Spring Framework Interveiw Questions and Answers: EJB Interview Questions

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EJB INTERVIEW QUESTIONS

How EJB Invocation happens?

• Retrieve Home Object reference from Naming Service via JNDI

• Return Home Object reference to the client

• Create me a new EJB Object through Home Object interface

• Create EJB Object from the Ejb Object

• Return EJB Object reference to the client

• Invoke business method using EJB Object reference

• Delegate request to Bean (Enterprise Bean).

Is it possible to share an HttpSession between a JSP and EJB? What

happens when I change a value in the HttpSession from inside an EJB?

You can pass the HttpSession as parameter to an EJB method, only if all

objects in session are serializable.This has to be consider as ?passed-by-value", that

means that it?s read-only in the EJB. If anything is altered from inside the EJB, it

won?t be reflected back to the HttpSession of the Servlet Container.The ?pass-byreference?

can be used between EJBs Remote Interfaces, as they are remote

references. While it IS possible to pass an HttpSession as a parameter to an EJB

object, it is considered to be ?bad practice ? in terms of object oriented design. This

is because you are creating an unnecessary coupling between back-end objects

(ejbs) and front-end objects (HttpSession). Create a higher-level of abstraction for

your ejb?s api. Rather than passing the whole, fat, HttpSession (which carries with it

a bunch of http semantics), create a class that acts as a value object (or structure)

that holds all the data you need to pass back and forth between front-end/back-end.

Consider the case where your ejb needs to support a non-http-based client. This

higher level of abstraction will be flexible enough to support it.

The EJB container implements the EJBHome and EJBObject classes. For

every request from a unique client, does the container create a separate

instance of the generated EJBHome and EJBObject classes?

The EJB container maintains an instance pool. The container uses these

instances for the EJB Home reference irrespective of the client request. while refering

the EJB Object classes the container creates a separate instance for each client

request. The instance pool maintainence is up to the implementation of the

container. If the container provides one, it is available otherwise it is not mandatory

for the provider to implement it. Having said that, yes most of the container

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providers implement the pooling functionality to increase the performance of the

application server. The way it is implemented is again up to the implementer.

Can the primary key in the entity bean be a Java primitive type such as int?

The primary key can't be a primitive type--use the primitive wrapper classes,

instead. For example, you can use java.lang.Integer as the primary key class, but

not int (it has to be a class, not a primitive)

Can you control when passivation occurs?

The developer, according to the specification, cannot directly control when

passivation occurs. Although for Stateful Session Beans, the container cannot

passivate an instance that is inside a transaction. So using transactions can be a a

strategy to control passivation.

The ejbPassivate() method is called during passivation, so the developer has

control over what to do during this exercise and can implement the require optimized

logic.

Some EJB containers, such as BEA WebLogic, provide the ability to tune the

container to minimize passivation calls.

Taken from the WebLogic 6.0 DTD -"The passivation-strategy can be either

"default" or "transaction". With the default setting the container will attempt to keep

a working set of beans in the cache. With the "transaction" setting, the container will

passivate the bean after every transaction (or method call for a non-transactional

invocation).

What is the advantage of using Entity bean for database operations, over

directly using JDBC API to do database operations? When would I use one

over the other?

Entity Beans actually represents the data in a database. It is not that Entity

Beans replaces JDBC API. There are two types of Entity Beans Container Managed

and Bean Mananged. In Container Managed Entity Bean - Whenever the instance of

the bean is created the container automatically retrieves the data from the

DB/Persistance storage and assigns to the object variables in bean for user to

manipulate or use them. For this the developer needs to map the fields in the

database to the variables in deployment descriptor files (which varies for each

vendor).

In the Bean Managed Entity Bean - The developer has to specifically make

connection, retrive values, assign them to the objects in the ejbLoad() which will be

called by the container when it instatiates a bean object. Similarly in the ejbStore()

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the container saves the object values back the the persistance storage. ejbLoad and

ejbStore are callback methods and can be only invoked by the container. Apart from

this, when you use Entity beans you dont need to worry about database transaction

handling, database connection pooling etc. which are taken care by the ejb

container. But in case of JDBC you have to explicitly do the above features. what

suresh told is exactly perfect. ofcourse, this comes under the database transations,

but i want to add this. the great thing about the entity beans of container managed,

whenever the connection is failed during the transaction processing, the database

consistancy is mantained automatically. the container writes the data stored at

persistant storage of the entity beans to the database again to provide the database

consistancy. where as in jdbc api, we, developers has to do manually.

What is EJB QL?

EJB QL is a Query Language provided for navigation across a network of

enterprise beans and dependent objects defined by means of container managed

persistence. EJB QL is introduced in the EJB 2.0 specification. The EJB QL query

language defines finder methods for entity beans with container managed

persistenceand is portable across containers and persistence managers. EJB QL is

used for queries of two types of finder methods: Finder methods that are defined in

the home interface of an entity bean and which return entity objects. Select

methods, which are not exposed to the client, but which are used by the Bean

Provider to select persistent values that are maintained by the Persistence Manager

or to select entity objects that are related to the entity bean on which the query is

defined.

Write a brief description about local interfaces

EJB was originally designed around remote invocation using the Java Remote

Method Invocation (RMI) mechanism, and later extended to support to standard

CORBA transport for these calls using RMI/IIOP. This design allowed for maximum

flexibility in developing applications without consideration for the deployment

scenario, and was a strong feature in support of a goal of component reuse in J2EE.

Many developers are using EJBs locally -- that is, some or all of their EJB calls are

between beans in a single container.

