EJB Interview Questions
Q: What are the different kinds of enterprise beans?
A:Stateless session bean- An instance of these non-persistent EJBs provides a
service without storing an interaction or conversation state between methods. Any
instance can be used for any client.
Stateful session bean- An instance of these non-persistent EJBs maintains state
across methods and transactions. Each instance is associated with a particular client.
Entity bean- An instance of these persistent EJBs represents an object view of the
data, usually rows in a database. They have a primary key as a unique identifier.
Entity bean persistence can be either container-managed or bean-managed.
Message-driven bean- An instance of these EJBs is integrated with the Java
Message Service (JMS) to provide the ability for message-driven beans to act as a
standard JMS message consumer and perform asynchronous processing between the
server and the JMS message producer.
A:Stateless session bean- An instance of these non-persistent EJBs provides a
service without storing an interaction or conversation state between methods. Any
instance can be used for any client.
Stateful session bean- An instance of these non-persistent EJBs maintains state
across methods and transactions. Each instance is associated with a particular client.
Entity bean- An instance of these persistent EJBs represents an object view of the
data, usually rows in a database. They have a primary key as a unique identifier.
Entity bean persistence can be either container-managed or bean-managed.
Message-driven bean- An instance of these EJBs is integrated with the Java
Message Service (JMS) to provide the ability for message-driven beans to act as a
standard JMS message consumer and perform asynchronous processing between the
server and the JMS message producer.
Q: What is Session Bean?
A:A session bean is a non-persistent object that implements some business logic
running on the server. One way to think of a session object is as a logical extension
of the client program that runs on the server.
Session beans are used to manage the interactions of entity and other session
beans,access resources, and generally perform tasks on behalf of the client.
There are two basic kinds of session bean: stateless and stateful.
Stateless session beans are made up of business methods that behave like
procedures; they operate only on the arguments passed to them when they are
invoked. Stateless beans are called stateless because they are transient; they do not
maintain business state between method invocations.Each invocation of a stateless
business method is independent from previous invocations. Because stateless
session beans are stateless, they are easier for the EJB container to manage, so they
tend to process requests faster and use less resources.
Stateful session beans encapsulate business logic and state specific to a client.
Stateful beans are called "stateful" because they do maintain business state between
method invocations, held in memory and not persistent. Unlike stateless session
beans, clients do not share stateful beans. When a client creates a stateful bean,
that bean instance is dedicated to service only that client. This makes it possible to
maintain conversational state, which is business state that can be shared by
methods in the same stateful bean.
Q:What is Entity Bean?
A:The entity bean is used to represent data in the database. It provides an objectoriented
interface to data that would normally be accessed by the JDBC or some
other back-end API. More than that, entity beans provide a component model that
allows bean developers to focus their attention on the business logic of the bean,
while the container takes care of managing persistence,transactions, and access
control.
There are two basic kinds of entity beans: container-managed ersistence (CMP)
andbean-managed persistence (BMP).
Container-managed persistence beans are the simplest 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. Vendor tools are used to map
the entity fields to the database and absolutely no database access code is written in
the bean class.
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.
Q: What are the methods of Entity Bean?
A:An entity bean consists of 4 groups of methods:
1.create methods: To create a new instance of a CMP entity bean, and therefore
insert data into the database, the create() method on the bean's home interface
must be invoked. They look like this: EntityBeanClass ejbCreateXXX(parameters),
where EntityBeanClass is an Entity Bean you are trying to instantiate,
ejbCreateXXX(parameters) methods are used for creating Entity Bean instances
according to the parameters specified and to some programmer-defined conditions.
A bean's home interface may declare zero or more create() methods, each of which
must have corresponding ejbCreate() and ejbPostCreate() methods in the bean
class. These creation methods are linked at run time, so that when a create()
method is invoked on the home interface, the container delegates the invocation to
the corresponding ejbCreate() and ejbPostCreate() methods on the bean class.
2: finder methods: The methods in the home interface that begin with "find" are
called the find methods. These are used to query the EJB server for specific entity
beans, based on the name of the method and arguments passed. Unfortunately,
there is no standard query language defined for find methods, so each vendor will
implement the find method differently. In CMP entity beans, the find methods are
not implemented with matching methods in the bean class; containers implement
them when the bean is deployed in a vendor specific manner. The deployer will use
vendor specific tools to tell the container how a particular find method should
behave. Some vendors will use object-relational mapping tools to define the behavior
of a find method while others will simply require the deployer to enter the
appropriate SQL command.
There are two basic kinds of find methods: single-entity and multi-entity. Singleentity
find methods return a remote reference to the one specific entity bean that
matches the find request. If no entity beans are found, the method throws an
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ObjectNotFoundException . Every entity bean must define the single-entity find
method with the method name findByPrimaryKey(), which takes the bean's primary
key type as an argument.
The multi-entity find methods return a collection ( Enumeration or Collection type) of
entities that match the find request. If no entities are found, the multi-entity find
returns an empty collection.
3. remove methods: These methods (you may have up to 2 remove methods, or
don't have them at all) allow the client to physically remove Entity beans by
specifying either Handle or a Primary Key for the Entity Bean.
4. home methods: These methods are designed and implemented by a developer,
and EJB specification doesn't have any requirements for them except the need to
throw a RemoteException is each home method.
