Dependency Injection in EJB 3

By Debu Panda

Enterprise JavaBeans (EJB) is a platform for building portable, reusable, and scalable business applications using the Java programming language. Since its initial incarnation, EJB has been touted as a component model or framework that lets you build enterprise Java applications without having to reinvent services such as transactions, security, and automated persistence for building an application.

EJB 3 greatly simplifies development by adopting a POJO programming model. As shown in the following figure an annotation transforms a simple POJO to an EJB.

EJB 3 not only simplifies development of session and message driven beans but it also radically simplifies the persistence model by implementing a simplified Object-Relational Mapping approach similar to Oracle TopLink and JBoss Hibernate as a part of the Java Persistence API. Note that JPA replaces EJB 2 CMP Entity beans in the EJB 3 spec, while being available outside of the Java EE container.

Following is an example of a simple EJB 3 Stateless session bean.

import javax.ejb.Stateless;

import ejb3inaction.example.persistence.Bid;

@Stateless

public class PlaceBidBean implements PlaceBid {

...

public PlaceBidBean() {}

public Bid addBid(Bid bid) {

System.out.println("Adding bid, bidder ID="

+

bid.getBidderID()

+ ", item ID=" + bid.getItemID()

+ ", bid amount="

+ bid.getBidAmount() + ".");

r

eturn save(bid);

}

...

}

import javax.ejb.Local;

import ejb3inaction.example.persistence.Bid;

@Local public interface PlaceBid {

Bid addBid(Bid bid);

}

​

With EJB 3, dependency injection has greatly simplified accessing both EJB resources"such as JDBC DataSource, JMS Objects, and JPA Entity Manager"and services"such as Timer, User Transaction, and Web Services. You will find this Refcard useful when building enterprise Java applications with EJB 3 and JPA. It lists all metadata annotations, describes them and provides examples. It also provides descriptions for XML elements that you can use for injection.

Most enterprise Java applications use external resources and services such as Data Source, EJB, or web services. EJB 3 makes using resources and services simpler by implementing dependency injection.

Dependency injection allows you to simply declare component dependencies and let the EJB container deal with the complexities of instantiating, initializing, and sequencing resources and supplying service or resource references to clients as required. Development frameworks like Spring framework originally popularized dependency injection.

In EJB 3, you may think of dependency injection as the inverse of JNDI. It is the responsibility of the container to inject an object based on the dependency declaration. The figure below compares dependency injection with JNDI.

You can use either metadata annotations or XML descriptors to use dependency injection.

Refer to the following Java EE 5 specifications and XML schema for more information on dependency injection.

JSR 220

Enterprise JavaBeans 3.0:

http://www.jcp.org/en/jsr/detail?id=220

JSR 224

Java API for XML-Based Web Services (JAX-WS) 2.0:

http://jcp.org/en/jsr/detail?id=224

JSR 250:

Common Annotations for the Java Platform:

http://jcp.org/en/jsr/detail?id=224

Schema for EJB 3 deployment descriptor:

http://java.sun.com/xml/ns/javaee/ejb-jar_3_0.xsd

Schema that defines common schema components:

http://java.sun.com/xml/ns/javaee/javaee_5.xsd

Dependency injection is supported only on managed classes and not on regular POJO.

The following table depicts what type of managed classes can inject what objects.

Java EE 5 introduced several metadata annotations as part of JSR 250. Although primarily geared toward EJB, these annotations also apply to Java EE components such as Servlets, JSF managed beans, and application clients.

The following annotations can be used in a field or a setter method for dependency injection. However, these annotations may also be applied at the class level for defining dependencies and then used with JNDI look up.

javax.annotation.Resource

The @Resource annotation is used to inject any of the following: JDBC data sources, JMS connection factories, JMS destinations, mail resource, environment entries, timer service, UserTransaction, and EJBContext.

The following table shows attributes for the @Resource annotation.

