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Latest revision as of 00:00, 16 June 2012

[edit] EJB Tutorials

[edit] Local Stateless Session Hello

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.

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-stateless-bean
Inject the bean into the application servlet

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

[edit] Files in this tutorial

File	Description
`WEB-INF/web.xml`	web.xml configuration
`WEB-INF/classes/example/Hello.java`	The local interface for the stateless session bean
`WEB-INF/classes/example/HelloBean.java`	The implementation for the stateless session bean
`WEB-INF/classes/example/HelloServlet.java`	The client for the stateless session bean

[edit] 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.

[edit] Hello.java

package example;

public interface Hello {
  public String hello();
}

[edit] Bean Implementation

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

[edit] HelloBean.java

package example;

import javax.ejb.Stateless;
import javax.ejb.TransactionAttribute;
import static javax.ejb.TransactionAttributeType.SUPPORTS;

import javax.inject.Named;

@Stateless
public class HelloBean implements Hello {
  @Inject @Named("greeting")
  private String _greeting;

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

[edit] @Stateless

The @Stateless annotation marks the bean as a stateless session bean. Resin will create a stub implementing Hello and store it in the Java Injection directory with type Hello and name @Name("HelloBean").

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

[edit] @Name

The @com.caucho.config.Name annotation tells Resin to lookup the greeting String in Java Injection directory using Resin's internal @Name binding "greeting" when the session bean is created.

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

[edit] Alternate Dependency Injection

In some cases, it may be clearer to configure the session bean directly, rather than using Java Injection injection. Instead of creating a separate <env-entry>, you can configure the greeting value using XML straight from the resin-web.xml file.

[edit] resin-web.xml

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

  <qa:TestBean xmlns:qa="urn:java:qa">
    <qa:greeting>Hello, World from web.xml</qa:greeting>
  </qa:TestBean>

</web-app>

[edit] @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. [doc|ejb-annotations.xtp#@TransactionAttribute @TransactionAttribute] marks the transaction boundary for each business method.

[edit] ==

@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.

TransactionAttribute	meaning
REQUIRED	Start a new transaction if necessary
SUPPORTS	Don't start a new transaction, but use one if it exists

[edit] Configuring the EJB stateless bean

<ee:Stateless> configure the session bean from the resin-web.xml. The <ee:Stateless> entry will look at the bean's annotations to enhance the class.

[edit] ejb-stateless-bean in web.xml

<web-app xmlns="http://caucho.com/ns/resin"
          xmlns:qa="urn:java:qa"
          xmlns:lang="urn:java:java.lang"
          xmlns:ee="urn:java:ee">

  <lang:String ee:Named="greeting">
    Hello, World
  </lang:String>

  <qa:TestBean>
    <ee:Stateless/>
  </qa:TestBean>
  
</web-app>

The <qa:TestBean> can optionally configure the bean directly with its properites as described in the alternate dependency injection section.

[edit] Client

[edit] HelloServlet.java

import javax.inject.Inject;

public class HelloServlet extends GenericServlet {
  @Inject private Hello _hello;

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

[edit] @EJB

The @Inject annotation tells Resin to look for a Hello component in the Java Injection repository.

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

[edit] alternative configuration

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

<servlet servlet-name="hello" servlet-class="example.HelloServlet">
  <init hello="${Hello}"/>
</servlet>

Revision as of 00:00, 15 June 2012 (view source) Rick (Talk \| contribs) (Created page with " =EJB Tutorials= =Local Stateless Session Hello= <p>Stateless sessions make database queries and updates robust by setting transaction boundaries at each business method. Th...")		Latest revision as of 00:00, 16 June 2012 (view source) Rick (Talk \| contribs)
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Resin 4 Java EE Application Server EJB Tutorial