Fresh BEA Weblogic Interview Questions & Answers:
1. Why does FOR UPDATE in Oracle 8 cause an ORA-01002 error?
The Oracle 8 server generates an ORA-01002:fetch out of sequence error message when you use a FOR UPDATE statement with AUTOCOMMIT turned on (which is the default state when using JDBC). This is known to happen on Oracle 8.0 and 8.1 on Solaris and on Oracle 8.1 on Windows NT. If you turn AUTOCOMMIT off, you will not receive this error. Because this problem is due to a change in the Oracle 8 server, you should contact Oracle support for more information.
2. How can I control on which WebLogic Server(s) my application will run?
A system administrator can specify on which WebLogic Server(s) applications will run by specifying targets when configuring connection factories. Each connection factory can be deployed on multiple WebLogic servers.
Note: If you use the default connection factory, you have no control over the WebLogic server on which the connection factory may be deployed. If you would like to target a particular WebLogic server, create a new connection factory and specify the appropriate JMS server target(s).
3. How do I look up an ORA SQLException?
If your WebLogic jDriver for Oracle application produces an SQLException such as:
java.sql.SQLException: ORA-12536: TNS: operation would block
You can look up an Oracle error by using the oerr command. For example, the description of error ORA-12536 can be found with the command:
> oerr ora 12536
4. How do I use Unicode codesets with the WebLogic jDriver for Oracle driver?
To use Unicode codesets:
1. Install the appropriate codeset when you install Oracle. If you did not do this in the original installation, you will need to re-run the Oracle installer and install the proper codeset.
2. Define the NLS_LANG variable in the environment where the JDBC driver is running. Do this by assigning the proper codeset to NLS_LANG in the shell from where you start the WebLogic Server.
The Developers Guide has more information about internationalization support.
Yes. Use the setAttribute() method for SAX mode parsing and the getAttribute() method for DOM mode parsing. Using these methods in a Servlet, however, is a WebLogic-specific feature. This means that the Servlet may not be fully portable to other Servlet engines, so use the feature with caution.
The WL-Proxy-Client-Cert header can be spoofed (used) by any client which has direct access to WebLogic Server. WebLogic Server takes the certificate information from that header, trusting that is came from a secure source (the plug-in) and use that information to authenticate the user. In previous releases of WebLogic Server, the default behavior was to always trust that header. Now you need to explicitly define trust of the WL-Proxy-Client-Cert header. A new parameter clientCertProxy allows WebLogic Server to on the implicit trust of the certificate header. If you need an additional level of security, use a connection filter to limit all connections into WebLogic Server (therefore allowing WebLogic Server to only accept connections from the machine on which the plug-in is running).
The clientCertProxy parameter has been added to the HTTPClusterServlet and Web applications.
For the HTTPClusterServlet, add the parameter to the web.xml file as follows:
<context-param>
<param-name>clientCertProxy</param-name>
<param-value>true</param-value>
</context-param>
For Web applications, add the parameter to the web.xml file as follows:
ServletRequestImpl context-param
<context-param>
<param-name>weblogic.http.clientCertProxy</param-name>
<param-value>true</param-value>
</context-param>
You can also use this parameter in a cluster as follows:
<Cluster ClusterAddress="127.0.0.1" Name="MyCluster"
ClientCertProxyHeader="true"/>
7. 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.
8. 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:
<!DOCTYPE mydoc PUBLIC "My public ID String"
"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:
<!DOCTYPE mydoc SYSTEM "http://foo.com/url/to/my/dtd">
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.
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.
10. 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=<port_for_debugger_to_connect>
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.
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