Java™ Application Development on Linux - Dator

21.3 What’s in a Name? An Introduction to JNDI489Example 21.2 is what we get when we run this program against one of theauthor’s DNS domains. 7Example 21.2 Running GetDomain against the multitool.net domain[mschwarz@cassidy simpleApp]$ java GetDomain multitool.netObject [baroni: java.lang.Object]Object [erik: java.lang.Object]Object [www: java.lang.Object]Object [class: java.lang.Object]Object [jboss: java.lang.Object]Object [penguin: java.lang.Object]Object [mail: java.lang.Object]Object [cvs: java.lang.Object]Object [stiletto: java.lang.Object]Object [penfold: java.lang.Object]Object [ns2: java.lang.Object]Object [ns1: java.lang.Object]Object [irc: java.lang.Object][mschwarz@cassidy simpleApp]$The GetDomain main() method. This is another “single class” programexample. In this case, the main() method creates an instance of the class,passing the first command-line argument to the constructor. We’ll cover theconstructor in the next section. By now, you will recognize that this is one ofthe purely pedagogical examples. Note the complete lack of input validationand error checking on the number and content of the command-line arguments.Establishing an initial context. For both naming and directory services, itis necessary to establish an initial context. A context is a collected set of names.A directory system is a connected set of contexts. Our example is for DNS.We must set an initial context for DNS. The class constructor (lines 11–15)does that.7. Note that directory operations in the JNDI DNS Service Provider Interface are done withDNS zone transfers. Many domains, especially large domains, disable zone transfers either forsecurity reasons, or because they generate a lot of network traffic and are a popular tool forDenial of Service (DoS) attacks on name servers. To put it simply: This program won’t workon a lot of domains, especially from outside.