/********************************************************************************** No 'Access-Control-Allow-Origin' header is present on the requested resource.* 说明：*     在php中使用ajax进行跨域访问的过程中遇到这个问题，梦真帮忙解决了。**                                       2016-12-14 深圳 南山平山村 曾剑锋********************************************************************************/一、参考文档：1. XmlHttpRequest error: Origin null is not allowed by Access-Control-Allow-Originhttp://stackoverflow.com/questions/3595515/xmlhttprequest-error-origin-null-is-not-allowed-by-access-control-allow-origin2. 跨域Ajax之ContentType:application/jsonhttp://www.foreverpx.cn/2016/06/22/cross_content_type/3. JQuery 的 ajax 出现Origin null is not allowed by Access-Control-Allow-Origin 解决方法http://blog.csdn.net/leon90dm/article/details/8120378

二、错误现象：Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'null' is therefore not allowed access. The response had HTTP status code 501.

三、跨域ajax访问代码：$.ajax({url: "http://192.168.1.20/data.php",type: "POST",contentType:"application/json; charset=utf-8",  //  ---->  问题就在这里了
    data: JSON.stringify(ajaxPostData),dataType:"json",success: function(data){//On ajax success do thisconsole.info("success.");if (data["status"] == "ok"){alert("Settings is Ok. The Machine is rebooting.");}},error: function(xhr, ajaxOptions, thrownError) {//On error do thisconsole.info("error.");if (xhr.status == 200) {alert(ajaxOptions);}else {alert(xhr.status);alert(thrownError);}}
});四、问题原因：在使用Ajax跨域请求时，如果设置Header的ContentType为application/json,会分两次发送请求。第一次先发送Method为OPTIONS的请求到服务器，这个请求会询问服务器支持哪些请求方法（GET,POST等），支持哪些请求头等等服务器的支持情况。等到这个请求返回后，如果原来我们准备发送的请求符合服务器的规则，那么才会继续发送第二个请求，否则会在Console中报错。五、解决办法：1. 在ajax访问中不指定contentType:"application/json; charset=utf-8",使用默认的就可以了；2. 如下代码：$.ajax({url: "http://192.168.1.20/data.php",type: "POST",data: JSON.stringify(ajaxPostData),dataType:"json",success: function(data){//On ajax success do thisconsole.info("success.");if (data["status"] == "ok"){alert("Settings is Ok. The Machine is rebooting.");}},error: function(xhr, ajaxOptions, thrownError) {//On error do thisconsole.info("error.");if (xhr.status == 200) {alert(ajaxOptions);}else {alert(xhr.status);alert(thrownError);}}});