The document details the architecture and functionality of Enterprise JavaBeans (EJBs) within Java EE applications, including various types such as stateful, stateless, and singleton session beans, as well as message-driven beans. It discusses transaction management, including how to handle transactions both by the application container and manually, and outlines design patterns like DAO and DTO for data management. The lecture emphasizes the role of EJBs in encapsulating business logic and managing interactions between components efficiently.

1. Business layer
Lecturer: Ondrej Mihályi

2. Contents
●Multilayered application architecture
●Enterprise Java Beans
●Transaction creation and handling
●Java EE architectural patterns

3. Layers of Java EE application

4. Traditional transactions
tx.begin();
try {
// … executing in transaction
tx.commit
} catch (Exception e) {
tx.rollback();
}

5. How simple can transactions be?
@Inject ServiceBean service;
// before transaction
service.execute(); // running in transaction
// after transaction

6. What are EJBs?
● Represent service layer
● Components encapsulating business logic
● Automatic handling of transactions
● Accessible remotely outside of application
● Security authorization

7. Types of EJBs
● Session beans:
– Stateless, Stateful, Singleton
– Accessed directly via a method call
● Message-driven beans
– Exexcuted when message is delivered

8. Stateful Session Bean
● Stateful instance unique to the client
● Holds state between method calls
● Only one client can access at a time
● Must support serialization (swapping)
● @Stateful

9. When Stateful?
● The bean's state represents the interaction
between the bean and a specific client.
● The bean needs to hold information about
the client across method invocations.
● Behind the scenes, the bean manages the
work flow of several enterprise beans.

10. Stateless Session Bean
● State is not held between method calls
● Can be reused by multiple clients
● More efficient than Stateful
– Reused using a pool of beans
● Can implement a web service
● @Stateless

13. When Stateless?
● The bean does not have state.
● The bean methods are independent.
● The bean implements a web service.
● To improve performance, whenever
Stateful is not necessary.

14. Singleton Session Bean
● Similar to stateless, but:
– Single instance is reused by all clients
– State is held between method calls
● Can be created on application startup
● @Singleton (javax.ejb.Singleton)

15. When Singleton?
● When some data is shared by multiple
clients
● To perform an action at application startup

16. Message-driven Bean
● Single method onMessage(Message msg)
● Called when message received
● Otherwise resembles Stateless
– No state, can be pooled, shared by clients
● @MessageDriven

18. When Message-driven?
● Asynchronous processing
– Distribute processing on multiple nodes
– Do not block other requests
– Queue execution after pending requests

19. Transaction management
● @TransactionManagement
● By container (default setting)
– Application server creates and closes transactions
– TransactionManagementType.CONTAINER
● By application
– Application calls methods on transaction object
– TransactionManagementType.BEAN

20. Container managed Tx
● @TransactionAttribute
● REQUIRED, REQUIRES_NEW,
NOT_SUPPORTED, …
● Configuration on class level, or for each
method

21. Application managed Tx
● @Inject UserTransaction tx;
● tx.begin(), tx.commit()

22. Asynchronous invocation
● Execute a business method in another
thread
● Runs in a new transaction
● Method either void or returns Future
● @Asynchronous

23. Remote business interface
● EJB can be accessible from outside of application
● Needs to implement an interface
– Marked be @Remote
● Exposed in JNDI registry
● Remote method invocation from outside of application
server

24. DAO pattern
● Data Access Object
● DAO objects are abstraction over persistence
layer
● Provide object-oriented encapsulation
● Single point of querying, entity creation and
persisting

25. DTO pattern
● Data Transfer Object
● Simple value objects to transfer data
between layers
● Used to transfer data specific to one layer
into another layer in various contexts and
forms

26. Service facade
● Groups multiple components
● Provides single access methods to execute
complicated business logic
● When multiple business components must run in
single transaction
● Represented by EJBs, which combine logic of multiple
simpler EJBs