code: public void setPriority(Integer priority) {        
          if (priority == null) {
              _properties.remove(AMQP_PROP_PRIORITY);
              return;
          }
          
          int priorityValue = priority.intValue();
          if ((priorityValue < 0) || (priorityValue > 9)) {
              String s = "AMQP 0-9-1 spec mandates 'priority' value to be between 0 and 9";
              throw new IllegalStateException(s);
          }
          
          _properties.put(AMQP_PROP_PRIORITY, priority);
      }
docs: Sets the value of "priority" property. If a null value is passed
  in, it indicates that the property is not set.
  
  @param  priority    value of "priority" property