Enterprise JavaBean
Encyclopedia
Enterprise software
Enterprise software, also known as enterprise application software , is software used in organizations, such as in a business or government, contrary to software chosen by individuals...
s.
The EJB specification is one of several Java API
Application programming interface
An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...
s in the Java EE specification. EJB is a server-side
Server-side
Server-side refers to operations that are performed by the server in a client–server relationship in computer networking.Typically, a server is a software program, such as a web server, that runs on a remote server, reachable from a user's local computer or workstation...
model that encapsulates
Encapsulation
- Chemistry :* Molecular encapsulation, in chemistry, the confinement of an individual molecule within a larger molecule* Capsule , in pharmacy, the enclosure of a medicine within a relatively stable shell for administration...
the business logic
Business logic
Business logic, or domain logic, is a non-technical term generally used to describe the functional algorithms that handle information exchange between a database and a user interface.- Scope of business logic :Business logic:...
of an application. The EJB specification was originally developed in 1997 by IBM and later adopted by Sun Microsystems
Sun Microsystems
Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...
(EJB 1.0 and 1.1) in 1999 and enhanced under the Java Community Process
Java Community Process
The Java Community Process or JCP, established in 1998, is a formalized process that allows interested parties to get involved in the definition of future versions and features of the Java platform....
as JSR 19 (EJB 2.0), JSR 153 (EJB 2.1), JSR 220 (EJB 3.0) and JSR 318 (EJB 3.1).
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' interface code). Such code addresses the same types of problems, and solutions to these problems are often repeatedly re-implemented by programmers. Enterprise JavaBeans are intended to handle such common concerns as persistence
Persistence (computer science)
Persistence in computer science refers to the characteristic of state that outlives the process that created it. Without this capability, state would only exist in RAM, and would be lost when this RAM loses power, such as a computer shutdown....
, transactional integrity
Data integrity
Data Integrity in its broadest meaning refers to the trustworthiness of system resources over their entire life cycle. In more analytic terms, it is "the representational faithfulness of information to the true state of the object that the information represents, where representational faithfulness...
, and security
Computer security
Computer security is a branch of computer technology known as information security as applied to computers and networks. The objective of computer security includes protection of information and property from theft, corruption, or natural disaster, while allowing the information and property to...
in a standard way, leaving programmers free to concentrate on the particular problem at hand.
Accordingly, the EJB specification details how an application server
Application server
An application server is a software framework that provides an environment in which applications can run, no matter what the applications are or what they do...
provides:
- Transaction processingTransaction processingIn computer science, transaction processing is information processing that is divided into individual, indivisible operations, called transactions. Each transaction must succeed or fail as a complete unit; it cannot remain in an intermediate state...
- Integration with the PersistencePersistence (computer science)Persistence in computer science refers to the characteristic of state that outlives the process that created it. Without this capability, state would only exist in RAM, and would be lost when this RAM loses power, such as a computer shutdown....
services offered by the Java Persistence API (JPA)Java Persistence APIThe Java Persistence API, sometimes referred to as JPA, is a Java programming language framework managing relational data in applications using Java Platform, Standard Edition and Java Platform, Enterprise Edition.... - Concurrency controlConcurrency controlIn information technology and computer science, especially in the fields of computer programming , operating systems , multiprocessors, and databases, concurrency control ensures that correct results for concurrent operations are generated, while getting those results as quickly as possible.Computer...
- EventsEvent-driven programmingIn computer programming, event-driven programming or event-based programming is a programming paradigm in which the flow of the program is determined by events—i.e., sensor outputs or user actions or messages from other programs or threads.Event-driven programming can also be defined as an...
using Java Message ServiceJava Message ServiceThe Java Message Service API is a Java Message Oriented Middleware API for sending messages between two or more clients. JMS is a part of the Java Platform, Enterprise Edition, and is defined by a specification developed under the Java Community Process as JSR 914... - Naming and directory serviceDirectory serviceA directory service is the software system that stores, organizes and provides access to information in a directory. In software engineering, a directory is a map between names and values. It allows the lookup of values given a name, similar to a dictionary...
s (JNDIJava Naming and Directory InterfaceThe Java Naming and Directory Interface is a Java API for a directory service that allows Java software clients to discover and look up data and objects via a name. Like all Java APIs that interface with host systems, JNDI is independent of the underlying implementation...