You can inject a resource at the field or setter method level.

The following example shows data source injection at the field level:

@Resource(name="jdbc/ActionBazaarDS")

private DataSource dataSource;

​

The following code shows data source injection at the setter method level:

private DataSource dataSource;

@Resource(name="jdbc/ActionBazaarDB")

public void setDataSource(DataSource dataSource) {

this.dataSource = dataSource;

}

​

EJB 3 allows you to explicitly specify a global JNDI name using the mappedName parameter of the @Resource annotation. For example, if you"re using the Oracle Application Server and you have a data source with a global JNDI name of jdbc/OracleDS, you can specify the resource mapping as follows:

@Resource(name="jdbc/ActionBazaarDS", mappedName="jdbc/ OracleDS")

private javax.jdbc.DataSource myDB;

​

XML Elements to Define Resource Injection

If you are using deployment descriptor (ejb-jar.xml) instead of annotations, then you must use resource-ref, resource-env-ref or env-entry XML elements to define dependencies.

resource-ref

The resource-ref is used to specify resource references. Example: data source and JMS connection factories.

<resource-ref>

<res-ref-name>jdbc/ActionBazaarDS</res-ref-name>

<res-type>javax.sql.DataSource</res-type>

<res-auth>Container</res-auth>

<res-sharing-scope>Shareable</res-sharing-scope>

<injection-target>

<injection-tar

get-class>

actionbazaar.buslogic.BidManagerBean</injection-target-class>

<injection-tar

get-name>dataSource</injection-target-name>

</injection-target>

</resource-ref>

​

resource-env-ref

The resource-env-ref is used to specify references to JMS destination resources such as a Queue or Topic.

<resource-env-ref>

<resource-env-ref-name>jms/OrderBillingQueue</resource-env-ref-name>

<resource-env-ref-type>

javax.jms.Destination

</resource-env-ref-type>

<injection-target>

<injection-target-class>

ejb3inaction.example.buslogic.PlaceOrderBean

</injection-target-class>

<injection-tar

get-name>billingQueue</injection-target-name>

</injection-target>

</resource-env-ref>

​

env-entry

The env-entry defines environment entries for an EJB.

The injection-target defines the name of a class and a name (field or property) within that class into which a resource, EJB, or entity manager should be injected as we saw in the above examples.

Like resource injection, you can inject Session bean references into another EJB or other managed class using annotations or deployment XML. If you prefer annotations you can use @javax.ejb.EJB annotations to inject either remote or local EJB references. Using deployment XML, you can inject remote EJB references with ejb-ref and local EJB references with ejb-local-ref.

@javax.ejb.EJB

Injects a session bean reference into a field or method.

@EJB(name="BidManagerRemote")

private BidManager bidManager;

​

ejb-local-ref

Used to specify a dependency on the local interface of a session bean.

ejb-ref

Used to specify a dependency on the remote interface of a session bean.

The JPA EntityManager is the bridge between the OO and relational worlds as depicted in the following figure.

JPA supports two types of EntityManager: container-managed and application-managed. As the name suggests the lifecycle of the container manages container-managed EntityManager whereas the lifecycle (creation, destruction) is performed by application. An application-managed EntityManager is created from an EntityManagerFactory.

Injecting Container-Managed EntityManager

You can inject a container-managed EntityManager (either transaction scoped or extended) using @javax.persistence.PersistenceContext annotation.

@javax.persistence.PersistenceContext

The following table defines the parameters/attributes of this annotation.

You can inject a transaction scoped EntityManager as follows:

@PersistenceContext(unitName="actionBazaar")

private EntityManager entityManager;

​

An extended scoped EntityManager can only used in a stateful session bean as in the following example:

@Stateful

@TransactionAttribute(TransactionAttributeType.NOT_SUPPORTED)

public class PlaceOrderBean implements PlaceOrder {

@PersistenceContext(unitName = "actionBazaar", type =

PersistenceContextType.EXTENDED)

EntityManager em;

​

The persistence-context-ref is similar to @PersistenceContext; it defines references to a container-managed entity manager.