With this feedback in mind, the EJB 2.0 expert group has created a local

interface mechanism. The local interface may be defined for a bean during

development, to allow streamlined calls to the bean if a caller is in the same

container. This does not involve the overhead involved with RMI like marshalling etc.

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This facility will thus improve the performance of applications in which co-location is

planned.

Local interfaces also provide the foundation for container-managed

relationships among entity beans with container-managed persistence.

What are the special design care that must be taken when you work with

local interfaces?

EIt is important to understand that the calling semantics of local interfaces

are different from those of remote interfaces. For example, remote interfaces pass

parameters using call-by-value semantics, while local interfaces use call-byreference.

This means that in order to use local interfaces safely, application developers

need to carefully consider potential deployment scenarios up front, then decide which

interfaces can be local and which remote, and finally, develop the application code

with these choices in mind.

While EJB 2.0 local interfaces are extremely useful in some situations, the

long-term costs of these choices, especially when changing requirements and

component reuse are taken into account, need to be factored into the design

decision.

What happens if remove( ) is never invoked on a session bean?

In case of a stateless session bean it may not matter if we call or not as in

both cases nothing is done. The number of beans in cache is managed by the

container.

In case of stateful session bean, the bean may be kept in cache till either the

session times out, in which case the bean is removed or when there is a requirement

for memory in which case the data is cached and the bean is sent to free pool.

What is the difference between Message Driven Beans and Stateless Session

beans?

In several ways, the dynamic creation and allocation of message-driven bean

instances mimics the behavior of stateless session EJB instances, which exist only for

the duration of a particular method call. However, message-driven beans are

different from stateless session EJBs (and other types of EJBs) in several significant

ways:

• Message-driven beans process multiple JMS messages asynchronously, rather

than processing a serialized sequence of method calls.

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• Message-driven beans have no home or remote interface, and therefore

cannot be directly accessed by internal or external clients. Clients interact

with message-driven beans only indirectly, by sending a message to a JMS

Queue or Topic.

Note: Only the container directly interacts with a message-driven bean by

creating bean instances and passing JMS messages to those instances as necessary.

The Container maintains the entire lifecycle of a message-driven bean; instances

cannot be created or removed as a result of client requests or other API calls.

How can I call one EJB from inside of another EJB?

EJBs can be clients of other EJBs. It just works. Use JNDI to locate the Home

Interface of the other bean, then acquire an instance reference, and so forth.

What is an EJB Context?

EJBContext is an interface that is implemented by the container, and it is also

a part of the bean-container contract. Entity beans use a subclass of EJBContext

called EntityContext. Session beans use a subclass called SessionContext. These

EJBContext objects provide the bean class with information about its container, the

client using the bean and the bean itself. They also provide other functions. See the

API docs and the spec for more details.

The EJB container implements the EJBHome and EJBObject classes. For

every request from a unique client, does the container create a separate

instance of the generated EJBHome and EJBObject classes?

The EJB container maintains an instance pool. The container uses these

instances for the EJB Home reference irrespective of the client request. While

refering the EJB Object classes the container creates a separate instance for each

client request. The instance pool maintainence is up to the implementation of the

container. If the container provides one, it is available otherwise it is not mandatory

for the provider to implement it. Having said that, yes most of the container

providers implement the pooling functionality to increase the performance of the

application server. The way it is implemented is again up to the implementer.

What is EJB?

Enterprise JavaBeans (EJB) technology is the server-side component

architecture for the Java 2 Platform, Enterprise Edition (J2EE) platform. EJB

technology enables rapid and simplified development of distributed, transactional,

secure and portable applications based on Java technology.

What are the different types of Enterprise JavaBeans?

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There are 3 types of enterprise beans, namely: Session bean, Entity beans

and Message driven beans.

What is Session Bean?

Session bean represents a single client inside the J2EE server. To access the

application deployed in the server the client invokes methods on the session bean.

The session bean performs the task shielding the client from the complexity of the

business logic.

Session bean components implement the javax.ejb.SessionBean interface.

Session beans can act as agents modeling workflow or provide access to special

transient business services. Session beans do not normally represent persistent

business concepts. A session bean corresponds to a client server session. The

session bean is created when a client requests some query on the database and

exists as long as the client server session exists.

What are different types of session bean ?

There are two types of session beans, namely: Stateful and Stateless.

What is a Stateful Session bean?

Stateful session bean maintain the state of the conversation between the

client and itself. When the client invokes a method on the bean the instance

variables of the bean may contain a state but only for the duration of the invocation.

A stateful session bean is an enterprise bean (EJB component) that acts as a

server-side extension of the client that uses it. The stateful session bean is created

by a client and will work for only that client until the client connection is dropped or

the bean is explicitly removed. The stateful session bean is EJB component that

implements the javax.ejb.SessionBean interface and is deployed with the declarative

attribute "stateful". Stateful session beans are called "stateful" because they

maintain a conversational state with the client. In other words, they have state or

instance fields that can be initialized and changed by the client with each method

invocation. The bean can use the conversational state as it process business

methods invoked by the client.

What is stateless session bean ?

Stateless session beans are of equal value for all instances of the bean. This

means the container can assign any bean to any client, making it very scalable.

A stateless session bean is an enterprise bean that provides a stateless

service to the client. Conceptually, the business methods on a stateless session bean

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are similar to procedural applications or static methods; there is no instance state,

so all the data needed to execute the method is provided by the method arguments.

The stateless session bean is an EJB component that implements the

javax.ejb.SessionBean interface and is deployed with the declarative attribute

"stateless". Stateless session beans are called "stateless" because they do not

maintain conversational state specific to a client session. In other words, the

instance fields in a stateless session bean do not maintain data relative to a client

session. This makes stateless session beans very lightweight and fast, but also limits

their behavior. Typically an application requires less number of stateless beans

compared to stateful beans.

