7
EJB Primer

After you have installed Oracle Application Server Containers for J2EE (OC4J) and configured the base server and default Web site, you can start developing J2EE applications. This chapter assumes that you have a working familiarity with simple J2EE concepts and a basic understanding for EJB development.

The following sections describe how to develop and deploy EJB applications with OC4J:

Develop EJBs--Developing and testing an EJB module within the standard J2EE specification.
Prepare the EJB Application for Assembly--Before deploying, you must modify an XML file that acts as a standard J2EE application descriptor file for the enterprise application.
Deploy the Enterprise Application to OC4J--Archive the enterprise Java application into an Enterprise ARchive (EAR) file and deploy it to OC4J.

This chapter uses a stateless session bean example to show you each development phase and deployment steps for an EJB. As an introduction to EJBs, a simple EJB with a basic OC4J-specific configuration is used. You can download the stateless session bean example from the OC4J sample code page on the OTN Web site.

For more information on EJBs in OC4J, see the Oracle Application Server Containers for J2EE Enterprise JavaBeans Developer's Guide.

Develop EJBs

You develop EJB components for the OC4J environment in the same way as in any other standard J2EE environment. Here are the steps to develop EJBs:

Create the Development Directory--Create a development directory for the enterprise application (as Figure 7-1 shows).
Implement the EJB--Develop your EJB with its home interfaces, component interfaces, and bean implementation.
Access the EJB--Develop the client to access the bean through the remote or local interface.
Create the Deployment Descriptor--Create the standard J2EE EJB deployment descriptor for all beans in your EJB application.
Archive the EJB Application--Archive your EJB files into a JAR file.

Create the Development Directory

Although you can develop your application in any manner, we encourage you to use consistent naming for locating your application easily. One method would be to implement your enterprise Java application under a single parent directory structure, separating each module of the application into its own subdirectory.

Our hello example was developed using the directory structure mentioned in "Creating the Development Directory". Notice in Figure 7-1 that the EJB and Web modules exist under the hello application parent directory and are developed separately in their own directory.

Figure 7-1 Hello Directory Structure

Text description of ejbprima.gif follows

Text description of the illustration ejbprima.gif

Note:

For EJB modules, the top of the module (ejb_module) represents the start of a search path for classes. As a result, classes belonging to packages are expected to be located in a nested directory structure beneath this point. For example, a reference to a package class 'myapp.Hello.class' is expected to be located in "...hello/ejb_module/myapp/Hello.class".

Implement the EJB

When you implement a session or entity EJB, create the following:

Note:

Message-driven beans have similar, but not the same, requirements as listed below. See the Oracle Application Server Containers for J2EE Enterprise JavaBeans Developer's Guide for more information.

The home interfaces for the bean. The home interface defines the create method for your bean. If the bean is an entity bean, it also defines the finder method(s) for that bean.
1. The remote home interface extends javax.ejb.EJBHome.
2. The local home interface extends javax.ejb.EJBLocalHome.
The component interfaces for the bean.
1. The remote interface declares the methods that a client can invoke remotely. It extends javax.ejb.EJBObject.
2. The local interface declares the methods that a collocated bean can invoke locally. It extends javax.ejb.EJBLocalObject.
The bean implementation includes the following:
1. The implementation of the business methods that are declared in the component interfaces.
2. The container callback methods that are inherited from either the javax.ejb.SessionBean or javax.ejb.EntityBean interfaces.
3. The ejb* methods that match the home interface create methods:
  - For stateless session beans, provide an ejbCreate method with no parameters.
  - For stateful session beans, provide an ejbCreate method with parameters matching those of the create method as defined in the home interfaces.
  - For entity beans, provide ejbCreate and ejbPostCreate methods with parameters matching those of the create method as defined in the home interfaces.

Creating the Home Interfaces

The home interfaces (remote and local) are used to create the bean instance; thus, they define the create method for your bean. Each type of EJB can define the create method in the following ways:

EJB Type	Create Parameters
Stateless Session Bean	Can have only a single `create` method, with no parameters.
Stateful Session Bean	Can have one or more `create` methods, each with its own defined parameters.
Entity Bean	Can have zero or more `create` methods, each with its own defined parameters. All entity beans must define one or more finder methods, where at least one is a `findByPrimaryKey` method.

For each create method, a corresponding ejbCreate method is defined in the bean implementation.

