JavaFX FXML controller - constructor vs initialize method

My Application class looks like this:

public class Test extends Application {

	private static Logger logger = LogManager.getRootLogger();
    
	@Override
	public void start(Stage primaryStage) throws Exception {

		String resourcePath = "/resources/fxml/MainView.fxml";
		URL location = getClass().getResource(resourcePath);
		FXMLLoader fxmlLoader = new FXMLLoader(location);

		Scene scene = new Scene(fxmlLoader.load(), 500, 500);

		primaryStage.setScene(scene);
		primaryStage.show();
	}
    
	public static void main(String[] args) {
		launch(args);
	}
}

The FXMLLoader creates an instance of the corresponding controller (given in the FXML file via fx:controller) by invoking first the default constructor and then the initialize method:

public class MainViewController {

	public MainViewController() {
		System.out.println("first");
	}

	@FXML
	public void initialize() {
		System.out.println("second");
	}
}

The output is:

first
second

So, why does the initialize method exist? What is the difference between using a constructor or the initialize method to initialize the controller required things?

Thanks for your suggestions!

In a few words: The constructor is called first, then any @FXML annotated fields are populated, then initialize() is called.

This means the constructor does not have access to @FXML fields referring to components defined in the .fxml file, while initialize() does have access to them.

Quoting from the Introduction to FXML:

> [...] the controller can define an initialize() method, which will be called once on an implementing controller when the contents of its associated document have been completely loaded [...] This allows the implementing class to perform any necessary post-processing on the content.

The initialize method is called after all @FXML annotated members have been injected. Suppose you have a table view you want to populate with data:

class MyController { 
    @FXML
    TableView<MyModel> tableView; 
    
    public MyController() {
        tableView.getItems().addAll(getDataFromSource()); // results in NullPointerException, as tableView is null at this point. 
    }

    @FXML
    public void initialize() {
        tableView.getItems().addAll(getDataFromSource()); // Perfectly Ok here, as FXMLLoader already populated all @FXML annotated members. 
    }
}

In Addition to the above answers, there probably should be noted that there is a legacy way to implement the initialization. There is an interface called Initializable from the fxml library.

import javafx.fxml.Initializable;

class MyController implements Initializable {
    @FXML private TableView<MyModel> tableView;

    @Override
    public void initialize(URL location, ResourceBundle resources) {
        tableView.getItems().addAll(getDataFromSource());
    }
}

Parameters:

location - The location used to resolve relative paths for the root object, or null if the location is not known.
resources - The resources used to localize the root object, or null if the root object was not localized. 

And the note of the docs why the simple way of using @FXML public void initialize() works:

NOTE This interface has been superseded by automatic injection of location and resources properties into the controller. FXMLLoader will now automatically call any suitably annotated no-arg initialize() method defined by the controller. It is recommended that the injection approach be used whenever possible.