LDAP and JNDI: Together forever

Learn how LDAP and JNDI combine to form a powerful directory and Java object storage facility

By Sameer Tyagi, JavaWorld.com, 03/24/00

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Figure 1. Screen shots of the server console after running the ADDSerialize example

Store a reference to the object

Instead of storing the entire serialized state of an object, you can store a reference to that object instead. JNDI's javax.naming.Reference class records address information about objects not directly bound to the directory service. The reference to an object contains the following information:

• The class name of the referenced object
• A vector of javax.naming.RefAddr objects that represents the addresses
• The name and location of the object factory to use during reconstruction

The javax.naming.RefAddr abstract class, seen in Figure 2 below, contains information indicating the ways in which you can contact the object (e.g., via a location in memory, a lookup on another machine, etc.) or recreate it with the same state. The class defines an association between content and type. The content (an object) stores information required to rebuild the object and the type (a string) identifies the purpose of the content.

Figure 2. The relation between a Reference, RefAddr, Type, and Content

RefAddr also overrides the java.lang.Object.equals(Object obj) and java.lang.Object.hashcode() methods to ensure that two references are equal if the content and type are equal. RefAddr has two concrete subclasses: javax.naming.StringRefAddr, which stores strings, and javax.naming.BinaryRefAddr, which stores an array of bytes. For example, a string reference address could be an IP, URL, hostname, or something similar.

Consider the example of a referenceable Apartment class. The ADDReference.java example creates a few instances of Apartment and stores them in the server. What happens internally? Since the object is referenceable, a reference is stored and not the serialized object. When the example tries to look up an apartment belonging to styagi, it gets a reference from the server that contains information about the factory class needed, the apartment size, and its location. It then requests that the factory create an Apartment object with the right size and location and return that object. All this happens transparently to the user.

Context ctx = new InitialContext(env);

ctx.bind("apartment=styagi,ou=JavaObjects,o=myserver.com",new
   Apartment("studio","Mill Complex"));

ctx.bind("apartment=mojoe,ou=JavaObjects,o=myserver.com", new Apartment("2 
   room","Farm House Apartments"));

ctx.bind("apartment=janedoe,ou=JavaObjects,o=myserver.com",new Apartment("1 
   room","Pond Side"));

ctx.bind("apartment=rogerp,ou=JavaObjects,o=myserver.com", new Apartment("3 
   room","Mill Complex"));

ctx.bind("apartment=jamesm,ou=JavaObjects,o=myserver.com", new 
   Apartment("studio","Fox Hill Apartments"));

ctx.bind("apartment=paulh,ou=JavaObjects,o=myserver.com",  new 
   Apartment("duplex","Woodbridge"));

ctx.bind("apartment=vkevink,ou=JavaObjects,o=myserver.com",new Apartment("1 
   room","Woodgate Apartments"));

Apartment apt = (Apartment)ctx.lookup("apartment=styagi,ou=
   JavaObjects,o=myserver.com");

System.out.println(apt);

The Apartment class would look something like: