Interview Questions and Answers

Enterprise java bean (EJB) is a server side component which runs on application server or we call container, developed for the purpose of distributed and enterprise level application.

J2EE defines three types of archives:

The entity bean is used to represent data in the database. It provides an object-oriented interface to ____.

An EJB container is a run-time environment that manages one or more enterprise beans. The EJB container manages the life cycles of enterprise bean objects, coordinates distributed transactions, and implements object security. Generally, each EJB container is provided by an EJB server and contains a set of enterprise beans that run on the server.

An EJB server is a high-level process or application that provides a run-time environment to support the execution of server applications that use enterprise beans. An EJB server provides a JNDI-accessible naming service, manages and coordinates the allocation of resources to client applications, provides access to system resources, and provides a transaction service. An EJB server could be provided by, for example, a database or application server.

EJB technology is the core of J2EE. It enables developers to write reusable and portable server-side business logic for the J2EE platform.

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.

Stateless session bean- An instance of these non-persistent EJBs provides a service without storing an interaction or conversation state between methods. Any instance can be used for any client.

Stateful session bean- An instance of these non-persistent EJBs maintains state across methods and transactions. Each instance is associated with a particular client.

Entity bean- An instance of these persistent EJBs represents an object view of the data, usually rows in a database. They have a primary key as a unique identifier. Entity bean persistence can be either container-managed or bean-managed.

Message-driven bean- An instance of these EJBs is integrated with the Java Message Service (JMS) to provide the ability for message-driven beans to act as a standard JMS message consumer and perform asynchronous processing between the server and the JMS message producer.

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

Container-managed persistence (CMP) and bean-managed persistence (BMP). With CMP, the container manages the persistence of the entity bean.

The bean class defines create methods that match methods in the home interface and business methods that match methods in the remote interface. The bean class also implements a set of callback methods that allow the container to notify the bean of events in its life cycle. The callback methods are defined in the javax.ejb.EntityBean interface that is implemented by all entity beans.The EntityBean interface has the following definition. Notice that the bean class implements these methods.

public interface javax.ejb.EntityBean {
public void setEntityContext();
public void unsetEntityContext();
public void ejbLoad();
public void ejbStore();
public void ejbActivate();
public void ejbPassivate();
public void ejbRemove();
}

The setEntityContext() method provides the bean with an interface to the container called the EntityContext. The EntityContext interface contains methods for obtaining information about the context under which the bean is operating at any particular moment. The EntityContext interface is used to access security information about the caller; to determine the status of the current transaction or to force a transaction rollback; or to get a reference to the bean itself, its home, or its primary key. The EntityContext is set only once in the life of an entity bean instance, so its reference should be put into one of the bean instance�s fields if it will be needed later.

The unsetEntityContext() method is used at the end of the bean�s life cycle before the instance is evicted from memory to dereference the EntityContext and perform any last-minute clean-up.

The ejbLoad() and ejbStore() methods in CMP entities are invoked when the entity bean�s state is being synchronized with the database. The ejbLoad() is invoked just after the container has refreshed the bean container-managed fields with its state from the database. The ejbStore() method is invoked just before the container is about to write the bean container-managed fields to the database. These methods are used to modify data as it�s being synchronized. This is common when the data stored in the database is different than the data used in the bean fields.

The ejbPassivate() and ejbActivate() methods are invoked on the bean by the container just before the bean is passivated and just after the bean is activated, respectively. Passivation in entity beans means that the bean instance is disassociated with its remote reference so that the container can evict it from memory or reuse it. It�s a resource conservation measure the container employs to reduce the number of instances in memory. A bean might be passivated if it hasn�t been used for a while or as a normal operation performed by the container to maximize reuse of resources. Some containers will evict beans from memory, while others will reuse instances for other more active remote references. The ejbPassivate() and ejbActivate() methods provide the bean with a notification as to when it�s about to be passivated (disassociated with the remote reference) or activated (associated with a remote reference).

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

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

If a developer doesn�t want a Container to manage transactions, it�s possible to implement all database operations manually.

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 or.

Transaction_read_uncommitted, Transaction_read_committed, Transaction_repeatable_read.

The main technologies in J2EE are: Enterprise JavaBeansTM (EJBsTM), JavaServer PagesTM (JSPsTM), Java Servlets, the Java Naming and Directory InterfaceTM (JNDITM), the Java Transaction API (JTA), CORBA, and the JDBCTM data access API.

EJB is a specification for J2EE server, not a product; Java beans may be a graphical component in IDE.

The process by which the contents in the memory of one physical m/c are replicated in all the m/c in the cluster is called in-memory replication.

Session and Entity EJBs consist of 4 and 5 parts respetively:

1. A remote interface (a client interacts with it),

2. A home interface (used for creating objects and for declaring business methods),

3. A bean object (an object, which actually performs business logic and EJB-specific operations).

4. A deployment descriptor (an XML file containing all information required for maintaining the EJB) or a set of deployment descriptors (if you are using some container-specific features).

5.A Primary Key class - is only Entity bean specific.

The main reason is because there is a clear division of roles and responsibilities between the two interfaces. The home interface is your way to communicate with the container, that is who is responsible 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.

If a developer doesn�t want a Container to manage transactions, it�s possible to implement all database operations manually by writing the appropriate JDBC code. This often leads to productivity increase, but it makes an Entity Bean incompatible with some databases and it enlarges the amount of code to be written. All transaction management is explicitly performed by a developer.

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.

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.

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.

Step 1: Retrieve Home Object reference from Naming Service via JNDI.
step 2: Return Home Object reference to the client.
step 3: Create me a new EJB Object through Home Object interface.
step 4: Create EJB Object from the Ejb Object
step 5: Return EJB Object reference to the client.
step 6: Invoke business method using EJB Object reference.
step 7: Delegate request to Bean (Enterprise Bean).

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 or component interface or when the method is invoked as the result of the arrival of a JMS message. (Sun�s EJB Specification) Below is a list of transactional attributes:

1. NotSupported - transaction context is unspecified.

2. Required - bean�s method invocation is made within a transactional context. If a client is not associated with a transaction, a new transaction is invoked automatically.

3. Supports - if a transactional context exists, a Container acts like the transaction attribute is Required, else - like NotSupported.

4. RequiresNew - a method is invoked in a new transaction context.

5. Mandatory - if a transactional context exists, a Container acts like the transaction attribute is Required, else it throws a javax.ejb.TransactionRequiredException.

6. Never - a method executes only if no transaction context is specified.

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.

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.

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-by-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 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.

A select method can return a persistent field (or a collection thereof) of a related entity bean. A finder method can return only a local or remote interface (or a collection of interfaces).

Because it is not exposed in any of the local or remote interfaces, a select method cannot be invoked by a client. It can be invoked only by the methods implemented within the entity bean class. A select method is usually invoked by either a business or a home method.

A select method is defined in the entity bean class. For bean-managed persistence, a finder method is defined in the entity bean class, but for container-managed persistence it is not.

A Java Bean is a software component written in the Java programming language that conforms to the JavaBeans component specification. The JavaBeans APIs became part of the �core� Java APIs as of the 1.1 release of the JDK.

The JavaBeans specification defines a Java-based software component model that adds a number of features to the Java programming language. Some of these features include:
.introspection
.customization
.events
.properties
.persistence

Enterprise JavaBeans (EJBs) are Java-based software components that are built to comply with Java�s EJB specification and run inside of an EJB container supplied by a J2EE provider. An EJB container provides distributed application functionality such as transaction support, persistence and lifecycle management for the EJBs.

Abstract schema is part of an entity bean�s deployment descriptor which defines the bean�s persistent fields and their relationship. Abstract schema is specified for entity beans with container managed persistence. We specify the name of the Abstract schema name in the deployment descriptor. The queries written in EJB QL for the finder methods references this name. The information provided in this Abstract Schema is used by the container for persistence management and relationship management.

An EJB can use local client view only if it is really guaranteed that other enterprise beans or clients will only address the bean within a single JVM. With local client view, you can do pass-by-reference, which means your bean, as well as the client, will work directly with one copy of the data. Any changes made by the bean will be seen by the client and vice versa. Pass-by-reference eliminates time/system expenses for copying data variables, which provides a performance advantage.

An MDB is essentially a message consumer that can listen to a message destination or a message endpoint and gets activated when a message arrives. By design, MDBs are anonymous in nature and hence cannot be directly invoked by a client. The only way to invoke an MDB is to send a message to the destination or endpoint to which it is listening. As MDBs are stateless in nature and are not related to any specific client, they can be pooled for concurrent processing of messages.

Container services are totally depends upon the �vendor implementation�. But more or less most of the vendors suppots the basic services like,
LifeCycle Management - It is Automatic.
Session Management - it is used by Developer coded callback methods.
Transaction Management - it is used by configuring deployment descriptor (DD) .
Security management - it is used by configuring deployment descriptor (DD) .
The other services, if any will be in advanced versions, and depends on Vendor specific.