What is an Entity Bean?

An entity bean represents a business object in a persistent storage

mechanism. An entity bean typically represents a table in a relational database and

each instance represents a row in the table. Entity bean differs from session bean

by: persistence, shared access, relationship and primary key. T

What are different types of entity beans?

There are two types of entity beans available. Container Managed Persistence

(CMP) , Bean managed persistence (BMP).

What is CMP (Container Managed Persistence)?

The term container-managed persistence means that the EJB container

handles all database access required by the entity bean. The bean's code contains no

database access (SQL) calls. As a result, the bean's code is not tied to a specific

persistent storage mechanism (database). Because of this flexibility, even if you

redeploy the same entity bean on different J2EE servers that use different databases,

you won't need to modify or recompile the bean's code. So, your entity beans are

more portable.

What is BMP (Bean managed persistence)?

Bean managed persistence (BMP) occurs when the bean manages its

persistence. Here the bean will handle all the database access. So the bean's code

contains the necessary SQLs calls. So it is not much portable compared to CMP.

Because when we are changing the database we need to rewrite the SQL for

supporting the new database.

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What is abstract schema?

In order to generate the data access calls, the container needs information

that you provide in the entity bean's abstract schema. It is a part of Deployment

Descriptor. It is used to define the bean's persistent fields and relation ships.

When we should use Entity Bean?

When the bean represents a business entity, not a procedure. we should use

an entity bean. Also when the bean's state must be persistent we should use an

entity bean. If the bean instance terminates or if the J2EE server is shut down, the

bean's state still exists in persistent storage (a database).

When to Use Session Beans?

At any given time, only one client has access to the bean instance. The state

of the bean is not persistent, existing only for a short period (perhaps a few hours).

The bean implements a web service. Under all the above circumstances we can use

session beans.

When to use Stateful session bean?

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. The bean mediates between the client and the other components of the

application, presenting a simplified view to the client. Under all the above

circumstances we can use a stateful session bean.

When to use a stateless session bean?

The bean's state has no data for a specific client. In a single method

invocation, the bean performs a generic task for all clients. For example, you might

use a stateless session bean to send an email that confirms an online order. The

bean fetches from a database a set of read-only data that is often used by clients.

Such a bean, for example, could retrieve the table rows that represent the products

that are on sale this month. Under all the above circumstance we can use a stateless

session bean.

How EJB Invocation happens?

• Retrieve Home Object reference from Naming Service via JNDI.

• Return Home Object reference to the client.

• Create me a new EJB Object through Home Object interface.

• Create EJB Object from the Ejb Object.

• Return EJB Object reference to the client.

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• Invoke business method using EJB Object reference. Delegate request to

Bean (Enterprise Bean).

Is it possible to share an HttpSession between a JSP and EJB? What

happens when I change a value in the HttpSession from inside an EJB?

You can pass the HttpSession as parameter to an EJB method, only if all

objects in session are serializable.This has to be consider as passed-by-value, that

means that it’s read-only in the EJB. If anything is altered from inside the EJB, it

won’t be reflected back to the HttpSession of the Servlet Container.The pass-byreference

can be used between EJBs Remote Interfaces, as they are remote

references. While it is possible to pass an HttpSession as a parameter to an EJB

object, it is considered to be bad practice in terms of object-oriented design. This is

because you are creating an unnecessary coupling between back-end objects (EJBs)

and front-end objects (HttpSession). Create a higher-level of abstraction for your

EJBs API. Rather than passing the whole, fat, HttpSession (which carries with it a

bunch of http semantics), create a class that acts as a value object (or structure)

that holds all the data you need to pass back and forth between front-end/back-end.

Consider the case where your EJB needs to support a non HTTP-based client. This

higher level of abstraction will be flexible enough to support it.

The EJB container implements the EJBHome and EJBObject classes. For

every request from a unique client, does the container create a separate

instance of the generated EJBHome and EJBObject classes?

The EJB container maintains an instance pool. The container uses these

instances for the EJB Home reference irrespective of the client request. while refering

the EJB Object classes the container creates a separate instance for each client

request. The instance pool maintenance is up to the implementation of the container.

If the container provides one, it is available otherwise it is not mandatory for the

provider to implement it. Having said that, yes most of the container providers

implement the pooling functionality to increase the performance of the application

server. The way it is implemented is, again, up to the implementer.

Can the primary key in the entity bean be a Java primitive type such as int?

The primary key can’t be a primitive type. Use the primitive wrapper classes,

instead. For example, you can use java.lang.Integer as the primary key class, but

not int (it has to be a class, not a primitive).

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Can you control when passivation occurs?

The developer, according to the specification, cannot directly control when

passivation occurs. Although for Stateful Session Beans, the container cannot

passivate an instance that is inside a transaction. So using transactions can be a a

strategy to control passivation. The ejbPassivate() method is called during

passivation, so the developer has control over what to do during this exercise and

can implement the require optimized logic. Some EJB containers, such as BEA

WebLogic, provide the ability to tune the container to minimize passivation calls.

Taken from the WebLogic 6.0 DTD -”The passivation-strategy can be either “default”

or “transaction”. With the default setting the container will attempt to keep a

working set of beans in the cache. With the “transaction” setting, the container will

passivate the bean after every transaction (or method call for a non-transactional

invocation).

What is the advantage of using Entity bean for database operations, over

directly using JDBC API to do database operations? When would I use one

over the other?

