code:     public Authentication authenticate(Authentication authentication) throws AuthenticationException {
          Assert.isInstanceOf(UsernamePasswordAuthenticationToken.class, authentication,
              messages.getMessage("LdapAuthenticationProvider.onlySupports",
                  "Only UsernamePasswordAuthenticationToken is supported"));
  
          final UsernamePasswordAuthenticationToken userToken = (UsernamePasswordAuthenticationToken)authentication;
  
          String username = userToken.getName();
          String password = (String) authentication.getCredentials();
  
          if (logger.isDebugEnabled()) {
              logger.debug("Processing authentication request for user: " + username);
          }
  
          if (!StringUtils.hasLength(username)) {
              throw new BadCredentialsException(messages.getMessage("LdapAuthenticationProvider.emptyUsername",
                      "Empty Username"));
          }
  
          Assert.notNull(password, "Null password was supplied in authentication token");
  
          DirContextOperations userData = doAuthentication(userToken);
  
          UserDetails user = userDetailsContextMapper.mapUserFromContext(userData, authentication.getName(),
                      loadUserAuthorities(userData, authentication.getName(), (String)authentication.getCredentials()));
  
          return createSuccessfulAuthentication(userToken, user);
      }
  
  