) - SecurityComputer securityComputer security is a branch of computer technology known as information security as applied to computers and networks. The objective of computer security includes protection of information and property from theft, corruption, or natural disaster, while allowing the information and property to...
(Java Cryptography Extension (JCE) and JAASJava Authentication and Authorization ServiceJava Authentication and Authorization Service, or JAAS, pronounced "Jazz", is a Java security framework for user-centric security to augment the Java code-based security...
) - DeploymentSoftware deploymentSoftware deployment is all of the activities that make a software system available for use.The general deployment process consists of several interrelated activities with possible transitions between them. These activities can occur at the producer site or at the consumer site or both...
of software components in an application server - Remote procedure callRemote procedure callIn computer science, a remote procedure call is an inter-process communication that allows a computer program to cause a subroutine or procedure to execute in another address space without the programmer explicitly coding the details for this remote interaction...
s using RMI-IIOPRMI-IIOPRMI-IIOP denotes the Java Remote Method Invocation interface over the Internet Inter-Orb Protocol , which delivers Common Object Request Broker Architecture distributed computing capabilities to the Java 2 platform...
. - Exposing business methods as Web ServiceWeb serviceA Web service is a method of communication between two electronic devices over the web.The W3C defines a "Web service" as "a software system designed to support interoperable machine-to-machine interaction over a network". It has an interface described in a machine-processable format...
s - Asynchronous method invocationAsynchronous method invocationIn object-oriented programming, asynchronous method invocation , also known as asynchronous method calls or asynchronous pattern is a design pattern for asynchronous invocation of potentially long-running methods of an object.It is equivalent to the IOU pattern described in 1996 by Allan...
- Timer service
Additionally, the Enterprise JavaBean specification defines the roles played by the EJB container and the EJBs as well as how to deploy the EJBs in a container. Note that the current EJB 3.1 specification does not detail how an application server provides persistence (a task delegated to the JPA specification), but instead details how business logic can easily integrate with the persistence services offered by the application server.
To deploy and run EJB beans, a Java EE Application server
Application server
An application server is a software framework that provides an environment in which applications can run, no matter what the applications are or what they do...
can be used, as these include an EJB container by default. Alternatively, a standalone container such as OpenEJB can be used.
Rapid adoption followed by criticism
The vision was persuasively presented by EJB advocates such as IBMIBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
and Sun Microsystems
Sun Microsystems
Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...
, 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. Some developers felt that the APIs of the EJB standard were far more complex than those developers were used to. An abundance of checked exceptions, required interface
Interface (Java)
An interface in the Java programming language is an abstract type that is used to specify an interface that classes must implement. Interfaces are declared using the interface keyword, and may only contain method signature and constant declarations...
s, and the implementation of the bean class as an abstract class were all unusual and counter-intuitive for many programmers. The problems that the EJB standard was attempting to address, such as object-relational mapping
Object-relational mapping
Object-relational mapping in computer software is a programming technique for converting data between incompatible type systems in object-oriented programming languages. This creates, in effect, a "virtual object database" that can be used from within the programming language...
and transactional integrity, were 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
Çorba
Chorba , ciorbă , shurpa , shorpo , or sorpa is one of various kinds of soup or stew found in national cuisines across Middle East...
(and optionally other protocols), even though the large majority of business applications actually do not require this distributed computing
Distributed computing
Distributed computing is a field of computer science that studies distributed systems. A distributed system consists of multiple autonomous computers that communicate through a computer network. The computers interact with each other in order to achieve a common goal...
functionality. The EJB 2.0 specification 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. It was introduced largely to address the performance problems that existed with EJB 1.0.
The complexity issue, however, continued to hinder EJB's acceptance. Although 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
Hibernate (Java)
Hibernate is an object-relational mapping library for the Java language, providing a framework for mapping an object-oriented domain model to a traditional relational database...
(for persistence and object-relational mapping) and Spring Framework (which provided an alternate and far less verbose way to encode business logic). Despite lacking the support of big businesses, these technologies grew in popularity and were adopted more and more by businesses who had become disillusioned with EJBs.
