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Different kind of enterrise beans are Stateless session bean, Stateful session bean, Entity bean, Message-driven bean.

A session bean is a type of enterprise bean; a type of EJB server-side component. Session bean components implement the javax.ejb.SessionBean interface and can be stateless or stateful. Stateless session beans are components that perform transient services; stateful session beans are components that are dedicated to one client and act as a server-side extension of that client.

Session beans can act as agents modeling workflow or provide access to special transient business services. As an agent, a stateful session bean might represent a customer's session at an online shopping site. As a transitive service, a stateless session bean might provide access to validate and process credit card orders.

Session beans do not normally represent persistent business concepts like Employee or Order. This is the domain of a different component type called an entity bean.

An entity bean is a type of enterprise bean; a type of EJB server-side component. Entity bean components implement the javax.ejb.EntityBean interface and can be container-managed (CMP) or bean-managed (BMP). Entity beans are designed to represent data in the database; they wrapper data with business object semantics and read and update data automatically.

Entity beans have identity, where each bean represents a unique set of data. An entity bean of type Customer, for example, would have thousands of identities -- one for each customer in the database. Entity beans can be safely shared by many clients, so that several applications can access the same bean identity at the concurrently.

An entity bean consists of 4 groups of methods. 1.) create methods 2.) finder methods 3.) remove methods 4.) home methods

Container-managed persistence beans are the simpler than  bean-managed persistence (BMP) enterprise bean for the bean developer to create and the most difficult for the EJB server to support. This is because all the logic for synchronizing the bean's state with the database is handled automatically by the container. This means that the bean developer doesn't need to write any data access logic, while the EJB server is supposed to take care of all the persistence needs automatically. With CMP, the container manages the persistence of the entity bean. A CMP bean developer doesn't need to worry about JDBC code and transactions, because the Container performs database calls and transaction management instead of the programmer. Vendor tools are used to map the entity fields to the database and absolutely no database access code is written in the bean class. All table mapping is specified in the deployment descriptor. Otherwise, a BMP bean developer takes the load of linking an application and a database on his shoulders.

The bean-managed persistence (BMP) enterprise bean manages synchronizing its state with the database as directed by the container. The bean uses a database API to read and write its fields to the database, but the container tells it when to do each synchronization operation and manages the transactions for the bean automatically. Bean-managed persistence gives the bean developer the flexibility to perform persistence operations that are too complicated for the container or to use a data source that is not supported by the container.BMP beans are not 100% database-independent, because they may contain database-specific code, but CMP beans are unable to perform complicated DML (data manipulation language) statements. EJB 2.0 specification introduced some new ways of querying database (by using the EJB QL - query language).

Callback methods allows the container to notify the bean of events in its life cycle. The callback methods are defined in the javax.ejb.EntityBean interface.

Can Entity Beans have no create() methods? – Yes. In some cases the data is inserted NOT using Java application,.

When an enterprise bean defines the boundaries of the transaction it is called bean-managed transaction. If we use bean managed transaction we are responsible for progrmming transaction logic into your application code. That is we are responsible for issuing a begin statement and ether a commit or an abort statement.

What are transaction attributes? – Transaction attributes determine the behavior of the methods within a transaction. The different Transaction Attributes are:-

Required, RequiresNew, Manadatory, Supports, Never, NotRequired

The following are some of the points to be remembered when specifying transaction attributes for Enterprise JavaBeansTM-technology based components (EJBTM):

-All methods of a session bean's remote interface (and super interfaces) must have specified transaction attributes.

-Session bean home interface methods should not have transaction attributes.

(All methods of an entity bean's remote and home interfaces (and super interfaces) must have specified transaction attributes, with the exception of: methods getEJBHome(), getHandle(), getPrimaryKey(), isIdentical() methods of the remote interface; and methods getEJBMetaData(), getHomeHandle() of the home interface.

-Use Required as the default transaction attribute, to ensure that methods are invoked within a Java Transaction API (JTA) transaction context. Required causes the enterprise bean to use existing transactional context if it exists, or to create one otherwise.

-Use RequiresNew when the results of the method must be committed regardless whether the caller's transaction succeeds. For example, a method that logs all attempted transactions, whether those transaction succeed or not, could use RequiresNew to add log entries. RequiresNew always creates a new transaction context before the method call, and commits or rolls back after the method call. Note that any existing client transaction context will be suspended until the method call returns.

