CDI and Weld

CDI .

Dependency Injection in JEE6

jens.augustsson@redpill-linpro.com

Consulting ● Development ● IT Operations ● Training ● Support ● Products

Today.

1. What it is
2. Features
3. Advices


next...

1. What it is


Example.
public class TextTranslator {

   private final SentenceParser sentenceParser;
   private final Translator sentenceTranslator;

   @Inject
   public TextTranslator(SentenceParser sentenceParser,
Translator sentenceTranslator) {

      this.sentenceParser = sentenceParser;
      this.sentenceTranslator = sentenceTranslator;
   }

   public String translate(String text) {
      StringBuilder sb = new StringBuilder();
      for (String sentence: sentenceParser.parse(text)) {
          sb.append(sentenceTranslator.translate(sentence));
      }
      return sb.toString();
   }
}


Example - 2.
@Stateless

public class SentenceTranslator implements Translator {

public String translate(String sentence) { ... }

}

public class SentenceParser {

public List<String> parse(String text) { ... }

}

Injection of: Managed Bean, EJB session beans
Injection to: MDB, interceptor, Servlet, JAX-WS SE, JSP (tag h / ev lis)


Martin Fowler.
Coined the term ”Dependency Injection”

«The fundamental choice is between
Service Locator and Dependency Injection»

http://martinfowler.com/articles/injection.html

«Inversion of Control» ... «The Hollywood principle»


CDI - JSR299.

”Context and Dependency Injection for the Java EE Platform”

«Loose coupling, strong typing»


but...

Why care?


JEE5 DI features.
Resource injection in JEE5
✔ @EJB, @Resource, @PersistenceContext,
@PersistenceUnit
✔ Servlets, JSF backing beans and other EJBs

Problems
✔ Cannot inject EJBs into Struts Actions
✔ Cannot inject DAOs or helper classes that
were not written as EJBs
✔ Hard to integrate anything but strictly
business components


Alternative DI frameworks.
PicoContainer
✔ Early one

HiveMind
✔ Howard Lewis Ship & Tapestry

Spring Core
✔ The de facto standard

Seam 2
✔ By the Hibernate Team

Guice
✔ Crazy Bob Lee @ Google


but I've heard the term.....
CDI
WebBeans
old name for... new name for...

JSR-299
implements... inspired...
Seam 2
Weld
part of...

includes...
uses... Java EE 6
D4J
name for...
Seam 3 JSR-330

created by... created by...

Spring Core
Guice


next...

2. CDI Features


Injection points.
Class constructor public class Checkout {
   private final ShoppingCart cart;

   @Inject
   public Checkout(ShoppingCart cart) {
      this.cart = cart;
   }
}

Initializer method public class Checkout {
private ShoppingCart cart;

@Inject
void setShoppingCart(ShoppingCart cart) {
this.cart = cart;
}
}

Direct field public class Checkout {
private @Inject ShoppingCart cart;
}


Bean types.
A user-defined class or interface
In a JEE module with a /META-INF/beans.xml

public class BookShop

extends Business

implements Shop<Book>, Auditable {

...

}

...but «there can be only one»...


Non-default qualifiers.
Custom annotations links...
@Qualifier
@Retention(RUNTIME)
@Target({TYPE, METHOD, FIELD, PARAMETER})
public @interface Asynchronous {}

...the qualified bean...
@Asynchronous
public class AsynchronousPaymentProcessor implements PaymentProcessor {
public void process(Payment payment) { ... }
}

...to declared injection points.
@Inject @Asynchronous PaymentProcessor asyncPaymentProcessor;
@Inject @Synchronous PaymentProcessor syncPaymentProcessor;


Producer methods.
Run time qualifier

public PaymentAction {
@Inject @Preferred PaymentStrategy paymentStrategy;
...
}

@Singleton @Managed
public PaymentServiceBean imlements PaymentService {

PaymentStrategy paymentStrategy = PaymentStrategy.CREDIT_CARD;

@Produces @Preferred @SessionScoped
public PaymentStrategy getPaymentStrategy(CreditCardPaymentStrategy ccps,
CheckPaymentStrategy cps,
PayPalPaymentStrategy ppps) {
switch (paymentStrategy) {
case CREDIT_CARD: return ccps;
case CHEQUE: return cps;
case PAYPAL: return ppps;
default: return null;
}
}


Scopes and Contexts.
Scope determines...
✔ When a new instance of any bean with that scope is created
✔ When an existing instance of any bean with that scope is destroyed
✔ Which injected references refer to any instance of a bean with that scope
✔ CDI features an extensible context model