Remote Invocation

Any remote client invokes the EJB through its remote interface. The client invokes the create method that is declared within the remote home interface. The container passes the client call to the ejbCreate method--with the appropriate parameter signature--within the bean implementation. You can use the parameter arguments to initialize the state of the new EJB object.

The remote home interface must extend the javax.ejb.EJBHome interface.
All create methods must throw the following exceptions:
- javax.ejb.CreateException
- javax.ejb.EJBException or another RuntimeException

Example 7-1 Remote Home Interface for Session Bean

The following code sample illustrates a remote home interface for a session bean called HelloHome.

package hello;

import javax.ejb.*;
import java.rmi.*;

public interface HelloHome extends EJBHome
{
  public Hello create() throws CreateException, RemoteException;

}

Local Invocation

An EJB can be called locally from a client that exists in the same container. Thus, a collocated bean, JSP, or servlet invokes the create method that is declared within the local home interface. The container passes the client call to the ejbCreate method--with the appropriate parameter signature--within the bean implementation. You can use the parameter arguments to initialize the state of the new EJB object.

The local home interface must extend the javax.ejb.EJBLocalHome interface.
All create methods may throw the following exceptions:
- javax.ejb.CreateException
- javax.ejb.EJBException or another RuntimeException

Example 7-2 Local Home Interface for Session Bean

The following code sample shows a local home interface for a stateless session bean called HelloLocalHome.

package hello;

import javax.ejb.*;

public interface HelloLocalHome extends EJBLocalHome
{
 public HelloLocal create() throws CreateException, EJBException;

}

Creating the Component Interfaces

The component interfaces define the business methods of the bean that a client can invoke.

Creating the Remote Interface

The remote interface defines the business methods that a remote client can invoke. Here are the requirements for developing the remote interface:

The remote interface of the bean must extend the javax.ejb.EJBObject interface, and its methods must throw the java.rmi.RemoteException exception.
You must declare the remote interface and its methods as public for remote clients.
The remote interface, all its method parameters, and return types must be serializable. In general, any object that is passed between the client and the EJB must be serializable, because RMI marshals and unmarshals the object on both ends.
Any exception can be thrown to the client, as long as it is serializable. Runtime exceptions, including EJBException and RemoteException, are transferred back to the client as remote runtime exceptions.

Example 7-3 Remote Interface Example for Hello Session Bean

The following code sample shows a remote interface called Hello with its defined methods, each of which will be implemented in the stateless session bean.

package hello;

import javax.ejb.*;
import java.rmi.*;

public interface Hello extends EJBObject
{
  public String sayHello(String myName) throws RemoteException;
}

Creating the Local Interface

The local interface defines the business methods of the bean that a local (collocated) client can invoke.

The local interface of the bean must extend the javax.ejb.EJBLocalObject interface.
You declare the local interface and its methods as public.

Example 7-4 Local Interface for Hello Session Bean

The following code sample contains a local interface called HelloLocal with its defined methods, each of which will be implemented in the stateless session bean.

package hello;

import javax.ejb.*;

public interface HelloLocal extends EJBLocalObject
{
  public String sayHello(String myName) throws EJBException;
}

Implementing the Bean

The bean contains the business logic for your application. It implements the following methods:

The signature for each of these methods must match the signature in the remote or local interface, except that the bean does not throw the RemoteException. Since both the local and the remote interfaces use the bean implementation, the bean implementation cannot throw the RemoteException.
The lifecycle methods are inherited from the SessionBean or EntityBean interface. These include the ejb<Action> methods, such as ejbActivate, ejbPassivate, and so on.
The ejbCreate methods that correspond to the create method(s) that are declared in the home interfaces. The container invokes the appropriate ejbCreate method when the client invokes the corresponding create method.
Any methods that are private to the bean or package used for facilitating the business logic. This includes private methods that your public methods use for completing the tasks requested of them.

Example 7-5 Hello Session Bean Implementation

The following code shows the bean implementation for the hello example.

Note:

You can download this example on OTN from the OC4J sample code page.

package hello;

import javax.ejb.*;

public class HelloBean implements SessionBean
{
  public SessionContext ctx;

  public HelloBean()
  {    // constructor
  }

  public void ejbCreate() throws CreateException
  {   // when bean is created
  }

  public void ejbActivate()
  {    // when bean is activated
  }

  public void ejbPassivate()
  {    // when bean is deactivated
  }

  public void ejbRemove()
  {   // when bean is removed
  }

  public void setSessionContext(SessionContext ctx)
  {    this.ctx = ctx;
  }

  public void unsetSessionContext()
  {    this.ctx = null;
  }

  public String sayHello(String myName) throws EJBException
  {
    return ("Hello " + myName);
  }
}

Note:

You can download this example on OTN from the OC4J sample code page.

