Lecture 8 - Session Beans and Jakarta Persistence (Intro to EJB)

Question 1

What is a Jakarta Enterprise Bean (EJB) and what services does it provide?

Answer 1

A bean that can be used for business logic:
- Concurrency
- Transaction management
- Persistence
- Security etc.

Question 2

What are the 3 types of EJBs?

Answer 2

Session beans (business behaviour, 1 client at a time)
@Stateless, @Stateful, @Singleton

Message-driven beans (async messages, methods are normally synchronous)
@MessageDriven

Entity beans (persist in DB, multiple clients at a time)
@Entity

Question 3

Why use EJB over CDI bean?

Answer 3

Transactions + security controls for methods

Automatic concurrency

Instance pooling -> stateless beans can be shared

Passivation -> Stateful beans can be temp stored in memory when not needed

Question 4

Difference between stateless and stateful session beans?

Answer 4

Stateless beans are short-lived and not persistent (independent method calls).

Stateful session beans hold client information between method calls.

NOTE: singleton beans are useful for application-wide shared resources. Must be thread-safe/concurrent

Question 5

What are entity beans?

Answer 5

EJB bean that has persistent state

can be used by multiple clients at a time

Replaced by jakarta persistence (don’t depend on EJB container, less boilerplate)

Question 6

What are EJB session beans?

Answer 6

Interact with single client at a time.

Stateful session beans should be serializable

Relatively short lived (represents chunk of application logic)

Question 7

What is Jakarta Persistence?

Answer 7

Java classes with annotations

EntityManager manages beans that are attached:
- create
- find
- update
- delete

changes to attached beans tracked and saved to DB

detached beans are regular java objects that can be sent across the network/application as data transfer objects

Question 8

What is Object-to-relational persistence?

Answer 8

mapping object fields to a relational database using EJB 3 standard - Object-Relational Mapping (ORM)

Fields annotated with @Id represent PK
(Normally have @Column annotation as well)

Fields annotated with @Column(name=”someName”) maps field to a column in DB

Question 9

An entity bean is defined for each table (T or F)

Answer 9

T
(Multiple rows are created and mapped to multiple instances)

(CRUD operations from Entity manager manipulate these instances)

Question 10

Annotations for Jakarta persistence (Entity bean, ID - primary key, Table, Column )

Answer 10

@jakarta.persistence.*

Question 11

What is persistence.xml?

Answer 11

Lives in META-INF
Specifies name of persistence unit (configure how application connects to DB)
Specifies reference to data source (inside <persistence-unit>)
May specify vendor-specific options</persistence-unit>