Entity Beans actually represents the data in a database. It is not that Entity

Beans replaces JDBC API. There are two types of Entity Beans Container Managed

and Bean Mananged. In Container Managed Entity Bean - Whenever the instance of

the bean is created the container automatically retrieves the data from the

DB/Persistance storage and assigns to the object variables in bean for user to

manipulate or use them. For this the developer needs to map the fields in the

database to the variables in deployment descriptor files (which varies for each

vendor). In the Bean Managed Entity Bean - The developer has to specifically make

connection, retrive values, assign them to the objects in the ejbLoad() which will be

called by the container when it instatiates a bean object. Similarly in the ejbStore()

the container saves the object values back the the persistance storage. ejbLoad and

ejbStore are callback methods and can be only invoked by the container. Apart from

this, when you use Entity beans you dont need to worry about database transaction

handling, database connection pooling etc. which are taken care by the ejb

container.

What is EJB QL?

EJB QL is a Query Language provided for navigation across a network of

enterprise beans and dependent objects defined by means of container managed

persistence. EJB QL is introduced in the EJB 2.0 specification. The EJB QL query

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language defines finder methods for entity beans with container managed

persistenceand is portable across containers and persistence managers. EJB QL is

used for queries of two types of finder methods: Finder methods that are defined in

the home interface of an entity bean and which return entity objects. Select

methods, which are not exposed to the client, but which are used by the Bean

Provider to select persistent values that are maintained by the Persistence Manager

or to select entity objects that are related to the entity bean on which the query is

defined.

Write a brief description about local interfaces?

EJB was originally designed around remote invocation using the Java Remote

Method Invocation (RMI) mechanism, and later extended to support to standard

CORBA transport for these calls using RMI/IIOP. This design allowed for maximum

flexibility in developing applications without consideration for the deployment

scenario, and was a strong feature in support of a goal of component reuse in J2EE.

Many developers are using EJBs locally, that is, some or all of their EJB calls are

between beans in a single container. With this feedback in mind, the EJB 2.0 expert

group has created a local interface mechanism. The local interface may be defined

for a bean during development, to allow streamlined calls to the bean if a caller is in

the same container. This does not involve the overhead involved with RMI like

marshalling etc. This facility will thus improve the performance of applications in

which co-location is planned. Local interfaces also provide the foundation for

container-managed relationships among entity beans with container-managed

persistence.

What are the special design care that must be taken when you work with

local interfaces?

It is important to understand that the calling semantics of local interfaces are

different from those of remote interfaces. For example, remote interfaces pass

parameters using call-by-value semantics, while local interfaces use call-byreference.

This means that in order to use local interfaces safely, application

developers need to carefully consider potential deployment scenarios up front, then

decide which interfaces can be local and which remote, and finally, develop the

application code with these choices in mind. While EJB 2.0 local interfaces are

extremely useful in some situations, the long-term costs of these choices, especially

when changing requirements and component reuse are taken into account, need to

be factored into the design decision.

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What happens if remove( ) is never invoked on a session bean?

In case of a stateless session bean it may not matter if we call or not as in

both cases nothing is done. The number of beans in cache is managed by the

container. In case of stateful session bean, the bean may be kept in cache till either

the session times out, in which case the bean is removed or when there is a

requirement for memory in which case the data is cached and the bean is sent to

free pool.

What is the difference between Message Driven Beans and Stateless Session

beans?

In several ways, the dynamic creation and allocation of message-driven bean

instances mimics the behavior of stateless session EJB instances, which exist only for

the duration of a particular method call. However, message-driven beans are

different from stateless session EJBs (and other types of EJBs) in several significant

ways: Message-driven beans process multiple JMS messages asynchronously, rather

than processing a serialized sequence of method calls. Message-driven beans have

no home or remote interface, and therefore cannot be directly accessed by internal

or external clients. Clients interact with message-driven beans only indirectly, by

sending a message to a JMS Queue or Topic. Only the container directly interacts

with a message-driven bean by creating bean instances and passing JMS messages

to those instances as necessary. The Container maintains the entire lifecycle of a

message-driven bean; instances cannot be created or removed as a result of client

requests or other API calls.

How can I call one EJB from inside of another EJB?

EJBs can be clients of other EJBs. It just works. Use JNDI to locate the Home

Interface of the other bean, then acquire an instance reference, and so forth.

What is an EJB Context?

EJBContext is an interface that is implemented by the container, and it is also

a part of the bean-container contract. Entity beans use a subclass of EJBContext

called EntityContext. Session beans use a subclass called SessionContext. These

EJBContext objects provide the bean class with information about its container, the

client using the bean and the bean itself. They also provide other functions. See the

API docs and the spec for more details.

What are the different kinds of enterprise beans?

Different kind of enterrise beans are Stateless session bean, Stateful session

bean, Entity bean, Message-driven bean.

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What is Session Bean?

A session bean is a non-persistent object that implements some business

logic running on the server. One way to think of a session object.

What are the methods of Entity Bean?

An entity bean consists of 4 groups of methods, create methods.

What is the difference between Container-Managed Persistent (CMP) bean

and Bean-Managed Persistent(BMP)?

Container-managed persistence (CMP) and bean-managed persistence (BMP).

With CMP, the container manages the persistence of the entity bean.

What are the callback methods in Entity beans?

Callback methods allows the container to notify the bean of events in its life

cycle. The callback methods are defined in the javax.ejb.EntityBean interface.

What is software architecture of EJB?

Session and Entity EJBs consist of 4 and 5 parts respectively, a remote

interface.

Can Entity Beans have no create() methods?

Yes. In some cases the data is inserted NOT using Java application,.

