How to abort a Task in JavaFX? - multithreading

Question

Is it possible to abort a Task in JavaFX? My Task could run into situations where I want to cancel the rest of the operations within it.
I would need to return a value, somehow, so I can handle the cause of the abort in the JFX Application Thread.
Most of the related answers I've seen refer to handling an already-canceled Task, but now how to manually cancel it from within the Task itself.
The cancel() method seems to have no effect as both messages below are displayed:
public class LoadingTask<Void> extends Task {
#Override
protected Object call() throws Exception {
Connection connection;
// ** Connect to server ** //
updateMessage("Contacting server ...");
try {
connection = DataFiles.getConnection();
} catch (SQLException ex) {
updateMessage("ERROR: " + ex.getMessage());
ex.printStackTrace();
cancel();
return null;
}
// ** Check user access ** //
updateMessage("Verifying user access ...");
try {
String username = System.getProperty("user.name");
ResultSet resultSet = connection.createStatement().executeQuery(
SqlQueries.SELECT_USER.replace("%USERNAME%", username));
// If user doesn't exist, block access
if (!resultSet.next()) {
}
} catch (SQLException ex) {
}
return null;
}
}
And example would be greatly appreciated.

Answer 1

Why not just let the task go into a FAILED state if it fails? All you need (I also corrected the errors with the type of the task and return type of the call method) is
public class LoadingTask extends Task<Void> {
#Override
protected Void call() throws Exception {
Connection connection;
// ** Connect to server ** //
updateMessage("Contacting server ...");
connection = DataFiles.getConnection();
// ** Check user access ** //
updateMessage("Verifying user access ...");
String username = System.getProperty("user.name");
ResultSet resultSet = connection.createStatement().executeQuery(
SqlQueries.SELECT_USER.replace("%USERNAME%", username));
// I am not at all sure what this is supposed to do....
// If user doesn't exist, block access
if (!resultSet.next()) {
}
return null;
}
}
Now if an exception is thrown by DataFiles.getConnection(), the call method terminates immediately with an exception (the remained is not executed) and the task enters a FAILED state. If you need access to the exception in the case that something goes wrong, you can do:
LoadingTask loadingTask = new LoadingTask();
loadingTask.setOnFailed(e -> {
Throwable exc = loadingTask.getException();
// do whatever you need with exc, e.g. log it, inform user, etc
});
loadingTask.setOnSucceeded(e -> {
// whatever you need to do when the user logs in...
});
myExecutor.execute(loadingTask);