Middleware Technology (Part VIII, Enterprise Java …Middleware Technology (Part VIII, Enterprise...

Middleware Technology(Part VIII, Enterprise Java

Beans)Andrei Popovici

Information and Communication SystemsResearch Group

Department of Computer ScienceETH Zürich

[email protected]://www.inf.ethz.ch/department/IS/iks/

2A. Popovici. Information and Communication Systems Research Group. ETH Zürich. Part VIII - EJB

Outline

ο EJB fundamentals: beans and containersο Containersο Beans

• session beans• entity beans

ο Exampleο EJB Internals:

⇒ Persistence⇒ Pooling⇒ Transactions

ο Conclusions and Outlook


The EJB Component ModelAhead towards the ideal middleware system:

ο portability⇒ Java⇒ components: pure application logic

ο Interoperability⇒ components have hooks to cooperate with the

environment (container)• EJB API and coding conventions

ο Integration⇒ Naming services⇒ Data Persistence⇒ Transactions⇒ Authentication & Authorization


EJB: Beans and ContainersTwo main concepts dominate the EJB paradigm:ο Beans: fit together in a standard way

⇒ contain pure application logic and hooks

ο Containers = middlewareο know how to communicate with the

beans and with the clientsο know how to create and destroy

beansο implement services developers don’t

want to worry aboutο all messages go through the

container

CONTAINER

clients

Java Virtual Machine

CONTAINER


Containersο A bean container is the server

application on which the beancode runs.⇒ container vendors: IBM,

SUN, Oracleο The container interposes itself

between the client and the bean.Server Host

ο Before a bean is able to live in a container, ithas to be adapted to a specific container.This process is called deployment.

ο Deployment is vendor specific... The client triggers the creation of beans inside the container,

but does not directly access the methods of the beans it hascreated. Instead, the container exports a remote interface tothe client.


EJB: The Big Picture

client

= the XML-formatted file created bythe deployer in which the type ofthe bean, its name, etc.. arespecified Deployment

descriptor

CONTAINER

deploy bean

JNDI: ejb/beans/myBeanH = bean code (app.logic)

Deploymentdescriptor

= bean home, generated by the deployer,published as jndi:ejb/beans/myBeanH

Step 1: install bean

(create Bean)ask home forbean instance

use bean

Step 2: use bean


Beans and Clientsο Beans are (independent) business objects. Some examples:

⇒ The small software piece behind a web site counter⇒ A currency converter⇒ A shopping cart in a web store may be implemented by a

bean. The items inside may be other beans.ο Scenarios:

⇒ The bean is not influenced by what the client did before(e.g., currency converter)

• Stateless session bean⇒ The bean is influenced by what the client did (e.g.,

shopping cart)• Stateful session bean

⇒ The bean has to survive a server crash (e.g., web sitecounter)

• Stateful, entity bean. All entity beans are stateful.


Bean Typesο Session Beans

⇒ are used by one client at a time⇒ are lost in a server crash⇒ Stateless: look the same to the client before and after the

operation⇒ Stateful: persistence

ο Entity Beans⇒ are the core business objects⇒ represent shared data in a database⇒ allow shared access from multiple clients⇒ survive a server crash


Example: Web Counter

Web server = client role

Pagerequest

container

ο Developer:

EJBObject

Counter

EntityBean

CounterBeanCounterHome

EJBHome

int click() create() ejbCreate()int click()

ο Deployer: <name>ejb/beans/myBeanH</name><home>CounterHome</home>...

(1) Counter.xml:

(2) bash$ deployejb counter.xml container

ο Java client (code running in the web server):

counterHome=ctx.lookup(“jndi:ejb/beans/myBeanH”);Counter c = counterHome.create();

app. logic

utility

ctx: JNDI Context


EJB and Persistence (1)ο Session beans have to use JDBC. Fortunately, beans do not

have to bother with the JDBC driver setup, which is the mainportability problem in Java applications which access adatabase. Instead, the database (data source) is published inthe JNDI naming service. The beans just specify the SQLcommand for retrieving the data from a database:⇒ Example stateless session bean:

⇒ Example stateful session bean:

ejbCreate() { /* ask container for data source*/}ejbRemove() { /* tell container we’re finished */ }

ejbCreate() {..}ejbRemove() {..}ejbActivate() { /* similar to ejbCreate */ }ejbPassivate() { /* similar to ejbRemove */}

(code from CounterBean.java)

(code from CounterBean.java)


EJB and Persistence (2)ο Entity beans usually correspond to a record in a relational database.

