BEA WebLogic Interview Questions and Answers 17

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
* An ORB and WebLogic Server-IIOP.

How can I debug the Java code that I have running in WebLogic Server?
You can use tools such as WebGain, JBuilder, NetBeans and JDB that rely on the Java Platform Debugger Architecture (JPDA) to debug your Java code running in WebLogic Server.
JPDA is integrated in the Java 2 Platform, Standard Edition (J2SE) SDK 1.3 on all platforms and SDK 1.2.2 for Linux. There is a download available from Sun to add JPDA support to the J2SE SDK 1.2.2 on Solaris and Microsoft Window platforms. If you are using J2SE SDK 1.2.2 on these platforms you must first get this download.
To allow a debugger to attach to the virtual machine that WebLogic runs you have to start WebLogic in debug mode. In order to start WebLogic in debug mode using a Sun virtual machine follow these steps (start with step one only if using a Solaris platform):
1. If using a Solaris platform, change the LD_LIBRARY_PATH environment variable to prepend $JAVA_home/lib/sparc:
export LD_LIBRARY_PATH=$JAVA_home/lib/sparc:$LD_LIBRARY_PATH
2. Add the following parameters to the java command line (before the "weblogic.Server" string) that launches WebLogic server:
-Xdebug
-Xnoagent
-Xrunjdwp:transport=dt_socket
server=y
address=
suspend=n
-Djava.compiler=NONE

Note that with the Hotspot Performance engine the -Xnoagent and -Djava.compiler=NONE options are no longer required, but are accepted and ignored for compatibility reasons.
If server=y and no address parameter is supplied, WebLogic Server chooses the transport address and prints it to the standard output stream. So, if a line such as:
Listening for transport dt_socket at address: 46666
prints in your standard output stream when the server starts, the number 46666 is the port number to be supplied to your tool's remote debugger in order to attach it to WebLogic's virutal machine.

How do I call a servlet with parameters in the URL?
The usual format of a servlet parameter is a name=value pair that comes after a question-mark (?) at the end of the URL. To access these parameters, call the getParameter() method on the HttpServletRequest object, then write code to test the strings. For example, if your URL parameters are "func=topic," where your URL appears as:

http://www.myserver.com/myservlet?func=topic
then you could parse the parameter as follows, where "req" is the HttpServletRequest object:
String func = req.getParameter("func");
if (func.equalsIgnoreCase("topic")) {
. . . do some work
}

Which of the following are the benefits of MDB (Message Driven Beans) over standard JMS consumers?
a. In case of a MDB, developer needs to create a MessageListener class that utilizes a server-wide session pool.
b. WebLogic Server container provides standard EJB services to MDBs.
c. MDBs benefit from the write-once, deploy-anywhere paradigm of EJBs.
d. MDBs can be associated with multiple Messaging Queues or Topics unlike standard JMS.

Choices B and C are correct. A message-driven bean is a special kind of EJB that acts as a message consumer in the WebLogic JMS messaging system. As with standard JMS message consumers, message-driven beans receive messages from a JMS Queue or Topic, and perform business logic based on the message contents. EJB deployers create listeners to a Queue or Topic at deployment time, and WebLogic Server automatically creates and removes message-driven bean instances as needed to process incoming messages.
Because message-driven beans are implemented as EJBs, they benefit from several key services that are not available to standard JMS consumers. Most importantly, message-driven bean instances are wholly managed by the WebLogic Server EJB container. Using a single message-driven bean class, WebLogic Server creates multiple EJB instances as necessary to process large volumes of messages concurrently. This stands in contrast to a standard JMS messaging system, where the developer must create a MessageListener class that utilizes a server-wide session pool. Thus choice A is incorrect.
WebLogic Server provides standard EJB services to MDBs, such as security services and automatic transaction management. Thus choice B is correct.
Being implemented as EJBs, MDBS benefit from the write-once, deploy-anywhere quality of EJBs. Whereas a JMS MessageListener is tied to specific session pools, Queues, or Topics, message-driven beans can be developed independently of available server resources. Thus Choice C is also correct.
Its not that MDBs are always advantageous as compared to standard JMS consumers. One limitation of MDBs compared to standard JMS listeners is that a given MDB deployment can be associated with only one Queue or Topic. If your application requires a single JMS consumer to service messages from multiple Queues or Topics, you must use a standard JMS consumer, or deploy multiple message-driven bean classes. Thus Choice D is incorrect.

Which of the statements below is true for a web application using session management?
a.) The session object is invalidated, when the session times out.
b.) The session object is invalidated, when sessionInvalidate() method of HttpSession is invoked.
a. Both of the above statements are true.
b. Only statement a.) is true.
c. Only statement b.) is true.
d. None of the above statements is true.

B is the correct choice. The session object will become invalid in either of the following scenarios:
a.) When the session times out.
b.) When invalidate() method of the HttpSession interface is invoked.
Please note that invalidate() and not sessionInvalidate() is the method of HttpSession interface. Thus choice B is correct.

How do I identify the document type of an XML document?
If the XML document has a Public ID, then that is its document type. For example, if an XML document contains the following DOCTYPE declaration:
"http://foo.com/url/to/my/dtd">

then its document type is My public ID String.

If the DOCTYPE declaration does not contain a Public ID, but specifies a System ID, then the document type is the System ID. For example, in the following DOCTYPE declaration:

the document type is http://foo.com/url/to/my/dtd.
Note: The System ID is of the DTD, not of the XML document itself. It can, however, still be used as a way to identify the XML document.
If the XML document does not specify a DOCTYPE declaration, then the document type can be either the root element name or the namespace URI, if it has one.

How can I run multiple instances of the same servlet class in the same WebLogic Server instance?
If you want to run multiple instances, your servlet will have to implement the SingleThreadModel interface. An instance of a class that implements the SingleThreadModel interface is guaranteed not to be invoked by multiple threads simultaneously. Multiple instances of a SingleThreadModel interface are used to service simultaneous requests, each running in a single thread.
When designing your servlet, consider how you use shared resources outside of the servlet class such as file and database access. Because there are multiple instances of servlets that are identical, and may use exactly the same resources, there are still synchronization and sharing issues that must be resolved, even if you do implement the SingleThreadModel interface.