For a Session bean - one EJB object for one bean instance.
For entity bean - it depends , if 2 users are accessing one row at time then one EJB object is used for both the beans other wise for each bean one EJB object.

Lazy loading means not creating an object until the first time it is accessed. Lazy loading typically looks like this:

public class Example {
private Vector data = null;
public Vector getData() {
if (this.data == null) {
this.data = new Vector();
// Load data into vector �
}
return this.data;
}
}

This technique is most useful when you have large hierarchies of objects (such as a product catalog). You can lazy-load subordinate objects as you navigate down the hierarchy, and thereby only create objects when you need them.

No, you cannot map more than one table to a single CMP Entity Bean. CMP has been, in fact, designed to map a single table.

No. Decorator design pattern, is the one which exhibits very low level runtime polymorphism, for the specific and single object (Instance of the class), but not for atleast for a class. It is the stuff to add specific functionality to a single & pointed object and leaves others like it unmodified. It is having close similarities like aspectJ stuff, but not with EJB stuff.

Since EnterpriseBeans live in a managed container, the container is free to call your EJB components methods at its leisure. The container houses the information like current status of bean,security credentials of the user currently accessing the bean in one object is called EJBContext Object. A context represents a way for beans to perform callbacks and modify their current status Sessioncontext is EJB context for session bean Entitycontext is EJB context for entity bean Message driven context is EJB context for message driven bean.

Stateless SessionBean create() method doesnot contain any parameters and the syntax is as follows:
public interface XSessionEJBHome extends EJBHome
{
XSessionEJB create() throws RemoteException, CreateException;
}

The bulk of the changes in EJB 2.0 are found in the definition of a new CMP component model. It�s radically different from the old CMP model because it introduces an entirely new participant, the persistence manager, and a completely new way of defining container-managed fields, as well as relationships with other beans and dependent objects.

There is a pure division of roles between the two .
Home Interface is the way to communicate with the container which is responsible for creating , locating and removing beans and Remote Interface is the link to the bean that allows acces to all methods and members.

An EJB client JAR file is an optional JAR file that can contain all the class files that a client program needs to use the client view of the enterprise beans that are contained in the EJB JAR file. If you decide not to create a client JAR file for an EJB module, all of the client interface classes will be in the EJB JAR file.

Socket and ServerSocket.
ServerSocket is used for normal two-way socket communication. Socket class allows us to read and write through the sockets. getInputStream() and getOutputStream() are the two methods available in Socket class.

The process of propagating the changes in the properties of a server group during runtime to all the associated clones is called Ripple Effect.

The copies of a server group are called Clones. But unlike a Server Group Clones are associated with a node and are real server process running in that node.

Following are some of the main features supported in EJB 2.0:

1. Integration of EJB with JMS,
2. Message Driven Beans,
3. Implement additional Business methods in Home interface which are not specific for bean instance, EJB QL.

We can use the Java 2 Platform, Enterprise Edition Client Access Services (J2EETM CAS) COM Bridge 1.0, currently downloadable from Sun.

Entity beans that have container-managed relationships with other entity beans, must be accessed in the same local scope as those related beans, and therefore typically provide a local client view. In order to be the target of a container-managed relationship, an entity bean with container-managed persistence must provide a local interface.

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)

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.

Yes. In some cases the data is inserted NOT using Java application, so you may only need to retrieve the information, perform its processing, but not create your own information of this kind.

An entity bean consists of 4 groups of methods:

1. create methods: To create a new instance of a CMP entity bean, and therefore insert data into the database, the create() method on the bean�s home interface must be invoked. They look like this: EntityBeanClass ejbCreateXXX(parameters), where EntityBeanClass is an Entity Bean you are trying to instantiate, ejbCreateXXX(parameters) methods are used for creating Entity Bean instances according to the parameters specified and to some programmer-defined conditions.

A bean�s home interface may declare zero or more create() methods, each of which must have corresponding ejbCreate() and ejbPostCreate() methods in the bean class. These creation methods are linked at run time, so that when a create() method is invoked on the home interface, the container delegates the invocation to the corresponding ejbCreate() and ejbPostCreate() methods on the bean class.

2. finder methods: The methods in the home interface that begin with �find� are called the find methods. These are used to query the EJB server for specific entity beans, based on the name of the method and arguments passed. Unfortunately, there is no standard query language defined for find methods, so each vendor will implement the find method differently. In CMP entity beans, the find methods are not implemented with matching methods in the bean class; containers implement them when the bean is deployed in a vendor specific manner. The deployer will use vendor specific tools to tell the container how a particular find method should behave. Some vendors will use object-relational mapping tools to define the behavior of a find method while others will simply require the deployer to enter the appropriate SQL command.

