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Component Basics

Java API

Components are fundamental building blocks that can be added to a window, providing user interface functionality and custom behavior. In webforJ, the Component class serves as the foundation for all components within the engine.

Lifecycle Management

Understanding the component lifecycle is essential for creating, managing, and utilizing components effectively. The following two lifecycle states have methods to manipulate their behavior. These methods should not explicitly be called by the user.

Create and Destroy Hooks

All classes that extend the Component class are responsible for implementing the functionality to be executed when the Component is created, and when it is destroyed. This is done by overriding the onCreate() and onDestroy() methods, respectively.


The onCreate() method called when the component is added to a window. Creating components involves setting up their initial state and functionality. This is where you define what the component should do when it's first created. Whether it's initializing variables, setting up event listeners, or performing any other setup, the onCreate() method is your entry point for customizing component behavior.

This hook receives a window instance which allows for the addition of components contained within the component.

protected void onCreate(Window window) {
TextField text = new TextField();
Button btn = new Button();

window.add(text, add);

The onCreate() method is where the component and any constituents should be added to the window.


Destroying components is an essential part of managing resources and ensuring proper cleanup. Destroying a component is necessary when it's no longer needed or when you want to release resources associated with it. It allows a developer to perform cleanup tasks, such as stopping timers, releasing memory, or detaching event listeners. It also allows the destroy() method to be called on any constituent components.


The onDestroy() method is responsible for calling the destroy() method on any constituent components. Otherwise, these components will still exist in the DOM, but will not be reachable via the API.

Asynchronous Attachment

The whenAttached() method allows for functionality to be executed after a component has been added to a window. This method returns a PendingResult, which allows for additional specified behavior to execute asynchronously once the component is attached in the DOM.


Unlike the previous three methods, whenAttached() is meant to be explicitly called by the user.

public class Demo extends App {
public void run() throws webforJException {
Frame window = new Frame();

Button button = new Button();

/* Explicit call to whenAttached() which will display a
message box when the button is attached to the Frame.*/
button.whenAttached().thenAccept( e -> msgbox("I'm attached!"));

// onCreate() method is called


Observers play a vital role in keeping track of component lifecycle events. Observers can be added and removed using the addLifecycleObserver() and removeLifecycleObserver() methods, and receive notifications about events such as creation and destruction of components.

By adding observers, you can take action when a component is created, or destroyed. This is particularly useful for implementing custom logic or handling specific scenarios based on component events.

Button button = new Button();
button.addLifecycleObserver((button, lifecycleEvent) -> {
if (lifecycleEvent == ComponentLifecycleObserver.LifecycleEvent.DESTROY) {
//implemented logic to execute when the Button is destroyed

Component Properties

Component Identifiers

Component IDs serve as unique identifiers for components, allowing you to interact with them and manage their state effectively.

Server-Side Component ID

Every component created from the Component class is assigned a server-side identifier automatically. Server-side IDs are essential for internal tracking and identification of components within the framework. You can retrieve the server-side component ID using the getComponentId() method.

This can be helpful in many situations where having a unique, server-size identifier is necessary, such as querying for a specific component within a container.

Client-Side Component ID

Client-Side IDs allow for the user to obtain the client representation of the server component created in Java. All provided webforJ components have an implementation of this ID provided. If you want to obtain access to and use the client-side component, you can execute object.get() with the client ID to obtain the desired client component.


This ID is not the ID attribute of the element in the DOM.

In the below sample, an onClick event is added to a button, which is then fired by calling the method on the client component after it is obtained using get object.get() method.

public void run() throws webforJException {
Frame frame = new Frame();
Button btn = new Button("Click me");
btn.onClick(e -> {

btn.whenAttached().thenAccept(e -> {
getPage().executeJs("objects.get('" + btn.getClientComponentId() + "').click()");

User Data

The Component class allows you to include additional information within the component using the setUserData() method. This information is accessible only on the server side of the component via the getUserData() method, and is not sent to the client.

This is quite useful when there is information that should be included with a component, and when that information should be accessible without making a trip to the client to retrieve it.