EJB Interview Questions and Answers
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Ques 66. 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.
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Ques 67. 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).
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Ques 68. 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).
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Ques 69. 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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Ques 70. 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.
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