- Are enterprise beans allowed to use Thread.sleep()? - Enterprise beans make use of the services provided by the EJB container, such as life-cycle management. To avoid conflicts with these services, enterprise beans are restricted from performing certain operations: Managing or synchronizing threads
- Is it possible to write two EJB's that share the same Remote and Home interfaces, and have different bean classes? if so, what are the advantages/disadvantages? - It's certainly possible. In fact, there's an example that ships with the Inprise Application Server of an Account interface with separate implementations for CheckingAccount and SavingsAccount, one of which was CMP and one of which was BMP.
- Is it possible to specify multiple JNDI names when deploying an EJB? - No. To achieve this you have to deploy your EJB multiple times each specifying a different JNDI name.
- Is there any way to force an Entity Bean to store itself to the db? I don't wanna wait for the container to update the db, I want to do it NOW! Is it possible? - Specify the transaction attribute of the bean as RequiresNew. Then as per section 11.6.2.4 of the EJB v 1.1 spec EJB container automatically starts a new transaction before the method call. The container also performs the commit protocol before the method result is sent to the client.
- I am developing a BMP Entity bean. I have noticed that whenever the create method is invoked, the ejbLoad() and the ejbStore() methods are also invoked. I feel that once my database insert is done, having to do a select and update SQL queries is major overhead. is this behavior typical of all EJB containers? Is there any way to suppress these invocations? - This is the default behaviour for EJB. The specification states that ejbLoad() will be called before every transaction and ejbStore() after every transaction. Each Vendor has optimizations, which are proprietary for this scenario.
- Can an EJB send asynchronous notifications to its clients? - Asynchronous notification is a known hole in the first versions of the EJB spec. The recommended solution to this is to use JMS, which is becoming available in J2EE-compliant servers. The other option, of course, is to use client-side threads and polling. This is not an ideal solution, but it's workable for many scenarios.
- How can I access EJB from ASP? - You can use the Java 2 Platform, Enterprise Edition Client Access Services (J2EETM CAS) COM Bridge 1.0, currently downloadable from Sun
- Is there a guarantee of uniqueness for entity beans? - There is no such guarantee. The server (or servers) can instantiate as many instances of the same underlying Entity Bean (with the same PK) as it wants. However, each instance is guaranteed to have up-to-date data values, and be transactionally consistent, so uniqueness is not required. This allows the server to scale the system to support multiple threads, multiple concurrent requests, and multiple hosts.
- How do the six transaction attributes map to isolation levels like "dirty read"? Will an attribute like "Required" lock out other readers until I'm finished updating? - The Transaction Attributes in EJB do not map to the Transaction Isolation levels used in JDBC. This is a common misconception. Transaction Attributes specify to the container when a Transaction should be started, suspended(paused) and committed between method invocations on Enterprise JavaBeans. For more details and a summary of Transaction Attributes refer to section 11.6 of the EJB 1.1 specification.
- I have created a remote reference to an EJB in FirstServlet. Can I put the reference in a servlet session and use that in SecondServlet? - Yes. The EJB client (in this case your servlet) acquires a remote reference to an EJB from the Home Interface; that reference is serializable and can be passed from servlet to servlet. If it is a session bean, then the EJB server will consider your web client's servlet session to correspond to a single EJB session, which is usually (but not always) what you want.
- Can the primary key in the entity bean be a Java primitive type such as int? - The primary key can't be a primitive type–use the primitive wrapper classes, instead. For example, you can use java.lang.Integer as the primary key class, but not int (it has to be a class, not a primitive)
- What's new in the EJB 2.0 specification? - Following are the main features supported in EJB 2.0: Integration of EJB with JMS, Message Driven Beans, Implement additional Business methods in Home interface which are not specific for bean instance, EJB QL.
- How many types of protocol implementations does RMI have? - RMI has at least three protocol implementations: Java Remote Method Protocol(JRMP), Internet Inter ORB Protocol(IIOP), and Jini Extensible Remote Invocation(JERI). These are alternatives, not part of the same thing, All three are indeed layer 6 protocols for those who are still speaking OSI reference model.
- What is the need of Remote and Home interfaces. Why can't there be one? - In a few words, I would say that the main reason is because there is a clear division of roles and responsabilities between the two interfaces. The home interface is your way to communicate with the container, that is who is responsable 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.
- What is the difference between Java Beans and EJB? - Java Beans are client-side objects and EJBs are server side object, and they have completely different development, lifecycle, purpose.
- Question With regard to Entity Beans, what happens if both my EJB Server and Database crash, what will happen to unsaved changes? Is there any transactional log file used? - Actually, if your EJB server crashes, you will not even be able to make a connection to the server to perform a bean lookup, as the server will no longer be listening on the port for incoming JNDI lookup requests. You will lose any data that wasn't committed prior to the crash. This is where you should start looking into clustering your EJB server. Any unsaved and uncommited changes are lost the moment your EJB Server crashes. If your database also crashes, then all the saved changes are also lost unless you have some backup or some recovery mechanism to retrieve the data. So consider database replication and EJB Clustering for such scenarios, though the occurence of such a thing is very very rare. Thx, Uma All databse have the concept of log files(for exampe oracle have redo log files concept). So if data bases crashes then on starting up they fill look up the log files to perform all pending jobs. But is EJB crashes, It depend upon the container how frequenlty it passivates or how frequesntly it refreshes the data with Database.
