Enterprise JavaBeans

Question 1

What is an Enterprise JavaBean (EJB)?

Answer 1

An Enterprise JavaBean is a component written in Java.

Question 2

(3 answers)

What are the types of component in v3 of Enterprise JavaBeans (EJB)?

Answer 2

• Session beans
• Message-driven beans
• Entity classes

Question 3

(4 answers)

What are session beans?

Answer 3

• Part of the javax.ejb package
• Provide business logic
• Run inside an EJB container (a runtime environment)
• Client communicates with the data via the session bean

Question 4

(2 answers)

What are the types of session bean?

Answer 4

• Stateful session beans
• Stateless session beans

Question 5

What are stateful session beans?

Answer 5

Stateful session beans keep state across multiple method calls.

They are either ready, non-existent or passive (passive when written to disk, goes back to ready when read from disk).

(e.g. shopping cart)

Question 6

What are stateless session beans?

Answer 6

Stateless session beans discard the state after each method call.

They are either ready or non-existent.

(e.g. a database query)

Question 7

(4 answers)

What are entity classes?

Answer 7

• Part of the javax.persistence package
• Provide business data
• Capture the persistent business entities stored in a database
• Correspond to a table in a database

Question 8

What are entities?

Answer 8

Entities are instances of an entity class.

Question 9

What are entity managers?

Answer 9

Entity managers are used to manage entities and are part of the javax.persistence package.

Question 10

(2 answers)

What are the types of entity manager?

Answer 10

• Application-managed
  
  • Explicitly created and closed by the application
• Container-managed
  
  • Implicitly created and closed by the EJB container

Question 11

What are transfer objects?

Answer 11

Transfer objects represent the client’s view of data and are suitable for data transfer between session beans and clients.

They contain all necessary information in their instance variables.

Question 12

What are the differences between entities and transfer objects?

Answer 12

Entities:

• Reflect the way data is organised in the database
• Suitable for data transfer between the database and session beans
• May contain confidential information that should not be accessible to clients

Transfer objects:

• Represent the client’s view of data
• Suitable for data transfer between session beans and clients
• Only include information that is relevant to clients
• Can aggregate information stored across different entities, to match the way a client views information