What is bean managed transaction?

If a developer doesn’t want a Container to manage transactions, it’s possible

to implement all database operations manually.

What are transaction attributes?

The transaction attribute specifies how the Container must manage

transactions for a method when a client invokes the method via the enterprise bean’s

home.

What are transaction isolation levels in EJB?

• Transaction_read_uncommitted

• Transaction_read_committed

• Transaction_repeatable_read.

Is it possible for an EJB client to marshal an object of class java.lang.Class

to an EJB?

Technically yes, spec. compliant NO! - The enterprise bean must not attempt

to query a class to obtain information about the declared members that are not

otherwise accessible to the enterprise bean because of the security rules of the Java

language.

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Is it legal to have static initializer blocks in EJB?

Although technically it is legal, static initializer blocks are used to execute

some piece of code before executing any constructor or method while instantiating a

class. Static initializer blocks are also typically used to initialize static fields - which

may be illegal in EJB if they are read/write - In EJB this can be achieved by including

the code in either the ejbCreate(), setSessionContext() or setEntityContext()

methods.

Is it possible to stop the execution of a method before completion in a

SessionBean?

Stopping the execution of a method inside a Session Bean is not possible

without writing code inside the Session Bean. This is because you are not allowed to

access Threads inside an EJB.

What is the default transaction attribute for an EJB?

There is no default transaction attribute for an EJB. Section 11.5 of EJB v1.1

spec says that the deployer must specify a value for the transaction attribute for

those methods having container managed transaction. In WebLogic, the default

transaction attribute for EJB is SUPPORTS.

What is the difference between session and entity beans? When should I

use one or the other?

An entity bean represents persistent global data from the database; a session

bean represents transient user-specific data that will die when the user disconnects

(ends his session). Generally, the session beans implement business methods (e.g.

Bank.transferFunds) that call entity beans (e.g. Account.deposit, Account.withdraw)

Is there any default cache management system with Entity beans? In other

words whether a cache of the data in database will be maintained in EJB?

Caching data from a database inside the Application Server are what Entity

EJB’s are used for.The ejbLoad() and ejbStore() methods are used to synchronize the

Entity Bean state with the persistent storage(database). Transactions also play an

important role in this scenario. If data is removed from the database, via an external

application - your Entity Bean can still be “alive” the EJB container. When the

transaction commits, ejbStore() is called and the row will not be found, and the

transaction rolled back.

Why is ejbFindByPrimaryKey mandatory?

An Entity Bean represents persistent data that is stored outside of the EJB

Container/Server. The ejbFindByPrimaryKey is a method used to locate and load an

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Entity Bean into the container, similar to a SELECT statement in SQL. By making this

method mandatory, the client programmer can be assured that if they have the

primary key of the Entity Bean, then they can retrieve the bean without having to

create a new bean each time - which would mean creating duplications of persistent

data and break the integrity of EJB.

Why do we have a remove method in both EJBHome and EJBObject?

With the EJBHome version of the remove, you are able to delete an entity

bean without first instantiating it (you can provide a PrimaryKey object as a

parameter to the remove method). The home version only works for entity beans.

On the other hand, the Remote interface version works on an entity bean that you

have already instantiated. In addition, the remote version also works on session

beans (stateless and stateful) to inform the container of your loss of interest in this

bean.

How can I call one EJB from inside of another EJB?

EJBs can be clients of other EJBs. It just works. Use JNDI to locate the Home

Interface of the other bean, then acquire an instance reference, and so forth.

What is the difference between a Server, a Container, and a Connector?

An EJB server is an application, usually a product such as BEA WebLogic, that

provides (or should provide) for concurrent client connections and manages system

resources such as threads, processes, memory, database connections, network

connections, etc. An EJB container runs inside (or within) an EJB server, and

provides deployed EJB beans with transaction and security management, etc. The

EJB container insulates an EJB bean from the specifics of an underlying EJB server by

providing a simple, standard API between the EJB bean and its container. A

Connector provides the ability for any Enterprise Information System (EIS) to plug

into any EJB server which supports the Connector architecture.

How is persistence implemented in enterprise beans?

Persistence in EJB is taken care of in two ways, depending on how you

implement your beans: container managed persistence (CMP) or bean managed

persistence (BMP) For CMP, the EJB container which your beans run under takes care

of the persistence of the fields you have declared to be persisted with the database -

this declaration is in the deployment descriptor. So, anytime you modify a field in a

CMP bean, as soon as the method you have executed is finished, the new data is

persisted to the database by the container. For BMP, the EJB bean developer is

responsible for defining the persistence routines in the proper places in the bean, for

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instance, the ejbCreate(), ejbStore(), ejbRemove() methods would be developed by

the bean developer to make calls to the database. The container is responsible, in

BMP, to call the appropriate method on the bean. So, if the bean is being looked up,

when the create() method is called on the Home interface, then the container is

responsible for calling the ejbCreate() method in the bean, which should have

functionality inside for going to the database and looking up the data.

What is an EJB Context?

EJBContext is an interface that is implemented by the container, and it is also

a part of the bean-container contract. Entity beans use a subclass of EJBContext

called EntityContext. Session beans use a subclass called SessionContext. These

EJBContext objects provide the bean class with information about its container, the

client using the bean and the bean itself. They also provide other functions. See the

API docs and the spec for more details.

Is method overloading allowed in EJB?

Yes you can overload methods

Should synchronization primitives be used on bean methods?

No. The EJB specification specifically states that the enterprise bean is not

allowed to use thread primitives. The container is responsible for managing

concurrent access to beans at runtime.

