HttpServletRequest req；
HttpServletResponse resp
/** * 对跨域提供支持 */
resp.setHeader("Access-Control-Allow-Origin",
req.getHeader("Origin"));
resp.setHeader("Access-Control-Allow-Methods", "GET,POST,OPTIONS,PUT,DELETE");
resp.setHeader("Access-Control-Allow-Headers", req.getHeader("Access-Control-Request-Headers"));
resp.addHeader("Access-Control-Allow-Credentials", "true"); // 跨域时会首先发送一个option请求，这里我们给option请求直接返回正常状态
