Access to XMLHttpRequest at ....... has been blocked by CORS policy: Response to preflight reques...

错误原因: ajax无法发送跨域请求
解决方法:
使用原生ajax

// 前端设置是否带cookie
xhr.withCredentials = true;
示例代码：

var xhr = new XMLHttpRequest(); // IE8/9需用window.XDomainRequest兼容

// 前端设置是否带cookie
xhr.withCredentials = true;

xhr.open('post', 'http://www.domain2.com:8080/login', true);
xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
xhr.send('user=admin');

xhr.onreadystatechange = function() {
    if (xhr.readyState == 4 && xhr.status == 200) {
        alert(xhr.responseText);
    }
};