Stateless sessions make database queries and updates robust by setting transaction boundaries at each business method. This stateless session bean example annotates a single business method with a SUPPORTS transaction attribute, marking the method as a read-only transaction boundary.

See also:

• The @TransactionAttribute reference.

Demo

A Hello, World example for EJB 3.0 is much simpler than for earlier versions of EJB. To implement the EJB you need to implement:

• A local interface
• The bean implementation

To configure Resin to be a server for the EJB you need to:

• Configure the ejb-server
• Configure the client

In this tutorial, a simple "Hello" EJB is created and deployed within Resin.

Files in this tutorial

Local Interface

The remote interface defines the client view of the bean. It declares all the business methods. Our only business method is the hello method.

package example;

public interface Hello {
  public String hello();
}

Bean Implementation

The second class for EJBs is the bean implementation class. It implements the functionality provided by the remote interface.

package example;

import static javax.ejb.TransactionAttributeType.SUPPORTS;

@javax.ejb.Stateless
public class HelloBean implements Hello {
  private String _greeting = "Default Hello";

  @javax.ejb.Resource
  public void setGreeting(String greeting)
  {
    _greeting = greeting;
  }

  @javax.ejb.TransactionAttribute(SUPPORTS)
  public String hello()
  {
    return _greeting;
  }
}

@Stateless

The @Stateless annotation marks the bean as a stateless session bean. Resin will create a stub implementing Hello and store it in JNDI at "java:comp/env/ejb/HelloBean".

The @Stateless annotation can have an optional name value which overrides the default name of "HelloBean".

@Inject

The @javax.ejb.Resource annotation tells Resin to lookup the greeting from JNDI when the session bean is created. The JNDI name will be java:comp/env/greeting.

In this example, the greeting is configured with an <env-entry> in the web.xml.

Alternate Dependency Injection

The EJB 3.0 draft spec's dependency injection is somewhat inflexible since the greeting is required to be in JNDI. Resin offers a more flexible dependency injection configuration based on the configuration file. By setting the value in the configuration file, Resin's alternate dependency injection adds more flexibility and some clarity.

<ejb-server jndi-name="java:comp/env/ejb">
  <bean type="qa.TestBean">
    <init greeting="Hello, World from web.xml"/>
  </bean>
</ejb-server>

@TransactionAttribute

Managing transactions is the primary purpose of stateless session beans. Transactions are a more powerful version of a synchronized lock used to protect database integrity. @TransactionAttribute marks the transaction boundary for each business method.

@javax.ejb.TransactionAttribute(SUPPORTS)
public String hello()

The hello() business method uses SUPPORTS because it's a read-only method. It doesn't need to start a new transaction on its own, but will participate in any transaction that already exists.

The REQUIRED transaction value starts up a new transaction if none already exists. It's used when updating database values.

Configuring the Resin EJB server

<ejb-server> configure the Resin EJB server. Typically it configures the EJB root using jndi-name and configures a number of EJB classes using <bean>. The <bean> entry will look at the bean's annotations to enhance the class.

<web-app xmlns="http://caucho.com/ns/resin">

  ...
  <env-entry env-entry-name="greeting"
             env-entry-type="java.lang.String"
             env-entry-value="Hello, World."/>

  <ejb-server jndi-name="java:comp/env/ejb">
    <bean type="qa.TestBean"/>
  </ejb-server>
    
  ...

</web-app>

The <bean> can optionally configure the bean instances with an <init> tag as described in the alternate dependency injection section.

Client

public class HelloServlet extends GenericServlet {
  private Hello _hello;

  @javax.ejb.EJB
  public void setHello(Hello hello)
  {
    _hello = hello;
  }

  public void service(HttpServletRequest req, HttpServletResponse res)
    throws IOException, ServletException
  {
    PrintWriter out = res.getWriter();
    
    out.println(_hello.hello());
  }
}

@EJB

The @EJB annotation tells Resin to lookup the session bean in JNDI with name "java:comp/env/ejb/HelloBean".

The servlet could also lookup the Hello bean with JNDI in the init() method or use an <init> configuration in the web.xml:

<servlet servlet-name="hello" servlet-class="example.HelloServlet">
  <init hello="\${jndi('java:comp/env/ejb/HelloBean')}"/>
</servlet>

Demo