EJBs were promoted by Sun's Java Pet Store demo Java BluePrints
Java BluePrints
Java BluePrints is Sun Microsystems' best practices for Enterprise Java development. This is Sun's official programming model for Java Platform, Enterprise Edition Software Development Kit . It began with Java Pet Store, the original reference application for the Java EE platform...
. The use of EJBs was controversial and influential Java EE programmers such as Rod Johnson took positions in response to Java Pet Store that sought to deemphasize EJB use. Sun itself produced an alternative called Java Data Objects. Later, EJBs, Java Data Forms, and many of the ideas underlying Hibernate were combined to form EJB 3.0 which included the Java Persistence API and Plain Old Java Objects (POJOs). EJB 3.0 was less heavy weight than EJB 2.0 and provided more choices to developers.
Reinventing EJBs
Gradually an industry consensus emerged that the original EJB specification's primary virtue — enabling transactional integrity over distributed applications — was of limited use to most enterprise applications, and the functionality delivered by simpler frameworks like Spring and HibernateHibernate (Java)
Hibernate is an object-relational mapping library for the Java language, providing a framework for mapping an object-oriented domain model to a traditional relational database...
was more useful. Accordingly, the EJB 3.0 specification (JSR
Java Community Process
The Java Community Process or JCP, established in 1998, is a formalized process that allows interested parties to get involved in the definition of future versions and features of the Java platform....
220) was a radical departure from its predecessors, following this new paradigm. It shows a clear influence from Spring in its use of POJO
Pojo
Pojo may refer to:* Pohja, the Swedish name for the Finnish municipality* POJO, abbreviation of Plain Old Java Object in computer programming...
s, and its support for dependency injection
Dependency injection
Dependency injection is a design pattern in object-oriented computer programming whose purpose is to improve testability of, and simplify deployment of components in very large software systems....
to simplify configuration and integration of heterogeneous systems. 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
Java Persistence API
The Java Persistence API, sometimes referred to as JPA, is a Java programming language framework managing relational data in applications using Java Platform, Standard Edition and Java Platform, Enterprise Edition....
, the replacement for entity beans
Entity Bean
An Entity Bean is a type of Enterprise JavaBean, a server-side J2EE component, that represents persistent data maintained in a database. An entity bean can manage its own persistence or can delegate this function to its EJB Container . An entity bean is identified by a primary key...
in EJB 3.0. The EJB 3.0 specification relies heavily on the use of annotations
Java annotation
An annotation, in the Java computer programming language, is a special form of syntactic metadata that can be added to Java source code. Classes, methods, variables, parameters and packages may be annotated...
, a feature added to the Java language with its 5.0 release and Convention over configuration
Convention over Configuration
Convention over configuration is a software design paradigm which seeks to decrease the number of decisions that developers need to make, gaining simplicity, but not necessarily losing flexibility....
, to enable a much less verbose coding style.
Accordingly, in practical terms EJB 3.0 is very nearly a completely new API, bearing little resemblance to the previous EJB specifications.
Example
The following shows a basic example of what an EJB looks like in code:@Local
public interface CustomerServiceLocal {
void addCustomer(Customer customer);
}
@Stateless
public class CustomerService implements CustomerServiceLocal {
@PersistenceContext
private EntityManager entityManager;
public void addCustomer(Customer customer) {
entityManager.persist(customer);
}
}
The above defines a local interface and the implementation of a service class for persisting a Customer object (via O/R mapping). The EJB takes care of managing the persistence context and the addCustomer method is transactional and thread-safe by default. As demonstrated, the EJB focuses only on business logic and persistence and knows nothing about any particular presentation.
Such an EJB can be used by a class in e.g. the web layer as follows:
@Named
@RequestScoped
public class CustomerBacking {
@EJB
private CustomerServiceLocal customerService;
public String addCustomer {
customerService.addCustomer(customer);
context.addMessage(...); // abbreviated for brevity
return "customer_overview";
}
}
The above defines a JavaServer Faces
JavaServer Faces
JavaServer Faces is a Java-based Web application framework intended to simplify development integration of web-based user interfaces....
(JSF) backing bean in which the EJB is injected by means of the @EJB annotation. Its addCustomer method is typically bound to some UI component, like a button. Contrary to the EJB, the backing bean does not contain any business logic or persistence code, but delegates such concerns to the EJB. The backing bean does know about a particular presentation, of which the EJB had no knowledge.
