BEA WebLogic Interview Questions and Answers 11

How do I create a producer pool?

The following is some pseudo-code

for a producer class.



class ProducerPool {

static Hashmap pSets = new Hashtable();

static Hashmap inUse = new Hashtable();



QueueSender get(String contextURL,

String connectionFactoryName,

String destinationName) {

String lookup = contextURL+";

"+connectionFactName+";"+destName;

synchronized(pSets) {

producer set = pSets.get(lookup);

if (set != null && set not empty)

qs = set.removeFirst();

}

if (producer == null) {

create ctx

get connect factory

create connection

create session

look up destination

qs = create queue sender

}

synchronized(inUse) {

inUse.put(qs, lookup);

}

return qs;

}



void put(QueueSender qs) {

String lookup;

synchronized(inUse) {

lookup = inUse.remove(p);

}

synchronzied(pSets) {

producer set = pSets.get(lookup);

if (set == null) {

producer set = new producer set

pSets.put(lookup, producer set);

}

producer set.add(qs);

}

}

}



Note: Static classes may be garbage collected if there are no references to them, so make sure the application server has a permanent pointer to them in some manner. One way is to reference it permanently from within a servlet or EJB when they are initialized at startup.

Here is an example of using the producer pool within the onMessage method.

onMessage() {

QueueSender qs = ProducerPool.get(...);

qs.send(...);

ProducerPool.put(qs);

}



You can pre-populate this pool by calling it from

a startup class or a load-on-start servlet class.

What are pending messages in the console?

Pending means the message could have been:

* sent in a transaction but not committed.

* received and not acknowledged.

* received and not committed.

* subject to a redelivery delay (as of WebLogic Server 6.1).

* subject to a delivery time (as of WebLogic Server 6.1).

A rolled back message remains pending until the transaction actually rolls back. Rolling it back multiple times does not cause double counting, nor does an exception that set a transaction as rollbackOnly followed by an actual rollback.

Current implies messages that are not pending.

Total implies total since server last started. The byte counts only consider the payload of messages which includes the properties and the body but not the header.

How do I use a less than or greater than on a message selector in ejb-jar.xml?

Enclose the selector in a CDATA section. That will prevent the XML parser from thinking that less than or greater than is a tag.



<jms-message-selector>

'user' ]]>

Is it better to have more or fewer sessions for a given number of subscribers?

Using N sessions for N subscribers gives you concurrency up to N simultaneous threads of execution provided you have as many threads to work with. Each Session gets its own thread as long as there are enough threads available. Otherwise, the sessions serially reuse the available threads.

One session for N subscribers serializes all subscribers through that one session. If the load is heavy they may not be able to keep up without the extra threads.

If you are using CLIENT_ACKNOWLEDGE, N sessions gives you N separate message streams that can be individually recovered. Having one session crosses the streams giving you less control.

Match the EJB functions given below with the functionality equivalent in SQL

A.) ejbStore() 1.) INSERT

B.) ejbLoad() 2.) UPDATE

C.) ejbCreate() 3.) SELECT

a. A->1, B->2, C->3

b. A->2, B->1, C->3

c. A->3, B->2, C->1

d. A->1, B->3, C->2

e. A->2, B->3, C->1

f. A->3, B->1, C->2



Choice E is correct. When the create() method on a home interface is invoked, the container delegates the create() method call to the bean instance's matching ejbCreate() method. The ejbCreate() methods are used to initialize the instance state before record is inserted into the database. The ejbCreate() method is analogous to INSERT. The ejbStore() method is invoked just before the container the container is about to write the bean container-managed fields to the database. It is analogous to the UPDATE . The ejbLoad() is invoked just after the container has refreshed the bean container-managed files with its state from the database. It is analogous to the SELECT. Thus choice E is correct and others are not.

A client invokes a method on a stateful session bean instance deployed in the WebLogic Server. While the method execution is in progress another method call arrives on the server. What will be the result?

a. RemoteException is thrown if the value of concurrency-strategy property is false

b. EJBException is thrown if the value of concurrency-strategy property is false

c. The EJB container blocks the concurrent method call and allows it to proceed when the previous call has completed if the value of allow-concurrent-calls is true

d. In all cases, RemoteException is thrown



Choice C is correct. By default, simultaneous access to a stateful session EJB results in a RemoteException. However, you can set the allow-concurrent-calls option in the WebLogic EJB deployment descriptor to specify that a stateful session bean instance will allow concurrent method calls. This access restriction on stateful session EJBs applies whether the EJB client is remote or internal to WebLogic Server. By default, allows-concurrent-calls is false. However, when this value is set to true, the EJB container blocks the concurrent method call and allows it to proceed when the previous call has completed.

The concurrency-strategy element determines ejbLoad() and ejbStore() behavior for entity EJB instances.