The Java EE 5 Tutorial

Chapter 20 Enterprise Beans

Enterprise beans are Java EE components that implement Enterprise JavaBeans (EJB) technology. Enterprise beans run in the EJB container, a runtime environment within the Application Server (see Container Types). Although transparent to the application developer, the EJB container provides system-level services such as transactions and security to its enterprise beans. These services enable you to quickly build and deploy enterprise beans, which form the core of transactional Java EE applications.

What Is an Enterprise Bean?

Written in the Java programming language, an enterprise bean is a server-side component that encapsulates the business logic of an application. The business logic is the code that fulfills the purpose of the application. In an inventory control application, for example, the enterprise beans might implement the business logic in methods called checkInventoryLevel and orderProduct. By invoking these methods, clients can access the inventory services provided by the application.

Benefits of Enterprise Beans

For several reasons, enterprise beans simplify the development of large, distributed applications. First, because the EJB container provides system-level services to enterprise beans, the bean developer can concentrate on solving business problems. The EJB container, rather than the bean developer, is responsible for system-level services such as transaction management and security authorization.

Second, because the beans rather than the clients contain the application’s business logic, the client developer can focus on the presentation of the client. The client developer does not have to code the routines that implement business rules or access databases. As a result, the clients are thinner, a benefit that is particularly important for clients that run on small devices.

Third, because enterprise beans are portable components, the application assembler can build new applications from existing beans. These applications can run on any compliant Java EE server provided that they use the standard APIs.

When to Use Enterprise Beans

You should consider using enterprise beans if your application has any of the following requirements:

The application must be scalable. To accommodate a growing number of users, you may need to distribute an application’s components across multiple machines. Not only can the enterprise beans of an application run on different machines, but also their location will remain transparent to the clients.
Transactions must ensure data integrity. Enterprise beans support transactions, the mechanisms that manage the concurrent access of shared objects.
The application will have a variety of clients. With only a few lines of code, remote clients can easily locate enterprise beans. These clients can be thin, various, and numerous.

Types of Enterprise Beans

Table 20–1 summarizes the two types of enterprise beans. The following sections discuss each type in more detail.

Table 20–1 Enterprise Bean Types


Enterprise Bean Type	Purpose
Session	Performs a task for a client; optionally may implement a web service
Message-Driven	Acts as a listener for a particular messaging type, such as the Java Message Service API

Note –

Entity beans have been replaced by Java Persistence API entities. For information about entities, see Chapter 24, Introduction to the Java Persistence API.

What Is a Session Bean?

A session bean represents a single client inside the Application Server. To access an application that is deployed on the server, the client invokes the session bean’s methods. The session bean performs work for its client, shielding the client from complexity by executing business tasks inside the server.

As its name suggests, a session bean is similar to an interactive session. A session bean is not shared; it can have only one client, in the same way that an interactive session can have only one user. Like an interactive session, a session bean is not persistent. (That is, its data is not saved to a database.) When the client terminates, its session bean appears to terminate and is no longer associated with the client.

For code samples, see Chapter 22, Session Bean Examples.

State Management Modes

There are two types of session beans: stateful and stateless.

Stateful Session Beans

The state of an object consists of the values of its instance variables. In a stateful session bean, the instance variables represent the state of a unique client-bean session. Because the client interacts (“talks”) with its bean, this state is often called the conversational state.

The state is retained for the duration of the client-bean session. If the client removes the bean or terminates, the session ends and the state disappears. This transient nature of the state is not a problem, however, because when the conversation between the client and the bean ends there is no need to retain the state.

Stateless Session Beans

A stateless session bean does not maintain a conversational state with the client. When a client invokes the methods of a stateless bean, the bean’s instance variables may contain a state specific to that client, but only for the duration of the invocation. When the method is finished, the client-specific state should not be retained. Clients may, however, change the state of instance variables in pooled stateless beans, and this state is held over to the next invocation of the pooled stateless bean. Except during method invocation, all instances of a stateless bean are equivalent, allowing the EJB container to assign an instance to any client. That is, the state of a stateless session bean should apply accross all clients.

Because stateless session beans can support multiple clients, they can offer better scalability for applications that require large numbers of clients. Typically, an application requires fewer stateless session beans than stateful session beans to support the same number of clients.

A stateless session bean can implement a web service, but other types of enterprise beans cannot.

When to Use Session Beans

In general, you should use a session bean if the following circumstances hold:

At any given time, only one client has access to the bean instance.
The state of the bean is not persistent, existing only for a short period (perhaps a few hours).
The bean implements a web service.

