Understanding the Constructor and initialize() Method in OpenJFX

Yatin BatraNovember 18th, 2025Last Updated: November 18th, 2025

0 139 5 minutes read

When developing JavaFX applications using OpenJFX, a common question is whether to place initialization code inside the constructor of a controller class or inside the initialize() method. Both approaches have their merits and understanding the differences between them can help you write more effective, maintainable JavaFX code. Let us delve into understanding the differences between JavaFX constructor vs initialize methods and their roles in the application lifecycle.

1. What Is OpenJFX?

OpenJFX is an open-source, next-generation client application platform for desktop, mobile, and embedded systems based on Java. It is the reference implementation for JavaFX, which provides a rich set of GUI components, hardware-accelerated graphics, and media playback capabilities.

1.1 Constructor vs initialize() in OpenJFX

When using JavaFX’s FXML to define user interfaces, the controller class can contain both a constructor and an initialize() method.

1.1.1 Constructor

The constructor is called when the controller instance is created by the FXMLLoader.
At constructor time, @FXML annotated fields are not yet injected and remain null.
Suitable for basic object setup that does not depend on FXML-injected elements.
Cannot safely access UI components declared in FXML because they are not initialized yet.

1.1.2 initialize() Method

This method is called automatically by the FXMLLoader after all @FXML fields have been injected.
It is the ideal place to configure UI elements, set event listeners, or initialize data that depends on FXML controls.
Ensures that all FXML elements are fully initialized and accessible for use.

1.1.3 Common Pitfalls When Using Constructor and initialize()

Accessing FXML fields in the constructor: Since FXML injection happens after the constructor executes, any attempt to access UI controls in the constructor results in NullPointerException.
Missing @FXML annotation on initialize method: If the initialize() method is not public, it must be annotated with @FXML to be invoked by the FXMLLoader. Otherwise, it will not be called, leading to unexpected behavior.
Putting heavy logic in the constructor: Constructors should be lightweight and only prepare the controller object itself. Placing resource-intensive tasks here can delay UI loading and lead to unresponsive applications.
Assuming initialize() is always called: If the controller is not loaded via FXMLLoader (for example, instantiated manually), the initialize() method will not be called automatically, requiring manual invocation if needed.

1.1.4 Real-World Use Cases for Constructor and initialize()

In practical JavaFX applications, understanding when to use the constructor versus the initialize() method is essential for clean, maintainable code. Here are some common real-world scenarios:

Dependency Injection and Service Setup: Use the constructor to inject or set up non-UI dependencies such as service classes, data repositories, or configuration objects. Since these dependencies are not related to FXML components, the constructor is a suitable place for their initialization.
UI Component Configuration: Use the initialize() method to configure UI controls defined in FXML, such as setting button labels, configuring table columns, or binding properties. This ensures that all UI elements are fully injected and ready for manipulation.
Event Handler Attachment: Attach event listeners or handlers (e.g., button clicks, mouse events) within the initialize() method, where you can safely reference UI controls.
Dynamic UI Adjustments: If your application needs to adjust the UI based on runtime data or user preferences, perform these tasks in the initialize() method or afterward, ensuring the UI components exist.
Complex Initialization Logic: For setup that requires both injected UI components and external services, coordinate the logic in initialize() by using fields set during construction combined with the injected controls.
FXML Loader Custom Controller Factory: When using FXMLLoader.setControllerFactory() to provide controllers with dependencies (e.g., via dependency injection frameworks), the constructor is used to receive those dependencies, while the initialize() method handles UI-specific initialization.

1.2 JavaFX Controller Lifecycle Overview

Understanding the lifecycle of a JavaFX controller is key to knowing when and where to place your initialization code. When an FXML file is loaded using FXMLLoader, the following sequence occurs:

The controller class is instantiated by calling its constructor. At this stage, @FXML annotated fields are still null because the FXML components have not yet been injected.
The FXMLLoader injects all UI components defined in the FXML into the corresponding @FXML fields of the controller.
After injection, the initialize() method is automatically called. This method is where you should place any code that depends on the injected UI components.

This lifecycle ensures that UI elements are fully ready for manipulation only after the initialize() method is invoked, making it the proper place to perform UI-related setup tasks.

2. Code Example

The following Java code demonstrates the practical difference between the constructor and the initialize() method in a JavaFX controller.

// ConstructorVsInitializeExample.java

import javafx.application.Application;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class ConstructorVsInitializeExample extends Application {
  @Override
  public void start(Stage primaryStage) throws Exception {
    FXMLLoader loader = new FXMLLoader(getClass().getResource("sample.fxml"));
    VBox root = loader.load();
    primaryStage.setScene(new Scene(root));
    primaryStage.setTitle("Constructor vs initialize() Demo");
    primaryStage.show();
  }

  public static void main(String[] args) {
    launch(args);
  }
}

// Controller class
public class SampleController {
  @FXML private Button myButton;

  // Constructor
  public SampleController() {
    System.out.println("Constructor called");
    // myButton is still null here, so accessing it will cause
    // NullPointerException System.out.println(myButton.getText()); // This
    // would throw NPE!
  }

  // initialize() method
  @FXML
  public void initialize() {
    System.out.println("initialize() called");
    // Now myButton is injected and can be accessed safely
    myButton.setText("Initialized Button");
    myButton.setOnAction(e -> System.out.println("Button clicked!"));
  }
}

2.1 Code Explanation

This JavaFX example demonstrates the difference between the constructor and the initialize() method in a controller class. The ConstructorVsInitializeExample class extends Application and loads an FXML file named sample.fxml to set up the UI, displaying it in the primary stage. The SampleController class, linked to the FXML, has a @FXML-annotated Button called myButton. When the controller is instantiated, the constructor is called first, but at this point myButton is still null, so trying to access it would cause a NullPointerException. After the FXML fields are injected, the initialize() method runs automatically, where myButton is fully initialized and can be safely accessed. In initialize(), the button’s text is set and an action handler is attached to print a message when clicked, illustrating the proper place to interact with FXML components in JavaFX.

2.2 Code Output

Constructor called
initialize() called

(Button click will print: "Button clicked!" in console)

The console output first shows “Constructor called” because the controller’s constructor executes when the FXMLLoader creates the controller instance. At this stage, the @FXML fields like myButton are not yet initialized. Next, the message “initialize() called” appears, indicating that the initialize() method has been invoked after the FXMLLoader has injected all the FXML components. On the user interface, the button’s label changes to “Initialized Button” as set in the initialize() method, and clicking the button triggers the attached event handler, which prints “Button clicked!” to the console. This sequence clearly shows why UI component setup should be done in initialize() rather than the constructor.

3. Conclusion

In OpenJFX, the constructor is useful for basic controller setup that does not depend on UI components. However, because FXML-injected fields are not available during construction, any UI initialization or event handling should be done inside the initialize() method. This method ensures all @FXML elements are fully injected and ready for use. Following this best practice avoids common errors like NullPointerException and keeps your JavaFX applications robust and clean.