Types
An EJB container holds two major types of beans:- Session Beans that can be either "Stateful", "Stateless" or "Singleton" and can be accessed via either a Local (same JVM) or Remote (different JVM) interface or directly without an interface, in which case local semantics apply. All session beans support asynchronous execution for all views (local/remote/no-interface).
- Message Driven Beans (also known as MDBs or Message Beans). MDBs also support asynchronous execution, but via a messaging paradigm.
Stateful Session Beans
Stateful Session Beans are business objects having stateState (computer science)
In computer science and automata theory, a state is a unique configuration of information in a program or machine. It is a concept that occasionally extends into some forms of systems programming such as lexers and parsers....
: that is, they keep track of which calling client they are dealing with throughout a session and thus access to the bean instance is strictly limited to only one client at a time. In case concurrent access to a single bean is attempted anyway the container serializes those requests, but via the @AccessTimeout annotation the container can throw an exception instead. Stateful session beans' state may be persisted (passivated) automatically by the container to free up memory after the client hasn't accessed the bean for some time. The JPA extended persistence context is explicitly supported by Stateful Session Beans.
Examples:
- Checking out in a web store might be handled by a stateful session bean that would use its state to keep track of where the customer is in the checkout process, possibly holding locks on the items the customer is purchasing (from a system architecture's point of view, it would be less ideal to have the client manage those locks)
Stateless Session Beans
Stateless Session Beans are business objects that do not have state associated with them. However, access to a single bean instance is still limited to only one client at a time and thus concurrentConcurrent computing
Concurrent computing is a form of computing in which programs are designed as collections of interacting computational processes that may be executed in parallel...
access to the bean is prohibited. In case concurrent access to a single bean is attempted anyway the container simply routes each request to a different instance. This makes a stateless session bean automatically thread-safe. Instance variables can be used during a single method call from a client to the bean, but the contents of those instance variables are not guaranteed to be preserved across different client method
Method (computer science)
In object-oriented programming, a method is a subroutine associated with a class. Methods define the behavior to be exhibited by instances of the associated class at program run time...
calls. Instances of Stateless Session beans are typically pooled. If a second client accesses a specific bean right after a method call on it made by a first client has finished, it might get the same instance. The lack of overhead to maintain a conversation with the calling client makes them less resource-intensive than stateful beans.
Examples:
- 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.
- A user of a website clicking on a "keep me informed of future updates" box may trigger a call to an asynchronous method of the session 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).
- Fetching multiple independent pieces of data for a website, like a list of products and the history of the current user might be handled by asynchronous methods of a session bean as well (these calls are asynchronous because they can execute in parallelParallel computingParallel computing is a form of computation in which many calculations are carried out simultaneously, operating on the principle that large problems can often be divided into smaller ones, which are then solved concurrently . There are several different forms of parallel computing: bit-level,...
that way, which potentially increases performance). In this case, the asynchronous method will return a Future instance.
Singleton Session Beans
Singleton Session Beans are business objects having a global shared state within a JVM. Concurrent access to the one and only bean instance can be controlled by the container (Container-managed concurrency, CMC) or by the bean itself (Bean-managed concurrency, BMC). CMC can be tuned using the @Lock annotation, that designates whether a read lock or a write lock will be used for a method call. Additionally, Singleton Session Beans can explicitly request to be instantiated when the EJB container starts up, using the @Startup annotation.Examples:
- Loading a global daily price list that will be the same for every user might be done with a singleton session bean, since this will prevent the application having to do the same query to a database over and over again.
Message driven beans
Message Driven Beans are business objects whose execution is triggered by messages instead of by method calls. The Message Driven Bean is used among others to provide a high level ease-of-use abstraction for the lower level JMS (Java Message ServiceJava Message Service
The Java Message Service API is a Java Message Oriented Middleware API for sending messages between two or more clients. JMS is a part of the Java Platform, Enterprise Edition, and is defined by a specification developed under the Java Community Process as JSR 914...
