Java Enterprise in a Nutshell, Second Edition

Looking Up Objects in a Context

Retrieving an object by name from a naming system or directory is called looking up the object. This is the job of the lookup( ) method of Context. Performing a lookup is analogous to getting the number of a friend from a telephone book by looking up his name. You can use JNDI to look up and retrieve an EJB home interface from an application server or a remote object from a remote RMI registry.

When you call lookup( ), you specify the name of the child of the Context you want to find. lookup( ) returns a java.lang.Object that represents the child. Here’s how we did it in the Lookup example:

Object obj = initialContext.lookup(name);

Calling lookup( ) retrieves an object from the underlying naming system. The JNDI service provider determines the Java representation of these objects, and we have no way of affecting the provider’s decision. Depending on the naming system and the design of the provider, the object you retrieve may or may not implement Context. For example, if you use the Sun filesystem provider, and your current context is a directory, looking up a child that is a file returns an instance of java.io.File. Looking up a directory, however, returns an instance of FSContext or RefFSContext, both of which implement Context. As another example, say you use Novell’s NDS provider, and the current context is an NDS tree. If you look up an organization, you get back an OrganizationDirContext that implements both Context and Novell’s NdsObject interface. ...