Are we allowed to change the transaction isolation property in middle of a

transaction?

No. You cannot change the transaction isolation level in the middle of

transaction.

For Entity Beans, What happens to an instance field not mapped to any

persistent storage, when the bean is passivated?

The specification infers that the container never serializes an instance of an

Entity bean (unlike stateful session beans). Thus passivation simply involves moving

the bean from the “ready” to the “pooled” bin. So what happens to the contents of

an instance variable is controlled by the programmer. Remember that when an entity

bean is passivated the instance gets logically disassociated from it’s remote object.

Be careful here, as the functionality of passivation/activation for Stateless Session,

Stateful Session and Entity beans is completely different. For entity beans the

ejbPassivate method notifies the entity bean that it is being disassociated with a

particular entity prior to reuse or for dereference.

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What is a Message Driven Bean, what functions does a message driven bean

have and how do they work in collaboration with JMS?

Message driven beans are the latest addition to the family of component bean

types defined by the EJB specification. The original bean types include session beans,

which contain business logic and maintain a state associated with client sessions,

and entity beans, which map objects to persistent data. Message driven beans will

provide asynchrony to EJB based applications by acting as JMS message consumers.

A message bean is associated with a JMS topic or queue and receives JMS messages

sent by EJB clients or other beans. Unlike entity beans and session beans, message

beans do not have home or remote interfaces. Instead, message driven beans are

instantiated by the container as required. Like stateless session beans, message

beans maintain no client-specific state, allowing the container to optimally manage a

pool of message-bean instances. Clients send JMS messages to message beans in

exactly the same manner as they would send messages to any other JMS

destination. This similarity is a fundamental design goal of the JMS capabilities of the

new specification. To receive JMS messages, message driven beans implement the

javax.jms.MessageListener interface, which defines a single “onMessage()” method.

When a message arrives, the container ensures that a message bean corresponding

to the message topic/queue exists (instantiating it if necessary), and calls its

onMessage method passing the client’s message as the single argument. The

message bean’s implementation of this method contains the business logic required

to process the message. Note that session beans and entity beans are not allowed to

function as message beans.

Does RMI-IIOP support code downloading for Java objects sent by value

across an IIOP connection in the same way as RMI does across a JRMP

connection?

Yes. The JDK 1.2 support the dynamic class loading.

The EJB container implements the EJBHome and EJBObject classes. For

every request from a unique client, does the container create a separate

instance of the generated EJBHome and EJBObject classes?

The EJB container maintains an instance pool. The container uses these

instances for the EJB Home reference irrespective of the client request. while refering

the EJB Object classes the container creates a separate instance for each client

request. The instance pool maintainence is up to the implementation of the

container. If the container provides one, it is available otherwise it is not mandatory

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for the provider to implement it. Having said that, yes most of the container

providers implement the pooling functionality to increase the performance of the

application server. The way it is implemented is again up to the implementer.

What is the advantage of putting an Entity Bean instance from the “Ready

State” to “Pooled state”?

The idea of the “Pooled State” is to allow a container to maintain a pool of

entity beans that has been created, but has not been yet “synchronized” or assigned

to an EJBObject. This mean that the instances do represent entity beans, but they

can be used only for serving Home methods (create or findBy), since those methods

do not relay on the specific values of the bean. All these instances are, in fact,

exactly the same, so, they do not have meaningful state. Jon Thorarinsson has also

added: It can be looked at it this way: If no client is using an entity bean of a

particular type there is no need for cachig it (the data is persisted in the database).

Therefore, in such cases, the container will, after some time, move the entity bean

from the “Ready State” to the “Pooled state” to save memory. Then, to save

additional memory, the container may begin moving entity beans from the “Pooled

State” to the “Does Not Exist State”, because even though the bean’s cache has

been cleared, the bean still takes up some memory just being in the “Pooled State”.

Can a Session Bean be defined without ejbCreate() method?

The ejbCreate() methods is part of the bean’s lifecycle, so, the compiler will

not return an error because there is no ejbCreate() method. However, the J2EE spec

is explicit: the home interface of a Stateless Session Bean must have a single

create() method with no arguments, while the session bean class must contain

exactly one ejbCreate() method, also without arguments. Stateful Session Beans can

have arguments (more than one create method) stateful beans can contain multiple

ejbCreate() as long as they match with the home interface definition. You need a

reference to your EJBObject to startwith. For that Sun insists on putting a method for

creating that reference (create method in the home interface). The EJBObject does

matter here. Not the actual bean.

Is it possible to share an HttpSession between a JSP and EJB? What

happens when I change a value in the HttpSession from inside an EJB?

You can pass the HttpSession as parameter to an EJB method, only if all

objects in session are serializable.This has to be consider as “passed-by-value”, that

means that it’s read-only in the EJB. If anything is altered from inside the EJB, it

won’t be reflected back to the HttpSession of the Servlet Container.The “pass-byEJB

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reference” can be used between EJBs Remote Interfaces, as they are remote

references. While it IS possible to pass an HttpSession as a parameter to an EJB

object, it is considered to be “bad practice (1)” in terms of object oriented design.

This is because you are creating an unnecessary coupling between back-end objects

(ejbs) and front-end objects (HttpSession). Create a higher-level of abstraction for

your ejb’s api. Rather than passing the whole, fat, HttpSession (which carries with it

a bunch of http semantics), create a class that acts as a value object (or structure)

that holds all the data you need to pass back and forth between front-end/back-end.

Consider the case where your ejb needs to support a non-http-based client. This

higher level of abstraction will be flexible enough to support it. (1) Core J2EE design

patterns (2001)

Is there any way to read values from an entity bean without locking it for

the rest of the transaction (e.g. read-only transactions)?