) specification. It may subscribe to JMS message queues or message topics, which typically happens via the activationConfig attribute of the @MessageDriven annotation. They were added in EJB to allow event-driven processing. Unlike session beans, an MDB does not have a client view (Local/Remote/No-interface), i. e. clients can not look-up an MDB instance. It just listens for any incoming message on, for example, a JMS queue or topic and processes them automatically. Only JMS support is required by the Java EE spec, but Message Driven Beans can support other messaging protocols. Such protocols may be asynchronous but can also be synchronous. Since session beans can also be synchronous or asynchronous, the prime difference between session- and message driven beans is not the synchronicity, but the difference between (object oriented) method
Method (computer science)
In object-oriented programming, a method is a subroutine associated with a class. Methods define the behavior to be exhibited by instances of the associated class at program run time...
calling
Calling convention
In computer science, a calling convention is a scheme for how subroutines receive parameters from their caller and how they return a result; calling conventions can differ in:...
and messaging
Message passing
Message passing in computer science is a form of communication used in parallel computing, object-oriented programming, and interprocess communication. In this model, processes or objects can send and receive messages to other processes...
.
Examples:
- Sending a configuration update to multiple nodes might be done by sending a JMS message to a 'message topic' and could be handled by a Message Driven Bean listening to this topic (the message paradigm is used here, since the sender does not need to have knowledge about the amount of consumers or their location or even their exact type).
- Submitting a job to a work cluster might be done by sending a JMS message to a 'message queue' and could also be handled by a Message Driven Bean, but this time listening to a queue (the message paradigm and the queue is used, since the sender doesn't have to care which worker executes the job, but it does need assurance that a job is only executed once).
- Processing timing events from the Quartz scheduler can be handled by a Message Driven Bean; when a Quartz trigger fires, the MDB is automatically invoked. Since Java EE doesn't know about Quartz by default, a JCAJava EE Connector ArchitectureJava EE Connector Architecture is a Java-based technology solution for connecting application servers and enterprise information systems as part of enterprise application integration solutions. While JDBC is specifically used to connect Java EE applications to databases, JCA is a more generic...
resource adapter would be needed and the MDB would be annotated with a reference to this.
Entity beans (deprecated)
Previous versions of EJB also used a type of bean known as an Entity BeanEntity Bean
An Entity Bean is a type of Enterprise JavaBean, a server-side J2EE component, that represents persistent data maintained in a database. An entity bean can manage its own persistence or can delegate this function to its EJB Container . An entity bean is identified by a primary key...
. 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). In EJB 3.0, Entity Beans were replaced by the Java Persistence API
Java Persistence API
The Java Persistence API, sometimes referred to as JPA, is a Java programming language framework managing relational data in applications using Java Platform, Standard Edition and Java Platform, Enterprise Edition....
, which was completely separated to its own specification to allow the EJB specification to focus only on the "core session bean and message-driven bean component models and their client API". Entity Beans are still available in EJB 3.1 for backwards compatibility, but they have been officially proposed to be removed from the specification (via a process called "pruning").
Other types of Enterprise Beans have been proposed. For instance, Enterprise Media Bean
Enterprise Media Bean
Enterprise Media Beans address the integration of multimedia objects in Java EE applications. Objects can be images and streaming media located on external servers.-External links:* Enterprise JavaBeans* * *...
s (JSR 86) address the integration of multimedia
Multimedia
Multimedia is media and content that uses a combination of different content forms. The term can be used as a noun or as an adjective describing a medium as having multiple content forms. The term is used in contrast to media which use only rudimentary computer display such as text-only, or...
objects in Java EE applications.
Execution
EJBs are deployed in an EJB container, typically but not necessarily, within an application serverApplication server
An application server is a software framework that provides an environment in which applications can run, no matter what the applications are or what they do...
. 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 package. (The package is a service provider interface
Service provider interface
Service Provider Interface is a software mechanism to support replaceable components.It is the implementer-side equivalent of an API; a set of hooks that can or must be overridden....
used only by EJB container implementations.)
Clients of EJB beans do not instantiate those beans directly via Java's new operator, but instead have to obtain a reference via the EJB container. This reference is then not a reference to the implementation bean itself, but to a proxy
Proxy pattern
In computer programming, the proxy pattern is a software design pattern.A proxy, in its most general form, is a class functioning as an interface to something else...
