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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.
There is no support for CDI interceptors in the client. Although this is planned in a future release.
Passivating scopes are not supported.
The JSR-299 SPI is not supported for client side code. Although writing extensions for the client side container is possible via the Errai IOC Extensions API.
When choosing RPC style invocations on beans, you basically rely on a typed java interface the CDI managed bean needs to expose. A GWT client component can then create an invocation proxy based on this interface. For more information see chapter on RPC mechanism .
If you choose publish/subscribe then your CDI bean needs to implement the
MessageCallback
interface, as described in chapter
Chapter 2, Messaging
. Any bean exposed in this way can be accessed through the
MessageBuilderAPI
.
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.
Example 6.1. Example dependent scoped bean
public void MyDependentScopedBean {
private final Date createdDate;
public MyDependentScopedBean {
createdDate = new Date();
}
}
Example 6.2. Example ApplicationScoped bean
@ApplicationScoped
public void MyClientBean {
@Inject MyDependentScopedBean bean;
// ... //
}
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.
Example 6.3. FraudClient.java
public class FraudClient extends LayoutPanel {
@Inject
private Event<AccountActivity> event; (1)
private HTML responsePanel;
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public FraudClient() {
super(new BoxLayout(BoxLayout.Orientation.VERTICAL));
}
@PostConstruct
public void buildUI() {
Button button = new Button("Create activity", new ClickHandler() {
public void onClick(ClickEvent clickEvent) {
event.fire(new AccountActivity());
}
});
responsePanel = new HTML();
add(button);
add(responsePanel);
}
public void processFraud(@Observes @Detected Fraud fraudEvent) { (2)
responsePanel.setText("Fraud detected: " + fraudEvent.getTimestamp());
}
}
Two things are noteworthy in this example:
Injection of an
Event
dispatcher proxy
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:
Example 6.4. AccountService.java
@ApplicationScoped
public class AccountService {
�
@Inject @Detected
private Event<Fraud> event;
public void watchActivity(@Observes AccountActivity activity) {
Fraud fraud = new Fraud(System.currentTimeMillis());
event.fire(fraud);
}
}
A server can address a single client in response to an event annotating event types as
@Conversational
. Consider a service that responds to a subscription event.
Example 6.5. SubscriptionService.java
@ApplicationScoped
public class SubscriptionService {
�
@Inject
private Event<Documents> welcomeEvent;
public void onSubscription(@Observes Subscription subscription) {
Document docs = createWelcomePackage(subscription);
welcomeEvent.fire(docs);
}
}
And the
Document
class would be annotated like so:
As such, when
Document
events are fired, they will be limited in scope to the initiating conversational contents – which are implicitly inferred by the caller. So only the client which fired the
Subscription
event will receive the fired
Document
event.
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:
Example 6.7. MyServerBean.java
@ApplicationScoped
public class MyServerBean {
@Inject
Event<MyResponseEvent> myResponseEvent;
public void myClientObserver(@Observes MyRequestEvent event) {
MyResponseEvent response;
if (event.isThankYou()) {
// aww, that's nice!
response = new MyResponseEvent("Well, you're welcome!");
}
else {
// how rude!
response = new MyResponseEvent("What? Nobody says 'thank you' anymore?");
}
myResponseEvent.fire(response);
}
}
Domain-model:
Example 6.8. MyRequestEvent.java
@Portable
public class MyRequestEvent {
private boolean thankYou;
public MyRequestEvent(boolean thankYou) {
setThankYou(thankYou);
}
public void setThankYou(boolean thankYou) {
this.thankYou = thankYou;
}
public boolean isThankYou() {
return thankYou;
}
}
Example 6.9. MyResponseEvent.java
@Portable
public class MyResponseEvent {
private String message;
public MyRequestEvent(String message) {
setMessage(message);
}
public void setMessage(String message) {
this.message = message;
}
public String getMessage() {
return message;
}
}
Client application logic:
Example 6.10. MyClientBean.java
@EntryPoint
public class MyClientBean {
@Inject
Event<MyRequestEvent> requestEvent;
public void myResponseObserver(@Observes MyResponseEvent event) {
Window.alert("Server replied: " + event.getMessage());
}
@PostConstruct
public void init() {
Button thankYou = new Button("Say Thank You!");
thankYou.addClickHandler(new ClickHandler() {
public void onClick(ClickEvent event) {
requestEvent.fire(new MyRequestEvent(true));
}
}
Button nothing = new Button("Say nothing!");
nothing.addClickHandler(new ClickHandler() {
public void onClick(ClickEvent event) {
requestEvent.fire(new MyRequestEvent(false));
}
}
VerticalPanel vPanel = new VerticalPanel();
vPanel.add(thankYou);
vPanel.add(nothing);
RootPanel.get().add(vPanel);
}
}
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.
Example 6.11. App.java
@EntryPoint
public class App {
�...
@Produces @Supported
public MyBaseWidget createWidget() {
return (Canvas.isSupported()) ? new MyHtml5Widget() : new MyDefaultWidget();
}
}
Example 6.12. MyComposite.java
@ApplicationScoped
public class MyComposite extends Composite {
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@Inject @Supported
public MyBaseWidget widget;
...
}
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.
In development mode we need to bootstrap the CDI environment on our own and make both Errai and CDI available through JNDI (common denominator across all runtimes). Since GWT uses Jetty, that only supports read only JNDI, we need to replace the default Jetty launcher with a custom one that will setup the JNDI bindings:
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>gwt-maven plugin</artifactId>
<version>${gwt.maven}</version>
<configuration>
...
<server>org.jboss.errai.cdi.server.gwt.JettyLauncher</server>
</configuration>
<executions>
...
</executions>
</plugin>Consequently, when starting Development Mode from within your IDE the following program argument has to be provided: -server org.jboss.errai.cdi.server.gwt.JettyLauncher
Once this is set up correctly, we can bootstrap the CDI container through a servlet listener:
<web-app>
...
<listener>
<listener-class>org.jboss.errai.container.CDIServletStateListener</listener-class>
</listener>
<resource-env-ref>
<description>Object factory for the CDI Bean Manager</description>
<resource-env-ref-name>BeanManager</resource-env-ref-name>
<resource-env-ref-type>javax.enterprise.inject.spi.BeanManager</resource-env-ref-type>
</resource-env-ref>
...
</web-app>Sounds terribly complicated, no? Don't worry we provide a maven archetype that takes care of all these setup steps and configuration details.
Deployment to servlet engine has basically the same requirements as running in development mode. You need to include the servlet listener that bootstraps the CDI container and make sure both Errai and CDI are accessible through JNDI. For Jetty you can re-use the artefacts we ship with the archetype. In case you want to run on tomcat, please consult the Apache Tomcat Documentation .
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
).
Since the discovery of service implementations (beans) is delegated to the CDI container, we need to disable Errai's own service discovery mechanism. In order to do so, simply turn off the auto-discovery feature in
ErraiService.properties
errai.auto_discover_services=false