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.

J2EE with EJB and Weblogic interview questions

1. What is the difference between URL instance and URLConnection instance? - A URL instance represents the location of a resource, and a URLConnection instance represents a link for accessing or communicating with the resource at the location.
2. What are the two important TCP Socket classes? - Socket and ServerSocket. ServerSocket is used for normal two-way socket communication. Socket class allows us to read and write through the sockets. getInputStream() and getOutputStream() are the two methods available in Socket class.
3. What technologies are included in J2EE? - The primary technologies in J2EE are: Enterprise JavaBeansTM (EJBsTM), JavaServer PagesTM (JSPsTM), Java Servlets, the Java Naming and Directory InterfaceTM (JNDITM), the Java Transaction API (JTA), CORBA, and the JDBCTM data access API.
4. What is the Java Authentication and Authorization Service (JAAS) 1.0? - The Java Authentication and Authorization Service (JAAS) provides a way for a J2EE application to authenticate and authorize a specific user or group of users to run it. JAAS is a Java programing language version of the standard Pluggable Authentication Module (PAM) framework that extends the Java 2 platform security architecture to support user-based authorization.
5. What’s the difference between JNDI lookup(), list(), listBindings(), and search()? - lookup() attempts to find the specified object in the given context. I.e., it looks for a single, specific object and either finds it in the current context or it fails. list() attempts to return an enumeration of all of the NameClassPair’s of all of the objects in the current context. I.e., it’s a listing of all of the objects in the current context but only returns the object’s name and the name of the class to which the object belongs. listBindings() attempts to return an enumeration of the Binding’s of all of the objects in the current context. I.e., it’s a listing of all of the objects in the current context with the object’s name, its class name, and a reference to the object itself. search() attempts to return an enumeration of all of the objects matching a given set of search criteria. It can search across multiple contexts (or not). It can return whatever attributes of the objects that you desire. It’s by far the most complex and powerful of these options but is also the most expensive.
6. Components of JNDI - Naming Interface- The naming interface organizes information hierarchically and maps human-friendly names to addresses or objects that are machine-friendly. It allows access to named objects through multiple namespaces. Directory Interface - JNDI includes a directory service interface that provides access to directory objects, which can contain attributes, thereby providing attribute-based searching and schema support. Service Provider Interface - JNDI comes with the SPI, which supports the protocols provided by third parties.
7. What is the Max amount of information that can be saved in a Session Object? - As such there is no limit on the amount of information that can be saved in a Session Object. Only the RAM available on the server machine is the limitation. The only limit is the Session ID length(Identifier), which should not exceed more than 4K. If the data to be store is very huge, then it’s preferred to save it to a temporary file onto hard disk, rather than saving it in session. Internally if the amount of data being saved in Session exceeds the predefined limit, most of the servers write it to a temporary cache on Hard disk.
8. Must my bean-managed persistence mechanism use the WebLogic JTS driver? - BEA recommend that you use the TxDataSource for bean-managed persistence.
9. Do EJBs have to be homogeneously deployed across a cluster? Why? - Yes. Beginning with WebLogic Server version 6.0, EJBs must be homogeneously deployed across a cluster for the following reasons:
   • To keep clustering EJBs simple
   • To avoid cross server calls which results in more efficiency. If EJBs are not deployed on all servers, cross server calls are much more likely.
   • To ensure that every EJB is available locally
   • To ensure that all classes are loaded in an undeployable way
   • Every server must have access to each EJB’s classes so that it can be bound into the local JNDI tree. If only a subset of the servers deploys the bean, the other servers will have to load the bean’s classes in their respective system classpaths which makes it impossible to undeploy the beans.
10. Is an XSLT processor bundled in WebLogic Server? - Yes, an XSLT processor, based on Apache’s Xalan 2.0.1 processor, in WebLogic Server 6.1.
11. I plugged in a version of Apache Xalan that I downloaded from the Apache Web site, and now I get errors when I try to transform documents. What is the problem? - You must ensure that the version of Apache Xalan you download from the Apache Web site is compatible with Apache Xerces version 1.3.1. Because you cannot plug in a different version of Apache Xerces , the only version of Apache Xerces that is compatible with WebLogic Server 6.1 is 1.3.1. The built-in parser (based on version 1.3.1 of Apache Xerces) and transformer (based on version 2.0.1 of Apache Xalan) have been modified by BEA to be compatible with each other.
12. How do I increase WebLogic Server memory? - Increase the allocation of Java heap memory for WebLogic Server. (Set the minimum and the maximum to the same size.) Start WebLogic Server with the -ms32m option to increase the allocation, as in this example:

    $ java ... -ms32m -mx32m ...
    

    This allocates 32 megabytes of Java heap memory to WebLogic Server, which improves performance and allows WebLogic Server to handle more simultaneous connections. You can increase this value if necessary.
13. What causes Java.io exceptions in the log file of WebLogic Server? - You may see messages like these in the log file:

    (Windows NT)
     java.io.IOException Connection Reset by Peer
     java.io.EOFException Connection Reset by Peer
    (Solaris)
     java.io.Exception: Broken pipe
    

    These messages occur when you are using servlets. A client initiates an HTTP request, and then performs a series of actions on the browser:
    • Click Stop or enter equivalent command or keystrokes
    • Click Refresh or enter equivalent command or keystrokes
    • Send a new HTTP request.
    The messages indicate that WebLogic Server has detected and recovered from an interrupted HTTP request.
14. What is the function of T3 in WebLogic Server? - T3 provides a framework for WebLogic Server messages that support for enhancements. These enhancements include abbreviations and features, such as object replacement, that work in the context of WebLogic Server clusters and HTTP and other product tunneling. T3 predates Java Object Serialization and RMI, while closely tracking and leveraging these specifications. T3 is a superset of Java Object. Serialization or RMI; anything you can do in Java Object Serialization and RMI can be done over T3. T3 is mandated between WebLogic Servers and between programmatic clients and a WebLogic Server cluster. HTTP and IIOP are optional protocols that can be used to communicate between other processes and WebLogic Server. It depends on what you want to do. For example, when you want to communicate between a browser and WebLogic Server-use HTTP, or an ORB and WebLogic Server-IIOP.
15. What are the enhancements in EJB 2.0 specification with respect to Asynchronous communication? - EJB 2.0 mandates integration between JMS and EJB. We have specified the integration of Enterprise JavaBeans with the Java Message Service, and have introduced message-driven beans. A message-driven bean is a stateless component that is invoked by the container as a result of the arrival of a JMS message. The goal of the message-driven bean model is to make developing an enterprise bean that is asynchronously invoked to handle the processing of incoming JMS messages as simple as developing the same functionality in any other JMS MessageListener.
16. What are the enhancements in EJB 2.0 with respect to CMP? - EJB 2.0 extends CMP to include far more robust modeling capability, with support for declarative management of relationships between entity EJBs. Developers no longer need to re-establish relationships between the various beans that make up their application — the container will restore the connections automatically as beans are loaded, allowing bean developers to navigate between beans much as they would between any standard Java objects.
    EJB 2.0 also introduces for the first time a portable query language, based on the abstract schema, not on the more complex database schema. This provides a database and vendor-independent way to find entity beans at run time, based on a wide variety of search criteria.
17. Can you briefly describe local interfaces? - EJB was originally designed around remote invocation using the Java Remote Method Invocation (RMI) mechanism, and later extended to support to standard CORBA transport for these calls using RMI/IIOP. This design allowed for maximum flexibility in developing applications without consideration for the deployment scenario, and was a strong feature in support of a goal of component reuse in J2EE. Many developers are using EJBs locally - that is, some or all of their EJB calls are between beans in a single container. With this feedback in mind, the EJB 2.0 expert group has created a local interface mechanism. The local interface may be defined for a bean during development, to allow streamlined calls to the bean if a caller is in the same container. This does not involve the overhead involved with RMI like marshalling etc. This facility will thus improve the performance of applications in which co-location is planned. Local interfaces also provide the foundation for container-managed relationships among entity beans with container-managed persistence.
18. What are the special design care that must be taken when you work with local interfaces? - It is important to understand that the calling semantics of local interfaces are different from those of remote interfaces. For example, remote interfaces pass parameters using call-by-value semantics, while local interfaces use call-by-reference. This means that in order to use local interfaces safely, application developers need to carefully consider potential deployment scenarios up front, then decide which interfaces can be local and which remote, and finally, develop the application code with these choices in mind. While EJB 2.0 local interfaces are extremely useful in some situations, the long-term costs of these choices, especially when changing requirements and component reuse are taken into account, need to be factored into the design decision.
19. What happens if remove( ) is never invoked on a session bean? - In case of a stateless session bean it may not matter if we call or not as in both cases nothing is done. The number of beans in cache is managed by the container. In case of stateful session bean, the bean may be kept in cache till either the session times out, in which case the bean is removed or when there is a requirement for memory in which case the data is cached and the bean is sent to free pool.
20. What is the difference between creating a distributed application using RMI and using a EJB architecture? - It is possible to create the same application using RMI and EJB. But in case of EJB the container provides the requisite services to the component if we use the proper syntax. It thus helps in easier development and lesser error and use of proven code and methodology. But the investment on application server is mandatory in that case. But this investment is warranted because it results in less complex and maintainable code to the client, which is what the end client wants. Almost all the leading application servers provide load balancing and performance tuning techniques. In case of RMI we have to code the services and include in the program the way to invoke these services.
21. Why would a client application use JTA transactions? - One possible example would be a scenario in which a client needs to employ two (or more) session beans, where each session bean is deployed on a different EJB server and each bean performs operations against external resources (for example, a database) and/or is managing one or more entity beans. In this scenario, the client’s logic could required an all-or-nothing guarantee for the operations performed by the session beans; hence, the session bean usage could be bundled together with a JTA UserTransaction object. In the previous scenario, however, the client application developer should address the question of whether or not it would be better to encapsulate these operations in yet another session bean, and allow the session bean to handle the transactions via the EJB container. In general, lightweight clients are easier to maintain than heavyweight clients. Also, EJB environments are ideally suited for transaction management.

