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31). 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.

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

32). 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).

33). How to call any EJB from a servlet/JSP/Java Client?

Context ctx = new InitialContext();

//get Home interface of bean
//narrow -retype
BeanHome lhome = (BeanHome) javax.rmi.PortableRemoteObject.narrow(ctx.lookup("cz.train.Bean"), BeanHome.class);

//get remote interface
Bean lbean = lhome.create();

//now you can call bussiness method on remote interface like
lbean.doSomething()

34). 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)

35). What are the methods of Entity Bean?

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.

36). What is the difference between Container-Managed Persistent (CMP) bean and Bean-Managed Persistent(BMP) ?

Container-managed persistence(CMP) 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).

37).Can Entity Beans have no create() methods?

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

38). 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 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.

39). What are transaction isolation levels in EJB?

  1. Transaction_read_uncommitted- Allows a method to read uncommitted data from a DB(fast but not wise).

    2. Transaction_read_committed- Guarantees that the data you are getting has been committed.

    3. Transaction_repeatable_read - Guarantees that all reads of the database will be the same during the transaction (good for read and update operations).

    4. Transaction_serializable- All the transactions for resource are performed serial.

40). What is the difference between ejbCreate() and ejbPostCreate

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.

When working with BMP entity beans, this is not necessary. You have full control over the entire process, so you can place all the necessary logic surrounding your INSERT statement directly in the ejbCreate() method.

Even if you are creating BMP entity beans, the recommendation would still be to include an empty ejbPostCreate() method. Although some application servers will not enforce it, the spec indicates that this placeholder should be there.