, which either dynamically implements the local or remote business interface that the client requested or dynamically implements a sub-type of the actual bean. The proxy can then be directly cast to the interface or bean. A client is said to have a 'view' on the EJB, and the local interface, remote interface and bean type itself respectively correspond with the local view, remote view and no-interface view.
This proxy is needed in order to give the EJB container the opportunity to transparently provide cross-cutting (AOP
Aspect-oriented programming
In computing, aspect-oriented programming is a programming paradigm which aims to increase modularity by allowing the separation of cross-cutting concerns...
-like) services to a bean like transactions, security, interceptions, injections, remoting, etc.
E.g. a client invokes a method on a proxy, which will then first start a transaction with the help of the EJB container and then call the actual bean method. When the actual bean method returns, the proxy ends the transaction (i.e. by committing it or doing a rollback) and transfers control back to the client.
Transactions
EJB containers must support both container managed ACIDACID
In computer science, ACID is a set of properties that guarantee database transactions are processed reliably. In the context of databases, a single logical operation on the data is called a transaction...
transactions and bean managed transactions.
Container-managed transactions (CMT) are by default active for calls to session beans. That is, no explicit configuration is needed. This behavior may be declaratively tuned by the bean via annotations and if needed such configuration can later be overridden in the deployment descriptor. Tuning includes switching off transactions for the whole bean or specific methods, or requesting alternative strategies for transaction propagation and starting or joining a transaction. Such strategies mainly deal with what should happen if a transaction is or isn't already in progress at the time the bean is called. The following variations are supported :
| Type | Explanation |
|---|---|
| MANDATORY | If the client has not started a transaction, an exception is thrown. Otherwise the client's transaction is used. |
| REQUIRED | If the client has started a transaction, it is used. Otherwise a new transaction is started. (this is the default when no explicit type has been specified) |
| REQUIRES_NEW | If the client has started a transaction, it is suspended. A new transaction is always started. |
| SUPPORTS | If the client has started a transaction, it is used. Otherwise, no transaction is used. |
| NOT_SUPPORTED | If the client has started a transaction, it is suspended. No new transaction is started. |
| NEVER | If the client has started a transaction, an exception is thrown. No new transaction is started. |
Alternatively, the bean can also declare via an annotation that it wants to handle transactions programmatically via the JTA
Java Transaction API
The Java Transaction API is one of the Java Enterprise Edition APIs allowing distributed transactions to be done across multiple XA resources in a Java environment. JTA is a specification developed under the Java Community Process as JSR 907...
API. This mode of operation is called Bean Managed Transactions (BMT), since the bean itself handles the transaction instead of the container.
Events
JMS (Java Message Service) is used to send messages from beans to clients, to let clients receive asynchronous messages from these beans.MDBs can be used to receive messages from clients asynchronously using either a JMS
JMS
- Buildings :*EverBank Field, a sports stadium in Jacksonville, Florida, home of the Jacksonville Jaguars. Formerly known as Jacksonville Municipal Stadium...
Queue or a Topic.
Naming and directory services
As an alternative to injection, clients of an EJB can obtain a reference to the session bean's proxy object (the EJB stub) using JNDI. This alternative can be used in cases where injection is not available, such as non-managed beans or standalone remote Java SE clients, or when it's necessary to programmatically determine which bean to obtain.JNDI names for EJB session beans are assigned by the EJB container via the following scheme :
| Scope | Name pattern |
|---|---|
| Global | java:global[/ |
| Application | java:app/ |
| Module | java:module/ |
(entries in square brackets denote optional parts)
A single bean can be obtained by any name matching the above patterns, depending on the 'location' of the client. Clients in the same module as the required bean can use the module scope and larger scopes, clients in the same application as the required bean can use the app scope and higher, etc.
E.g. code running in the same module as the CustomerService bean (as given by the example shown earlier in this article) would use the following code to obtain a (local) reference to it:
CustomerServiceLocal customerService =
(CustomerServiceLocal) new InitialContext.lookup("java:module/CustomerService");
Remoting/distributed execution
EJB session beans have elaborate support for remoting.For communication with a client that's also written in the Java programming language a session bean can expose a remote-view via an @Remote annotated interface. This allows those beans to be called from clients in other JVMs which themselves may be located on other (remote) systems. From the point of view of the EJB container, any code in another JVM is remote.