Access the EJB

All EJB clients perform the following steps to instantiate a bean, invoke its methods, and destroy the bean:

Look up the home interface through a JNDI lookup. Follow JNDI conventions for retrieving the bean reference, including setting up JNDI properties if the bean is remote to the client.
Narrow the returned object from the JNDI lookup to the home interface, as follows:
1. When accessing the remote interface, use the PortableRemoteObject.narrow method to narrow the returned object.
2. When accessing the local interface, cast the returned object with the local home interface type.
Create instances of the bean in the server through the returned object. Invoking the create method on the home interface causes a new bean to be instantiated and returns a bean reference.

Note:
For entity beans that are already instantiated, you can retrieve the bean reference through one of its finder methods.
Invoke business methods, which are defined in the component (remote or local) interface.

After you are finished, invoke the remove method. This will either remove the bean instance or return it to a pool. The container controls how to act on the remove method.

Important:

In order to access EJBs, the client-side must download oc4j_client.zip file from http://otn.oracle.com/ software/products/ias/devuse.html. Unzip the JAR into a directory that is in your CLASSPATH. This JAR contains the classes necessary for client interaction. If you download this JAR into a browser, you must grant certain permissions. See the "Granting Permissions" section of the Security chapter in the Oracle Application Server Containers for J2EE Enterprise JavaBeans Developer's Guide for a list of these permissions.

If the client is not on an Oracle Application Server installation, you must also download the optic.jar file, which is located in ORACLE_HOME/opmn/lib on your Oracle Application Server installation. This JAR file must be in the CLASSPATH.

Example 7-6 A Servlet Acting as a Local Client

The following example is executed from a servlet that is collocated with the Hello bean. Thus, the session bean uses the local interface, and the JNDI lookup does not require JNDI properties.

Note:

The JNDI name is specified in the <ejb-local-ref> element in the session bean EJB deployment descriptor as follows:

<ejb-local-ref> <ejb-ref-name>ejb/HelloBean</ejb-ref-name> <ejb-ref-type>Session</ejb-ref-type> <local-home>hello.HelloLocalHome</local-home> <local>hello.HelloLocal</local> </ejb-local-ref>

package hello;

import javax.servlet.http.*;
import javax.servlet.*;
import javax.ejb.*;
import javax.naming.*;
import java.io.IOException;

public class HelloServlet extends HttpServlet
{
  HelloLocalHome helloHome;
  HelloLocal hello;

  public void init() throws ServletException
  {
    try {
     // 1. Retreive the Home Interface using a JNDI Lookup
     // Retrieve the initial context for JNDI. 

     // No properties needed when local
      Context context = new InitialContext();

      // Retrieve the home interface using a JNDI lookup using
      // the java:comp/env bean environment variable 

      // specified in web.xml
      helloHome = (HelloLocalHome)

             context.lookup("java:comp/env/ejb/HelloBean");

     //2. Narrow the returned object to be an HelloHome object.

     // Since the client is local, cast it to the correct object type.
     //3. Create the local Hello bean instance, return the reference 
      hello = (HelloLocal)helloHome.create();

    } catch(NamingException e) {
      throw new ServletException("Error looking up home", e);
    } catch(CreateException e) {
      throw new ServletException("Error creating local hello bean", e);
    }
  }

  public void doGet(HttpServletRequest request, 

                      HttpServletResponse response)
    throws ServletException, IOException
  {
    response.setContentType("text/html");
    ServletOutputStream out = response.getOutputStream();
    try
    {
      out.println("<html>");
      out.println("<body>");
     //4. Invoke a business method on the local interface reference.
      out.println(hello.sayHello("James Earl"));
      out.println("</body>");
      out.println("</html>");
    } catch(EJBException e) {
      out.println("EJBException error: " + e.getMessage());
    } catch(IOException e) {
      out.println("IOException error: " + e.getMessage());
    } finally {
      out.close();
    }
  }
}

Note:

You can download this example on OTN from the OC4J sample code page.

Example 7-7 A Java Client as a Remote Client

The following example is executed from a pure Java client that is a remote client. Any remote client must set up JNDI properties before retrieving the object, using a JNDI lookup.

