How to retrieve null-valued attributes from a LDAP server?

Question

(I posted this in the ES board but then thought this is more of a programmer's question, sorry for the duplication).
I am using JNDI api to do search operations on a Java Directory Server( part of SunOne).
However, I found all the attributes that do not have values are automatically filtered out from the search result.
NamingEnumeration answer = ctx.search(ctxName, filterExpr, cons);
               while(answer.hasMore()){
                    SearchResult sr = (SearchResult)answer.next();
                    Attributes attrs = sr.getAttributes();
                    for(NamingEnumeration ne = attrs.getIDs();ne.hasMore();){
                         System.out.println("ids:"+ne.next());
                    System.out.println("-------------------------------------------------------");
                   for (NamingEnumeration ae = sr.getAttributes().getAll(); ae.hasMore();) {
                       Attribute attr = (Attribute)ae.next();
                       System.out.println("attrName:"+attr.getID());
                       //System.out.println("attribute: " + attr.getID());
                       NamingEnumeration e = attr.getAll();
                       while(e.hasMore()){
                             System.out.println("  attrVal:"+e.next());
                   }Is there anything I did wrong here?
Here are a couple of things I noticed,
1. in a Softerra LDAP browser, those no-valued attributes are not present either. But in JXplorer, I can see the full list that includes the attributes that do not have a value.
2. I had Schema disabled in the server console.
Thank you in advance.

Answer 1

There are only two ways to read data from Directory Server:
1. a. just fetch the entry
b. display the content
2. a. fetch the entry
b. parse the entry and figure what object classes it is of
c. lookup each object class definition in the schema and retrieve the attribute list
d. combine the attributes of the entry with all the "possible" attributes of its object classe(s)
e. display the content
Here's for an easy example we can relate to:
I have the following entry in my DS
  cn=the_duuuuuude,dc=forum,dc=sun,dc=com
  objectClass: person
  cn: the_duuuuuude
  sn: arnaudIf you use method 1, you will get just what is stored in the db. That is:
  cn=the_duuuuuude,dc=forum,dc=sun,dc=com
  objectClass: person
  cn: the_duuuuuude
  sn: arnaudif you use method 2, you will get:
  cn=the_duuuuuude,dc=forum,dc=sun,dc=com
  objectClass: person
  cn: the_duuuuuude
  sn: arnaud
  description:
  seeAlso:
  telephoneNumber:
  userPassword:because when you looked up the 'person' object class you got this:
objectClasses: ( 2.5.6.6 NAME 'person' DESC 'Standard LDAP objectclass' SUP top MUST ( sn $ cn ) MAY ( description $ seeAlso $ telephoneNumber $ userPassword ) X-ORIGIN 'RFC 2256' )Now the important thing to note is that physically in the database, the attributes description, seeAlso, telephoneNumber and userPassword are NOT stored. It's not that they have a 'null' value. They're just not there. It doesn't stop you from looking up the schema.
Optimally, in your client, you would fetch the whole server schema and cache it so you have to do the extra round trip for every entry you process.
The difference you observe with various LDAP browsers might simply be that one uses method 1 and the other method 2.
Hope this helps wrap your mind around this.
-=arnaud=-