We have a key-value map bean which deadlocks during some concurrent

reads. Isolation levels seem to affect the database only, and we need to work within

a transaction.

The only thing that comes to (my) mind is that you could write a ‘group

accessor’ - a method that returns a single object containing all of your entity bean’s

attributes (or all interesting attributes). This method could then be placed in a

‘Requires New’ transaction. This way, the current transaction would be suspended for

the duration of the call to the entity bean and the entity bean’s fetch/operate/commit

cycle will be in a separate transaction and any locks should be released immediately.

Depending on the granularity of what you need to pull out of the map, the group

accessor might be overkill.

What is the difference between a “Coarse Grained” Entity Bean and a “Fine

Grained” Entity Bean?

A ‘fine grained’ entity bean is pretty much directly mapped to one relational

table, in third normal form. A ‘coarse grained’ entity bean is larger and more

complex, either because its attributes include values or lists from other tables, or

because it ‘owns’ one or more sets of dependent objects. Note that the coarse

grained bean might be mapped to a single table or flat file, but that single table is

going to be pretty ugly, with data copied from other tables, repeated field groups,

columns that are dependent on non-key fields, etc. Fine grained entities are

generally considered a liability in large systems because they will tend to increase

the load on several of the EJB server’s subsystems (there will be more objects

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exported through the distribution layer, more objects participating in transactions,

more skeletons in memory, more EJB Objects in memory, etc.)

What is EJBDoclet?

EJBDoclet is an open source JavaDoc doclet that generates a lot of the EJB

related source files from custom JavaDoc comments tags embedded in the EJB

source file.

Are enterprise beans allowed to use Thread.sleep()?

Enterprise beans make use of the services provided by the EJB container,

such as life-cycle management. To avoid conflicts with these services, enterprise

beans are restricted from performing certain operations: Managing or synchronizing

threads

Is it possible to write two EJB’s that share the same Remote and Home

interfaces, and have different bean classes? if so, what are the

advantages/disadvantages?

It’s certainly possible. In fact, there’s an example that ships with the Inprise

Application Server of an Account interface with separate implementations for

CheckingAccount and SavingsAccount, one of which was CMP and one of which was

BMP.

Is it possible to specify multiple JNDI names when deploying an EJB?

No. To achieve this you have to deploy your EJB multiple times each

specifying a different JNDI name.

Is there any way to force an Entity Bean to store itself to the db? I don’t

wanna wait for the container to update the db, I want to do it NOW! Is it

possible?

Specify the transaction attribute of the bean as RequiresNew. Then as per

section 11.6.2.4 of the EJB v 1.1 spec EJB container automatically starts a new

transaction before the method call. The container also performs the commit protocol

before the method result is sent to the client.

I am developing a BMP Entity bean. I have noticed that whenever the create

method is invoked, the ejbLoad() and the ejbStore() methods are also

invoked. I feel that once my database insert is done, having to do a select

and update SQL queries is major overhead. is this behavior typical of all EJB

containers? Is there any way to suppress these invocations?

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This is the default behaviour for EJB. The specification states that ejbLoad()

will be called before every transaction and ejbStore() after every transaction. Each

Vendor has optimizations, which are proprietary for this scenario.

Can an EJB send asynchronous notifications to its clients?

Asynchronous notification is a known hole in the first versions of the EJB spec.

The recommended solution to this is to use JMS, which is becoming available in

J2EE-compliant servers. The other option, of course, is to use client-side threads and

polling. This is not an ideal solution, but it’s workable for many scenarios.

How can I access EJB from ASP?

You can use the Java 2 Platform, Enterprise Edition Client Access Services

(J2EETM CAS) COM Bridge 1.0, currently downloadable from Sun

Is there a guarantee of uniqueness for entity beans?

There is no such guarantee. The server (or servers) can instantiate as many

instances of the same underlying Entity Bean (with the same PK) as it wants.

However, each instance is guaranteed to have up-to-date data values, and be

transactionally consistent, so uniqueness is not required. This allows the server to

scale the system to support multiple threads, multiple concurrent requests, and

multiple hosts.

How do the six transaction attributes map to isolation levels like “dirty

read”? Will an attribute like “Required” lock out other readers until I’m

finished updating?

The Transaction Attributes in EJB do not map to the Transaction Isolation

levels used in JDBC. This is a common misconception. Transaction Attributes specify

to the container when a Transaction should be started, suspended(paused) and

committed between method invocations on Enterprise JavaBeans. For more details

and a summary of Transaction Attributes refer to section 11.6 of the EJB 1.1

specification.

I have created a remote reference to an EJB in FirstServlet. Can I put the

reference in a servlet session and use that in SecondServlet?

Yes. The EJB client (in this case your servlet) acquires a remote reference to

an EJB from the Home Interface; that reference is serializable and can be passed

from servlet to servlet. If it is a session bean, then the EJB server will consider your

web client’s servlet session to correspond to a single EJB session, which is usually

(but not always) what you want.

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Can the primary key in the entity bean be a Java primitive type such as int?

The primary key can’t be a primitive type–use the primitive wrapper classes,

instead. For example, you can use java.lang.Integer as the primary key class, but

not int (it has to be a class, not a primitive)

What’s new in the EJB 2.0 specification?

Following are the main features supported in EJB 2.0: Integration of EJB with

JMS, Message Driven Beans, Implement additional Business methods in Home

interface which are not specific for bean instance, EJB QL.

How many types of protocol implementations does RMI have?

