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.
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 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 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.
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)
How EJB Invocation happens?
Step
1: Retrieve Home Object reference from Naming Service via JNDI.
step 2: Return Home Object reference to the client.
step 3: Create me a new EJB Object through Home Object
interface.
step 4: Create EJB Object from the Ejb
Object
step 5: Return EJB Object reference to the client.
step 6: Invoke business method using EJB Object
reference.
step 7: Delegate request to Bean (Enterprise Bean).
What are transaction attributes?
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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