Stateful session beans are appropriate if any of the following conditions are true:

The bean’s state represents the interaction between the bean and a specific client.
The bean needs to hold information about the client across method invocations.
The bean mediates between the client and the other components of the application, presenting a simplified view to the client.
Behind the scenes, the bean manages the work flow of several enterprise beans. For an example, see the AccountControllerBean session bean in Chapter 37, The Duke’s Bank Application.

To improve performance, you might choose a stateless session bean if it has any of these traits:

The bean’s state has no data for a specific client.
In a single method invocation, the bean performs a generic task for all clients. For example, you might use a stateless session bean to send an email that confirms an online order.

What Is a Message-Driven Bean?

A message-driven bean is an enterprise bean that allows Java EE applications to process messages asynchronously. It normally acts as a JMS message listener, which is similar to an event listener except that it receives JMS messages instead of events. The messages can be sent by any Java EE component (an application client, another enterprise bean, or a web component) or by a JMS application or system that does not use Java EE technology. Message-driven beans can process JMS messages or other kinds of messages.

For a simple code sample, see Chapter 23, A Message-Driven Bean Example. For more information about using message-driven beans, see Using the JMS API in a Java EE Application and Chapter 32, Java EE Examples Using the JMS API.

What Makes Message-Driven Beans Different from Session Beans?

The most visible difference between message-driven beans and session beans is that clients do not access message-driven beans through interfaces. Interfaces are described in the section Defining Client Access with Interfaces. Unlike a session bean, a message-driven bean has only a bean class.

In several respects, a message-driven bean resembles a stateless session bean.

A message-driven bean’s instances retain no data or conversational state for a specific client.
All instances of a message-driven bean are equivalent, allowing the EJB container to assign a message to any message-driven bean instance. The container can pool these instances to allow streams of messages to be processed concurrently.
A single message-driven bean can process messages from multiple clients.

The instance variables of the message-driven bean instance can contain some state across the handling of client messages (for example, a JMS API connection, an open database connection, or an object reference to an enterprise bean object).

Client components do not locate message-driven beans and invoke methods directly on them. Instead, a client accesses a message-driven bean through, for example, JMS by sending messages to the message destination for which the message-driven bean class is the MessageListener. You assign a message-driven bean’s destination during deployment by using Application Server resources.

Message-driven beans have the following characteristics:

They execute upon receipt of a single client message.
They are invoked asynchronously.
They are relatively short-lived.
They do not represent directly shared data in the database, but they can access and update this data.
They can be transaction-aware.
They are stateless.

When a message arrives, the container calls the message-driven bean’s onMessage method to process the message. The onMessage method normally casts the message to one of the five JMS message types and handles it in accordance with the application’s business logic. The onMessage method can call helper methods, or it can invoke a session bean to process the information in the message or to store it in a database.

A message can be delivered to a message-driven bean within a transaction context, so all operations within the onMessage method are part of a single transaction. If message processing is rolled back, the message will be redelivered. For more information, see Chapter 23, A Message-Driven Bean Example and Chapter 33, Transactions.

When to Use Message-Driven Beans

Session beans allow you to send JMS messages and to receive them synchronously, but not asynchronously. To avoid tying up server resources, do not to use blocking synchronous receives in a server-side component, and in general JMS messages should not be sent or received synchronously. To receive messages asynchronously, use a message-driven bean.

Defining Client Access with Interfaces

The material in this section applies only to session beans and not to message-driven beans. Because they have a different programming model, message-driven beans do not have interfaces that define client access.

A client can access a session bean only through the methods defined in the bean’s business interface. The business interface defines the client’s view of a bean. All other aspects of the bean (method implementations and deployment settings) are hidden from the client.

Well-designed interfaces simplify the development and maintenance of Java EE applications. Not only do clean interfaces shield the clients from any complexities in the EJB tier, but they also allow the beans to change internally without affecting the clients. For example, if you change a session bean from a stateless to a stateful session bean, you won’t have to alter the client code. But if you were to change the method definitions in the interfaces, then you might have to modify the client code as well. Therefore, it is important that you design the interfaces carefully to isolate your clients from possible changes in the beans.

Session beans can have more than one business interface. Session beans should, but are not required to, implement their business interface or interfaces.

When you design a Java EE application, one of the first decisions you make is the type of client access allowed by the enterprise beans: remote, local, or web service.

Remote Clients

A remote client of an enterprise bean has the following traits:

It can run on a different machine and a different Java virtual machine (JVM) than the enterprise bean it accesses. (It is not required to run on a different JVM.)
It can be a web component, an application client, or another enterprise bean.
To a remote client, the location of the enterprise bean is transparent.

To create an enterprise bean that allows remote access, you must do one of the following:

Decorate the business interface of the enterprise bean with the @Remote annotation:
```
@Remote
public interface InterfaceName { ... }
```
Decorate the bean class with @Remote, specifying the business interface or interfaces:
```
@Remote(InterfaceName.class)
public class BeanName implements InterfaceName { ... }
```

The remote interface defines the business and life cycle methods that are specific to the bean. For example, the remote interface of a bean named BankAccountBean might have business methods named deposit and credit. Figure 20–1 shows how the interface controls the client’s view of an enterprise bean.

