Issue
Normally I handle my exceptions by showing some custom Alert (JavaFX) with details, but JavaFX runtime is not initialized at all when the static initializer of my class runs.
Is there any way to handle such exception without printing its content to output like an animal?
public class MyStaticInitializedClass {
static {
try {
//do the things that may throw exception
} catch(Exception ex) {
ExceptionHandler.showException(ex);
}
}
}
public class ExceptionHandler {
public static void showException(Exception ex) {
//constructs JavaFX alert with exception details
alert.show();
}
}
Solution
First consder if you shouldn't let the application simply crash and log the reason. A failure in a static initializer typically means there's something seriously wrong with the environment, which is not likely something you can recover from. Also, as far as I know, once a class fails to load it can't ever be loaded by the same ClassLoader
again.
That said, if you want to show errors to your user in an alert, even if the error occurs before the JavaFX runtime has been initialized, then you need to save the error somewhere. Then, once you launch JavaFX, check wherever you stored the error(s) and show them. For example:
Main.java:
import javafx.application.Application;
public class Main {
public static void main(String[] args) {
// an "error" before JavaFX is launched
App.notifyUserOfError(new RuntimeException("OOPS!"));
Application.launch(App.class, args);
}
}
App.java:
import java.io.PrintWriter;
import java.io.StringWriter;
import java.util.ArrayDeque;
import java.util.Objects;
import java.util.Queue;
import javafx.application.Application;
import javafx.application.Platform;
import javafx.scene.Scene;
import javafx.scene.control.Alert;
import javafx.scene.control.Alert.AlertType;
import javafx.scene.control.Label;
import javafx.scene.control.TextArea;
import javafx.scene.layout.Priority;
import javafx.scene.layout.StackPane;
import javafx.scene.layout.VBox;
import javafx.scene.text.Font;
import javafx.stage.Stage;
public class App extends Application {
private static Queue<Throwable> errorQueue;
private static App appInstance;
public static synchronized void notifyUserOfError(Throwable throwable) {
Objects.requireNonNull(throwable);
if (appInstance == null) {
if (errorQueue == null) {
errorQueue = new ArrayDeque<>();
}
errorQueue.add(throwable);
} else {
if (Platform.isFxApplicationThread()) {
appInstance.showErrorAlert(throwable);
} else {
Platform.runLater(() -> appInstance.showErrorAlert(throwable));
}
}
}
private static synchronized Queue<Throwable> setAppInstance(App instance) {
if (appInstance != null) {
throw new IllegalStateException();
}
appInstance = instance;
var queue = errorQueue;
errorQueue = null; // no longer needed
return queue;
}
private Stage primaryStage;
@Override
public void start(Stage primaryStage) {
this.primaryStage = primaryStage;
var scene = new Scene(new StackPane(new Label("Hello, World!")), 600, 400);
primaryStage.setScene(scene);
primaryStage.show();
var errors = setAppInstance(this);
if (errors != null) {
// if non-null then should be non-empty
do {
showErrorAlert(errors.remove());
} while (!errors.isEmpty());
// possibly exit the application if you can't recover
}
}
private void showErrorAlert(Throwable error) {
var alert = new Alert(AlertType.ERROR);
alert.initOwner(primaryStage);
alert.setContentText(error.toString());
var sw = new StringWriter();
error.printStackTrace(new PrintWriter(sw));
var area = new TextArea(sw.toString());
area.setEditable(false);
area.setFont(Font.font("Monospaced", 12));
var details = new VBox(5, new Label("Stack trace:"), area);
VBox.setVgrow(area, Priority.ALWAYS);
alert.getDialogPane().setExpandableContent(details);
alert.showAndWait();
}
}
The above puts the error in a queue if JavaFX has not been initialized yet. At the end of the start
method the queue is checked for any errors and they're displayed to the user one after the other. If JavaFX has already been initialized then the error is immediately shown to the user.
Answered By - Slaw
Answer Checked By - David Marino (JavaFixing Volunteer)