Weblogic Interview questions -4

Why do I get an exception when trying to find a connection factory?
The exception is usually something like java.io.InvalidClassException or java.lang.NoClassDefFoundError.
Make sure weblogic.jar is in the CLASSPATH of the client. Also make sure you have the correct Java run-time jar files included (i.e., you might need rt.jar).

What can I do when I get java.lang.OutOfMemoryError because producers are faster than consumers?
Quotas can be used to help this situation. Your sender will then receive ResourceAllocationExceptions and the server will stay up. WLS 6.X does not support paging of messages out of memory.
As of WLS 6.1 SP02 or later, you can use the Message Paging feature, which can free up valuable virtual memory during peak message load periods by swapping out messages from virtual memory to persistent storage when message loads reach a specified threshold.

Which of the following is NOT true about deploying EJBs in the WebLogic Server?
a. The weblogic/config/examples/applications directory acts as an automatic deployment directory for EJB 2.0 .jar files and EJB .jar deployment directories
b. The automatic redeployment feature of the WebLogic Server can only redeploy an EJB's implementation classes, you cannot redeploy an EJB's public interfaces
c. Before deploying a packaged jar file containing uncompiled EJB classes and interfaces, we have to use weblogic.ejbc on the packaged .jar file to generate WebLogic Server container classes.

Choice C is correct because it is NOT true. A and B are true about deploying EJBs in the WebLogic server. The weblogic/config/examples/applications directory acts as an automatic deployment directory for EJB 2.0 .jar files and EJB .jar deployment directories. When you start WebLogic Server, it automatically deploys any valid EJB 2.0 .jar files or .jar directories that reside in the applications directory. WebLogic Server also checks the contents of applications every ten seconds to determine whether an EJB deployment has changed. If a deployment has changed, it is automatically redeployed using the dynamic deployment feature.
If you change the contents of a compiled EJB .jar file in applications, WebLogic Server automatically attempts to redeploy the .jar using the dynamic deployment feature. Since the automatic redeployment feature uses dynamic deployment, WebLogic Server can only redeploy an EJB's implementation classes. You cannot redeploy an EJB's public interfaces. You create compiled EJB 2.0 .jar files by Compiling your EJB classes and interfaces, packaging the EJB classes and interfaces into a valid .jar file and then Using weblogic.ejbc on the .jar file to generate WebLogic Server container classes. An uncompiled EJB .jar file has the same structure as a compiled file, but you do not have to compile individual class files and interfaces and you do not have to use weblogic.ejbc on the packaged .jar file to generate WebLogic Server container classes. So C is not true.

How do I increase WebLogic Server memory?
Increase the allocation of Java heap memory for WebLogic Server. (Set the minimum and the maximum to the same size.) Start WebLogic Server with the -ms32m option to increase the allocation, as in this example:
$ java ... -ms32m -mx32m ...
This allocates 32 megabytes of Java heap memory to WebLogic Server, which improves performance and allows WebLogic Server to handle more simultaneous connections. You can increase this value if necessary.

What is error "ORA-6502?"
The default length of a string bound to an OUTPUT parameter of a CallableStatement is 128 characters. If the value you assign to the bound parameter exceeds that length, you will get this error.
You can adjust the length of the value of the bound parameter by passing an explicit length with the scale argument to the CallableStatement.registerOutputParameter() method.

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).

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.

How do I restrict access to servlets and JSPs?
The Java Servlet API Specification v2.2 allows you to declaratively restrict access to specific Servlets and JSPs using the Web Application Deployment descriptor. Section 13.3.2 of the specification has an example deployment descriptor that uses declarative security. For more information, see Programming WebLogic HTTP Servlets.

How do I protect WebLogic Server from security attacks from bogus clients using the WL-Proxy-Client-Cert header?
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:

<param-name>clientCertProxy
<param-value>true


For Web applications, add the parameter to the web.xml file as follows:

ServletRequestImpl context-param

<param-name>weblogic.http.clientCertProxy
<param-value>true


You can also use this parameter in a cluster as follows:
ClientCertProxyHeader="true"/>

Which XML parser comes with WebLogic Server 6.1?
We bundle a parser, based on Apache's Xerces 1.3.1 parser, in WebLogic Server 6.1. In addition, we include a WebLogic proprietary high-performance non-validating parser that you can use for small to medium sized XML documents. The WebLogic XML Registry allows you to configure the parser you want to use for specific document types.