Built-in scopes
✔ (@Dependent)
✔ @RequestScoped
✔ @SessionScoped
✔ @ApplicationScoped
✔ @ConversationScoped


Conversation Scope.
@ConversationScoped @Stateful
public class OrderBuilder {
   private Order order;
   private @Inject Conversation conversation;
   private @PersistenceContext EntityManager em;

   public Order createOrder() {
      order = new Order();
      conversation.begin();
      return order;
   }

   public void addLineItem(Product product, int quantity) {
      order.add(new LineItem(product, quantity));
   }

   public void saveOrder() {
      em.persist(order);
      conversation.end();
   }

   @Remove
   public void destroy() {}
}


Interceptors.

business method interception
lifecycle callback interception
timeout method interception (ejb3)


Interceptors - 2.

Binding

@InterceptorBinding
@Target({METHOD, TYPE})
@Retention(RUNTIME)
public @interface MySecurity {}

public class ShoppingCart {
@MySecurity public void checkout() { ... }
}


Interceptors - 3.

Implementation - business method:
@MySecurity @Interceptor
public class MySecurityInterceptor {

@AroundInvoke
public Object manageSecurity(InvocationContext ctx) throws Exception { ... }

}

Implementation - lifecycle: @PostConstruct, @PreDestroy...

Implementation - timeout: @AroundTimeout


Decorators .
Interceptors capture orthogonal application concerns
§
The reverse is true of decorators
@Decorator
public abstract class LargeTransactionDecorator implements Account {

@Inject @Delegate @Any Account account;
@PersistenceContext EntityManager em;

public void withdraw(BigDecimal amount) {
account.withdraw(amount);
if ( amount.compareTo(LARGE_AMOUNT)>0 ) {
em.persist( new LoggedWithdrawl(amount) );
}
}

public void deposit(BigDecimal amount);
account.deposit(amount);
if ( amount.compareTo(LARGE_AMOUNT)>0 ) {
em.persist( new LoggedDeposit(amount) );
}
}

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Events.
Become observable....
@Inject @Any Event<Document> documentEvent;
...

documentEvent.fire(document);

Become observer....

public void onAnyDocumentEvent(@Observes Document document) { ... }

Many conditional observations...
public void refreshOnDocumentUpdate(@Observes(receive = IF_EXISTS) @Updated Document d

public void addProduct(@Observes(during = AFTER_SUCCESS) @Created Product product) { .. }


Predefine scope and interceptors Stereotypes.
Declare Predefined by CDI:
@Interceptor, @Decorator
@Stateless and @Model
@Transactional(requiresNew=true)
@Secure
@Target(TYPE) @Named
@Retention(RUNTIME) @RequestScoped
@Stereotype @Documented
public @interface BusinessLogic {} @Stereotype
@Target(TYPE,METHOD,FIELD)
@Retention(RUNTIME)
Use public @interface Model {}
@BusinessLogic
public class UserService { ... }


JEE comp env resources.
@Produces @Resource(lookup="java:global/env/jdbc/CustomerDatasource")
@CustomerDatabase Datasource customerDatabase;

@Produces @PersistenceUnit(unitName="CustomerDatabase")
@CustomerDatabase EntityManagerFactory customerDatabasePersistenceUnit;

@Produces @WebServiceRef(lookup="java:app/service/Catalog")
Catalog catalog;

@Produces @EJB(ejbLink="../their.jar#PaymentService")
PaymentService paymentService;


next...

3. Advices


Similarities – with Seam2.
Spring IoC users: CDI more type-safe and
annotation-driven

Guice users: CDI more geared towards
enterprise development

Seam users: CDI has a lot more advanced
features


Similarities – with D4J .
Only five annotations!

@Inject
@Qualifier
@Named
@Scope
@Singleton


Personal experiences.

Good stuff
Seam improvement – no outjection, method-time injection etc.
Great for use with other frameworks – like jBPM
XML-hell is /actually/ gone

Be careful
Start off with managed beans – switch when needed
Avoid injection from ”thinner” context – use @Dependent
Weld documentation not finished
Avoid ”upgrade” JBoss AS 5.x
XML Configuration in Seam 3 Module
Annotation Frustration...


Get started!.
In JBoss 6.0.0.Final (Weld 1.1.0.Beta2)

In GlassFish Server 3.1 (Weld 1.1.0.Final)

Embed Weld in Tomcat, Jetty... Android almost :-)

And read more!

Dan Allens slideshare: Google ”Dan Allen slideshare cdi”
Gavin King and Bob Lee flamewar: Google ”Gavin King Bob Lee jsr"


End.

jens.augustsson@redpill-linpro.com


CDI and Weld

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