Enterprise JavaBeans (EJB) :Introduction

Enterprise JavaBeans (EJB) technology is the server-side component architecture for Java Platform, Enterprise Edition (Java EE). EJB technology enables rapid and simplified development of distributed, transactional, secure and portable applications based on Java technology.



The EJB specification intends to provide a standard way to implement the back-end 'business' code typically found in enterprise applications (as opposed to 'front-end' user-interface code). Such code was frequently found to reproduce the same types of problems, and it was found that solutions to these problems are often repeatedly re-implemented by programmers. Enterprise Java Beans were intended to handle such common concerns as persistence, transactional integrity, and security in a standard way, leaving programmers free to concentrate on the particular problem at hand.

EJB types
Stateful Session Beans are distributed objects having state: that is, they keep track of which calling program they are dealing with throughout a session. For example, checking out in a web store might be handled by a stateful session bean, which would use its state to keep track of where the customer is in the checkout process. On the other hand, sending an e-mail to customer support might be handled by a stateless bean, since this is a one-off operation and not part of a multi-step process. Stateful session beans' state may be persisted, but access to the bean instance is limited to only one client. Stateless Session Beans are distributed objects that do not have state associated with them thus allowing concurrent access to the bean. The contents of instance variables are not guaranteed to be preserved across method calls. The lack of overhead to maintain a conversation with the calling program makes them less resource-intensive than stateful beans.

Message Driven Beans were introduced in the EJB 2.0 specification. which is supported by Java 2 Platform, Enterprise Edition 1.3 or higher. The message bean represents the integration of JMS (Java Message Service) with EJB to create an entirely new type of bean designed to handle asynchronous JMS messages. Message Driven Beans are distributed objects that behave asynchronously. That is, they handle operations that do not require an immediate response. For example, a user of a website clicking on a "keep me informed of future updates" box may trigger a call to a Message Driven Bean to add the user to a list in the company's database. (This call is asynchronous because the user does not need to wait to be informed of its success or failure.) These beans subscribe to JMS (Java Message Service) message queues or message topics. They were added in the EJB 2.0 specification to allow event-driven processing inside EJB Container. Unlike other types of beans, MDB does not have a client view (Remote/Home interfaces), i.e. clients can not look-up an MDB instance. It just listens for any incoming message on a JMS queue (or topic) and processes them automatically.

Previous versions of EJB also used a type of bean known as an Entity Bean. These were distributed objects having persistent state. Beans in which their container managed the persistent state were said to be using Container-Managed Persistence (CMP), whereas beans that managed their own state were said to be using Bean-Managed Persistence (BMP). Entity Beans were replaced by the Java Persistence API in EJB 3.0, though as of 2007, CMP 2.x style Entity beans are still available for backward compatibility.

Other types of Enterprise Beans have been proposed. For instance, Enterprise Media Beans (JSR 86) address the integration of multimedia objects in Java EE applications.

Java Struts:ActionServlet,Action Class,ActionForm

ActionServlet:-The class org.apache.struts.action.ActionServlet is the called the ActionServlet. In the the Jakarta Struts Framework this class plays the role of controller. All the requests to the server goes through the controller. Controller is responsible for handling all the requests.
Action Class:-The Action is part of the controller. The purpose of Action Class is to translate the HttpServletRequest to the business logic. To use the Action, we need to Subclass and overwrite the execute() method. The ActionServlet (commad) passes the parameterized class to Action Form using the execute() method. There should be no database interactions in the action. The action should receive the request, call business objects (which then handle database, or interface with J2EE, etc) and then determine where to go next. Even better, the business objects could be handed to the action at runtime (IoC style) thus removing any dependencies on the model. The return type of the execute method is ActionForward which is used by the Struts Framework to forward the request to the file as per the value of the returned ActionForward object.
ActionForm:-An ActionForm is a JavaBean that extends org.apache.struts.action.ActionForm. ActionForm maintains the session state for web application and the ActionForm object is automatically populated on the server side with data entered from a form on the client side.

Java Struts:What is in MVC?

he Model portion of an MVC-based system can be often be divided into two major subsystems -- the internal state of the system and the actions that can be taken to change that state.

The View portion of a Struts-based application is most often constructed using JavaServer Pages (JSP) technology. JSP pages can contain static HTML (or XML) text called "template text", plus the ability to insert dynamic content based on the interpretation (at page request time) of special action tags. The JSP environment includes a set of standard action tags, such as whose purpose is described in the JavaServer Pages Specification. In addition to the built-in actions, there is a standard facility to define your own tags, which are organized into "custom tag libraries."

Struts provides the Controller portion of the application. The Controller is focused on receiving requests from the client (typically a user running a web browser), deciding what business logic function is to be performed, and then delegating responsibility for producing the next phase of the user interface to an appropriate View component. The primary component of the Controller in the framework is a servlet of class ActionServlet. This servlet is configured by defining a set of ActionMappings. An ActionMapping defines a path that is matched against the request URI of the incoming request and usually specifies the fully qualified class name of an Action class. All Actions are subclassed from [org.apache.struts.action.Action]. Actions encapsulate calls to business logic classes, interpret the outcome, and ultimately dispatch control to the appropriate View component to create the response. While the framework dispatches to a View, actually rendering the View is outside its scope.

Java Struts : Struts Libraries

Question: What are the various Struts tag libraries?
Answer: Struts is very rich framework and it provides very good and user friendly way to develop web application forms. Struts provide many tag libraries to ease the development of web applications. These tag libraries are:
* Bean tag library - Tags for accessing JavaBeans and their properties.
* HTML tag library - Tags to output standard HTML, including forms, text boxes, checkboxes, radio buttons etc..
* Logic tag library - Tags for generating conditional output, iteration capabilities and flow management
* Tiles or Template tag library - For the application using tiles
* Nested tag library - For using the nested beans in the application

Java Struts:Flow Diagram

Struts:Componants of Struts

What are the components of Struts?

Answer: Struts is based on the MVC design pattern. Struts components can be categories into Model, View and Controller.
Model: Components like business logic / business processes and data are the part of Model.
View: JSP, HTML etc. are part of View
Controller: Action Servlet of Struts is part of Controller components which works as front controller to handle all the requests.

Java Struts:MVC Architecture

What is Model-View-Controller (MVC) Architecture?

Model-View-Controller architecture is all about dividing application components into three different categories Model, View and the Controller. Components of the MVC architecture has unique responsibility and each component is independent of the other component. Changes in one component will have no or less impact on other component. Responsibilities of the components are:

Model : Model is responsible for providing the data from the database and saving the data into the data store. All the business logic are implemented in the Model. Data entered by the user through View are check in the model before saving into the database. Data access, Data validation and the data saving logic are part of Model.

View : View represents the user view of the application and is responsible for taking the input from the user, dispatching the request to the controller and then receiving response from the controller and displaying the result to the user. HTML, JSPs, Custom Tag Libraries and Resources files are the part of view component.

Controller : Controller is intermediary between Model and View. Controller is responsible for receiving the request from client. Once request is received from client it executes the appropriate business logic from the Model and then produce the output to the user using the View component. ActionServlet, Action, ActionForm and struts-config.xml are the part of Controller.

Java Struts:Introduction

1. What is Jakarta Struts Framework? - Jakarta Struts is open source implementation of MVC (Model-View-Controller) pattern for the development of web based applications. Jakarta Struts is robust architecture and can be used for the development of application of any size. Struts framework makes it much easier to design scalable, reliable Web applications with Java.