-Use Supports for methods that either do not change the database (directly or indirectly); or update atomically, and it does not matter whether or not the update occurs within a transaction. Supports uses the client transaction context if it exists, or no transaction context otherwise.

-Use Mandatory when the method absolutely requires an existing transaction. Mandatory causes RemoteException to be thrown unless the client is associated with a transaction context.

-Use Never to ensure that a transactional client does not access methods that are not capable of participating in the transaction. Never causes RemoteException to be thrown if the client is associated with a transactional context.

-Use NotSupported when an enterprise bean accesses a resource manager that either does not support external transaction coordination, or is not supported by the J2EE product. In this case, the bean must have container-managed transaction demarcation, and all its methods must be marked NotSupported. NotSupported will result in the method being called outside of any transaction context, whether or not one exists.

Enterprise beans that implement interface SessionSynchronization must have either the Required, RequiresNew, or Mandatory transaction

What are transaction isolation levels in EJB? – Transaction_read_uncommitted, Transaction_read_committed, Transaction_repeatable_read.

How EJB Invocation happens? – Retrieve Home Object reference from Naming Service via JNDI. Return Home Object reference to the client. Create me a new EJB Object through Home Object interface. Create EJB Object from the Ejb Object. Return EJB Object reference to the client. Invoke business method using EJB Object reference. Delegate request to Bean (Enterprise Bean).

Is it possible to share an HttpSession between a JSP and EJB? What happens when I change a value in the HttpSession from inside an EJB? – You can pass the HttpSession as parameter to an EJB method, only if all objects in session are serializable.This has to be consider as passed-by-value, that means that it’s read-only in the EJB. If anything is altered from inside the EJB, it won’t be reflected back to the HttpSession of the Servlet Container.The pass-by-reference can be used between EJBs Remote Interfaces, as they are remote references. While it is possible to pass an HttpSession as a parameter to an EJB object, it is considered to be bad practice in terms of object-oriented design. This is because you are creating an unnecessary coupling between back-end objects (EJBs) and front-end objects (HttpSession). Create a higher-level of abstraction for your EJBs API. Rather than passing the whole, fat, HttpSession (which carries with it a bunch of http semantics), create a class that acts as a value object (or structure) that holds all the data you need to pass back and forth between front-end/back-end. Consider the case where your EJB needs to support a non HTTP-based client. This higher level of abstraction will be flexible enough to support it.

The EJB container implements the EJBHome and EJBObject classes. For every request from a unique client, does the container create a separate instance of the generated EJBHome and EJBObject classes? – The EJB container maintains an instance pool. The container uses these instances for the EJB Home reference irrespective of the client request. while refering the EJB Object classes the container creates a separate instance for each client request. The instance pool maintenance is up to the implementation of the container. If the container provides one, it is available otherwise it is not mandatory for the provider to implement it. Having said that, yes most of the container providers implement the pooling functionality to increase the performance of the application server. The way it is implemented is, again, up to the implementer.

Can the primary key in the entity bean be a Java primitive type such as int? – The primary key can’t be a primitive type. Use the primitive wrapper classes, instead. For example, you can use java.lang.Integer as the primary key class, but not int (it has to be a class, not a primitive).

Can you control when passivation occurs? – The developer, according to the specification, cannot directly control when passivation occurs. Although for Stateful Session Beans, the container cannot passivate an instance that is inside a transaction. So using transactions can be a a strategy to control passivation. The ejbPassivate() method is called during passivation, so the developer has control over what to do during this exercise and can implement the require optimized logic. Some EJB containers, such as BEA WebLogic, provide the ability to tune the container to minimize passivation calls. Taken from the WebLogic 6.0 DTD -”The passivation-strategy can be either “default” or “transaction”. With the default setting the container will attempt to keep a working set of beans in the cache. With the “transaction” setting, the container will passivate the bean after every transaction (or method call for a non-transactional invocation).

