Tour around Weld 3

2017-5-19   Matej Novotny

This post briefly describes all the main CDI 2.0 features and elaborates on Weld-specific features we added on top of that. It is not intended as a deep-dive but rather to give you the overall idea of what is going on and what can the new release offer. So, enough talk, let’s get the show on the road!

Async Events and Notification Options

Up until now, the only way to send events was to do so synchronously. That meant stopping the work of your current thread to instead occupy it with observer resolution and subsequent notification of observer methods. Once all that was done, the thread resumed it’s work.

With CDI 2.0 there are asynchronous events - a 'fire & forget' way of handling things. Here is how to fire such and event and how to observe it:

[ source, java ]

// nothing new here, plain old Event is used
@Inject Event<Payload> event;

public void sendAsyncEvent() {
    // we use a new fireAsync method for asynchronous events
    CompletionStage<Payload> eventResult = event.fireAsync(new Payload()).thenAccept(...);

public void asyncObserver (@ObservesAsync Payload payload){ … }

Few things to note here - first of all, the return value of fireAsync is CompletionStage allowing you to chain the work for when the event is done. You can follow-up with more tasks, react on exceptional return value and so on. Another noticeable detail is that observers which are to be notified have to have @ObservesAsync annotation. As you might have guessed, an observer can be notified of either synchronous (@Observes) or asynchronous (@ObservesAsync) events but never both!

Apart from this very basic usage, async events allow you to specify NotificationOptions - an option allowing you to tweak the way notification works. From CDI perspective, there is currently only one standardized option and that enables usage of custom executors which will be used for notifications. But the API was designed in a generic way so that implementation can enrich it with its own options. Weld currently offers two more options - timeout and parallel execution - both of which are well described in Weld docs. And here is a snippet showing it in action:

[ source, java ]

public void sendAsyncEventWithOptions() {
    // we use a secondary version of fireAsync method with NotificationOptions parameter
    event.fireAsync(new Payload(), NotificationOptions.of("weld.async.notification.timeout", 2000));

Configurators API

Another of the key features are Configurators. They are here to ease our work when doing small tweaks in extensions. Here is a snippet showing how easy it can be to register a new bean in AfterBeanDiscovery using configurator. You don’t have to create a new class implementing Bean<X> anymore:

[ source, java ]

public void addABean(@Observes AfterBeanDiscovery event) {
    // get an instance of BeanConfigurator
      // set the desired data
      //finally, add a callback to tell CDI how to instantiate this bean
      .produceWith(obj -> new Foo());

Following configurators were added:

  • AnnotatedTypeConfigurator

  • InjectionPointConfigurator

  • BeanAttributesConfigurator

  • BeanConfigurator

  • ObserverMethodConfigurator

  • ProducerConfigurator

On top of that, Weld adds one additional configurator - InterceptorConfigurator. This one allows you to observe WeldAfterBeanDiscovery and then use this configurator to create and register a custom interceptor from scratch.

SE Bootstrap API

A big change in CDI 2.0 is the SE support. Of course, Weld fans have had this for years now, but it has gone official, so that still counts, right?

The official CDI API is very similar to that of Weld, here is how it looks like:

[ source, java ]

public void bootBasicContainer() {
    SeContainerInitializer initializer = SeContainerInitializer.newInstance();
    try (SeContainer container = initializer.initialize()) {

This is the very basic way; you can also opt to use synthetic bean archive where you cherry-pick all the beans/interceptors/…​ in your archive. CDI spec describes this fairly well, so how about we instead shift our attention to what Weld SE offers on top of that?

A small reminder of how to boot SE container using pure Weld API:

[ source, java ]

public void bootWeldSeContainer() {
    Weld weld = new Weld();
    try (WeldContainer container = weld.initialize()) {;

One of the things Weld makes easier, is when you want to create a quick extension, but don’t really want to write a whole new class which you then need to place in META-INF/services or register on bootstrap. You can easily create a 'synthetic extension' programatically:

[ source, java ]

Extension testExtension = ContainerLifecycleObserver.extensionBuilder()
    .add(afterBeanDiscovery((e) -> System.out.println("Bean discovery completed!")))
    .add(processAnnotatedType().notify((e) -> {
            if (e.getAnnotatedType().getJavaClass().getName().startsWith("")) {

try (WeldContainer container = new Weld().addExtension(testExtension).initialize()) {
    // Use the container...

What we just did was to create an extension with two container lifecycle observer methods. It follows a familiar builder pattern and the body of the observer methods is specified as lambda expression. Before booting SE container, we register this extension as we would any other. For more information about this, don’t hesitate to check our older news post.

On-demand Request Context Activation

Especially in SE (although not only there) you might want to activate a RequestContext manually for certain period of time. There are two ways to achieve that, first of which is an interceptor - @ActivateRequestContext. You can use that on either a method or a type (enabling it for all methods). As you might expect, it will activate the context before executing your method and shut it down afterwards. The other way is through means of built-in bean named RequestContextController. This bean can be injected as any other CDI bean and offers to self-explanatory methods: activate and deactivate. The obvious advantage of this approach is that you can enable the context for an extended period of time.

Observer Method Ordering

A small but noteworthy improvement to observer methods. You can not leverage @Priority annotation in observer methods hence ordering them as you wish.

[ source, java ]

public void observeFirst (@Observes @Priority(1) EventPayload payload) {...}

public void observeSecond (@Observes @Priority(2) EventPayload payload) {...}

Intercepting Produced Beans

Up until now, anything you created using producers could not be easily intercepted. CDI 2.0 allows this for @AroundInvoke interceptors in two ways. There is a new method on BeanManager named createInterceptionFactory but most of the time you will rather use a built-in bean InterceptionFactory which can be injected as a producer method parameter. Here is a snippet:

[ source, java ]

public ProducedBean produceFoo(InterceptionFactory<ProducedBean> factory) {
    return factory.createInterceptedInstance(new ProducedBean());

To explain this a bit, we first inject the built-in bean with the type equal to the produced type we wish to intercept. Then we configure() it, which returns an AnnotatedTypeConfigurator allowing us to add interceptor binding. Finally, we invoke InterceptionFactory.createInterceptedInstance() which takes a the object we produce as a parameter.

You can read more about this in this CDI spec chapter.

Trimmed Bean Archives

Last but not least feature we will mention are so called 'trimmed' bean archives. You can mark an explicit bean archive as trimmed in beans.xml by adding the </trim> element. Such bean archive will perform an annotated type discovery as with bean-discovery-mode="all" but all types that don’t have a bean defining annotation or any scope annotation are then removed from the set of discovered types.

Even in this case, Weld allows you to go one step further and veto types based on regular expression. It works on a similar principle but affects your whole application - it processess all types from all bean archives. Your archives will be scanned as they would be with bean discovery mode all and ProcessAnnotatedType will be fired for all found annotated types. Then, based on the regular expression you provide, annotated types which do not have a bean defining annotation and match the regular expression will be vetoed.