Java™ Application Development on Linux - Dator

490Chapter 21Introduction to Enterprise JavaBeansSo what is going on here? This constructor is a bit unusual, isn’t it? TheInitialDirContext is a “context factory.” It takes an “environment,” whichis a Hashtable, that provides the information needed to make the context.And what is that information? At a minimum, the constant value associatedwith Context.INITIAL_CONTEXT_FACTORY must be associated with the classname of the real context factory for the directory system—in this case,com.sun.jndi.dns.DnsContextFactory. If you are from a C/C++background, think of this as a function pointer. 8We now have an initial directory context, which we can use to search.Going from the initial context to a DNS entry. Let’s now consider a usecase for this little program. The program begins at main(), line 21. We createan instance of our class and an instance of NamingEnumeration (which wewill discuss in a moment). We do some very lazy error handling by enclosingthe entire process in a simple try/catch block 9 and treating all exceptions thesame. The first thing we do is construct an instance of our class, passing in thefirst command-line argument 10 as the domain name to use for setting the initialcontext.Next, we get an enumeration of all the names in that context. This is donethrough a method in our class that simply wraps the actual JNDI call that obtainsthis enumeration. The real event is on line 18. The list() method ofthe directory context returns a NamingEnumeration, which is an extension ofthe classic Enumeration Java class. With it, you can iterate over the contentsof the context—which we do in lines 29–33. We rely on our old Objectmethod, toString(), to make these names readable for us. Of course, each8. Of course, it is not. What really happens here is that the code in InitialDirContextuses the Class class to load the specified class by name. All JNDI context factory classes implementthe Context interface, so InitialDirContext uses the ability of Class to load theclass by its name as an instance of Context.9. JNDI has a particularly rich collection of exceptions. When a naming or directory operationfails, it is usually possible to determine exactly how and why it failed from the type ofException thrown. All JNDI Exceptions extend NamingException, so they also make itquite easy to handle them in a lazy manner. In a real production application, you should atleast make some effort to differentiate between failures where the network is not working andfailures where the network is working fine, but the named resource does not exist. Believe us,if you have to support your application in production you will care about the difference.10. Again, very bad production coding. Note that no attempt is made to check the number ofarguments passed or their contents.