| Local Stateless Session Hello |
| Tutorials |
| Security |
Find this tutorial in:
/usr/local/resin/webapps/resin-doc/ejb3/tutorial/stateless
Try the Tutorial
Stateless sessions make database queries and updates robust by setting transaction boundaries at each business method. This bean example annotates a single business method with a SUPPORTS transaction attribute, marking the method as a read-only transaction boundary.
See also:
- The Resin Transaction example for a more detailed description of method transaction enhancement.
- The CMP Transaction example for a traditional session bean pattern.
- The @TransactionAttribute reference.
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 |
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 |
| 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.Inject
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.Inject 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.
| TransactionAttribute | meaning |
|---|---|
| REQUIRED | Start a new transaction if necessary |
| SUPPORTS | Don't start a new transaction, but use one if it exists |
| 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:lookup('java:comp/env/ejb/HelloBean')}"/>
</servlet>
|
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