Ajax.request 帮忙

先上代码:

function CreateMenu(url)
{
  var P='';
  Ext.Ajax.request({
    disableCaching:true,
    url:url,
    success:function(){P="GGGGG"}
  })
  alert(P)
}

为什么alert的P还是空的?而不是'GGGGG'呢?要怎样才能让P等于'GGGGG'呢?
谢谢~~