03 Message Driven Beans

8/12/2019 03 Message Driven Beans

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Message Driven Beans

Daniel Stollenwerk


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SAP AG 2004 / Daniel Stollenwerk / Java Message Service & Message Driven Beans /Message Driven Beans

Af ter completing th is topic, you will be able to:

state the dif ferences between Message Driven Beans

(MDBs) and other Enterprise Java Beans

explain how messages are passed to MDBs

implement an MDB

Message Driven Beans: Topic Objectives


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Message Driven Beans add a new functionality to the Enterprise Java Beans:

asynchronous message receiving


Enterprise Java Beans Overview

Session

Beans

Entity

Beans

EJB

Stateless Statefull Container

Managed

Persistence

Bean

Managed

Persistence

Asynchronous

Communication

Message Driven

Beans

Synchronous Communication


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Sometimes Message Driven Beans are referred to as JMS Clients. This might be

because in a way they receive messages. But in fact the EJB Container acts as a

client and receives messages which it then distributes along the different Beans

respectively their instances

Message Driven Beans do not have any interfaces for direct access to their

business methods they can only be called via messages


Message Driven Beans

came new with EJB 2.0

are handled by the EJB Container, which acts as a JMS Client MDBs can consume messages (and optionally producethem)

will be assigned to one Topic or Queue

are server-side components (like other EJBs) and consist of

the bean class

two deployment descriptors

no interfaces (home, remote)

MDBs are accessed by sending messages to the accordingQueue or Topic

are stateless


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When developing an MDB you do not have to care about other methods thanonMessage( ) . You can of course put coding into the other methods if you need

to be executed at the according hooks but to keep it simple you may also just putthe needed business logic into onMessage( ) .


The Dual Nature of MDBs

EnterpriseBean

ExampleMDB

MessageDrivenBean

MessageListener

The business logic is put

into the onMessage() method.

javax.ejb.MessageDrivenBean

by this MDBs inherit themethods the container needs to

handle them like other EJBs

javax.jms.MessageListener

provide an asynchronousreceipt of JMS messages


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Transactions

MDBs do not use the usual JMS transaction model; they can make

use of distributed t ransactions

container-managed transaction demarcation

The onMessage() method gets one the following attributes

Required: The message receipt and theonMessage() method

run within the same, new transaction.

NotSupported: The onMessage() method runs without a

transaction.

bean-managed t ransaction demarcation

The onMessage() method starts and commits transactions

explicit.

The usual way of transaction handling in JMS applications is to specify which

messages should be sent / received as a transaction. MDBs do not care about

messages in transactions but rather make sure that their business logic is

executed in a transactional mode.

container-managed

If you choose this option you can choose one of the following:

Required: Every onMessage() execution, including all commands executed in the method, will be

handled as a transaction.

NotSupported: The onMessage( ) execution runs without a transaction

bean-managed

Arbitrary parts of the code in the onMessage() method may be put into a transaction


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The EJB Container automatically instantiates new objects of a MDB if there are

more messages to be processed than instances available.

Because the EJB Container delivers an incoming message to an arbitrary MDB

instance which is capable of processing the message there is no guarantee about

the order of the messages.


Parallel Processing

The EJB Container handles several instances of one MDB at one

time server-side concurrent processing

Al l instances are equivalent

Each MDB instance runs within its own thread.

There is no guarantee that messages are delivered

in exact the same order as they are sent.

JMS ProviderEJB

Container

MDB

Instance 1 Thread A

MDB

Instance 2 Thread B

MDB

Instance 3 Thread C

MDB


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voi d set MessageDr i venCont ext ( MessageDr i venCont ext ct x)

is called by the EJB Container after the creation of a MDB instance, providingthe MDB instance with a reference to its MessageDr i venCont ext object.

voi d ej bCr eat e( )

is called by the EJB Container after the creation of a MDB instance (after theset MessageDr i venCont ext ( ) method call).

In this method resources needed for the beans life can be obtained.

voi d ej bRemove( )

is called by the EJB Container before deleting the MDB instance.

Resources should be released here.

if the onMessage( ) method throws a Runt i meExcept i on the bean will return tothe state does not exist; the ej bRemove method will then not be called.


Lifecycle

does not

exist

method-ready

pool

ejbRemove()//create a new instance

setMessageDrivenContext()

ejbCreate()

onMessage()

called before

deletion

invoked on

message

receipt

called after

creation


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The ej b- j ar . xml is the J2EE standard deployment descriptor.


Deployment Descriptorsejb-jar.xml / 1

MDBs are declared in the ejb-jar.xml with in the tag

EJB related data / Messaging related data

EJB name

bean class name

transaction demarcation type

environment entries used by the MDB

The JMS destination type the MDB l istens to

EJB related

Messaging

related


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Deployment Descriptorsejb-jar.xml / 2

Name / class

Destination type

Environment


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Deployment Descriptorsejb-j2ee-engine.xml

Vendor specific descriptor

contains additional information JMS information

the concrete destination of the MDB

the concrete ConnectionFactory used


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You should now be able to:

state the dif ferences between Message Driven Beans

(MDBs) and other Enterprise Java Beans

explain how messages are passed to MDBs

implement an MDB

Message Driven Beans: Topic Summary

Documents

03 Message Driven Beans