public class MainWindow {

    @UsesTextChanger
    private JButton btn1;

    @UsesTextChanger
    private JLabel lb1;

    public void ChangeTexts() {

        for (Field field: MainWindow.class.getDeclaredFields()) {
            field.setAccessible(true);
            UsesTextChanger usesTextChanger = field.getAnnotation(UsesTextChanger.class);
            if (usesTextChanger != null){

                try {
                    Method method = field.getType().getMethod("setText", new Class[]{String.class});
                    method.invoke(field, "my new text");

                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

java.lang.IllegalArgumentException: object is not an instance of declaring class

for (Component component: this.frame.getContentPane().getComponents()) {
    try {
        boolean componentUsesTextChangerAnnotation = true; // Is there a way to check if an annotation exists in an instanced object?
        if (componentUsesTextChangerAnnotation) {
            Method method = component.getClass().getMethod("setText", new Class[]{String.class});
            method.invoke(component, "my new text");
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
}