How to wait for a JavaFX Service to finish before returning data in caller?

Question

Working on my first Java project I can't seem to get around this probably basic problem: In a JavaFX application I have a DAO class, which starts a service to get values from a mysql db, builds an object from it and returns the object to the caller. But the object never gets build, because the return happens before the service has succeeded.

public IQA getQA(int id) throws SQLException {      
        try {           
        GetQuizService getQuizService = new GetQuizService();
        getQuizService.restart();
        getQuizService.setId(id);
        getQuizService.setOnSucceeded(e -> {
            this.quiz = getQuizService.getValue();
        });         
    } catch (Exception e) {
        System.err.println(e);
    }
    return quiz;
}

The service works fine, inside the onSucceeded action the object is present, but how can I make the return wait until the service has finished?

As requested here's a minimal version of the GetQuizService

public class GetQuizService extends Service<Quiz> {
    private int id;
    private Quiz quiz;
    public void setId(int id) {
        this.id = id;
    }

    @Override
    protected Task<Quiz> createTask() {
        return new Task<Quiz>() {
            @Override
            protected Quiz call() throws Exception {
                // Severall calls to db here, Quiz object gets constructed
                return quiz;
            }
        };
    }
}

GetQuizService code is 100 + lines long... also it works fine. When I do a syso inside getQuizService.setOnSucceeded I get this.quiz just fine. Its only on the return statement that I get a null. — rookie_senior, Mar 10 '19 at 18:02
[How to create an MCVE](https://stackoverflow.com/help/mcve). — SedJ601, Mar 10 '19 at 18:04

score 1 · Accepted Answer · answered Mar 10 '19 at 22:17

The problem in your code is, that you service methods are executed asynchronously.

You should return the Task<Quiz> instead of quiz and use that to update your frontend if the result is received (I have to few information to create an appropriate example for you).

Another option is to pass a callback to your service, which is invoked when the result is received instead of returning the quiz.

public void getQA(int id, QuizReceiver callback) throws SQLException {
    try {
        GetQuizService getQuizService = new GetQuizService();
        getQuizService.restart();
        getQuizService.setId(id);
        getQuizService.setOnSucceeded(e -> {
            callback.quizReceived(getQuizService.getValue());
        });         
    } catch (Exception e) {
        System.err.println(e);
    }
    return quiz;
}

public interface OuizReceiver {
    void quizReceived(IQA quiz);
}

Thanks! In the end I think this points to an design problem in the project, which I should address. — rookie_senior, Mar 11 '19 at 14:50
Thanks again Samuel! For anyone interested I have found [this thread](https://stackoverflow.com/questions/30249493/using-threads-to-make-database-requests) addresses the situation I'm facing pretty well — rookie_senior, Mar 11 '19 at 21:50

How to wait for a JavaFX Service to finish before returning data in caller?

1 Answers1