Interview questions for tech companies » EJB interview questionsEJB interview
questionsIs is possible for an EJB client to marshal an object of class
java.lang.Class to an EJB? - Technically yes, spec. compliant NO! - The
enterprise bean must not attempt to query a class to obtain information about
the declared members that are not otherwise accessible to the enterprise bean
because of the security rules of the Java language.
Is it legal to have static initializer blocks in EJB? - Although technically
it is legal, static initializer blocks are used to execute some piece of code
before executing any constructor or method while instantiating a class. Static
initializer blocks are also typically used to initialize static fields - which
may be illegal in EJB if they are read/write - In EJB this can be achieved by
including the code in either the ejbCreate(), setSessionContext() or
setEntityContext() methods.
Is it possible to stop the execution of a method before completion in a
SessionBean? - Stopping the execution of a method inside a Session Bean is not
possible without writing code inside the Session Bean. This is because you are
not allowed to access Threads inside an EJB.
What is the default transaction attribute for an EJB? - There is no default
transaction attribute for an EJB. Section 11.5 of EJB v1.1 spec says that the
deployer must specify a value for the transaction attribute for those methods
having container managed transaction. In WebLogic, the default transaction
attribute for EJB is SUPPORTS.
What is the difference between session and entity beans? When should I use one
or the other? - An entity bean represents persistent global data from the
database; a session bean represents transient user-specific data that will die
when the user disconnects (ends his session). Generally, the session beans
implement business methods (e.g. Bank.transferFunds) that call entity beans
(e.g. Account.deposit, Account.withdraw)
Is there any default cache management system with Entity beans ? In other
words whether a cache of the data in database will be maintained in EJB ? -
Caching data from a database inside the Application Server are what Entity
EJB's are used for.The ejbLoad() and ejbStore() methods are used to
synchronize the Entity Bean state with the persistent storage(database).
Transactions also play an important role in this scenario. If data is removed
from the database, via an external application - your Entity Bean can still be
"alive" the EJB container. When the transaction commits, ejbStore() is called
and the row will not be found, and the transaction rolled back.
Why is ejbFindByPrimaryKey mandatory? - An Entity Bean represents persistent
data that is stored outside of the EJB Container/Server. The
ejbFindByPrimaryKey is a method used to locate and load an Entity Bean into
the container, similar to a SELECT statement in SQL. By making this method
mandatory, the client programmer can be assured that if they have the primary
key of the Entity Bean, then they can retrieve the bean without having to
create a new bean each time - which would mean creating duplications of
persistent data and break the integrity of EJB.
Why do we have a remove method in both EJBHome and EJBObject? - With the
EJBHome version of the remove, you are able to delete an entity bean without
first instantiating it (you can provide a PrimaryKey object as a parameter to
the remove method). The home version only works for entity beans. On the other
hand, the Remote interface version works on an entity bean that you have
already instantiated. In addition, the remote version also works on session
beans (stateless and stateful) to inform the container of your loss of
interest in this bean.
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 the difference between a Server, a Container, and a Connector? - An
EJB server is an application, usually a product such as BEA WebLogic, that
provides (or should provide) for concurrent client connections and manages
system resources such as threads, processes, memory, database connections,
network connections, etc. An EJB container runs inside (or within) an EJB