Enterprise JavaBean
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Enterprise Java Bean (EJB) is a managed, server-sided component for modular construction of enterprise applications.
The Enterprise JavaBeans specification is one of the several Java APIs in the Java Platform, Enterprise Edition. EJB is a server-side component that encapsulates the business logic of an application. The EJB specification was originally developed in 1997 by IBM and later adopted by Sun Microsystems (EJB 1.0 and 1.1) and enhanced under the Java Community Process as JSR 19 (EJB 2.0), JSR 153 (EJB 2.1) and JSR 220 (EJB 3.0).
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[edit] History
[edit] Rapid adoption followed by criticism
This vision was persuasively presented by EJB advocates such as IBM and Sun Microsystems, and Enterprise JavaBeans were quickly adopted by large companies. Problems were quick to appear, however, and the reputation of EJBs began to suffer as a result. For starters, the APIs of the standard were far more complex than what typical developers are used to. An abundance of checked exceptions, required interfaces, and the implementation of the bean class as an abstract class were all unusual and counter-intuitive for many programmers. Granted, the problems that the EJB standard was attempting to address, such as object-relational mapping and transactional integrity, are complex. However many programmers found the APIs to be just as difficult if not more so, leading to a widespread perception that EJBs introduced complexity without delivering real benefits.
In addition, businesses found that using EJBs to encapsulate business logic brought a performance penalty. This is because the original specification only allowed for remote method invocation through CORBA (and optionally other protocols), even though the large majority of business applications actually do not require this distributed computing functionality. The EJB 2.1 standard addressed this concern by adding the concept of Local interfaces which could be called directly without performance penalties by applications that were not distributed over multiple servers.
The complexity issue, however, continued to hinder EJB's acceptance. Although high-quality developer tools made it easy to create and use EJBs by automating most of the repetitive tasks, these tools did not make it any easier to learn how to use the technology. Moreover, a counter-movement had grown up on the grass-roots level among programmers. The main products of this movement were the so-called 'lightweight' (i.e. in comparison to EJB) technologies of Hibernate (for persistence and object-relational mapping) and Spring Framework (which provided an alternate and far less verbose way to encode business logic). Despite their lacking the backing of big businesses that Enterprise JavaBeans had, these technologies grew in popularity and were adopted more and more by businesses who had become disillusioned with Enterprise JavaBeans.
[edit] Reinventing EJBs
Gradually an industry consensus emerged that the original EJB specification's primary virtue--enabling transactional integrity over distributed applications--was not of use to the majority of enterprise applications. The functionality delivered by Spring and Hibernate was what enterprise applications needed. The EJB 3.0 specification was a radical departure from its predecessors, and illustrates this new understanding. It shows a clear influence from Spring in its use of POJOs, and its support for dependency injection to simplify configuration and integration of heterogeneous systems. Hibernate's influence is even more clear as Gavin King, the creator of Hibernate, participated in the EJB 3.0 process and is an outspoken advocate of the technology. Many features originally in Hibernate were incorporated in the Java Persistence API, the replacement for entity beans in EJB 3.0. The EJB 3.0 specification relies heavily on the use of annotations, a feature added to the Java language with its 5.0 release, to enable a much less verbose coding style.
Accordingly, although all versions of the EJB spec seek to deliver a similar functionality, in practical terms EJB 3.0 is very nearly a completely new API, bearing little resemblance to the previous EJB specifications.
[edit] EJB types
An EJB container can hold four 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 companies 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.
[edit] Execution Of EJB
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 Enterprise JavaBean 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:
- methods that were not tied to a specific instance, such as those used to create an EJB instance or to find an existing entity EJB (see EJB Types, below). These were declared by the Home interface.
- methods that were 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 enterprise Java bean 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 enterprise Java beans 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
- Java interface for the Home interface
- Java interface for the object
- Persistent store
- 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.