Figure 20–1 Interfaces for an Enterprise Bean with Remote Access

Diagram showing a remote client accessing an enterprise
bean's methods through its remote interface.

Local Clients

A local client has these characteristics:

It must run in the same JVM as the enterprise bean it accesses.
It can be a web component or another enterprise bean.
To the local client, the location of the enterprise bean it accesses is not transparent.

The local business interface defines the bean’s business and life cycle methods. If the bean’s business interface is not decorated with @Local or @Remote, and the bean class does not specify the interface using @Local or @Remote, the business interface is by default a local interface. To build an enterprise bean that allows only local access, you may, but are not required to do one of the following:

Annotate the business interface of the enterprise bean as a @Local interface. For example:
```
@Local
public interface InterfaceName { ... }
```
Specify the interface by decorating the bean class with @Local and specify the interface name. For example:
```
@Local(InterfaceName.class)
public class BeanName implements InterfaceName { ... }
```

Deciding on Remote or Local Access

Whether to allow local or remote access depends on the following factors.

Tight or loose coupling of related beans: Tightly coupled beans depend on one another. For example, if a session bean that processes sales orders calls a session bean that emails a confirmation message to the customer, these beans are tightly coupled. Tightly coupled beans are good candidates for local access. Because they fit together as a logical unit, they typically call each other often and would benefit from the increased performance that is possible with local access.
Type of client: If an enterprise bean is accessed by application clients, then it should allow remote access. In a production environment, these clients almost always run on different machines than the Application Server. If an enterprise bean’s clients are web components or other enterprise beans, then the type of access depends on how you want to distribute your components.
Component distribution: Java EE applications are scalable because their server-side components can be distributed across multiple machines. In a distributed application, for example, the web components may run on a different server than do the enterprise beans they access. In this distributed scenario, the enterprise beans should allow remote access.
Performance: Due to factors such as network latency, remote calls may be slower than local calls. On the other hand, if you distribute components among different servers, you may improve the application’s overall performance. Both of these statements are generalizations; actual performance can vary in different operational environments. Nevertheless, you should keep in mind how your application design might affect performance.

If you aren’t sure which type of access an enterprise bean should have, choose remote access. This decision gives you more flexibility. In the future you can distribute your components to accommodate the growing demands on your application.

Although it is uncommon, it is possible for an enterprise bean to allow both remote and local access. If this is the case, either the business interface of the bean must be explicitly designated as a business interface by being decorated with the @Remote or @Local annotations, or the bean class must explicitly designate the business interfaces by using the @Remote and @Local annotations. The same business interface cannot be both a local and remote business interface.

Web Service Clients

A web service client can access a Java EE application in two ways. First, the client can access a web service created with JAX-WS. (For more information on JAX-WS, see Chapter 16, Building Web Services with JAX-WS.) Second, a web service client can invoke the business methods of a stateless session bean. Message beans cannot be accessed by web service clients.

Provided that it uses the correct protocols (SOAP, HTTP, WSDL), any web service client can access a stateless session bean, whether or not the client is written in the Java programming language. The client doesn’t even “know” what technology implements the service: stateless session bean, JAX-WS, or some other technology. In addition, enterprise beans and web components can be clients of web services. This flexibility enables you to integrate Java EE applications with web services.

A web service client accesses a stateless session bean through the bean’s web service endpoint implementation class. By default, all public methods in the bean class are accessible to web service clients. The @WebMethod annotation may be used to customize the behavior of web service methods. If the @WebMethod annotation is used to decorate the bean class’s methods, only those methods decorated with @WebMethod are exposed to web service clients.

For a code sample, see A Web Service Example: helloservice.

Method Parameters and Access

The type of access affects the parameters of the bean methods that are called by clients. The following topics apply not only to method parameters but also to method return values.

Isolation

The parameters of remote calls are more isolated than those of local calls. With remote calls, the client and bean operate on different copies of a parameter object. If the client changes the value of the object, the value of the copy in the bean does not change. This layer of isolation can help protect the bean if the client accidentally modifies the data.

In a local call, both the client and the bean can modify the same parameter object. In general, you should not rely on this side effect of local calls. Perhaps someday you will want to distribute your components, replacing the local calls with remote ones.

As with remote clients, web service clients operate on different copies of parameters than does the bean that implements the web service.

Granularity of Accessed Data

Because remote calls are likely to be slower than local calls, the parameters in remote methods should be relatively coarse-grained. A coarse-grained object contains more data than a fine-grained one, so fewer access calls are required. For the same reason, the parameters of the methods called by web service clients should also be coarse-grained.

