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Chapter 6. Errai CDI

6.1. CDI Overview
6.1.1. RPC Style Invocations on CDI beans
6.1.2. Publish/Subscribe with CDI managed components
6.2. Beans and Scopes
6.3. Events
6.3.1. Conversational events
6.3.2. Client-Server Event Example
6.4. Producers
6.5. Deploying Errai CDI
6.5.1. Deployment in Development Mode
6.5.2. Deployment to a Servlet Engine
6.5.3. Deployment to an Application Server
6.5.4. Configuration Options

CDI (Contexts and Dependency Injection) is the Jave EE standard (JSR-299) for handling dependency injection. In addition to dependency injection, the standard encompasses component lifecycle, application configuration, call-interception and a decoupled, type-safe eventing specification.

The Errai CDI extension implements a subset of the specification for use inside of client-side applications within Errai, as well as additional capabilities such as distributed eventing.

Errai CDI does not currently implement all life cycles specified in JSR-299 or interceptors. These deficiencies may be addressed in future versions.


The Errai CDI extension itself is implemented on top of the Errai IOC Framework (see Chapter 3, Dependency Injection ), which itself implements the JSR-330 specification. Inclusion of the CDI module your GWT project will result in the extensions automatically being loaded and made available to your application.

Beans that are deployed to a CDI container will automatically be registered with Errai and exposed to your GWT client application. So, you can use Errai to communicate between your GWT client components and your CDI backend beans.

Errai CDI based applications use the same annotation-driven programming model as server-side CDI components, with some notable limitations.

In Errai CDI, all client types are valid bean types if they are default constructable or can have construction dependencies satisfied. These unqualified beans belong the dependent pseudo-scope. See: Dependent Psuedo-Scope from CDI Documentation

Additionally, beans may be qualified as @ApplicationScoped , @Singleton or @EntryPoint . Although these three scopes are supported for completeness and conformance to the specification, within the client they effectively result in behavior that is identical.

Any CDI managed component may produce and consume events . This allows beans to interact in a completely decoupled fashion. Beans consume events by registering for a particular event type and optional qualifiers. The Errai CDI extension simply extends this concept into the client tier. A GWT client application can simply register an Observer for a particular event type and thus receive events that are produced on the server-side. Likewise and using the same API, GWT clients can produce events that are consumed by a server-side observer.

Let's take a look at an example.

Two things are noteworthy in this example:

  1. Injection of an Event dispatcher proxy

  2. Creation of an Observer method for a particular event type

The event dispatcher is responsible for sending events created on the client-side to the server-side event subsystem (CDI container). This means any event that is fired through a dispatcher will eventually be consumed by a CDI managed bean, if there is an corresponding Observer registered for it on the server side.

In order to consume events that are created on the server-side you need to declare an client-side observer method for a particular event type. In case an event is fired on the server this method will be invoked with an event instance of type you declared.

To complete the example, let's look at the corresponding server-side CDI bean:

A key feature of the Errai CDI framework is the ability to federate the CDI eventing bus between the client and the server. This permits the observation of server produced events on the client, and vice-versa.

Example server code:


Client application logic:

Producer methods and fields act as sources of objects to be injected. They are useful when additional control over object creation is needed before injections can take place e.g. when you need to make a decision at runtime before an object can be created and injected.

For more information on CDI producers, see the CDI specification and the WELD reference documentation .

If you do not care about the deployment details for now and just want to get started take a look at the ERRAI:CDI Quickstart Guide.

The CDI integration is a plugin to the Errai core framework and represents a CDI portable extension. Which means it is discovered automatically by both Errai and the CDI container. In order to use it, you first need to understand the different runtime models involved when working GWT, Errai and CDI.

Typically a GWT application lifecycle begins in Development Mode and finally a web application containing the GWT client code will be deployed to a target container (Servlet Engine, Application Server). This is no way different when working with CDI components to back your application.

What's different however is availability of the CDI container across the different runtimes. In GWT development mode and in a pure servlet environment you need to provide and bootstrap the CDI environment on your own. While any Java EE 6 Application Server already provides a preconfigured CDI container. To accomodate these differences, we need to do a little trickery when executing the GWT Development Mode and packaging our application for deployment.

We provide integration with the JBoss Application Server , but the requirements are basically the same for other vendors. When running a GWT client app that leverages CDI beans on a Java EE 6 application server, CDI is already part of the container and accessible through JNDI ( java:/BeanManager ).