Stateless- and Singleton session beans may also expose a "web service client view" for remote communication via WSDL and SOAP
SOAP
SOAP, originally defined as Simple Object Access Protocol, is a protocol specification for exchanging structured information in the implementation of Web Services in computer networks...
or plain XML. This follows the JAX-RPC
JAX-RPC
Java API for XML-based RPC allows a Java application to invoke a Java-based Web Service with a known description while still being consistent with its WSDL description. It can be seen as Java RMIs over Web services. JAX-RPC 2.0 was renamed JAX-WS 2.0 . JAX-RPC 1 is deprecated with Java EE 6...
and JAX-WS
JAX-WS
The Java API for XML Web Services is a Java programming language API for creating web services. It is part of the Java EE platform from Sun Microsystems. Like the other Java EE APIs, JAX-WS uses annotations, introduced in Java SE 5, to simplify the development and deployment of web service clients...
specifications. JAX-RPC support however is proposed for future removal. To support JAX-WS, the session bean is annotated with the @WebService annotation, and methods that are to be exposed remotely with the @WebMethod annotation.
Although the EJB specification does not mention exposure as RESTful web services in any way and has no explicit support for this form of communication, the JAX-RS
JAX-RS
JAX-RS: Java API for RESTful Web Services is a Java programming language API that provides support in creating web services according to the Representational State Transfer architectural style...
specification does explicitly support EJB. Following the JAX-RS spec, Stateless- and Singleton session beans can be root resources via the @Path annotation and EJB business methods can be mapped to resource methods via the @GET, @PUT, @POST and @DELETE annotations. This however does not count as a "web service client view", which is used exclusively for JAX-WS and JAX-RPC.
Communication via web services is typical for clients not written in the Java programming language, but is also convenient for Java clients who have trouble reaching the EJB server via a firewall. Additionally, web service based communication can be used by Java clients to circumvent the arcane and ill-defined requirements for the so-called "client-libraries"; a set of jar files that a Java client must have on its class-path in order to communicate with the remote EJB server. These client-libraries potentially conflict with libraries the client may already have (for instance, if the client itself is also a full Java EE server) and such a conflict is deemed to be a very hard or impossible to resolve.
Message Driven Beans have no specific support for remoting, but being listeners to end-points (e.g. JMS queues) they are implicitly remote components by virtue of the properties of whatever type of end-point they are listening to.
Security
The EJB Container is responsible for ensuring the client code has sufficient access rights to an EJB. Security aspects can be declaratively applied to an EJB bean via annotations.Home interfaces and required business interface
With EJB 2.1 and earlier, each EJB had to provide a Java implementation classClass (computer science)
In object-oriented programming, a class is a construct that is used as a blueprint to create instances of itself – referred to as class instances, class objects, instance objects or simply objects. A class defines constituent members which enable these class instances to have state and behavior...
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 Remote 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
Factory method pattern
The factory method pattern is an object-oriented design pattern to implement the concept of factories. Like other creational patterns, it deals with the problem of creating objects without specifying the exact class of object that will be created.The creation of an object often requires complex...
) or to find an existing entity EJB (see EJB Types, above). These were declared by the Home interface.
Instance methods : These are methods tied to a specific instance. They are placed in the Remote interface.
Required deployment descriptor
With EJB 2.1 and earlier, the EJB specification required a deployment descriptor to be present. This was needed to implement a mechanism that allowed EJBs to be deployedSoftware deployment
Software deployment is all of the activities that make a software system available for use.The general deployment process consists of several interrelated activities with possible transitions between them. These activities can occur at the producer site or at the consumer site or both...
in a consistent manner regardless of the specific EJB platform that was 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.) had to be specified in the deployment descriptor.
The deployment descriptor
Deployment Descriptor
A deployment descriptor refers to a configuration file for an artifact that is deployed to some container/engine.In the Java Platform, Enterprise Edition, a deployment descriptor describes how a component, module or application should be deployed...
is an XML
XML
Extensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....
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
- Stateful or Stateless (for Session Beans)
Old EJB containers from many vendors required more deployment information than that in the EJB specification. They would require the additional information as separate XML files, or some other configuration file format. An EJB platform vendor generally provided their own tools that would read this deployment descriptor, and possibly generated a set of classes that would implement the now deprecated Home and Remote interfaces.
