EJB Interview Questions and Answers for 10 years experience

1. What is EJB?

   • Answer: Enterprise Java Beans (EJB) is a server-side component architecture for creating distributed, transactional, and secure enterprise applications in Java. It simplifies the development of complex, scalable applications by providing a standardized framework for handling common enterprise concerns like transactions, persistence, and security.

2. What are the different types of EJBs?

   • Answer: There are three main types: 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 calls from the same client. The bean instance is tied to a specific client, and its state is preserved between method invocations. They are ideal for scenarios requiring context-sensitive operations.

4. Explain Stateless Session Beans.

   • Answer: Stateless Session Beans do not maintain conversational state. Each method call is treated independently. They are highly scalable and easier to manage due to their stateless nature, making them suitable for simple, short-lived operations.

5. Explain Singleton Session Beans.

   • Answer: Singleton Session Beans ensure only one instance of the bean exists within the EJB container. They are useful for managing shared resources or providing global application services. Concurrency control mechanisms must be carefully considered.

6. What are Message-Driven Beans (MDBs)?

   • Answer: MDBs are asynchronous beans that receive messages from Java Message Service (JMS) destinations (queues or topics). They are ideal for handling asynchronous tasks and decoupling components in a message-driven architecture.

7. What is the role of the EJB container?

   • Answer: The EJB container is the runtime environment that manages the lifecycle of EJBs, handles transactions, security, concurrency, and resource pooling. It provides a robust infrastructure for deploying and managing enterprise beans.

8. Explain EJB deployment descriptors.

   • Answer: Deployment descriptors (typically `ejb-jar.xml`) provide metadata about EJBs, including their names, types, transaction attributes, security roles, and resource references. They instruct the EJB container on how to deploy and manage the beans.

9. What are transaction attributes in EJBs?

   • Answer: Transaction attributes define how EJB methods interact with transactions. Common attributes include Required, RequiresNew, Mandatory, Supports, NotSupported, Never. They specify how a method joins or starts its own transaction.

10. Explain the difference between container-managed transactions and bean-managed transactions.

    • Answer: Container-managed transactions (CMT) let the EJB container manage transaction boundaries automatically based on deployment descriptor settings. Bean-managed transactions (BMT) require the bean to explicitly control transaction boundaries using JTA APIs.

11. How does EJB handle security?

    • Answer: EJB utilizes declarative security using roles and security constraints defined in the deployment descriptor. It integrates with authentication and authorization mechanisms to control access to EJB methods.

12. What are EJB interceptors?

    • Answer: EJB interceptors are pluggable pieces of code that can intercept method calls on EJBs before or after execution. They are useful for implementing cross-cutting concerns such as logging, auditing, or security.

13. What are EJB timers?

    • Answer: EJB timers allow scheduling the execution of EJB methods at specific times or intervals. This enables the implementation of scheduled tasks and background processes.

14. Explain EJB remote interfaces.

    • Answer: Remote interfaces define the methods accessible to clients located on different JVMs or machines. They allow distributed access to EJBs.

15. Explain EJB local interfaces.

    • Answer: Local interfaces define the methods accessible to clients within the same JVM as the EJB. They offer better performance compared to remote interfaces due to absence of network communication overhead.

16. What is a home interface in EJB (older versions)?

    • Answer: In older EJB versions, the home interface provided methods for creating and removing bean instances. This has largely been simplified in newer versions.

17. What is dependency injection in EJB?

    • Answer: EJB supports dependency injection via annotations like `@Inject`. This mechanism allows external resources and dependencies to be provided to the bean without hardcoding them.

18. How do you handle concurrency in EJBs?

    • Answer: Concurrency is handled using various mechanisms like using stateless session beans (naturally thread-safe), synchronization mechanisms, and container-managed concurrency.

19. What are the advantages of using EJB?

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

20. What are the disadvantages of using EJB?

    • Answer: Disadvantages can include complexity, potential performance overhead (especially with remote calls), and steeper learning curve compared to simpler frameworks.

21. How does EJB integrate with other technologies?

    • Answer: EJB integrates well with JMS for message-driven architectures, JPA for persistence, JTA for transaction management, and various security frameworks.

22. Explain the lifecycle of a stateless session bean.

    • Answer: A stateless session bean's lifecycle is managed by the container. It's created when needed, pooled for reuse, and passivated when idle, then destroyed when no longer needed.

23. Explain the lifecycle of a stateful session bean.

    • Answer: A stateful session bean's lifecycle is also container-managed but is tied to a specific client. It's created when the client first uses it, its state is maintained throughout the client's session, and it's removed when the session ends.

24. What is the role of the @PostConstruct and @PreDestroy annotations?

    • Answer: `@PostConstruct` is called after the bean is created and initialized, while `@PreDestroy` is called before the bean is destroyed, allowing for custom initialization and cleanup.

25. How do you handle exceptions in EJBs?

    • Answer: Exceptions are handled using standard Java exception handling mechanisms, often with custom exception classes for better error management and reporting.

26. What are the different ways to deploy EJBs?

    • Answer: EJBs are typically deployed using application servers like JBoss, GlassFish, or WebLogic. Deployment involves packaging the EJBs in an EAR file and deploying it to the server.

27. What are the best practices for designing EJBs?

    • Answer: Best practices include using appropriate bean types for the task, proper transaction management, clear separation of concerns, and utilizing dependency injection.

28. How do you test EJBs?

    • Answer: EJBs can be tested using various approaches, including unit testing with mocking frameworks, integration testing with a test container, and end-to-end testing with a full application server deployment.

29. What are some common performance considerations when working with EJBs?

    • Answer: Performance considerations include choosing appropriate bean types, minimizing remote calls, efficient database access (using JPA correctly), and proper resource management.

