Java J2ee Planet

An message-driven EJB is used to receive and process asynchronous messages using JMS. Message-driven EJBs are never directly invoked by other EJBs. However, they in turn can invoke methods of session and entity beans and send JMS messages to be processed by other message-driven EJBs. The topics listed below discuss development of message-driven beans. What are Message-Driven Beans? Message-driven beans are server-side objects used only to process JMS messages. These beans are stateless, in that each method invocation is independent from the next. Unlike session and entity beans, message-driven beans are not invoked by other beans or client applications. Instead a message-driven bean responds to a JMS message. Because message-driven beans are not invoked by other EJBs or clients, these beans do not have interfaces. For each message-driven bean a single method, onMessage , is defined to process a JMS message. Although message-driven beans cannot be invoked by other EJBs, the...

A Java Bean is defined as an instance of a class that contains private attributes (data), and getter & setter methods. If you have: private String myString; in your class, you should have the methods public String getMyString (); and public void setMyString (String settingString); defined in your code. Although, I have found that it is not absolutely nessary to have everything defined; just don't be surprised if something breaks! An EJB (Enterprise Java Bean) is much more complex They only reside in application servers that handle EJBs (Tomcat doesn't hold EJBs). There are 3 types of EJBs: 1) Session Usually contain some business logic. 2) Entity Usually interface with a data store (such as a database). 3) Message-Driven Receives messages from JMS

17)What’s the relationship between Enterprise JavaBeans component architecture and CORBA? The Enterprise JavaBeans specification is intended to support compliance with the range of CORBA standards, current and proposed. A Bean’s remote and home interfaces are RMI compliant, and thus can interact with CORBA objects via RMI/IIOP, Sun and IBM’s forthcoming adaptation of RMI that conforms with the CORBA-standard IIOP protocol. As a companion to the Enterprise JavaBeans specification, Sun Microsystems has defined a standard mapping from Enterprise Java Beans API to CORBA IDL. JTA, the transaction API prescribed by the Enterprise JavaBeans specification for bean-managed transactions, is designed to layer easily over the OMG OTS transaction standard. 18)What’s the relationship between Enterprise JavaBeans component architecture and XML technology? The two technologies are complementary: Enterprise JavaBeans defines a standard for portable business logic and XML technology defines a stand...

These topics show how to develop Enterprise JavaBeans. WebLogic Workshop provides you with the tools to make EJB development much easier, taking care of many implementation details for you and allowing you to focus on design. Note . WebLogic Workshop 8.1 supports the development, importing, and building of EJB2.0 compliant Enterprise JavaBeans. In addition, the WebLogic Platform supports the deployment only of EJB1.1 compliant Enterprise JavaBeans. Getting Started with EJB Project When you are building WebLogic platform applications, EJB project is the development environment for Enterprise JavaBeans. EJB project provides a number of tools that facilitate the development of the EJBs which encompass the business logic for your enterprise application. This topic provides an overview of EJBs and EJB project in platform applications. It includes the following sections: What is an Enterprise JavaBean? What is EJB Project? EJB Project and ejbgen Tags Building and Deploying E...

Session beans are used to execute business tasks for a client on the server. A session bean typically implements a certain kind of activity, such as ordering products or signing up for courses, and in executing the business rules typically invokes entity beans. For instance, ordering products is likely to involve stored information about products, customers, and credit cards, while signing up for courses is likely going to require invoking entity beans representing students and courses.   Stateful and Stateless There are two types of session beans, stateful and stateless. A stateful session bean maintains conversational state. In other words, a stateful session bean remembers the calling client application from one method to the next. For a stateful session bean, the results produced by one method might be co-dependent on the results of its prior methods invoked by the same client. A stateful session bean maintains this conversation with the client until the conversatio...

The following figure shows the life cycle of an entity bean. An entity bean has the following three states:   Does Not Exist . In this state, the bean instance simply does not exist. Pooled . When WebLogic server is first started, several bean instances are created and placed in the pool. A bean instance in the pooled state is not tied to particular data, that is, it does not correspond to a record in a database table . Additional bean instances can be added to the pool as needed, and a maximum number of instances can be set Ready . A bean instance in the ready state is tied to particular data, that is, it represents an instance of an actual business object. The various state transitions as well as the methods available during the various states are discussed below.

What are the design goals of the Enterprise JavaBeansTM architecture? The Enterprise JavaBeans specification defines a standard architecture for implementing the business logic of multi-tier applications as reusable components. In addition to Enterprise JavaBeans components, the architecture defines three other entities:servers,containers, and clients. This architecture incorporates several design goals: Enterprise JavaBeans servers are designed to wrap around legacy systems to provide fundamental services for "containers and the components they contain" Enterprise JavaBeans containers are designed to handle details of "component" life-cycle, transaction, and security management Component developers are free to focus on business logic, since containers provide services automatically by interceding in component method calls. A simple set of callback interfaces are all that a developer needs to implement to participate in container provided services. A client’s view o...