The Contents of an Enterprise Bean

To develop an enterprise bean, you must provide the following files:

Enterprise bean class: Implements the methods defined in the business interface and any life cycle callback methods.
Business Interfaces: The business interface defines the methods implemented by the enterprise bean class.
Helper classes: Other classes needed by the enterprise bean class, such as exception and utility classes.

You package the files in the preceding list into an EJB JAR file, the module that stores the enterprise bean. An EJB JAR file is portable and can be used for different applications. To assemble a Java EE application, you package one or more modules (such as EJB JAR files) into an EAR file, the archive file that holds the application. When you deploy the EAR file that contains the bean’s EJB JAR file, you also deploy the enterprise bean to the Application Server. You can also deploy an EJB JAR that is not contained in an EAR file. Figure 20–2 shows the contents of an EJB JAR file.

Figure 20–2 Structure of an Enterprise Bean JAR

Diagram showing the structure and contents of an enterprise
bean JAR file.

Naming Conventions for Enterprise Beans

Because enterprise beans are composed of multiple parts, it’s useful to follow a naming convention for your applications. Table 20–2 summarizes the conventions for the example beans in this tutorial.

Table 20–2 Naming Conventions for Enterprise Beans


Item	Syntax	Example
Enterprise bean name	`nameBean`	`AccountBean`
Enterprise bean class	`nameBean`	`AccountBean`
Business interface	`name`	`Account`

The Life Cycles of Enterprise Beans

An enterprise bean goes through various stages during its lifetime, or life cycle. Each type of enterprise bean (stateful session, stateless session, or message-driven) has a different life cycle.

The descriptions that follow refer to methods that are explained along with the code examples in the next two chapters. If you are new to enterprise beans, you should skip this section and run the code examples first.

The Life Cycle of a Stateful Session Bean

Figure 20–3 illustrates the stages that a session bean passes through during its lifetime. The client initiates the life cycle by obtaining a reference to a stateful session bean. The container performs any dependency injection and then invokes the method annotated with @PostConstruct, if any. The bean is now ready to have its business methods invoked by the client.

Figure 20–3 Life Cycle of a Stateful Session Bean

Diagram showing the life cycle of a stateful session
bean.

While in the ready stage, the EJB container may decide to deactivate, or passivate, the bean by moving it from memory to secondary storage. (Typically, the EJB container uses a least-recently-used algorithm to select a bean for passivation.) The EJB container invokes the method annotated @PrePassivate, if any, immediately before passivating it. If a client invokes a business method on the bean while it is in the passive stage, the EJB container activates the bean, calls the method annotated @PostActivate, if any, and then moves it to the ready stage.

At the end of the life cycle, the client invokes a method annotated @Remove, and the EJB container calls the method annotated @PreDestroy, if any. The bean’s instance is then ready for garbage collection.

Your code controls the invocation of only one life-cycle method: the method annotated @Remove. All other methods in Figure 20–3 are invoked by the EJB container. See Chapter 34, Resource Connections for more information.

The Life Cycle of a Stateless Session Bean

Because a stateless session bean is never passivated, its life cycle has only two stages: nonexistent and ready for the invocation of business methods. Figure 20–4 illustrates the stages of a stateless session bean.

Figure 20–4 Life Cycle of a Stateless Session Bean

Diagram showing the life cycle of a stateless session
bean.

The client initiates the life cycle by obtaining a reference to a stateless session bean. The container performs any dependency injection and then invokes the method annotated @PostConstruct, if any. The bean is now ready to have its business methods invoked by the client.

At the end of the life cycle, the EJB container calls the method annotated @PreDestroy, if any. The bean’s instance is then ready for garbage collection.

The Life Cycle of a Message-Driven Bean

Figure 20–5 illustrates the stages in the life cycle of a message-driven bean.

Figure 20–5 Life Cycle of a Message-Driven Bean

Diagram showing the life cycle of a message-driven bean.

The EJB container usually creates a pool of message-driven bean instances. For each instance, the EJB container performs these tasks:

If the message-driven bean uses dependency injection, the container injects these references before instantiating the instance.
The container calls the method annotated @PostConstruct, if any.

Like a stateless session bean, a message-driven bean is never passivated, and it has only two states: nonexistent and ready to receive messages.

At the end of the life cycle, the container calls the method annotated @PreDestroy, if any. The bean’s instance is then ready for garbage collection.

Further Information about Enterprise Beans

For more information on Enterprise JavaBeans technology, see:

Enterprise JavaBeans 3.0 specification:

http://java.sun.com/products/ejb/docs.html
The Enterprise JavaBeans web site:

http://java.sun.com/products/ejb