J2EE EJB interview questions

1. What is the relationship between local interfaces and container-managed relationships? - 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.
2. What does a remove method do for different cases of beans? - Stateless Session : Does not do anything to the bean as moving the bean from free pool to cache are managed by the container depending on load. Stateful Session: Removes the bean from the cache. Entity Bean: Deletes the bean (data) from persistent storage
3. How does a container-managed relationship work? - An entity bean accesses related entity beans by means of the accessor methods for its container-managed relationship fields, which are specified by the cmr-field elements of its abstract persistence schema defined in the deployment descriptor. Entity bean relationships are defined in terms of the local interfaces of the related beans, and the view an entity bean presents to its related beans is defined by its local home and local interfaces. Thus, an entity bean can be the target of a relationship from another entity bean only if it has a local interface.
4. What is the new basic requirement for a CMP entity bean class in 2.0 from that of ejb 1.1? - It must be abstract class. The container extends it and implements methods which are required for managing the relationships
5. What are the basic classes required in the client for invoking an EJB? - The home and the remote interfaces, the implementation of the Naming Context Factory, the stubs and skeletons. In some App servers the stubs and the skeletons can be dynamically downloaded from the server
6. 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:
   1. Message-driven beans process multiple JMS messages asynchronously, rather than processing a serialized sequence of method calls.
   2. 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.
   3. Only the container directly interacts with a message-driven bean by creating bean instances and passing JMS messages to those instances as necessary.
   4. 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.
7. What is the need for Clustering? - To scale the application so that it is highly available and has high throughput.
8. What are the benefits of Clustering and workload management? - They are
   1. It balances client processing requests, allowing incoming work requests to be distributed according to a configured Workload Management selection policy.
   2. It provides fail over capability by redirecting client requests to a running server when one or more servers are unavailable. This improves the availability of applications and administrative services.
   3. It enables systems to be scaled up to serve a higher client load than provided by the basic configuration. With server groups and clones additional instances of servers can easily be added to the configuration.
   4. It enables servers to be transparently maintained and upgraded while applications remain available for users.
   5. It centralizes administration of application servers and other objects.
9. What are the types of Scaling? - There are two types of scaling: Vertical Scaling and Horizontal Scaling.
10. What is 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.
11. What is 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.
12. What is a Server Group? - 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.
13. What is a Clone? - 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.
14. What is Ripple Effect? - The process of propagating the changes in the properties of a server group during runtime to all the associated clones is called Ripple Effect.
15. What level of Load Balancing is possible with EJBs? - The workload management service provides load balancing for the following types of enterprise beans: Homes of entity or session beans, Instances of entity beans, Instances of stateless session beans.
16. What is the basic requirement for in-memory replication in Weblogic? - The data in session should consist only of Serialized objects. Only setAttribute function should be used to set objects in session.
17. How JDBC services can be used in clustered environment? - Identical DataSource has to be created in each clustered server instances and configure to use different connection pools.
18. What are the services that should not be used in a Clustered Environment? - Non-clustered services:
    1. File Services
    2. Time services
    3. Weblogic events
    4. Weblogic Workspaces (In WebLogic 5.1)
19. Mention some tools to cluster Web Servers? - Web Servers can be clustered using Edge Server or DNS.
20. What is in-memory replication? - 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.
21. Difference Between Abstraction and Encapsulation - Abstraction is removing some distinctions between objects, so as to show their commonalities. Encapsulation is hiding the details of the implementation of an object so that there are no external dependencies on the particular implementation.