Implement custom annotation in Spring - spring

Question

I want to implement an annotation which registers classes (not instances of classes) with a factory as soon as the application is started. I am using Spring Framework 4.2.7.
Consider a system with a dashboard and multiple widgets. The dashboard has a configuration file which contains a list of widgets to display for the current user. When displayed it reads the configuration and creates the widgets. The widgets will receive additional parameters from the configuration.
Here is a bit of code illustrating this:
public class TestDashboard implements Dashboard {
public void dashboardPreDisplay() {
List<String> widgets = getWidgetList(/* current user in session */);
for (String widgetId : widgets) {
// create instance of DashboardWidget with given ID
DashboardWidget x = widgetFactory.createWidget(widgetId);
}
}
public List<String> getWidgetList(String user) {
// load list of IDs of DashboardWidgets to be displayed for the user
}
#Autowired
private WidgetFactory widgetFactory;
}
#Service
public class WidgetFactory {
public DashboardWidget createWidget(String widgetId) {
// look up Class<> of DashboardWidget with given id in widgetClasses
// construct and initialize DashboardWidget
}
private HashMap<String, Class<?>> widgetClasses;
}
When implementing my widgets I don't want to deal with registering the widget with the factory class. Ideally I would just annotate the widget like that:
#DashboardWidget(id = "uniqueId")
public class DashboardWidgetA implements DashboardWidget {
// ...
}
When the application starts it should scan the classpath for #DashboardWidget annotations and register the classes with the factory, so that the widgets can be constructed by giving the createWidget-method the id of the widget.
At the moment I am a little bit confused. I think Spring has every tool on board to achieve this behavior. But I cannot think of a way how to do it.
Do you have some advice for me?

Answer 1

Nothing prevents you to create your custom annotation:
#Retention(RetentionPolicy.RUNTIME)
#Target(ElementType.TYPE)
public #interface DashboardWidget {}
Then you can annotate your Widget's classes and make them spring beans. You have to keep in mind if you want to have them as singletons (scope=singleton) , or separate instances per user (scope=prototype).
You have to implement:
public class WidgetInitializationListener implements ApplicationListener<ContextRefreshedEvent> {
#Override
public void onApplicationEvent(ContextRefreshedEvent event) {
ApplicationContext context = event.getApplicationContext();
String[] beanDefinitionNames = context.getBeanDefinitionNames();
for (String beanDefinitionName : beanDefinitionNames) {
String originalClassName = getOriginalClassName(beanDefinitionName, event);
if (originalClassName != null) {
Class<?> clazz = Class.forName(originalClassName);
if (hasWidgetAnnotation(clazz)) {
registerSomewhereYourWidget(context, beanDefinitionName, originalClassName);
}
}
}
}
private String getOriginalClassName(String name, ContextRefreshedEvent event) {
try {
ConfigurableListableBeanFactory factory =
(ConfigurableListableBeanFactory)event.getApplicationContext().getAutowireCapableBeanFactory();
BeanDefinition beanDefinition = factory.getBeanDefinition(name);
return beanDefinition.getBeanClassName();
} catch (NoSuchBeanDefinitionException e) {
LOG.debug("Can't get bean definition for : " + name);
return null;
}
}
So mostly here is nothing to do with spring except you just run through your beans to find annotated ones.