- Question Can you control when passivation occurs? - The developer, according to the specification, cannot directly control when passivation occurs. Although for Stateful Session Beans, the container cannot passivate an instance that is inside a transaction. So using transactions can be a a strategy to control passivation. The ejbPassivate() method is called during passivation, so the developer has control over what to do during this exercise and can implement the require optimized logic. Some EJB containers, such as BEA WebLogic, provide the ability to tune the container to minimize passivation calls. Taken from the WebLogic 6.0 DTD - "The passivation-strategy can be either "default" or "transaction". With the default setting the container will attempt to keep a working set of beans in the cache. With the "transaction" setting, the container will passivate the bean after every transaction (or method call for a non-transactional invocation)."
- Does RMI-IIOP support dynamic downloading of classes? - No, RMI-IIOP doesn't support dynamic downloading of the classes as it is done with CORBA in DII (Dynamic Interface Invocation).Actually RMI-IIOP combines the usability of Java Remote Method Invocation (RMI) with the interoperability of the Internet Inter-ORB Protocol (IIOP).So in order to attain this interoperability between RMI and CORBA,some of the features that are supported by RMI but not CORBA and vice versa are eliminated from the RMI-IIOP specification.
- Does EJB 1.1 support mandate the support for RMI-IIOP ? What is the meaning of "the client API must support the Java RMI-IIOP programming model for portability, but the underlying protocol can be anything" ? - EJB1.1 does mandate the support of RMI-IIOP. There are 2 types of implementations that an EJB Server might provide: CORBA-based EJB Servers and Proprietry EJB Servers. Both support the RMI-IIOP API but how that API is implemented is a different story. (NB: By API we mean the interface provided to the client by the stub or proxy). A CORBA-based EJB Server actually implements its EJB Objects as CORBA Objects (it therefore encorporates an ORB and this means that EJB's can be contacted by CORBA clients (as well as RMI-IIOP clients) A proprietry EJB still implements the RMI-IIOP API (in the client's stub) but the underlying protocol can be anything. Therefore your EJB's CANNOT be contacted by CORBA clients. The difference is that in both cases, your clients see the same API (hence, your client portability) BUT how the stubs communicate with the server is different.
- The EJB specification says that we cannot use Bean Managed Transaction in Entity Beans. Why? - The short, practical answer is… because it makes your entity beans useless as a reusable component. Also, transaction management is best left to the application server - that's what they're there for. It's all about atomic operations on your data. If an operation updates more than one entity then you want the whole thing to succeed or the whole thing to fail, nothing in between. If you put commits in the entity beans then it's very difficult to rollback if an error occurs at some point late in the operation.
- Can I invoke Runtime.gc() in an EJB? - You shouldn't. What will happen depends on the implementation, but the call will most likely be ignored. You should leave system level management like garbage collection for the container to deal with. After all, that's part of the benefit of using EJBs, you don't have to manage resources yourself.
- What is clustering? What are the different algorithms used for clustering? - Clustering is grouping machines together to transparantly provide enterprise services.The client does not now the difference between approaching one server or approaching a cluster of servers.Clusters provide two benefits: scalability and high availability. Further information can be found in the JavaWorld article J2EE Clustering.
- What is the advantage of using Entity bean for database operations, over directly using JDBC API to do database operations? When would I use one over the other? - Entity Beans actually represents the data in a database. It is not that Entity Beans replaces JDBC API. There are two types of Entity Beans Container Managed and Bean Mananged. In Container Managed Entity Bean - Whenever the instance of the bean is created the container automatically retrieves the data from the DB/Persistance storage and assigns to the object variables in bean for user to manipulate or use them. For this the developer needs to map the fields in the database to the variables in deployment descriptor files (which varies for each vendor). In the Bean Managed Entity Bean - The developer has to specifically make connection, retrive values, assign them to the objects in the ejbLoad() which will be called by the container when it instatiates a bean object. Similarly in the ejbStore() the container saves the object values back the the persistance storage. ejbLoad and ejbStore are callback methods and can be only invoked by the container. Apart from this, when you use Entity beans you dont need to worry about database transaction handling, database connection pooling etc. which are taken care by the ejb container. But in case of JDBC you have to explicitly do the above features. what suresh told is exactly perfect. ofcourse, this comes under the database transations, but i want to add this. the great thing about the entity beans of container managed, whenever the connection is failed during the transaction processing, the database consistancy is mantained automatically. the container writes the data stored at persistant storage of the entity beans to the database again to provide the database consistancy. where as in jdbc api, we, developers has to do manually.
- What is the role of serialization in EJB? - A big part of EJB is that it is a framework for underlying RMI: remote method invocation. You're invoking methods remotely from JVM space 'A' on objects which are in JVM space 'B' – possibly running on another machine on the network. To make this happen, all arguments of each method call must have their current state plucked out of JVM 'A' memory, flattened into a byte stream which can be sent over a TCP/IP network connection, and then deserialized for reincarnation on the other end in JVM 'B' where the actual method call takes place. If the method has a return value, it is serialized up for streaming back to JVM A. Thus the requirement that all EJB methods arguments and return values must be serializable. The easiest way to do this is to make sure all your classes implement java.io.Serializable.
- What is EJB QL? - EJB QL is a Query Language provided for navigation across a network of enterprise beans and dependent objects defined by means of container managed persistence. EJB QL is introduced in the EJB 2.0 specification. The EJB QL query language defines finder methods for entity beans with container managed persistenceand is portable across containers and persistence managers. EJB QL is used for queries of two types of finder methods: Finder methods that are defined in the home interface of an entity bean and which return entity objects. Select methods, which are not exposed to the client, but which are used by the Bean Provider to select persistent values that are maintained by the Persistence Manager or to select entity objects that are related to the entity bean on which the query is defined.
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