Note:

The JNDI name is specified in the <ejb-ref> element in the client's application-client.xml file--as follows:

<ejb-ref> <ejb-ref-name>ejb/HelloBean</ejb-ref-name> <ejb-ref-type>Session</ejb-ref-type> <home>hello.HelloHome</home> <remote>hello.Hello</remote> </ejb-ref>

The jndi.properties file for this client is as follows:

java.naming.factory.initial=

        com.evermind.server.ApplicationClientInitialContextFactory
java.naming.provider.url=opmn:ormi://opmnhost:oc4j_inst1/helloworld
java.naming.security.principal=admin
java.naming.security.credentials=welcome

The pure Java client that invokes Hello remotely is as follows:

package hello;

import javax.ejb.*;
import javax.naming.*;
import javax.rmi.PortableRemoteObject;
import java.io.*;
import java.util.*;
import java.rmi.RemoteException;

/*
 * A simple client for accessing an EJB.
 */

public class HelloClient
{
  public static void main(String[] args)
  {
    System.out.println("client started...");
    try {
      // Initial context properties are set in the jndi.properties file
      //1. Retrieve remote interface using a JNDI lookup*/
      Context context = new InitialContext();

     // Lookup the HelloHome object. The reference is retrieved from the
     // application-local context (java:comp/env). The variable is
     // specified in the application-client.xml).
      Object homeObject = context.lookup("java:comp/env/Helloworld");

     //2. Narrow the reference to HelloHome. Since this is a remote 

     //   object, use the PortableRemoteObject.narrow method.
      HelloHome home = (HelloHome) PortableRemoteObject.narrow

                                        (homeObject, HelloHome.class);

     //3. Create the remote object and narrow the reference to Hello.
      Hello remote =
        (Hello) PortableRemoteObject.narrow(home.create(), Hello.class);

     //4. Invoke a business method on the remote interface reference.
      System.out.println(remote.sayHello("James Earl"));

    } catch(NamingException e) {
      System.err.println("NamingException: " + e.getMessage());
    } catch(RemoteException e) {
      System.err.println("RemoteException: " + e.getMessage());
    } catch(CreateException e) {
      System.err.println("FinderException: " + e.getMessage());
    }
  }
}

Note:

You can download this example on OTN from the OC4J sample code page.

Create the Deployment Descriptor

After implementing and compiling your classes, you must create the standard J2EE EJB deployment descriptor for all beans in the module. The XML deployment descriptor (defined in the ejb-jar.xml file) describes the EJB module of the application. It describes the types of beans, their names, and attributes. The structure for this file is mandated in the DTD file, which is provided at " http://java.sun.com/dtd/ejb-jar_2_0.dtd".

Any EJB container services that you want to configure is also designated in the deployment descriptor. For information about data sources and JTA, see the Oracle Application Server Containers for J2EE Services Guide. For information about security, see the Oracle Application Server Containers for J2EE Security Guide.

After creation, place the deployment descriptors for the EJB application in the META-INF directory that is located in the same directory as the EJB classes. See Figure 7-1 for more information.

The following example shows the sections that are necessary for the Hello example, which implements both a remote and a local interface.

Example 7-8 XML Deployment Descriptor for Hello Bean

The following is the deployment descriptor for a version of the hello example that uses a stateless session bean. This example defines both the local and remote interfaces. You do not have to define both interface types; you may define only one of them.

<?xml version="1.0"?>
<!DOCTYPE ejb-jar PUBLIC "-//Sun Microsystems, Inc.//DTD Enterprise 
JavaBeans 1.1//EN" "http://java.sun.com/j2ee/dtds/ejb-jar_1_1.dtd">

<ejb-jar>
   <display-name>hello</display-name>
   <description>
      An EJB app containing only one Stateless Session Bean
   </description>
   <enterprise-beans>
      <session>
         <description>no description</description>
         <display-name>HelloBean</display-name>
         <ejb-name>HelloBean</ejb-name>
         <home>hello.HelloHome</home>
         <remote>hello.Hello</remote>
         <local-home>hello.HelloLocalHome</local-home>
         <local>hello.HelloLocal</local>
         <ejb-class>hello.HelloBean</ejb-class>
         <session-type>Stateless</session-type>
         <transaction-type>Container</transaction-type>
      </session>
   </enterprise-beans>

   <assembly-descriptor>
      <container-transaction>
         <method>
            <ejb-name>HelloBean</ejb-name>
            <method-name>*</method-name>
         </method>
         <trans-attribute>Supports</trans-attribute>
      </container-transaction>
      <security-role>
         <role-name>users</role-name>
      </security-role>
   </assembly-descriptor>
</ejb-jar>

Note:

You can download this example on OTN from the OC4J sample code page.