There are two basic kinds of find methods: single-entity and multi-entity. Single-entity find methods return a remote reference to the one specific entity bean that matches the find request. If no entity beans are found, the method throws an ObjectNotFoundException . Every entity bean must define the single-entity find method with the method name findByPrimaryKey(), which takes the bean�s primary key type as an argument.

The multi-entity find methods return a collection ( Enumeration or Collection type) of entities that match the find request. If no entities are found, the multi-entity find returns an empty collection.

3. remove methods: These methods (you may have up to 2 remove methods, or don�t have them at all) allow the client to physically remove Entity beans by specifying either Handle or a Primary Key for the Entity Bean.

4. home methods: These methods are designed and implemented by a developer, and EJB specification doesn�t have any requirements for them except the need to throw a RemoteException is each home method.

As per the specifications, there can be �ZERO� or �MORE� create() methods defined in an Entity Bean. In cases where create() method is not provided, the only way to access the bean is by knowing its primary key, and by acquiring a handle to it by using its corresponding finder method. In those cases, you can create an instance of a bean based on the data present in the table. All one needs to know is the primary key of that table. i.e. a set a columns that uniquely identify a single row in that table. Once this is known, one can use the �getPrimaryKey()� to get a remote reference to that bean, which can further be used to invoke business methods.

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).

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)

The purpose of ejbPostCreate() is to perform clean-up database operations after SQL INSERTs (which occur when ejbCreate() is called) when working with CMP entity beans. ejbCreate() is called before database INSERT operations. You need to use ejbPostCreate() to define operations, like set a flag, after INSERT completes successfully.

WHERE and ORDERBY clauses are optional in EJB QL where as SELECT and FROM are required clauses.

You shouldn�t. What will happen depends on the implementation, but the call will most likely be ignored.

Purpose of Static initializer block is to initialize some static fields before any execution of constructor or method or we can say creation of object. According to EJB Spec it�s a violation if static field are used and they are non final .

"EJB Spec":
Enterprise bean are not allowed to read or write the non final fields.

But technically,from the point of java its correct but if in EJB we use static initializer block to initialize static field then because EJB components are used in distributed environment mean to say if single JVM then it will be ok but if run on different JVM then it will be a problem if we change or update the value in on environment then only the instance running on same JVM have new value .that�s why static blocks are avoided and also all static field should be final.
And same thing we can achieve in ejbCreate(), setSessionContext() or setEntityContext() methods.

No, Not possible because EJBs are created and managed by container and if in ejbs we allow threading containers life cycle methods will be interrupted by us and also the purpose of EJB is to perform business logic not to controlling a system or implementation level functioning so container will manage the thread and developer can concentrate on business logic.

Connection pooling is one of the Advance feature of container which enhanced our application performance .Using connection pooling mechanism client are not required to create every time connection object to interact with database .Here in pool objects are already available Whenever a client request for a database connection then an instance is picked from the connection pool to get an access to database and when user complete with his work instance is returned to connection pool. There is a limit specified by App server administrator for the availability of number of connections and beyond a specified limit a predefined number increases numbers of connection pool instances. When demand goes back to normal then access amount of connection pool instances are removed.

Session Fa�ade is one of the design pattern which is used in EJB component in this pattern session beans are used as a wrapper classes for entity bean and using session bean we interact with Entity bean .it will give clean approach towards client access to bean components and also reduce network calls so as to make the whole system of high performance.

Transaction is group of operation or work which should be performed either completely or none to maintain the data integrity. All transaction must have ACID property(atomicity ,consistency,integrity,durability) so transaction can be said completed if we commit on successful execution and rollback on unsuccessful execution.
There are two ways of transaction management.
� Declarative Transaction Mang.
� Programmatic Transaction Mang.
Now we see what the transactions Attribute are
Transaction Attribute conveys to the container the intended transactional behavior of the associated EJB component's method.

Required: if required attribute is associated with Method then new Transaction context may or may not be created, means if method is already associated with transaction context then no new transaction context should be created.

Requires New: if Requires New attribute is associated with Method then always new Transaction context may be created.

NotSupported:if Method is Associated with this Attribute then method is a not a part of transaction.

Supported:if a Method is Associated with this transaction Attribute then method will act as Not supported if calling component is not associated with transaction and if calling component is associated with transaction then act as a required Attribute.

Mandatory:if a method is Associated with this attribute then always be called from calling component transaction context.

