33. If my session bean with single method insert record into 2 entity beans, how can know that the process is done in same transaction (the attributes for these beans are Required)?
If your session bean is using bean-managed transactions, you can ensure that the calls are handled in the same transaction by :
javax.transaction.UserTransaction tran= null;
try {
tran=ctx.getUserTransaction();
tran.begin();
myBeanHome1.create(....);
myBeanHome2.create(...);
tran.commit();
}
catch(...) {
// some thing goes here
}
You may want to check if you're already running in a transaction by calling tran.getStatus().
34. When should I adopt BMP and when I should use CMP?
You can use CMP and BMP beans in the same application... obviously, a bean can be BMP or CMP, not both at the same time (they are mutually exclusive).
There is a common approach that is normally used and considered a good one. You should start developing CMP beans, unless you require some kind of special bean, like multi-tables, that cannot be completely realized with a single bean. Then, when you realize that you need something more or that you would prefer handling the persistence (performance issue are the most common reason)
35. Is it legal to have static initializer blocks in EJB?
Although technically it is legal, static initializer blocks are used to execute some piece of code before executing any constructor or method while instantiating a class. Static initializer blocks are also typically used to initialize static fields - which may be illegal in EJB if they are read/write - In EJB this can be achieved by including the code in either the ejbCreate(), setSessionContext() or setEntityContext() methods.
36. What is the difference between session and entity beans? When should I use one or the other?
An entity bean represents persistent global data from the database; a session bean represents transient user-specific data that will die when the user disconnects (ends his session). Generally, the session beans implement business methods (e.g. Bank.transferFunds) that call entity beans (e.g. Account. deposit, Account.withdraw)
37. Is there any default cache management system with Entity beans? In other words whether a cache of the data in database will be maintained in EJB?
Caching data from a database inside the Application Server are what Entity EJB’s are used for the ejbLoad( ) and ejbStore( ) methods are used to synchronize the Entity Bean state with the persistent storage (database).
Transactions also play an important role in this scenario. If data is removed from the database, via an external application - your Entity Bean can still be “alive” the EJB container. When the transaction commits, ejbStore( ) is called and the row will not be found, and the transaction rolled back.
38. Why is ejbFindByPrimaryKey () mandatory?
An Entity Bean represents persistent data that is stored outside of the EJB Container/Server.
The ejbFindByPrimaryKey () is a method used to locate and load an Entity Bean into the container, similar to a SELECT statement in SQL. By making this method mandatory, the client programmer can be assured that if they have the primary key of the Entity Bean, then they can retrieve the bean without having to create a new bean each time - which would mean creating duplications of persistent data and break the integrity of EJB.
39. Why do we have a remove () method in both EJBHome and EJBObject?
With the EJBHome version of the remove, you are able to delete an entity bean without first instantiating it (you can provide a Primary Key object as a parameter to the remove method).
The home version only works for entity beans. On the other hand, the Remote interface version works on an entity bean that you have already instantiated. In addition, the remote version also works on session beans (stateless and stateful) to inform the container of your loss of interest in this bean.
40. How can I call one EJB from inside of another EJB?
EJBs can be clients of other EJBs. It just works. Use JNDI to locate the Home Interface of the other bean, then acquire an instance reference and so forth.
41. Is method overloading allowed in EJB? - Yes you can overload methods
42. Are we allowed to change the transaction isolation property in middle of a transaction?
No. You cannot change the transaction isolation level in the middle of transaction.
43. What is Java Messaging Service (JMS)?
JMS (JAVA MESSAGING SERVICE) is an API .It is a specification with one package javax.jms. It is a technology which can pass the information synchronously or asynchronously.
If your session bean is using bean-managed transactions, you can ensure that the calls are handled in the same transaction by :
javax.transaction.UserTransaction tran= null;
try {
tran=ctx.getUserTransaction();
tran.begin();
myBeanHome1.create(....);
myBeanHome2.create(...);
tran.commit();
}
catch(...) {
// some thing goes here
}
You may want to check if you're already running in a transaction by calling tran.getStatus().
34. When should I adopt BMP and when I should use CMP?
You can use CMP and BMP beans in the same application... obviously, a bean can be BMP or CMP, not both at the same time (they are mutually exclusive).
There is a common approach that is normally used and considered a good one. You should start developing CMP beans, unless you require some kind of special bean, like multi-tables, that cannot be completely realized with a single bean. Then, when you realize that you need something more or that you would prefer handling the persistence (performance issue are the most common reason)
35. Is it legal to have static initializer blocks in EJB?
Although technically it is legal, static initializer blocks are used to execute some piece of code before executing any constructor or method while instantiating a class. Static initializer blocks are also typically used to initialize static fields - which may be illegal in EJB if they are read/write - In EJB this can be achieved by including the code in either the ejbCreate(), setSessionContext() or setEntityContext() methods.
36. What is the difference between session and entity beans? When should I use one or the other?
An entity bean represents persistent global data from the database; a session bean represents transient user-specific data that will die when the user disconnects (ends his session). Generally, the session beans implement business methods (e.g. Bank.transferFunds) that call entity beans (e.g. Account. deposit, Account.withdraw)
37. Is there any default cache management system with Entity beans? In other words whether a cache of the data in database will be maintained in EJB?
Caching data from a database inside the Application Server are what Entity EJB’s are used for the ejbLoad( ) and ejbStore( ) methods are used to synchronize the Entity Bean state with the persistent storage (database).
Transactions also play an important role in this scenario. If data is removed from the database, via an external application - your Entity Bean can still be “alive” the EJB container. When the transaction commits, ejbStore( ) is called and the row will not be found, and the transaction rolled back.
38. Why is ejbFindByPrimaryKey () mandatory?
An Entity Bean represents persistent data that is stored outside of the EJB Container/Server.
The ejbFindByPrimaryKey () is a method used to locate and load an Entity Bean into the container, similar to a SELECT statement in SQL. By making this method mandatory, the client programmer can be assured that if they have the primary key of the Entity Bean, then they can retrieve the bean without having to create a new bean each time - which would mean creating duplications of persistent data and break the integrity of EJB.
39. Why do we have a remove () method in both EJBHome and EJBObject?
With the EJBHome version of the remove, you are able to delete an entity bean without first instantiating it (you can provide a Primary Key object as a parameter to the remove method).
The home version only works for entity beans. On the other hand, the Remote interface version works on an entity bean that you have already instantiated. In addition, the remote version also works on session beans (stateless and stateful) to inform the container of your loss of interest in this bean.
40. How can I call one EJB from inside of another EJB?
EJBs can be clients of other EJBs. It just works. Use JNDI to locate the Home Interface of the other bean, then acquire an instance reference and so forth.
41. Is method overloading allowed in EJB? - Yes you can overload methods
42. Are we allowed to change the transaction isolation property in middle of a transaction?
No. You cannot change the transaction isolation level in the middle of transaction.
43. What is Java Messaging Service (JMS)?
JMS (JAVA MESSAGING SERVICE) is an API .It is a specification with one package javax.jms. It is a technology which can pass the information synchronously or asynchronously.