Since EJB 3.0 (JSR 220), the XML descriptor is replaced by Java annotation
Java annotation
An annotation, in the Java computer programming language, is a special form of syntactic metadata that can be added to Java source code. Classes, methods, variables, parameters and packages may be annotated...
s set in the Enterprise Bean implementation (at source level), although it is still possible to use an XML descriptor instead of (or in addition to) the annotations. If an XML descriptor and annotations are both applied to the same attribute within an Enterprise Bean, the XML definition overrides the corresponding source-level annotation, although some XML elements can also be additive (e.g., an activation-config-property in XML with a different name than already defined via an @ActivationConfigProperty annotation will be added instead of replacing all existing properties).
Container variations
Starting with EJB 3.1, the EJB specification defines two variants of the EJB container; a full version and a limited version. The limited version adheres to a proper subset of the specification called EJB 3.1 Lite and is part of Java EE 6's web profile (which is itself a subset of the full Java EE 6 specification).EJB 3.1 Lite excludes support for the following features :
- Remote interfaces
- RMI-IIOP Interoperability
- JAX-WS Web Service Endpoints
- EJB Timer Service (@Schedule, @Timeout)
- Asynchronous session bean invocations (@Asynchronous)
- Message-driven beans
EJB 3.1, final release (2009-12-10)
JSR 318. The purpose of the Enterprise JavaBeans 3.1 specification is to further simplify the EJB architecture by reducing its complexity from the developer's point of view, while also adding new functionality in response to the needs of the community:- Local view without interface (No-interface view)
- .war packaging of EJB components
- EJB Lite: definition of a subset of EJB
- Portable EJB Global JNDI Names
- Singletons (Singleton Session Beans)
- Application Initialization and Shutdown Events
- EJB Timer Service Enhancements
- Simple Asynchrony (@Asynchronous for session beans)
EJB 3.0, final release (2006-05-11)
JSR 220 - Major changes:This release made it much easier to write EJBs, using 'annotations' rather than the complex 'deployment descriptors' used in version 2.x. The use of home and remote interfaces and the ejb-jar.xml file were also no longer required in this release, having been replaced with a business interface and a bean that implements the interface.
EJB 2.1, final release (2003-11-24)
JSR 153 - Major changes:- Web serviceWeb serviceA Web service is a method of communication between two electronic devices over the web.The W3C defines a "Web service" as "a software system designed to support interoperable machine-to-machine interaction over a network". It has an interface described in a machine-processable format...
support (new): stateless session beans can be invoked over SOAPSOAPSOAP, originally defined as Simple Object Access Protocol, is a protocol specification for exchanging structured information in the implementation of Web Services in computer networks...
/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 JMSJava Message ServiceThe Java Message Service API is a Java Message Oriented Middleware API for sending messages between two or more clients. JMS is a part of the Java Platform, Enterprise Edition, and is defined by a specification developed under the Java Community Process as JSR 914...
. - 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 DTDsDocument Type DefinitionDocument Type Definition is a set of markup declarations that define a document type for SGML-family markup languages...
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 JavaJava (programming language)Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
. - 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 JavaJava (programming language)Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
. 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 JavaJava (programming language)Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
APIs. - Provide interoperability between enterprise Beans and Java EE components as well as non-Java programming language applications.
- Be compatible with the CORBA protocols (RMI-IIOP).
EJB 1.1, final release (1999-12-17)
Major changes:- XML deployment descriptors
- Default JNDI contexts
- RMI over IIOP
- 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.
EJB 1.0 (1998-03-24)
Announced at JavaOne 1998JavaOne
JavaOne is an annual conference inaugurated in 1996 by Sun Microsystems to discuss Java technologies, primarily among Java developers. JavaOne is held in San Francisco, California typically running from Monday to Thursday. Technical sessions on a variety of topics are held during the day. In the...
, 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 Beans.
External links
- Oracle's EJB Product main page
- EJB 3.0 API Javadocs
- The EJB 3.0 Specification
- Sun's EJB 3.0 Tutorial
- EJB (3.0) Glossary
- EJB FAQ