通常，所有AJAX请求都有 X-Requested-With 标题集。你可以检查这个头是否被设置并且呈现所需的响应格式：

  if（request.getHeader（'X-Requested- '））{
 //呈现响应为JSON 
} else {
 //渲染HTML页面
} 
 $或者（正如Martin Hauner在评论中指出的那样）使用 request.xhr 属性，它们基本相同并且返回 true 如果当前请求是AJAX： 

  if（request.xhr ）{
 //将响应呈现为JSON 
}其他{
 //渲染HTML页面
} 
 
  request 是表示当前请求的对象。 在Grails文档中阅读更多关于它的内容。 
 
I want to know if there is a way in Grails to return HTML or JSON depending if I make a GET to an action or if I just call an action through Ajax.

For example, if I make an Ajax call to "controller/action", is there a way to return JSON and if I go to the same "controller/action" trough a link, make it render an HTML page? or i have to define two diferent actions?
Typically all AJAX requests have X-Requested-With header set. You can check if this header is set and render desired format of response:
if (request.getHeader('X-Requested-With')) {
    // render response as JSON
} else {
    // render HTML page
}
Or (as Martin Hauner pointed out in comments) use request.xhr property which do basically the same and returns true if current request is an AJAX:
if (request.xhr) {
    // render response as JSON
} else {
    // render HTML page
}
request is an object representing current request. Read more about it in Grails documentation.