Using threads to make database requests

Question

I\'m trying to understand how threads works in java. This is a simple database request that returns a ResultSet. I\'m using JavaFx.

    package application;
         


        

Answer 1

Exception in thread "Thread A" java.lang.IllegalStateException: Not on FX application thread; currentThread = Thread A

The exception is trying to tell you that you are trying to access JavaFX scene graph outside the JavaFX application thread. But where ??

courseCodeLbl.setText(rs.getString(1)); // <--- The culprit

If I can't do this how do I use a background thread?

The are different approaches which leads to similar solutions.

Wrap you Scene graph element with Platform.runLater

There easier and most simple way is to wrap the above line in Plaform.runLater, such that it gets executed on JavaFX Application thread.

Platform.runLater(() -> courseCodeLbl.setText(rs.getString(1)));

Use Task

The better approach to go with these scenarios is to use Task, which has specialized methods to send back updates. In the following example, I am using updateMessage to update the message. This property is bind to courseCodeLbl textProperty.

Task task = new Task() {
    @Override
    public Void call() {
        String courseName = "";
        Course c = new Course();
        c.setCCode(Integer.valueOf(courseId.getText()));
        mController = new ModelController(c);
        try {
            ResultSet rs = mController.get();
            if(rs.next()) {
                // update message property
                updateMessage(rs.getString(1));
            }
        } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return null;
    }
}

public void getCourseNameOnClick(){
    try {
        Thread t = new Thread(task);
        // To update the label
        courseCodeLbl.textProperty.bind(task.messageProperty());
        t.setDaemon(true); // Imp! missing in your code
        t.start();
    } catch (NumberFormatException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}