Units 7 & 8

Question 1

Remote Method Invocation (RMI)

Answer 1

object-orientated counterpart of RPC

raised the naming, locating + binding problems of distributed systems

a technology that can be used by itself

Question 2

Registry

Answer 2

a naming service that associates objects to unique names

Question 3

Layered architecture of remote method calls

Answer 3

1. server application binds the server object to a symbolic name in the registry
2. client object looks up the symbolic name in the registry + obtains a reference to the server object
3. client object uses the reference to make a remote method call

Question 4

Remote method

Answer 4

a method that can be called from outside the JVM where is is executing

(local in same JVM)

Question 5

Remote object

Answer 5

an object with at least one remote method

Question 6

Remote interface

Answer 6

specifies methods available to client objects

objects of classes implementing this interface will be accessed remotely

Question 7

Remote Method Invocation (RMI) proxy later

Answer 7

aka stub/skeleton layer

stub - client-side proxy object

skeleton - server-side proxy object

Question 8

Location transparency (RMI)

Answer 8

the actual location of the stub object is unknown to the client

Question 9

Enterprise JavaBeans (EJB)

Answer 9

a component model defined by JavaEE for building enterprise applications

Question 10

What is a bean?

Answer 10

a component written in Java

types - session beans (business logic) + entity classes (business data)

Question 11

Primary key

Answer 11

uniquely identify a record

Question 12

Foreign key

Answer 12

uniquely identifies a record in another table

Question 13

How can a remote object, remote interface + a remote method be detected in the server-side code?

Answer 13

a remote interface extends Remote

a remote method throws a RemoteException

a remote object’s class usually extends UnicastRemoteObject

Question 14

How must the registry be set up?

Answer 14

a registry service is started on a port known to the client + server applications

Question 15

how does the server make the remote object available to clients

Answer 15

server application creates a remote object + exports it so that it is ready to receive incoming calls

server application then asks the registry to bind a name to a remote reference - this name is made known to clients

Question 16

How does a client obtain a reference to the remote object?

Answer 16

client looks up the name in the registry

registry returns (using serialisation) a copy of the stub object

client now has a local reference to the stub object residing on the same JVM heap

Question 17

Entity classes

Answer 17

classes that represent the business entities stored persistently in the relational db

their corresponding instances are called entities

@Entity annotation to state it is an entity class

Question 18

Persistence context

Answer 18

a set of entities for which the runtime system knows where the data is in persistent storage

only when an entity is within a persistence context can any previous changes to it be made persistent

Question 19

Entity manager

Answer 19

manage an entity’s lifecycle

javax.persistence.EntityManager interface provides methods to put entities into persistent contexts

Question 20

Entity lifecycle stages (4)

Answer 20

New
Managed
Detached
Removed

new + detached outside persistent context

managed + removed inside persistent context

Question 21

Session bean

Answer 21

access + modify entities

encapsulates an interactive session of a single client with the application server

an interaction consists of clients calling the one of the bean’s methods + possibly getting a result back

a session is therefore a sequence of one or more method calls + returns between client + session bean

session beans are not persistent + are not shared among clients

Question 22

Stateless session bean

Answer 22

annotated with @Stateless

access to the db can be done be:

1. beans’s constructor creates an entity manager
2. each bean’s method calls the necessary entity
   manager methods to retrieve, change + store
   entities
3. entity manager is closed when the bean is no
   longer necessary

Question 23

Dependency injection

Answer 23

an object (e.g. session bean) indicates that it depends on the existence of another object (e.g, entity manager) + the container ‘injects’ such an object into the application at runtime

Question 24

Stateful session bean

Answer 24

retain info across multiple method calls

annotated with @Stateful

typically provides:

• one or more constructors to initialise the session
• one or more methods to use during the conversation
• one method to signal the end of the session
  (annotated with @Remote)

Question 25

What are the 2 types of entity manager?

Answer 25

1. application-managed

2. container-managed

Question 26

Transfer objects

Answer 26

contain all the necessary info in their instance variables

fewer methods required + their signatures concise + fewer remote method calls have to be made

represent the clients view of data while entities represent the server’s view

suitable for data transfer between clients + session beans

if client remote transfer objects must be declared as Serializable + they are copied in method calls + returns - this means any change to the TO by the client is not reflected in the server

Question 27

Summarise the rationale for using transfer objects

Answer 27

shield entities from clients
simplify business interfaces
reduce the number of methods + calls necessary to transfer data between clients + session beans