It is common that after a migration to Java, specially coming from legacy platforms like LINC or COBOL, that our clients want to take advantage of new technologies. So it happens that they are now authenticating against an Active Directory or another LDAP server. And thanks to the new platforms it is really easy for us to help them integrate this new functionality.
This is sample program that show how to authenticate with for example a Windows Active Directory.

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.Hashtable;

import javax.naming.Context;
import javax.naming.NamingEnumeration;
import javax.naming.NamingException;
import javax.naming.directory.Attributes;
import javax.naming.directory.SearchControls;
import javax.naming.directory.SearchResult;
import javax.naming.ldap.InitialLdapContext;
import javax.naming.ldap.LdapContext;

public class LDAPTest
{
       static class LDAP
      {
            static String ATTRIBUTE_FOR_USER = "sAMAccountName";
            public Attributes authenticateUser(String username, String password, String _domain, String host, String dn)
            {

                  String returnedAtts[] ={ "sn", "givenName", "mail" };
                  String searchFilter = "(&(objectClass=user)(" + ATTRIBUTE_FOR_USER + "=" + username + "))";
                  //Create the search controls

                  SearchControls searchCtls = new SearchControls();
                  searchCtls.setReturningAttributes(returnedAtts);
                  //Specify the search scope

                  searchCtls.setSearchScope(SearchControls.SUBTREE_SCOPE);
                  String searchBase = dn;
                  Hashtable environment = new Hashtable();
                  environment.put(Context.INITIAL_CONTEXT_FACTORY, "com.sun.jndi.ldap.LdapCtxFactory");
                  //Using starndard Port, check your instalation

                  environment.put(Context.PROVIDER_URL, "ldap://" + host + ":389");
                  environment.put(Context.SECURITY_AUTHENTICATION, "simple");

                  environment.put(Context.SECURITY_PRINCIPAL, username + "@" + _domain);
                  environment.put(Context.SECURITY_CREDENTIALS, password);
                  LdapContext ctxGC = null;
                  try
                  {
                        ctxGC = new InitialLdapContext(environment, null);
                        //    Search for objects in the GC using the filter

                        NamingEnumeration answer = ctxGC.search(searchBase, searchFilter, searchCtls);
                        while (answer.hasMoreElements())
                        {
                              SearchResult sr = (SearchResult)answer.next();
                              Attributes attrs = sr.getAttributes();
                              if (attrs != null)
                              {
                                    return attrs;
                              }
                        }

                   }
                  catch (NamingException e)
                  {
                        System.out.println("Just reporting error");
                        e.printStackTrace();
                  }
                  return null;
            }
     }

      public static void main(String[] args) throws Exception
      {
            InputStreamReader converter = new InputStreamReader(System.in);
            BufferedReader in = new BufferedReader(converter);
            System.out.println("Please type username:");
            String username = in.readLine();
            System.out.println("Please type password:");
            String password = in.readLine();
            LDAP ldap = new LDAP();

            //Yo specify in the authenticate user the attributes that you want returned

            //Some companies use standard attributes like 'description' to hold an employee ID

            //The ActiveDirectory data can be enhanced to add custom attributes like

            //printer

            // Some instalations usually have several ACtiveDirectoryServers, lets say

            // 192.150.0.8, 192.150.0.7 y 192.150.0.9 and they use a

            // DNS round robin to balance the load

            Attributes att = ldap.authenticateUser(username, password, "mydomain.com", "myactivedirectoryhost.com", "DC=mydomain,DC=com");
            if (att == null)
            {
                  System.out.println("Sorry your use is invalid or password incorrect");
            }
            else
            {
                  String s = att.get("givenName").toString();
                  System.out.println("GIVEN NAME=" + s);
            }
      }
}