Archive the EJB Application

After you have finalized your implementation and created the deployment descriptors, archive your EJB application into a JAR file. The JAR file should include all EJB application files and the deployment descriptor.

Note:

If you have included a Web application as part of this enterprise Java application, follow the instructions for building the Web application in the Oracle Application Server Containers for J2EE User's Guide.

For example, to archive your compiled EJB class files and XML files for the Hello example into a JAR file, perform the following in the ../hello/ejb_module directory:

% jar cvf helloworld-ejb.jar .

This archives all files contained within the ejb_module subdirectory within the JAR file.

Prepare the EJB Application for Assembly

To prepare the application for deployment, you do the following:

Modify the application.xml file with the modules of the enterprise Java application.
Archive all elements of the application into an EAR file.

These steps are described in the following sections:

Modify the Application.xml File

The application.xml file acts as the standard J2EE application descriptor file for the application and contains a list of the modules that are included within your enterprise application. You use each <module> element defined in the application.xml file to designate what comprises your enterprise application. Each module describes one of three things: EJB JAR, Web WAR, or any client files. Respectively, designate the <ejb>, <web>, and <java> elements in separate <module> elements.

The <ejb> element specifies the EJB JAR filename.
The <web> element specifies the Web WAR filename in the <web-uri> element, and its context in the <context> element.
The <java> element specifies the client JAR filename, if any.

As Figure 7-2 shows, the application.xml file is located under a META-INF directory under the parent directory for the application. The JAR, WAR, and client JAR files should be contained within this directory. Because of this proximity, the application.xml file refers to the JAR and WAR files only by name and relative path--not by full directory path. If these files were located in subdirectories under the parent directory, then these subdirectories must be specified in addition to the filename.

Figure 7-2 Archive Directory Format

Text description of ejbprim2.gif follows

Text description of the illustration ejbprim2.gif

For example, the following example modifies the <ejb>, <web>, and <java> module elements within application.xml for the Hello EJB application that also contains a servlet that interacts with the EJB.

<?xml version="1.0"?>
<!DOCTYPE application PUBLIC "-//Sun Microsystems, Inc.//DTD J2EE 
Application 1.2//EN" "http://java.sun.com/j2ee/dtds/application_1_
2.dtd">
<application>
  <display-name>helloworld j2ee application</display-name>
  <description>
     A sample J2EE application that uses a Helloworld Session Bean
     on the server and calls from java/servlet/JSP clients.
  </description>
  <module>
    <ejb>helloworld-ejb.jar</ejb>
  </module>
  <module>
    <web>
      <web-uri>helloworld-web.war</web-uri>
      <context-root>/helloworld</context-root>
    </web>
  </module>
  <module>
    <java>helloworld-client.jar</java>
  </module>
</application>

Create the EAR File

Create the EAR file that contains the JAR, WAR, and XML files for the application. Note that the application.xml file serves as the EAR application descriptor file.

To create the helloworld.ear file, execute the following in the hello directory contained in Figure 7-2:

% jar cvf helloworld.ear .

This step archives the application.xml, the helloworld-ejb.jar, the helloworld-web.war, and the helloworld-client.jar files into the helloworld.ear file.

Deploy the Enterprise Application to OC4J

After archiving your application into an EAR file, deploy the application to OC4J. See "Deploying Applications" for information on how to deploy your application.

7EJB Primer

Develop EJBs

Create the Development Directory

Figure 7-1 Hello Directory Structure

Implement the EJB

Creating the Home Interfaces

Remote Invocation

Example 7-1 Remote Home Interface for Session Bean

Local Invocation

Example 7-2 Local Home Interface for Session Bean

Creating the Component Interfaces

Creating the Remote Interface

Example 7-3 Remote Interface Example for Hello Session Bean

Creating the Local Interface

Example 7-4 Local Interface for Hello Session Bean

Implementing the Bean

Example 7-5 Hello Session Bean Implementation

Access the EJB

Example 7-6 A Servlet Acting as a Local Client

Example 7-7 A Java Client as a Remote Client

Create the Deployment Descriptor

Example 7-8 XML Deployment Descriptor for Hello Bean

Archive the EJB Application

Prepare the EJB Application for Assembly

Modify the Application.xml File

Figure 7-2 Archive Directory Format

Create the EAR File

Deploy the Enterprise Application to OC4J

7
EJB Primer