EJB Interview Questions and Answers for 2 years experience

1. What is EJB?

   • Answer: EJB stands for Enterprise Java Beans. It's a server-side component architecture for building distributed, transactional, and secure Java applications. It simplifies enterprise application development by providing a framework for managing complex business logic and interactions with databases and other resources.

2. What are the different types of EJBs?

   • Answer: The main types are Session Beans (Stateful, Stateless, Singleton), Message-Driven Beans (MDBs), and Entity Beans (now largely replaced by JPA).

3. Explain Stateful Session Beans.

   • Answer: Stateful Session Beans maintain conversational state across multiple method invocations from a single client. Each client gets its own instance of the bean, preserving data between calls. This is suitable for applications needing to remember client-specific information.

4. Explain Stateless Session Beans.

   • Answer: Stateless Session Beans don't maintain conversational state. Each method invocation is independent of others. This allows for better scalability and concurrency as multiple clients can share the same bean instance.

5. What are Singleton Session Beans?

   • Answer: Singleton Session Beans provide only one instance of the bean for the entire application. They are useful for managing shared resources or maintaining application-wide state.

6. Explain Message-Driven Beans (MDBs).

   • Answer: MDBs are asynchronous beans that process messages from JMS (Java Message Service) queues or topics. They are ideal for handling asynchronous operations and decoupling components in a system.

7. What is the role of the EJB container?

   • Answer: The EJB container manages the lifecycle of EJBs, provides security, transactions, concurrency control, and resource management. It handles deployment, instantiation, and destruction of beans, abstracting away much of the underlying infrastructure.

8. What are the benefits of using EJBs?

   • Answer: Benefits include simplified development, improved scalability and performance, enhanced security, transaction management, resource pooling, and simplified deployment.

9. Explain EJB deployment descriptors.

   • Answer: Deployment descriptors (typically `ejb-jar.xml`) are XML files that provide metadata about EJBs, including their names, interfaces, transaction attributes, security roles, and other deployment-related information.

10. What are local and remote interfaces in EJBs?

    • Answer: Local interfaces are used for clients running in the same JVM as the EJB container. Remote interfaces are used for clients running in different JVMs, requiring communication over a network. Remote interfaces typically use RMI (Remote Method Invocation).

11. Explain EJB transactions.

    • Answer: EJBs support transactions using declarative or programmatic approaches. Declarative uses annotations or deployment descriptors to specify transaction attributes (e.g., Required, RequiresNew, Mandatory). Programmatic approach uses the `UserTransaction` interface for more fine-grained control.

12. What are EJB security roles?

    • Answer: EJB security roles define groups of users or principals with specific access rights to EJBs. Security is managed through annotations or deployment descriptors, controlling which users can access specific methods.

13. What is the difference between `@Local` and `@Remote` annotations?

    • Answer: `@Local` specifies a local interface accessible only within the same JVM, while `@Remote` specifies a remote interface accessible across JVMs using RMI or other communication protocols.

14. How do you handle concurrency in EJBs?

    • Answer: Concurrency is handled by the container using techniques like pooling and thread management. Stateless session beans naturally handle concurrency well due to their stateless nature. For stateful beans, the container manages concurrency based on the bean's scope and configuration.

15. Explain the lifecycle of a stateless session bean.

    • Answer: The container creates instances as needed and pools them for reuse. Beans are not explicitly created or destroyed by the client. The container manages their lifecycle, focusing on efficient resource utilization.

16. Explain the lifecycle of a stateful session bean.

    • Answer: Stateful session beans are created when a client invokes a method for the first time. They remain active as long as the client maintains interaction. The container passivates (stores state to secondary storage) and reactivates them as needed to manage resources.

17. What is the purpose of the `@PostConstruct` and `@PreDestroy` annotations?

    • Answer: `@PostConstruct` is called after the bean is created and initialized by the container. `@PreDestroy` is called before the bean is destroyed by the container, allowing for cleanup operations.

18. How do you inject dependencies into EJBs?

    • Answer: Dependency injection is done using `@EJB` annotation. This annotation injects an instance of another EJB into the current bean.

19. What are interceptors in EJBs?

    • Answer: Interceptors allow for cross-cutting concerns to be implemented without modifying the business logic of the EJB. They can be used for logging, security, or other aspects that apply across multiple methods.

20. What is a timer service in EJB?

    • Answer: The EJB timer service allows scheduling the execution of methods at specific times or intervals. This is useful for creating scheduled tasks and background processes.

21. How do you handle exceptions in EJBs?

    • Answer: Exceptions are handled using standard Java exception handling mechanisms (try-catch blocks). The container may also handle exceptions in specific ways based on transaction attributes and configuration.

22. What are the different deployment options for EJBs?

    • Answer: EJBs can be deployed to application servers like GlassFish, JBoss, WebLogic, and WildFly. Deployment involves packaging the EJBs into an EAR (Enterprise Archive) or WAR (Web Application Archive) file and deploying it to the server.

23. What are the differences between EJB 2.x and EJB 3.x?

    • Answer: EJB 3.x significantly simplified development with the introduction of annotations, reduced boilerplate code, and improved integration with other Java EE technologies. EJB 2.x relied heavily on XML deployment descriptors and had more complex configurations.

24. What is CMP and BMP in EJB (Entity Beans)?

    • Answer: CMP (Container-Managed Persistence) and BMP (Bean-Managed Persistence) were persistence mechanisms for Entity Beans. CMP was managed by the container, while BMP required developers to handle database interactions directly. Both are largely obsolete due to JPA.