Never: if a method is Associated with this attribute then never be called from calling component transaction context.
The default transaction value for EJB is Support.

Both of these methods are used in context of entity bean for the purpose of synchronizing the instance variable to corresponding value store in the database. ejbLoad method load or refresh the instance variable from the database and ejbStore method writes variables value back to the database.

JNDI context: provides mechanism to lookup resources on network.

Initial Context: it provides initial context to resources.

Session Context:it has all the information available which is required to session bean from container.

EjbContext:it contain information which is required to both session and entity bean.

There are two types of scaling: Vertical Scaling and Horizontal Scaling.
Vertical Scaling - When multiple server clones of an application server are defined on the same physical m/c, it is called Vertical Scaling. The objective is to use the processing power of that m/c more efficiently.
Horizontal Scaling - When Clones of an application server are defined on multiple physical m/c, it is called Horizontal Scaling. The objective is to use more than one less powerful m/c more efficiently.

A server group is a template of an Application Server(and its contents) i.e, it is a logical representation of the application server. It has the same structure and attributes as the real Application Server, but it is not associated with any node, and does not correspond to any real server process running on any node.

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.

Container-managed persistence beans are the simplest for the bean developer to create and the most difficult for the EJB server to support. This is because all the logic for synchronizing the bean�s state with the database is handled automatically by the container. This means that the bean developer doesn�t need to write any data access logic, while the EJB server is supposed to take care of all the persistence needs automatically. With CMP, the container manages the persistence of the entity bean. A CMP bean developer doesn�t need to worry about JDBC code and transactions, because the Container performs database calls and transaction management instead of the programmer. Vendor tools are used to map the entity fields to the database and absolutely no database access code is written in the bean class. All table mapping is specified in the deployment descriptor. Otherwise, a BMP bean developer takes the load of linking an application and a database on his shoulders.

The bean-managed persistence (BMP) enterprise bean manages synchronizing its state with the database as directed by the container. The bean uses a database API to read and write its fields to the database, but the container tells it when to do each synchronization operation and manages the transactions for the bean automatically. Bean-managed persistence gives the bean developer the flexibility to perform persistence operations that are too complicated for the container or to use a data source that is not supported by the container.BMP beans are not 100% database-independent, because they may contain database-specific code, but CMP beans are unable to perform complicated DML (data manipulation language) statements. EJB 2.0 specification introduced some new ways of querying database (by using the EJB QL - query language).

You can use CMP and BMP beans in the same applicatioN... obviously, a bean can be BMP or CMP, not both at the same time (they are mutually exclusive).

There is a common approach that is normally used and considered a good one. You should start developing CMP beans, unless you require some kind of special bean, like multi-tables, that cannot be completely realized with a single bean. Then, when you realize that you need something more or that you would prefer handling the persistence (performanbce issue are the most common reason), you can change the bean from a CMP to a BMP.

Static variables are only ok if they are final. If they are not final, they will break the cluster. What that means is that if you cluster your application server (spread it across several machines) each part of the cluster will run in its own JVM.

Say a method on the EJB is invoked on cluster 1 (we will have two clusters - 1 and 2) that causes value of the static variable to be increased to 101. On the subsequent call to the same EJB from the same client, a cluster 2 may be invoked to handle the request. A value of the static variable in cluster 2 is still 100 because it was not increased yet and therefore your application ceases to be consistent. Therefore, static non-final variables are strongly discouraged in EJBs.

If we define the entity bean as being reentrant, multiple clients can connect to the Entity bean & execute methods within the entity bean concurrently. Container takes care of synchronization. If we define the entity bean as non-reentrant and many clients connect to it concurrently to execute a method, exception is thrown .

The remote client view specification is only available in EJB 2.0. The remote client view of an enterprise bean is location independent. A client running in the same JVM as a bean instance uses the same API to access the bean as a client running in a different JVM on the same or different machine.
Remote interface: The remote interface specifies the remote business methods that a client can call on an enterprise bean.
Remote home interface: The remote home interface specifies the methods used by remote clients for locating, creating, and removing instances of enterprise bean classes.

The local client view specification is only available in EJB 2.0. Unlike the remote client view, the local client view of a bean is location dependent. Local client view access to an enterprise bean requires both the local client and the enterprise bean that provides the local client view to be in the same JVM. The local client view therefore does not provide the location transparency provided by the remote client view. Local interfaces and local home interfaces provide support for lightweight access from enterprise bean that are local clients. Session and entity beans can be tightly couple with their clients, allowing access without the overhead typically associated with remote method calls.