Q: What are the methods of Entity Bean?What is the difference between Container-Managed Persistent (CMP) bean and Bean-Managed Persistent(BMP) ?
A:Container-managed persistence beans are the simplest 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% databaseindependent,
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).
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% databaseindependent,
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).
Q:What are the callback methods in Entity beans?
A:The bean class defines create methods that match methods in the home interface
and business methods that match methods in the remote interface. The bean class
also implements a set of callback methods that allow the container to notify the bean
of events in its life cycle. The callback methods are defined in the
javax.ejb.EntityBean interface that is implemented by all entity beans.The
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EntityBean interface has the following definition. Notice that the bean class
implements these methods.
public interface javax.ejb.EntityBean {
public void setEntityContext();
public void unsetEntityContext();
public void ejbLoad();
public void ejbStore();
public void ejbActivate();
public void ejbPassivate();
public void ejbRemove();
}
The setEntityContext() method provides the bean with an interface to the
container called the EntityContext. The EntityContext interface contains methods for
obtaining information about the context under which the bean is operating at any
particular moment. The EntityContext interface is used to access security information
about the caller; to determine the status of the current transaction or to force a
transaction rollback; or to get a reference to the bean itself, its home, or its primary
key. The EntityContext is set only once in the life of an entity bean instance, so its
reference should be put into one of the bean instance's fields if it will be needed
later.
The unsetEntityContext() method is used at the end of the bean's life cycle before
the instance is evicted from memory to dereference the EntityContext and perform
any last-minute clean-up.
The ejbLoad() and ejbStore() methods in CMP entities are invoked when the entity
bean's state is being synchronized with the database. The ejbLoad() is invoked just
after the container has refreshed the bean container-managed fields with its state
from the database. The ejbStore() method is invoked just before the container is
about to write the bean container-managed fields to the database. These methods
are used to modify data as it's being synchronized. This is common when the data
stored in the database is different than the data used in the bean fields.
The ejbPassivate() and ejbActivate() methods are invoked on the bean by the
container just before the bean is passivated and just after the bean is activated,
respectively. Passivation in entity beans means that the bean instance is
disassociated with its remote reference so that the container can evict it from
memory or reuse it. It's a resource conservation measure the container employs to
reduce the number of instances in memory. A bean might be passivated if it hasn't
been used for a while or as a normal operation performed by the container to
maximize reuse of resources. Some containers will evict beans from memory, while
others will reuse instances for other more active remote references. The
ejbPassivate() and ejbActivate() methods provide the bean with a notification as to
when it's about to be passivated (disassociated with the remote reference) or
activated (associated with a remote reference).
Q:What is software architecture of EJB?
A:Session and Entity EJBs consist of 4 and 5 parts respetively:
1. A remote interface (a client interacts with it),
2. A home interface (used for creating objects and for declaring business methods),
3. A bean object (an object, which actually performs business logic and EJB-specific
operations).
4. A deployment descriptor (an XML file containing all information required for
maintaining the EJB) or a set of deployment descriptors (if you are using some
container-specific features).
5.A Primary Key class - is only Entity bean specific.
Q:Can Entity Beans have no create() methods?
A:Yes. In some cases the data is inserted NOT using Java application, so you may only
need to retrieve the information, perform its processing, but not create your own
information of this kind.
Q:What is bean managed transaction?
A:If a developer doesn't want a Container to manage transactions, it's possible to
implement all database operations manually by writing the appropriate JDBC code.
This often leads to productivity increase, but it makes an Entity Bean incompatible
with some databases and it enlarges the amount of code to be written. All
transaction management is explicitly performed by a developer
.
Q:What are transaction attributes?
A:The transaction attribute specifies how the Container must manage transactions for a
method when a client invokes the method via the enterprise bean’s home or
component interface or when the method is invoked as the result of the arrival of a
JMS message. (Sun's EJB Specification) Below is a list of transactional attributes:
1.NotSupported - transaction context is unspecified.
2.Required - bean's method invocation is made within a transactional context. If a
client is not associated with a transaction, a new transaction is invoked
automatically.
3.Supports - if a transactional context exists, a Container acts like the transaction
attribute is Required, else - like NotSupported.
4.RequiresNew - a method is invoked in a new transaction context.
5.Mandatory - if a transactional context exists, a Container acts like the transaction
attribute is Required, else it throws a javax.ejb.TransactionRequiredException.
6.Never - a method executes only if no transaction context is specified.
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Q:What are transaction attributes?
A:The transaction attribute specifies how the Container must manage transactions for a
method when a client invokes the method via the enterprise bean’s home or
component interface or when the method is invoked as the result of the arrival of a
JMS message. (Sun's EJB Specification) Below is a list of transactional attributes:
1.NotSupported - transaction context is unspecified.
2.Required - bean's method invocation is made within a transactional context. If a
client is not associated with a transaction, a new transaction is invoked
automatically.
3.Supports - if a transactional context exists, a Container acts like the transaction
attribute is Required, else - like NotSupported.
4.RequiresNew - a method is invoked in a new transaction context.
5.Mandatory - if a transactional context exists, a Container acts like the transaction
attribute is Required, else it throws a javax.ejb.TransactionRequiredException.
6.Never - a method executes only if no transaction context is specified.
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