 An EJB container can hold three major categories of beans:
Session Beans
Stateless Session Beans
Stateful Session Beans
Entity Beans
Message Driven Beans (MDBs or Message Beans)
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.
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 bean's state may be persisted, but access to the bean instance is limited to only one client.
Entity Beans are distributed objects having persistent state. The persistent state may or may not be managed by the bean itself. Beans in which their container manages the persistent state are said to be using Container-Managed Persistence (CMP), whereas beans that manage their own state are said to be using Bean-Managed Persistence (BMP).
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.
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.
EJBs are deployed in an EJB container within the application server. The specification describes how an EJB interacts with its container and how client code interacts with the container/EJB combination. The EJB classes used by applications are included in the javax.ejb package. (The javax.ejb.spi package is a service provider interface used only by EJB container implementations.)
With EJB 2.1 and earlier, each EJB had to provide a Java implementation class and two Java interfaces. The EJB container created instances of the Java implementation class to provide the EJB implementation. The Java interfaces were used by client code of the EJB.
The two interfaces, referred to as the Home and the Component interface, specified the signatures of the EJB's remote methods. The methods were split into two groups:
class methods - not tied to a specific instance, such as those used to create an EJB instance (factory method) or to find an existing entity EJB (see EJB Types, below). These were declared by the Home interface.
instance methods, i.e. methods tied to a specific instance. These are placed in the Component interface.
Because these are merely Java interfaces and not concrete classes, the EJB container must generate classes for these interfaces that will act as a proxy in the client. Client code invokes a method on the generated proxies, which in turn places the method arguments into a message and sends the message to the EJB server.
[edit] Remote Communication
The EJB specification requires that EJB containers support accessing the EJBs using RMI-IIOP. EJBs may be accessed from any CORBA application or provide Web Services.
[edit] Transactions
EJB containers must support both container managed ACID transactions and bean managed transactions. Container-managed transactions use a declarative syntax for specifying transactions in the deployment descriptor.
[edit] Events
JMS is used to send messages from the beans to client objects, to let clients receive asynchronous messages from these beans. MDB can be used to receive messages from client applications asynchronously using either a JMS Queue or a Topic.
[edit] Naming and Directory Services
Clients of the EJB locate the Home Interface implementation object using JNDI. The Home interface may also be found using the CORBA name service. From the home interface, client code can find entity beans, as well as create and delete existing EJBs.
[edit] Security
The EJB Container is responsible for ensuring the client code has sufficient access rights to an EJB.
[edit] Deploying EJBs
The EJB Specification also defines a mechanism that lets EJBs be deployed in a similar manner regardless of the specific EJB platform that is chosen. Information about how the bean should be deployed (such as the name of the Home or Remote interfaces, whether and how to store the bean in a database, etc.) are specified in the deployment descriptor.
The deployment descriptor is an XML document having an entry for each EJB to be deployed. This XML document specifies the following information for each EJB
Name of the Home interface
Java class for the Bean (business object)
Java interface for the Home interface
Java interface for the business object
Persistent store (only for Entity Beans)
Security roles and permissions
EJB containers from many vendors require more deployment information than that in the EJB specification. They will require the additional information as separate XML files, or some other configuration file format. An EJB platform vendor generally provides their own tools that will read this deployment descriptor, and possibly generate a set of classes that will implement the Home and Remote interfaces.
Since EJB3.0 (JSR 220), the XML descriptor is replaced by Java annotations setted in the Enterprise Bean implementation (at source level), although it is still possible to use an XML descriptor instead.
[edit] Version History
[edit] EJB 3.0, final release (2006-05-02)
JSR 220 - Major changes:
Annotations, alternative to deployment descriptors
[edit] EJB 2.1, final release (2003-11-24)
JSR 153 - Major changes:
Web service support (new): stateless session beans can be invoked over SOAP/HTTP. Also, an EJB can easily access a Web service using the new service reference.
EJB timer service (new): Event-based mechanism for invoking EJBs at specific times.
Message-driven beans accepts messages from sources other than JMS.
Message destinations (the same idea as EJB references, resource references, etc.) has been added.
EJB query language (EJB-QL) additions: ORDER BY, AVG, MIN, MAX, SUM, COUNT, and MOD.
XML schema is used to specify deployment descriptors, replaces DTDs
[edit] EJB 2.0, final release (2001-08-22)
JSR 19 - Major changes: Overall goals:
The standard component architecture for building distributed object-oriented business applications in Java.
Make it possible to build distributed applications by combining components developed using tools from different vendors.
Make it easy to write (enterprise) applications: Application developers will not have to understand low-level transaction and state management details, multi-threading, connection pooling, and other complex low-level APIs.
Will follow the Write Once, Run Anywhere - philosophy of Java. An enterprise Bean can be developed once, and then deployed on multiple platforms without recompilation or source code modification.
Address the development, deployment, and runtime aspects of an enterprise application’s life cycle.
Define the contracts that enable tools from multiple vendors to develop and deploy components that can interoperate at runtime.
Be compatible with existing server platforms. Vendors will be able to extend their existing products to support EJBs.
Be compatible with other Java APIs.
Provide interoperability between enterprise Beans and Java 2 Platform Enterprise Edition (J2EE) components as well as non-Java programming language applications.
Be compatible with the CORBA protocols (RMI-IIOP).
[edit] EJB 1.1, final release (1999-12-17)
Major changes: '* XML deployment descriptors'* Default JNDI contexts* RMIoverIIOP
Security - role driven, not method driven
Entity Bean support - mandatory, not optional
Goals for Release 1.1:
Provide better support for application assembly and deployment.
Specify in greater detail the responsibilities of the individual EJB roles.
[edit] EJB 1.0 (1998-03-24)
Announced at JavaOne 1998, Sun's third Java developers conference (March 24 through 27) Goals for Release 1.0:
Defined the distinct “EJB Roles” that are assumed by the component architecture.
Defined the client view of enterprise Beans.
Defined the enterprise Bean developer’s view.
Defined the responsibilities of an EJB Container provider and server provider; together these make up a system that supports the deployment and execution of enterprise Bea
related resource: home security
|
