outside of the fx application thread. I switched my code over from using SwingWorker to a Task.

At first, that worked until I added a change listener to the table's observableList. I then received the error "Not on FX application thread;"

The error happened inside the onChanged method when I attempted to update a Label's value. I resolved this by wrapping it inside Platform.runLater().

I'm just confused as to why changing the label says it wasn't on the application thread. On what thread was this running? Also, am I adding rows to my table correctly by using a task? In my actual application, I could be adding 50k rows hence why the separate thread so as to not lock up the UI.

public class Temp extends Application{  
    private ObservableList<String> libraryList = FXCollections.observableArrayList();

    public void start(Stage stage) {

        Label statusLabel = new Label("stuff goes here");

        TableView<String> table = new TableView<String>(libraryList);
        table.setColumnResizePolicy(TableView.CONSTRAINED_RESIZE_POLICY);

        TableColumn<String, String> col = new TableColumn<String, String>("Stuff");
        col.setCellValueFactory(cellData -> new ReadOnlyStringWrapper(cellData.getValue()));
        table.getColumns().add(col);

        libraryList.addListener(new ListChangeListener<String>() {
            public void onChanged(Change change) {
                // Problem was caused by setting the label's text (prior to adding the runLater)
                Platform.runLater(()->{
                    statusLabel.setText(libraryList.size()+" entries");
                });                 
            }               
        });

        // dummy stuff
        libraryList.add("foo");
        libraryList.add("bar");

        Button b = new Button("Press Me");
        b.setOnAction(new EventHandler<ActionEvent>() {
            public void handle(ActionEvent e) {
                FileTask task = new FileTask();
                new Thread(task).start();
            }
        });

        BorderPane mainBody = new BorderPane();

        mainBody.setTop(statusLabel);
        mainBody.setCenter(table);
        mainBody.setBottom(b);
        Scene scene = new Scene(mainBody);
        stage.setScene(scene);
        stage.show();       
    }


    class FileTask extends Task<Boolean>{

        public FileTask(){

        }

        protected Boolean call() throws Exception{

            Random rand = new Random();
            for(int i = 0; i < 5; i++) {
                String s = ""+rand.nextInt(Integer.MAX_VALUE);
                libraryList.add(s);
            }

            return true;
        }   
    }     

    public static void main(String[] args) {
        Application.launch(args);

    }
}

Answer 1

It's working as expected, you have the application thread and the task thread, they kind of look like this:

App ------\ ----------------------
Task       \-label.setText() Exception

You can't do any UI work on anything but the App thread, so adding your RunLater does this:

App ----\ -------------/ RunLater(label.setText()) ----------
Task     \-add to list/

which works well. There are a few ways to manage this based on what you want to do:

• If you want to update the Table list within the Task, you can move the RunLater call to inside the task, rather than inside the handler, this way it will still get you back to the App thread. This way if you're actually on the app thread, there is no need to call RunLater within the handler.

App ---\ -----------------------/ label.setText() ----------
Task    \-RunLater(add to list)/

• Another option is to just use a Task> which will run on the other thread, and return the full list of strings that are going to be added. This is more likely what you want if you're making network calls in the task, get a list of items, then add them once they are all downloaded to the table.

App -----\ ------------------------------/ label.setText() ---/ add to table list-------
Task      \-build list, update progress /- return final list /

Hopefully the formatting stays.

Answer 2

Consider encapsulating the information needed by the view in a separate class (typically referred to as model). The view should respond to changes in the model by means of listener or binding.
You can use a thread or threads, to update the model:

import java.util.Random;
import javafx.application.Application;
import javafx.application.Platform;
import javafx.beans.binding.Bindings;
import javafx.beans.property.IntegerProperty;
import javafx.beans.property.ReadOnlyStringWrapper;
import javafx.beans.property.SimpleIntegerProperty;
import javafx.collections.FXCollections;
import javafx.collections.ListChangeListener;
import javafx.collections.ObservableList;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.control.TableColumn;
import javafx.scene.control.TableView;
import javafx.scene.layout.BorderPane;
import javafx.stage.Stage;

public class Temp extends Application{

    @Override
    public void start(Stage stage) {

        Model model = new Model();

        Label statusLabel = new Label("stuff goes here");

        TableView<String> table = new TableView<>(model.getLibraryList());
        table.setColumnResizePolicy(TableView.CONSTRAINED_RESIZE_POLICY);

        TableColumn<String, String> col = new TableColumn<>("Stuff");
        col.setCellValueFactory(cellData -> new ReadOnlyStringWrapper(cellData.getValue()));
        table.getColumns().add(col);
        statusLabel.textProperty().bind(Bindings.concat(model.sizeProperty.asString(), " entries"));

        // dummy stuff
        model.add("foo");  model.add("bar");

        Button b = new Button("Press Me");
        b.setOnAction(e -> {
            FileTask task = new FileTask(model);
            new Thread(task).start();
        });

        BorderPane mainBody = new BorderPane();

        mainBody.setTop(statusLabel);
        mainBody.setCenter(table);
        mainBody.setBottom(b);
        Scene scene = new Scene(mainBody);
        stage.setScene(scene);
        stage.show();
    }

    class Model {

        private final ObservableList<String> libraryList;
        private final IntegerProperty sizeProperty;

        Model(){
            libraryList = FXCollections.observableArrayList();
            sizeProperty = new SimpleIntegerProperty(0);
            libraryList.addListener((ListChangeListener<String>) change -> {
                Platform.runLater(()->sizeProperty.set(libraryList.size()));
            });
        }

        //synchronize if you want to use multithread
        void add(String string) {
            Platform.runLater(()->sizeProperty.set(libraryList.add(string)));
        }

        ObservableList<String> getLibraryList() {
            return libraryList;
        }

        IntegerProperty getSizeProperty() {
            return sizeProperty;
        }
    }

    class FileTask implements Runnable{

        private final  Model model;

        public FileTask(Model model){
            this.model = model;
        }

        @Override
        public void run() {
            Random rand = new Random();
            for(int i = 0; i < 5; i++) {
                String s = ""+rand.nextInt(Integer.MAX_VALUE);
                model.add(s);
            }
        }
    }

    public static void main(String[] args) {
        Application.launch(args);
    }
}