Creating a component extension

In this tutorial we create a simple extension that can be attached to a PasswordField, displaying a floating notification if the user’s Caps Lock seems to be enabled. We assume the reader is already familiar with the Creating a UI extension tutorial.

This extension has almost no server-side functionality; the whole Extension class is as follows:

public class CapsLockWarning extends AbstractExtension {
  protected CapsLockWarning(PasswordField field) {
    // Non-public constructor to discourage direct instantiation

  public static CapsLockWarning warnFor(PasswordField field) {
    return new CapsLockWarning(field);

When there’s nothing to configure for the extension, users just want to enable it for some component and be done with it. By defining a static factory method, the user only needs to do something like CapsLockWarning.warnFor(myPasswordField); to make myPasswordField get the new functionality.

The client side is not overly complicated, either. We override the extend method, called by the framework when the client-side extension connector is attached to its target the client-side counterpart of the connector to which the server-side extension instance is attached in this case, PasswordFieldConnector.

We add a key press handler to the password widget, checking if the input looks like Caps Lock might be enabled. The Caps Lock state cannot be directly queried in GWT/JavaScript, so we use a trick: check if either

  • the shift key was not held but the entered character was uppercase, or

  • the shift key was held but the entered character was lowercase.

If this is the case, we show a warning in the form of a floating widget (VOverlay). This demonstrates how an extension may make use of UI elements even though it is not a part of the layout hierarchy. A frequent use case for extensions is showing different types of floating overlay elements that are temporary in character.

public class CapsLockWarningConnector extends AbstractExtensionConnector {
  protected void extend(ServerConnector target) {
    final Widget passwordWidget = ((ComponentConnector) target).getWidget();

    final VOverlay warning = new VOverlay();
    warning.add(new HTML("Caps Lock is enabled!"));

    passwordWidget.addDomHandler(new KeyPressHandler() {
      public void onKeyPress(KeyPressEvent event) {
        if (isEnabled() && isCapsLockOn(event)) {
        } else {
    }, KeyPressEvent.getType());

  private boolean isCapsLockOn(KeyPressEvent e) {
    return e.isShiftKeyDown() ^ Character.isUpperCase(e.getCharCode());

To use the Caps Lock warning, compile your widgetset and extend a PasswordField with something like this

PasswordField field = new PasswordField("Enter your password");