25. Explain JPA and its relationship with EJBs.

    • Answer: JPA (Java Persistence API) is the standard for persistence in Java EE. It replaced Entity Beans' CMP and BMP. EJBs can seamlessly integrate with JPA for database access, leveraging JPA's features for object-relational mapping.

26. How do you manage transactions across multiple EJBs?

    • Answer: Transactions can span multiple EJBs using the container's transaction management capabilities. Declarative transaction attributes can ensure atomicity and consistency across multiple beans.

27. What are some common performance considerations when using EJBs?

    • Answer: Efficient database access (using JPA optimization), minimizing network calls (especially with remote interfaces), proper resource pooling, and efficient concurrency handling are vital for performance.

28. How can you monitor and debug EJB applications?

    • Answer: Application servers provide monitoring tools to track resource usage, transaction performance, and other metrics. Debugging can be done using IDE debuggers or logging frameworks integrated with the application.

29. What are some best practices for designing EJB applications?

    • Answer: Follow design patterns (like DAO, Facade), keep beans loosely coupled, use appropriate EJB types based on needs, leverage container services effectively, and write clean, maintainable code.

30. Describe a situation where you used EJBs to solve a problem.

    • Answer: [This requires a specific example from your experience. Describe a project where you used EJBs, the problem you solved, and how EJB features helped.]

31. Explain your experience with different EJB containers.

    • Answer: [Describe your experience with specific application servers like GlassFish, JBoss, WebLogic, etc. Mention any challenges or successes you encountered.]

32. How familiar are you with different transaction management strategies in EJBs?

    • Answer: [Discuss your understanding of declarative and programmatic transaction management. Mention different transaction attributes (Required, RequiresNew, etc.) and when you might use them.]

33. How do you handle security in your EJB applications?

    • Answer: [Describe your experience with EJB security roles, annotations, and security policies. Mention how you've implemented authorization and authentication.]

34. What are some common challenges you faced while working with EJBs, and how did you overcome them?

    • Answer: [Provide concrete examples of challenges like debugging complex transactions, configuring security properly, or handling performance issues. Explain your problem-solving approach.]

35. What are your preferred tools and technologies for developing and deploying EJB applications?

    • Answer: [Mention IDEs (Eclipse, IntelliJ), build tools (Maven, Gradle), application servers, and any other relevant technologies.]

36. What are your plans for continuing your education and skill development in EJB and related technologies?

    • Answer: [Discuss your commitment to learning new technologies, including specific areas like microservices, cloud computing, or other relevant advancements.]

37. Explain the concept of container-managed relationships in EJBs.

    • Answer: [This relates to Entity Beans and is largely obsolete with JPA. However, you can discuss the concept of how the container managed relationships between entities, simplifying persistence logic.]

38. How does EJB handle connection pooling?

    • Answer: The EJB container manages connection pooling, efficiently managing database connections to improve performance and resource utilization. Developers don't typically interact directly with connection pools.

39. Explain the differences between optimistic and pessimistic locking in the context of EJBs.

    • Answer: Both relate to concurrency control in JPA (which EJBs often use). Optimistic locking assumes conflicts are rare and checks for them during updates, while pessimistic locking prevents conflicts by locking resources during access.

40. How would you design an EJB application for high availability and scalability?

    • Answer: [Discuss strategies like using stateless session beans, clustering, load balancing, and using a message queue for asynchronous communication to achieve high availability and scalability.]

41. What are some common design patterns used with EJBs?

    • Answer: [Mention patterns like Data Access Object (DAO), Facade, Session Facade, and others. Explain how these patterns improve modularity, maintainability, and testability of EJB applications.]

42. How do you handle logging in your EJB applications?

    • Answer: [Discuss using logging frameworks like Log4j or SLF4j to log application events and errors. Explain the importance of logging for debugging and monitoring.]

43. What is your experience with testing EJBs?

    • Answer: [Describe your approaches to unit testing, integration testing, and potentially mocking dependencies when testing EJBs. Mention tools used.]

44. Explain your understanding of the Java EE architecture.

    • Answer: [Demonstrate a good understanding of the overall Java EE architecture, showing how EJBs fit within the broader context of other technologies like Servlets, JSPs, JMS, and JPA.]

45. How familiar are you with CDI (Contexts and Dependency Injection)?

    • Answer: [Discuss your knowledge of CDI, how it complements EJBs, and how it simplifies dependency injection and managing scopes in Java EE applications.]

46. What is your experience with using JMS with EJBs?

    • Answer: [Discuss your experience using JMS with MDBs to handle asynchronous communication and decouple components in enterprise applications.]

47. How do you ensure the security of sensitive data handled by your EJBs?

    • Answer: [Discuss strategies like encryption, secure coding practices, using appropriate security contexts, and adhering to security best practices for data handling.]

48. Describe your experience working with different database systems and their interaction with EJBs.

    • Answer: [Mention your familiarity with databases like Oracle, MySQL, PostgreSQL, etc., and how you've integrated them using JPA within EJB applications.]

49. How would you approach troubleshooting a performance bottleneck in an EJB application?

    • Answer: [Outline your debugging process, using profiling tools, examining logs, investigating database performance, and optimizing code or configurations to address the bottleneck.]

50. What are your thoughts on the future of EJBs in the context of microservices?

    • Answer: [Discuss the changing landscape, acknowledging the rise of microservices and considering how EJBs might be used or replaced in modern architectures. You may mention using EJBs in a monolith or as part of a microservice architecture.]

51. What is your preferred method for managing configuration data in an EJB application?

    • Answer: [Discuss approaches like using configuration files, environment variables, or JNDI for managing application configuration data, emphasizing the importance of externalizing configuration.]

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