struts1的几个对象---哪位大哥帮忙解释下

?
显示的结果：?
ActionMessages.GLOBAL_MESSAGE:org.apache.struts.action.GLOBAL_MESSAGE
ActionErrors.GLOBAL_MESSAGE:org.apache.struts.action.GLOBAL_MESSAGE
Globals.MESSAGE_KEY:org.apache.struts.action.ACTION_MESSAGE
Globals.ERROR_KEY:org.apache.struts.action.ERROR