    Context c = new InitialContext(); UserTransaction ut = (UserTransaction)
    c.lookup("java:comp/UserTransaction");
    ut.begin();
     // perform multiple operations...
    ut.commit() ...
    

22. Can the bean class implement the EJBObject class directly? If not why? - It is better not to do it will make the Bean class a remote object and its methods can be accessed without the containers? security, and transaction implementations if our code by mistake passed it in one of its parameters. Its just a good design practice.
23. What does isIdentical() method return in case of different type of beans? - Stateless - true always. Stateful - depends whether the references point to the same session object. Entity - Depends whether the primary key is the same and the home is same.
24. How should you type cast a remote object? Why? - A client program that is intended to be interoperable with all compliant EJB Container implementations must use the javax.rmi.PortableRemoteObject.narrow(…) method to perform type-narrowing of the client-side representations of the remote home and remote interfaces. Programs using the cast operator for narrowing the remote and remote home interfaces are likely to fail if the Container implementation uses RMI-IIOP as the underlying communication transport.
25. What should you do in a passive method? - You try to make all nontransient variables, which are not one of the following to null. For the given list the container takes care of serializing and restoring the object when activated. Serializable objects, null, UserTransaction, SessionContext, JNDI contexts in the beans context, reference to other beans, references to connection pools.
    Things that must be handled explicitly are like a open database connection etc. These must be closed and set to null and retrieved back in the activate method.