What is the advantage of using Entity bean for database operations, over directly using JDBC API to do database operations? When would I use one over the other? – Entity Beans actually represents the data in a database. It is not that Entity Beans replaces JDBC API. There are two types of Entity Beans Container Managed and Bean Mananged. In Container Managed Entity Bean – Whenever the instance of the bean is created the container automatically retrieves the data from the DB/Persistance storage and assigns to the object variables in bean for user to manipulate or use them. For this the developer needs to map the fields in the database to the variables in deployment descriptor files (which varies for each vendor). In the Bean Managed Entity Bean – The developer has to specifically make connection, retrive values, assign them to the objects in the ejbLoad() which will be called by the container when it instatiates a bean object. Similarly in the ejbStore() the container saves the object values back the the persistance storage. ejbLoad and ejbStore are callback methods and can be only invoked by the container. Apart from this, when you use Entity beans you dont need to worry about database transaction handling, database connection pooling etc. which are taken care by the ejb container.

What is EJB QL? – EJB QL is a Query Language provided for navigation across a network of enterprise beans and dependent objects defined by means of container managed persistence. EJB QL is introduced in the EJB 2.0 specification. The EJB QL query language defines finder methods for entity beans with container managed persistenceand is portable across containers and persistence managers. EJB QL is used for queries of two types of finder methods: Finder methods that are defined in the home interface of an entity bean and which return entity objects. Select methods, which are not exposed to the client, but which are used by the Bean Provider to select persistent values that are maintained by the Persistence Manager or to select entity objects that are related to the entity bean on which the query is defined.

Brief description about local interfaces? – EJB was originally designed around remote invocation using the Java Remote Method Invocation (RMI) mechanism, and later extended to support to standard CORBA transport for these calls using RMI/IIOP. This design allowed for maximum flexibility in developing applications without consideration for the deployment scenario, and was a strong feature in support of a goal of component reuse in J2EE. Many developers are using EJBs locally, that is, some or all of their EJB calls are between beans in a single container. With this feedback in mind, the EJB 2.0 expert group has created a local interface mechanism. The local interface may be defined for a bean during development, to allow streamlined calls to the bean if a caller is in the same container. This does not involve the overhead involved with RMI like marshalling etc. This facility will thus improve the performance of applications in which co-location is planned. Local interfaces also provide the foundation for container-managed relationships among entity beans with container-managed persistence.

What are the special design care that must be taken when you work with local interfaces? – It is important to understand that the calling semantics of local interfaces are different from those of remote interfaces. For example, remote interfaces pass parameters using call-by-value semantics, while local interfaces use call-by-reference. This means that in order to use local interfaces safely, application developers need to carefully consider potential deployment scenarios up front, then decide which interfaces can be local and which remote, and finally, develop the application code with these choices in mind. While EJB 2.0 local interfaces are extremely useful in some situations, the long-term costs of these choices, especially when changing requirements and component reuse are taken into account, need to be factored into the design decision.

What happens if remove( ) is never invoked on a session bean? – In case of a stateless session bean it may not matter if we call or not as in both cases nothing is done. The number of beans in cache is managed by the container. In case of stateful session bean, the bean may be kept in cache till either the session times out, in which case the bean is removed or when there is a requirement for memory in which case the data is cached and the bean is sent to free pool.

What is the difference between Message Driven Beans and Stateless Session beans? – In several ways, the dynamic creation and allocation of message-driven bean instances mimics the behavior of stateless session EJB instances, which exist only for the duration of a particular method call. However, message-driven beans are different from stateless session EJBs (and other types of EJBs) in several significant ways: Message-driven beans process multiple JMS messages asynchronously, rather than processing a serialized sequence of method calls. Message-driven beans have no home or remote interface, and therefore cannot be directly accessed by internal or external clients. Clients interact with message-driven beans only indirectly, by sending a message to a JMS Queue or Topic. Only the container directly interacts with a message-driven bean by creating bean instances and passing JMS messages to those instances as necessary. The Container maintains the entire lifecycle of a message-driven bean; instances cannot be created or removed as a result of client requests or other API calls.

How can I call one EJB from inside of another EJB? – EJBs can be clients of other EJBs. It just works. Use JNDI to locate the Home Interface of the other bean, then acquire an instance reference, and so forth.

What is an EJB Context? – EJBContext is an interface that is implemented by the container, and it is also a part of the bean-container contract. Entity beans use a subclass of EJBContext called EntityContext. Session beans use a subclass called SessionContext. These EJBContext objects provide the bean class with information about its container, the client using the bean and the bean itself. They also provide other functions. See the API docs and the spec for more details.