<persistence-context-ref>

<persistence-conte

xt-ref-name>

A

ctionBazaar

</persistence-conte

xt-ref-name>

<persistence-unit-name>actionBazaar</persistence-unit-name>

<persistence-conte

xt-type>Transaction</persistence-context-type>

<injection-tar

get>

<injection-target-class>

ejb3inaction.example.buslogic.PlaceBidBean

</injection-target-class>

<injection-target-name>em</injection-target-name>

</injection-tar

get>

</persistence-context-ref>

​

Injecting EntityManagerFactory

@javax.persistence.PersistenceUnit

@Stateless

public class ItemManagerBean implements ItemManager {

@PersistenceUnit

private EntityManagerFactory entityManagerFactory;

private EntityManager entityManager;

@PostConstruct

public void initialize() {

entityManager = entityManagerFactory.createEntityManager();

}

}

​

The persistence-unit-ref is similar to @PersistenceUnit that is used to define references to a persistence unit (i.e., entity manager factory).

@javax.xml.ws.WebServiceRef

You can inject an endpoint interface as follows:

@WebServiceRef(TrackDeliveryService.class)

private TrackDeliverySEI deliveryService;

​

You can inject a service interface as follows:

@WebServiceRef

private TrackingService service;

​

service-ref

The service-ref XML element is used to specify dependency on a web service if you are using deployment descriptor. The following table contains only the elements that are used from EJB clients.

The detailed schema for the service-ref can be found online at java.sun.com/xml/ns/javaee/javaee_web_services_client_1_2.xsd.

The Spring Framework is one of the driving forces behind popularizing the POJO programming model and dependency injection. In this section we will examine how you can inject and use Spring POJOs into EJB 3 Session beans or Message Driven Beans.

The Spring framework provides factory classes based on the following abstract classes that you use to develop Spring-enabled EJBs.

To use a Spring factory class to access a Spring bean, your EJB 3 bean class must implement the onEjbCreate() method.

Below is the PlaceBidBean EJB 3 example transformed into a Spring-enabled stateless session bean. Here the PlaceBidBean acts as a fa"ade and delegates the actual business logic to the PlaceBidServiceBean. The PlaceBidServiceBean is a Spring POJO that may use the full power of the Spring framework.

@Stateless(name = "PlaceBid")

public class PlaceBidBean extends AbstractStatelessSessionBean

implements PlaceBid {

private BidServiceBean bidService;

public PlaceBidBean() {

}

protected void onEjbCreate() {

bidService =

(BidServiceBean) getBeanFactory().getBean("bidService");

}

public Long addBid(String userId, Long itemId, Double bidPrice) {

return bidService.addBid(userId, itemId, bidPrice);

}

​

When an EJB instance is created (when a client invokes an EJB), the onEjbCreate method is invoked automatically. A JNDI lookup is performed to obtain the path for the bean factory by using an environment entry named ejb/BeanFactoryPath. So you have to define it in the EJB deployment descriptor for the EJB:

<session>

<display-name>PlaceBid</display-name>

<ejb-name>PlaceBid</ejb-name>

<env-entry>

<env-entry-name>ejb/BeanFactoryPath</env-entry-name>

<env-entry-type>java.lang.String</env-entry-type>

<env-entry-value>/actionBazaar-service.xml</env-entry-value>

</env-entry>

</session>

​

Although EJB 3 made deployment descriptor optional there are a few cases where you still have to use it. In our example we"ve set the env-entry-value for the ejb/BeanFactoryPath environment variable at /actionBazaar-service.xml. So you have to package the EJB classes, Spring classes, and Spring configuration file into your ejb-jar package. The Spring bean factory (configuration file) defines the Spring beans. Refer to the Spring Configuration Refcard for details about Spring configuration.