Therefore, it has a unique identifier which survives over time and overclient accesses: the primary key (PK). The bean developer must create aspecial PK Java class.

ο Clients get references to entity beans by specifying the PKο The persistence of an entity bean may be:

⇒ bean managed (like in session beans, using JDBC). In this case, thebean developer has to provide the proper code for storing andretrieving the state of the bean (ejbLoad, ejbStore)

⇒ container managed: the container automatically stores in thedatabase all attributes of an entity bean instance. Container managedpersistence makes life easier (no SQL statements) for the developer.The developer still has to implement some special methods.

ο What kind of persistence mechanisms the bean uses is specified in thedeployment descriptor:

<persistence-type>container</persistence-type>


EJB and Poolingο If a container has many clients, it cannot afford to keep all

beans associated to the clients in the main memory.Therefore⇒ it reuses the stateless session beans living in the

container⇒ it temporarily stores the data of a stateful session bean in

persistent storage (activation and passivation)ο As a consequence, the developer of a bean has to implement

methods like ejbActivate, ejbPassivate, ejbCreate,ejbRemove to react to the container’s decision about a bean.

ο On the other hand, the container has to consistently callthese methods every time it creates, destroys, or stores abean.


EJB and Transactionsο A business method implemented by a bean (e.g.

‘doReservation(hotel,flight)’ has to be executed atomically,like a transaction

ο The deployer can specify this in the deploymentdescriptor,like this:

ο The transaction management in EJB is based the JTA (Java Transaction API)ο The transaction management (like persistence) may be

⇒ implicit - container uses JTA, developer writes pureapplication logic

⇒ explicit - developer writes JTA and JDBC code

<container-transaction> <method>doReservation</method> <trans-attribute>required</trans-attribute></container-transaction>


Practical Considerationsο Shall I choose Corba or EJB?

⇒ Corba provides at least the same services as EJB.⇒ EJB application is easier to write and deploy.⇒ The EJB framework is complex.

ο Which container vendor shall I choose?⇒ Sun’s specifications are ahead of the vendor’s

implementation.ο Current trend: everybody is reshaping their applications

towards EJBο World Wide Web: Java Server Pages (JSP) or Servlets work

well together with EJBο Performance: Java is still slower its counterparts for WWW

⇒ however, EJB provides good scalability.


Conclusions and Outlookο Enterprise Java Beans are easy to writeο But the EJB architecture is complex and is closely tied to

other Java packages and specifications, like JNDI, JDBC,JTA or RMI. A bean developer will have to understand themas well.

ο By separating deployment and development, reusing ofbeans becomes a reality. Most of the services needed bysuch components can be transparently managed by thecontainer. The almost complete separation between servicesand business logic is one of the main advantages of EJB.

ο SUN’s specifications are ahead of the products supplied bycontainer vendors.

ο EJB can be used in conjunction with other server-side javatechnologies like servlets or Java Server Pages (JSP). That’swhy many E-commerce companies are shifting their productstowards EJB and Java-based www technologies.


Appendix: ContainerBean.java

Class CounterBean implements EntityBean { public String counterId; public int counterValue; public CoutnerPK ejbCreate(String id, int cval) {..} public ejbPostCreate(String id,int cval) {..} public setEntityContext(EntityContext ctx) {..} public unsetEntityContext() {..} public void ejbActivate() {..} public void ejbPassivate() {..} public void ejbLoad() {..} public void ejbStore() {..} public void click() { } public int getValue();}

This Java class is an example ofabout half the methods whichhave to be implemented in aBean. Sometimes, the methodsmay be left empty, e.g., if thepersistence is container-managed.

Middleware Technology (Part VIII, Enterprise Java …Middleware Technology (Part VIII, Enterprise...

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