IBM Core Java Campus Interview Question Answer

What is an applet?

An applet is a small Internet-based program written in Java, a programming language for the Web, which can be downloaded by any computer. The applet is also able to run in HTML. The applet is usually embedded in an HTML page on a Web site and can be executed from within a browser.

Can I use a "native" two-tier driver for a browser applet?

No. Within an unsigned applet, you cannot load native libraries over the wire, access the local file system, or connect to any host except the host from which you loaded the applet. The applet security manager enforces these restrictions on applets as protection against applets being able to do unsavory things to unsuspecting users.
If you are trying to use jDriver for Oracle from an applet, then you are violating the first restriction. Your applet will fail when it attempts to load the native (non-Java layer) library that allows jDriver for Oracle to make calls into the non-Java Oracle client libraries. If you look at the exception that is generated, you will see that your applet fails in java.lang.System.loadLibrary, because the security manager determined that you were attempting to load a local library and halted the applet.
You can, however, use the WebLogic JTS or Pool driver for JDBC connectivity in applets. When you use one of these WebLogic multitier JDBC drivers, you need one copy of WebLogic jDriver for Oracle (or any other two-tier JDBC driver) for the connection between the WebLogic Server and the DBMS.

How do clients learn about new WebLogic Server instances?

Once a client has done a JNDI lookup and begins using an object reference, it finds out about new server instances only after the cluster-aware stub has updated its list of available servers.

What happens when a failure occurs and the stub cannot connect to a Web Logic Server instance?

When the failure occurs, the stub removes the failed server instance from its list. If there are no servers left in its list, the stubb uses DNS again to find a running server and obtain a current list of running instances. Also, the stub periodically refreshes its list of available server instances in the cluster; this allows the stub to take advantage of new servers as they are added to the cluster.

How do stubs work in a Web Logic Server cluster?

Clients that connect to a WebLogic Server cluster and look up a clustered object obtain a replica-aware stub for the object. This stub contains the list of available server instances that host implementations of the object. The stub also contains the load balancing logic for distributing the load among its host servers.

What's causing ClassFormatErrors with my applet?

Problem: I downloaded your distribution and copied the classes to my HTTP server DocumentRoot. I created an applet that I ran successfully from my Netscape server. I placed it in the server directory /webz/ns-home/classes/applets/myapp.class and called it with the following:

<APPLET CODEBASE=http://myserver.com/webz/ns-home/classes CODE=applets.myapp.class>

Then I set my attributes in the Administration Console to listen on port 7001, and I started the WebLogic Server on the HTTP machine so I could use my applet with WebLogic JDBC, like this:

<APPLET CODEBASE=t3://myserver.com:7001/webz/ns-home/classes CODE=applets.myapp.class>

When I changed the CODEBASE tag to point to the WebLogic Server, I started getting ClassFormatErrors.

A. There are several problems with your setup. The most obvious have to do with your CODEBASE:

1. The CODEBASE tag in your applet should point to your HTTP server, not to WebLogic Server.

2. The directory path referenced in your CODEBASE tag is not an absolute directory path on the HTTP server; it is a configured path that originates from the HTTP Document Root. You are using the absolute path in your CODEBASE tag. If your class "myapp" is in the "applets" package, then the correct CODEBASE for your setup would be:

<APPLET CODEBASE=http://myserver.com/classes CODE=applets.myapp.class>

In addition, if you are getting a ClassFormatError, it signals a problem with your HTTP server configuration. It could be that you haven't loaded the WebLogic or applet classes in the correct directory on the HTTP server, or you are specifying the CODEBASE or the CODE incorrectly in your APPLET tag.

Remember, too, that if you installed the WebLogic distribution on the machine from which you are running the applet, your applet will first look for the WebLogic classes in your local CLASSPATH, which may obscure the fact that you haven't properly installed the classes for serving from the HTTP server. To test your HTTP configuration properly, you need to temporarily rename the WebLogic classes in your local CLASSPATH or try your applet from another machine.

How many Web Logic Servers can I have on a multi-CPU machine?

There are many possible configurations and each has its own advantages and disadvantages. BEA WebLogic Server has no built-in limit for the number of server instances that can reside in a cluster. Large, multi-processor servers such as Sun Microsystems, Inc. Sun Enterprise 10000, therefore, can host very large clusters or multiple clusters. In most cases, WebLogic Server clusters scale best when deployed with one WebLogic Server instance for every two CPUs. However, as with all capacity planning, you should test the actual deployment with your target web applications to determine the optimal number and distribution of server instances. See Performance Considerations for Multi-CPU Machines for additional information.

What are the alternatives to using applets?

BEA supports the use of server-side applications with HTTP servlets and Java Server Pages (JSPs) as part of the J2EE platform. We recommend that before you develop new applications, consider using either servlets or JSPs. A well-designed series of interactive Web pages using servlets and Java Server Pages (JSPs) generally yield a faster and more reliable Web site.If you are currently using applets, you may find that most can be converted to Java applications using Java Web Start and you can continue using WebLogic Server. For information, go to Sun's Java Web Start site.

What are the Applet?s information methods?

The following are the Applet?s information methods: getAppletInfo() method: Returns a string describing the applet, its author, copyright information, etc. getParameterInfo( ) method: Returns an array of string describing the applet?s parameters.

What?s the applet class loader, and what does it provide?

example, the applet viewer?s applet class loader is implemented by the class sun.applet.AppletClassLoader.The class loader enforces the Java name space hierarchy. The class loader guarantees that a unique namespace exists for classes that come from the local file system, and that a unique namespace exists for each network source. When a browser loads an applet over the net, that applet?s classes are placed in a private namespace associated with the applet?s origin. Thus, applets loaded from different network sources are partitioned from each other. Also, classes loaded by the class loader are passed through the verifier. The verifier checks that the class file conforms to the Java language specification ? it doesn?t assume that the class file was produced by a ?friendly? or ?trusted? compiler. On the contrary, it checks the class files for purposeful violations of the language type rules and name space restrictions. The verifier ensures that:
a. There are no stack overflows or underflows.
b. All register accesses and stores are valid
c. The parameters to all byte code instructions are correct.
d. There is no illegal data conversion.
e. The verifier accomplishes that by doing a data-flow analysis of the byte code instruction stream, along with checking the class file format, object signatures, and special analysis of finally clauses that are used for Java exception handling.