Task doesn't update local variable - javafx

Question

I'm using a Task to read a text file, the task is invoked when the user clicks "open file" menu, it is supposed to read the text file, and then update the local variable "text", the problem occurs at the first try, if i open a file, nothing happens, and the value of the text string stays as it is, if i open any file again, everything works as expected, i couldn't find the cause of this.
The method that has the task
private void readFile(File file){
Task<String> task = new Task<String>() {
#Override
protected String call() {
List<String> list = EditorUtils.readFromFile(file);
String str = list.stream().collect(Collectors.joining("\n"));
return str;
}
};
task.setOnSucceeded(new EventHandler<WorkerStateEvent>() {
#Override
public void handle(WorkerStateEvent t) {
setCurrentText(task.getValue());
}
});
task.setOnFailed(e -> setCurrentText("FAILED"));
Thread t = new Thread(task);
t.setDaemon(true);
t.start();
}
SetCurrentText
private void setCurrentText(String text){
this.text = text;
}
The method of the controller
#FXML
void openMenuItemClick(ActionEvent event) {
fileChooser.setTitle("title");
fileChooser.getExtensionFilters().add
(new FileChooser.ExtensionFilter("TXT files (*.txt)", "*.txt"));
File file = fileChooser.showOpenDialog(open.getParentPopup().getScene().getWindow());
if (file != null){
readFile(file);
System.out.println(text); //prints null since "text" isn't initialized yet
}
}
EditorUtils#readFromFile
public static List<String> readFromFile(File file){
List<String> lines = new ArrayList<>();
try {
lines = Files.readAllLines(Paths.get(file.getPath()), StandardCharsets.UTF_8);
} catch (IOException e) {
e.printStackTrace();
}
return lines;
}

Answer 1

Your readFile method creates a Task, gives it to a Thread, launches the Thread, and returns. You then try to immediately print out the value of text. There's no guarantee your Task will have completed by the time you call println(text). In fact, it's very likely your Task has not completed yet. But that's not the only problem.
The call to readFile and println are both done on the same thread—in this case, the JavaFX Application Thread. The problem here is that the EventHandler you pass to setOnSucceeded will be invoked on the FX thread as well. The way this is achieved internally is with a Platform.runLater call which schedules the action with the FX thread to be ran some time in the future. This can't happen while the FX thread is executing openMenuItemClick and must wait until the method returns.
What this all means is that setCurrentText will never run until after the call to println. But by the second time openMenuItemClick is invoked the text will have been set1. So what you're seeing the second time is actually the result of the first Task.
If you want to do something with text once the Task completes then you should do it inside the onSucceeded or onFailed handler. Or you can make text a StringProperty and observe it for changes.
1. Technically, it may have been set. There's still no guarantee the Task has completed by then.

Answer 2

This is perfectly normal behaviour when using multiple threads. You access the file from a task running on background thread. On completion this task triggers an update on the JavaFX application thread.
By the time readFile returns the task may not have been completed. The fact that Task uses Platform.runLater to execute the onSucceeded handler results in this handler never being invoked before the openMenuItemClick method completes, even if the file is read before System.out.println is reached.
If you need to update the GUI based on the result of the Task, you should do so from the event handler. The code updating the text field runs after the System.out.println(text); statement. The second time you start the task, you print results of the task started the first time the menu item was clicked, not the new one. You can verify this by moving the println to the beginning of the openMenuItemClick method.