跨域问题"Access to XMLHttpRequest at 'url' from origin 'null' has been blocked by CORS policy"解决

Request header field content-type is not allowed by Access-Control-Allow-Headers in preflight response

预运行响应中的访问控制允许头不允许请求头字段内容类型，也就是我们的请求head中包含了不允许的字段内容。

原因：

包含自定义header字段的跨域请求，浏览器会先向服务器发送OPTIONS请求，探测该服务器是否允许自定义的跨域字段。如果允许，则继续实际的POST／GET正常请求，否则，返回标题所示错误。

解决方案：

需要在系统的过滤器（filter）中设置响应头包含该字段

 HttpServletResponse rep = (HttpServletResponse) servletResponse;
 rep.setHeader("Access-Control-Allow-Origin", "*");
 rep.setHeader("Access-Control-Allow-Credentials", "true");
 // 允许的访问方法
 rep.setHeader("Access-Control-Allow-Methods","POST, GET, PUT, OPTIONS, DELETE, PATCH");
 // Access-Control-Max-Age 用于 CORS 相关配置的缓存
 rep.setHeader("Access-Control-Max-Age", "3600");
 rep.setHeader("Access-Control-Allow-Headers","Access-Control-Allow-Headers, content-type,x-requested-with,Authorization, x-ui-request,lang");