26. 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.
27. 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)
28. 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 EJBs 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.
29. 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 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.
30. 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.
31. 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.
32. 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. See Sun’s J2EE Connectors for more in-depth information on Connectors.
33. 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 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.
34. 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.
35. 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.
36. 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.
37. 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 its 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.
38. 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.
39. 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 supports the dynamic class loading. The EJB container implements the EJBHome and EJBObject classes. For every request from a unique client,
40. 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 referring 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.
41. 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.
42. What is the need of Remote and Home interface. Why can’t it be in one?
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 that 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.
43. Can I develop an Entity Bean without implementing the create() method in the home interface?
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.
What is the difference between Context, InitialContext and Session Context? How they are used? javax.naming.Context is an interface that provides methods for binding a name to an object. It’s much like the RMI Naming.bind() method.
javax.naming.InitialContext is a Context and provides implementation for methods available in the Context interface.
Where as SessionContext is an EJBContext objects that is provided by the EJB container to a SessionBean in order for the SessionBean to access the information and/or services or the container.
There is EntityContext too which is also and EJBContext object that’ll be provided to an EntityBean for the purpose of the EntityBean accessing the container details. In general, the EJBContext (SessionContext and EntityContext), AppletContext and ServletContext help the corresponding Java objects in knowing about its ‘context’ [environment in which they run], and to access particular information and/or service. Whereas, the javax.naming.Context is for the purpose of ‘NAMING’ [by the way of referring to] an object.
44. Why an onMessage call in Message-driven bean is always a separate transaction?
EJB 2.0 specification: “An onMessage call is always a separate transaction, because there is never a transaction in progress when the method is called.” When a message arrives, it is passed to the Message Driven Bean through the onMessage() method, that is where the business logic goes. Since there is no guarantee when the method is called and when the message will be processed, is the container that is responsible of managing the environment, including transactions.
45. Why are ejbActivate() and ejbPassivate() included for stateless session bean even though they are never required as it is a no conversational bean?
To have a consistent interface, so that there is no different interface that you need to implement for Stateful Session Bean and Stateless Session Bean. Both Stateless and Stateful Session Bean implement javax.ejb.SessionBean and this would not be possible if stateless session bean is to remove ejbActivate and ejbPassivate from the interface.
46. Static variables in EJB should not be relied upon as they may break in clusters. Why?
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.
47. If I throw a custom ApplicationException from a business method in Entity bean which is participating in a transaction, would the transaction be rolled back by container?
EJB Transaction is automatically rolled back only when a SystemException (or a subtype of it) is thrown. Your ApplicationException can extend from javax.ejb.EJBException, which is a sub class of RuntimeException. When an EJBException is encountered the container rolls back the transaction. EJB Specification does not mention anything about Application exceptions being sub-classes of EJBException. You can tell container to rollback the transaction, by using setRollBackOnly on SessionContext/EJBContext object as per type of bean you are using.
48. Does Stateful Session bean support instance pooling?
Stateful Session Bean conceptually doesn’t have instance pooling.
49. Can I map more than one table in a CMP?
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.