• RMI has at least three protocol implementations:

• Java Remote Method Protocol (JRMP)

• Internet Inter ORB Protocol (IIOP)

• Jini Extensible Remote Invocation(JERI)

These are alternatives, not part of the same thing, All three are indeed layer

6 protocols for those who are still speaking OSI reference model.

What is the need of Remote and Home interfaces. Why can’t there be one?

In a few words, I would say that the main reason is because there is a clear

division of roles and responsabilities between the two interfaces. The home interface

is your way to communicate with the container, that is who is responsable of

creating, locating even removing one or more beans. The remote interface is your

link to the bean, that will allow you to remotely access to all its methods and

members. As you can see there are two distinct elements (the container and the

beans) and you need two different interfaces for accessing to both of them.

What is the difference between Java Beans and EJB?

Java Beans are client-side objects and EJBs are server side object, and they

have completely different development, lifecycle, purpose.

What happens if both my EJB Server and Database crash, what will happen

to unsaved changes? Is there any transactional log file used?

Actually, if your EJB server crashes, you will not even be able to make a

connection to the server to perform a bean lookup, as the server will no longer be

listening on the port for incoming JNDI lookup requests. You will lose any data that

wasn’t committed prior to the crash. This is where you should start looking into

clustering your EJB server. Any unsaved and uncommited changes are lost the

moment your EJB Server crashes. If your database also crashes, then all the saved

changes are also lost unless you have some backup or some recovery mechanism to

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retrieve the data. So consider database replication and EJB Clustering for such

scenarios, though the occurence of such a thing is very very rare. Thx, Uma All

databse have the concept of log files(for exampe oracle have redo log files concept).

So if data bases crashes then on starting up they fill look up the log files to perform

all pending jobs. But is EJB crashes, It depend upon the container how frequenlty it

passivates or how frequesntly it refreshes the data with Database.

Can you control when passivation occurs?

The developer, according to the specification, cannot directly control when

passivation occurs. Although for Stateful Session Beans, the container cannot

passivate an instance that is inside a transaction. So using transactions can be a a

strategy to control passivation. The ejbPassivate() method is called during

passivation, so the developer has control over what to do during this exercise and

can implement the require optimized logic. Some EJB containers, such as BEA

WebLogic, provide the ability to tune the container to minimize passivation calls.

Taken from the WebLogic 6.0 DTD - “The passivation-strategy can be either “default”

or “transaction”. With the default setting the container will attempt to keep a

working set of beans in the cache. With the “transaction” setting, the container will

passivate the bean after every transaction (or method call for a non-transactional

invocation).”

Does RMI-IIOP support dynamic downloading of classes?

No, RMI-IIOP doesn’t support dynamic downloading of the classes as it is

done with CORBA in DII (Dynamic Interface Invocation).Actually RMI-IIOP combines

the usability of Java Remote Method Invocation (RMI) with the interoperability of the

Internet Inter-ORB Protocol (IIOP).So in order to attain this interoperability between

RMI and CORBA,some of the features that are supported by RMI but not CORBA and

vice versa are eliminated from the RMI-IIOP specification.

Does EJB 1.1 support mandate the support for RMI-IIOP ? What is the

meaning of “the client API must support the Java RMI-IIOP programming

model for portability, but the underlying protocol can be anything”?

EJB1.1 does mandate the support of RMI-IIOP. There are 2 types of

implementations that an EJB Server might provide: CORBA-based EJB Servers and

Proprietry EJB Servers. Both support the RMI-IIOP API but how that API is

implemented is a different story. (NB: By API we mean the interface provided to the

client by the stub or proxy). A CORBA-based EJB Server actually implements its EJB

Objects as CORBA Objects (it therefore encorporates an ORB and this means that

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EJB’s can be contacted by CORBA clients (as well as RMI-IIOP clients) A proprietry

EJB still implements the RMI-IIOP API (in the client’s stub) but the underlying

protocol can be anything. Therefore your EJB’s CANNOT be contacted by CORBA

clients. The difference is that in both cases, your clients see the same API (hence,

your client portability) BUT how the stubs communicate with the server is different.

The EJB specification says that we cannot use Bean Managed Transaction in

Entity Beans. Why?

The short, practical answer is… because it makes your entity beans useless as

a reusable component. Also, transaction management is best left to the application

server - that’s what they’re there for. It’s all about atomic operations on your data. If

an operation updates more than one entity then you want the whole thing to succeed

or the whole thing to fail, nothing in between. If you put commits in the entity beans

then it’s very difficult to rollback if an error occurs at some point late in the

operation.

Can I invoke Runtime.gc() in an EJB?

You shouldn’t. What will happen depends on the implementation, but the call

will most likely be ignored. You should leave system level management like garbage

collection for the container to deal with. After all, that’s part of the benefit of using

EJBs, you don’t have to manage resources yourself.

What is clustering? What are the different algorithms used for clustering?

Clustering is grouping machines together to transparantly provide enterprise

services.The client does not now the difference between approaching one server or

approaching a cluster of servers.Clusters provide two benefits: scalability and high

availability. Further information can be found in the JavaWorld article J2EE

Clustering.

What is the advantage of using Entity bean for database operations, over

directly using JDBC API to do database operations? When would I use one

over the other?

Entity Beans actually represents the data in a database. It is not that Entity

Beans replaces JDBC API. There are two types of Entity Beans Container Managed

and Bean Mananged. In Container Managed Entity Bean - Whenever the instance of

the bean is created the container automatically retrieves the data from the

DB/Persistance storage and assigns to the object variables in bean for user to

manipulate or use them. For this the developer needs to map the fields in the

database to the variables in deployment descriptor files (which varies for each

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