code: public static String getProperty( Properties props, String key, String def )
      {
          String value = props.getProperty( key, def );
          if( value == null )
          {
              return null;
          }
          if( "".equals( value ) )
          {
              return value;
          }
          value = PropertyResolver.resolve( props, value );
          return value;
      }
docs: Return the value of a property.
  
  @param props the property file
  @param key   the property key to lookup
  @param def   the default value
  
  @return the resolve value