Introduction
Application Overview
Code Snippet
Sample Application Files
Setting up the Sample Application
Additional References

Introduction

• Prerequisite
  
  
• Technical Overview

Prerequisite

To work your way through this Sample Application, a basic understanding of the fundamental concepts of servlets, JSP, JDBC and EJB(Session and Entity Bean) are required. For more information please refer Additional References.

Technical Overview

Servlets provide a component-based, platform-independent method for building Web-based applications. They are generally used for generating dynamic Web content. In this sample, the servlet gets a request from a client such as a Web browser, interacts with the EJB to process the request, returns the response back to the client and displays the retrieved information on the client machine.

Enterprise JavaBeans (EJB) is a specification created by Sun Microsystems that defines a framework for server-side Java components. Implementations of the EJB specification can typically be found in application servers, which support the creation of e-business solutions. EJB makes distributed object technology more accessible and easier to use by offering an higher level of abstraction and therefore more efficient for software development. To know more about EJB refer the Additional References section at the end.

Application Overview 

This sample shows how a servlet can communicate with an EJB component to process a user request. In the current sample user enters employee id, this id will be passed to ClientServlet servlet. ClientServlet will locate EmployeeSession Session bean. EmployeeSession Session bean will locate Employee Entity bean to fetch the employee information from the database and to create employeeDetails object. EmployeeDetail object is a Synchronised object, which can be used to encapsulate employee information and to transfer employee information over the network.

Employee Entity bean is local to the session bean, hence EmployeeSession bean locates local home of the Employee Entity bean. The employeeDetails object will be passed back to the session bean. The Session bean will further pass employeeDetails object to the ClientServlet servlet. Finally the ClientServlet servlet performs the responsibility to display the employee information to the client. Following figure shows the application architecture.

                                            
                                                                            Architecture Diagram

Code Snippet

• Calling Session bean from the servlet
• Calling Entity bean from the Session bean

Calling Session bean from the servlet

Servlet need to perform the following steps to call session bean's function.
a) Acquire a JNDI Initial context.
b) Locate the Home object using JNDI.
c) Use home object to creat a the remote object of the session bean.
d) Make a call to the getEmployeeDetails(...) function by using the remote object.

Following code snippet illustrates, how to call session bean's method from the servlet.

ClientServlet.getEmployeeDetails( ... )

 
    private EmployeeDetails getEmployeeDetails(String id)
                throws EmployeeException, Exception {
        // create the reference to EmployeeSession session bean
        EmployeeSession employeeMgr = null;

        // create the reference to EmployeeDetails object
        EmployeeDetails detail = null;

        // create the primary key
        EmployeePK pk = new EmployeePK(id);

        try {
            // obtain the JNDI initial context
            InitialContext ctx = new InitialContext();

            // get a reference to a home object - the factory to the EmployeeSession
            // remote object
            Object obj = ctx.lookup("java:comp/env/EmployeeSession");

            // Home object are RMI-IIOP object so they must be cast to  
            //RMI-IIOP object 
            EmployeeSessionHome home = (EmployeeSessionHome) PortableRemoteObject.narrow(obj,
                    EmployeeSessionHome.class);

            // use factory to create the remote object.
            employeeMgr = home.create();

            // call getEmployeeDetails(...) method on the EJB object. The EJB object 
            // will delegate the call to the bean, receive the employee details 
            // and set it to the detail reference variable.
            detail = employeeMgr.getEmployeeDetails(pk);
        } catch (EmployeeException ex) {
            // throw EmployeeException exception
            throw ex;
        } catch (Exception ex) {
            // throw Exception exception
            throw ex;
        }

        // return EmployeeDetails object
        return detail;
    }
    

Calling Entity bean from the Session bean

Now the session bean need to perform the following steps to call entity bean's function.
a) Acquire a JNDI Initial context.
b) Locate the LocalHome object of the Employee Entity bean using JNDI.
c) Use local home object to find and to creat the local object of the Employee Entity bean for the given employee id.
d) Make a call to the getEmployeeDetails(...) function by using the local object.

Following code snippet illustrates, how to call entity bean's method from the session bean.

EmployeeSessionBean.getEmployeeDetails( ... )

 
    public EmployeeDetails getEmployeeDetails(EmployeePK pk)
                throws RemoteException, EmployeeException, FinderException {
        // create the reference variable of EmployeeLocal and EmployeeDetails class.                                                
        EmployeeLocal employee = null;
        EmployeeDetails detail = null;

        try {
            //get the reference to the home object of Employee Entity bean.
            EmployeeLocalHome home = getEmployeeLocalHome();

            // find the employee for the given employee's primary key
            employee = home.findByPrimaryKey(pk);

            // get the employee details.
            detail = employee.getEmployeeDetails();
        } catch (FinderException ex) {
            // if there is no employee for the given primary key ,throw 
            // FinderException exception.
            throw new FinderException("Employee Id " + pk.toString() +
                " is not found");
        } catch (EmployeeException ex) {
            // throw EmployeeException exception
            throw new EmployeeException("Datasource in not located");
        } catch (Exception ex) {
            // throw EmployeeException exception
            ex.printStackTrace();
            throw new EmployeeException(ex.toString());
        }

        // return  employee details.
        return detail;
    }
    

Sample Application Files 

Readme file and Stylesheets

Java Server Page files for the Application

ANT build Script

Configuration files for the Application

Java Source files for the Application

JDeveloper project files.

Setting up the Sample Application 

Additional References

• Java Servlet Technology tutorial
• JavaServer Pages
• Java Servlet2.1 APIs Package Index
• Introduction to EJB's
• Sample code on EJB's

We hope you find this README file helpful. Please enter your comments in the OTN Sample Code Discussion Forum.