Error handling / logging with asynchronous calls

Question

Hi,
I want to log every error my app might throw. I thought it would be sufficient to surround either the launch method or the start method with a try...catch. But it doesn't work as excepted.
If an error occurs in an asynchronous call, e.g. in a button click, the exception is never caught.
Do you have an idea, how to solve this?
Example:
import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;
public class TestApp4 extends Application {
    public static void main(String[] args) {
        launch(args);
    public void start(final Stage stage) throws Exception {
        try {
            final VBox root = new VBox();
            Button button = new Button("Click");
            button.setOnAction(new EventHandler<ActionEvent>() {
                @Override
                public void handle(ActionEvent actionEvent) {
                    int i = 0;
                    int a = 5 / i;
            root.getChildren().add(button);
            Scene scene = new Scene(root);
            stage.setScene(scene);
            stage.show();
        } catch (Exception e) {
            // do logging here
            e.printStackTrace();
}

Answer 1

I'm not 100% sure how it works, but what I am pretty sure is that these Exceptions in EventHandlers are caught by JavaFX itself inside the JavaFX thread. They have to, because if they let the exception bubble up to the top, the FX thread would end (and with it, all response in the UI). It is probably caught at the level where Events are triggered somewhere (my guess is inside the QuantumToolKit).
Anyway, these exceptions are printed to System.err -- if you can redirect that stream to a log file or whatever you can have them all in one place. The error stream can be set in your application simply by calling System.setErr().
Note also that exceptions occuring in a Task are stored in the exception property -- sometimes these can be hard to find as they are put there silently -- you'll need to check your Tasks for exceptions and print them.