30. How do you handle resource injection in EJBs?

    • Answer: Resource injection is done using `@Resource` annotation to inject resources defined in the deployment descriptor, such as database connections or JMS queues.

31. Describe your experience with different EJB application servers.

    • Answer: [Candidate should detail their experience with specific application servers, such as JBoss, GlassFish, WebSphere, WebLogic, etc., including deployment strategies and troubleshooting.]

32. Explain your experience with EJB security configurations.

    • Answer: [Candidate should describe their experience in configuring roles, security constraints, authentication, authorization, and integration with other security frameworks.]

33. How have you optimized EJB applications for performance?

    • Answer: [Candidate should describe specific techniques used to improve performance, like using caching, connection pooling, efficient database queries, and minimizing remote calls.]

34. Describe a challenging problem you encountered while working with EJBs and how you solved it.

    • Answer: [Candidate should describe a specific challenge and the steps taken to resolve it. This demonstrates problem-solving skills and experience.]

35. Explain your understanding of EJB transactions and their different isolation levels.

    • Answer: [Candidate should explain the various transaction isolation levels (e.g., Read Uncommitted, Read Committed, Repeatable Read, Serializable) and how they impact database concurrency.]

36. How do you handle asynchronous operations using EJBs?

    • Answer: [Candidate should describe using Message-Driven Beans (MDBs) with JMS for asynchronous processing.]

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

    • Answer: [Candidate should describe significant differences, such as the introduction of annotations, simplification of deployment descriptors, and the shift away from home and remote interfaces in EJB 3.x.]

38. How do you monitor and troubleshoot EJB applications?

    • Answer: [Candidate should discuss tools and techniques used for monitoring, such as application server logging, JMX, and performance monitoring tools.]

39. What are your thoughts on the future of EJB technology?

    • Answer: [Candidate should discuss their perspective on the continued relevance of EJB in the context of newer technologies and frameworks.]

40. Explain your experience with using EJBs in a microservices architecture.

    • Answer: [Candidate should describe their experience, if any, of using EJBs in a microservices context, acknowledging the challenges and potential trade-offs.]

41. How do you handle distributed transactions across multiple EJBs?

    • Answer: [Candidate should explain the use of JTA (Java Transaction API) and two-phase commit protocols for managing distributed transactions.]

42. What are your preferred methods for managing EJB application configuration?

    • Answer: [Candidate should discuss using application server configuration tools, environment variables, and other techniques for managing configurations.]

43. How do you handle database connection pooling within an EJB application?

    • Answer: [Candidate should discuss utilizing the application server's connection pooling features and configuring appropriate connection parameters.]

44. How would you approach designing a highly available and scalable EJB application?

    • Answer: [Candidate should outline strategies such as clustering, load balancing, failover mechanisms, and database replication for high availability and scalability.]

45. Explain your experience with different EJB deployment strategies, such as hot deployment and rolling updates.

    • Answer: [Candidate should discuss the use of hot deployment, rolling updates, and other deployment techniques to minimize downtime and improve deployment efficiency.]

46. How familiar are you with the different Java EE design patterns relevant to EJB development?

    • Answer: [Candidate should mention patterns like Singleton, Facade, Data Access Object (DAO), and others applicable to EJB applications.]

47. Describe your experience with using EJB timers for scheduling tasks.

    • Answer: [Candidate should explain their experience in configuring and using EJB timers to schedule periodic or one-time tasks.]

48. How have you used logging effectively in your EJB applications?

    • Answer: [Candidate should discuss the use of logging frameworks like Log4j or SLF4j to capture relevant information for debugging and monitoring.]

49. How do you handle data integrity and consistency in an EJB application?

    • Answer: [Candidate should describe their use of transactions, appropriate database constraints, and validation techniques to ensure data integrity.]

50. What are some common security vulnerabilities associated with EJB applications and how to mitigate them?

    • Answer: [Candidate should mention common vulnerabilities, such as SQL injection, cross-site scripting (XSS), and others, along with the mitigation strategies.]

51. Explain your experience with integrating EJB applications with external systems.

    • Answer: [Candidate should discuss techniques such as using web services (SOAP/REST), messaging systems (JMS), and other integration methods.]

52. How do you handle different environments (development, testing, production) for EJB applications?

    • Answer: [Candidate should explain their experience with configuration management, deployment automation, and environment-specific settings for EJB applications.]

53. How familiar are you with using CDI (Contexts and Dependency Injection) with EJBs?

    • Answer: [Candidate should discuss their experience with using CDI for dependency injection and managing bean lifecycles.]

54. Explain your experience with performance tuning and profiling EJB applications.

    • Answer: [Candidate should describe techniques used for performance analysis and optimization, such as using profiling tools and analyzing application server logs.]

55. How would you design an EJB application for scalability across multiple servers?

    • Answer: [Candidate should outline a scalable design, addressing issues like load balancing, session replication, and database scaling.]

56. Describe your experience with integrating EJBs with other Java EE technologies like Servlets and JSPs.

    • Answer: [Candidate should discuss techniques for integrating EJBs with other technologies within a Java EE application.]

57. What strategies do you use for managing data persistence in your EJB applications?

    • Answer: [Candidate should discuss the use of JPA (Java Persistence API) and other persistence strategies.]

58. Describe your experience with implementing different concurrency control mechanisms in EJB applications.

    • Answer: [Candidate should discuss using optimistic locking, pessimistic locking, and other concurrency control mechanisms within EJBs.]

59. How do you handle resource cleanup and release in your EJB applications?

    • Answer: [Candidate should discuss using `@PreDestroy` and ensuring proper resource handling to prevent resource leaks.]

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