解决跨域问题之预检请求

预检请求（Preflight Request）是跨域资源共享（CORS）中用于安全检查的一种机制。

它是由浏览器自动发起的一个OPTIONS请求，目的是在实际跨域请求之前，询问服务器是否允许这次跨域操作。

当浏览器发起一个跨域请求（即跨域资源请求）时，如果请求方法不是简单请求（例如 GET 或 POST），或者请求头包含非简单头部（例如自定义请求头），浏览器会先发送一个预检请求。

@Configuration
public class CorsConfig {private static final String MAX_AGE = "18000L";@Beanpublic WebFilter corsFilter(){return(ServerWebExchange serverWebExchange, WebFilterChain webFilterChain) ->{//1.从serverWebExchange获得request请求ServerHttpRequest request = serverWebExchange.getRequest();//2.判断是否是跨域请求，不是直接return回去if(!CorsUtils.isCorsRequest(request)){return webFilterChain.filter(serverWebExchange);}//3.获取请求头HttpHeaders httpHeaders = request.getHeaders();//4.从serverWebExchange获得responseServerHttpResponse response = serverWebExchange.getResponse();//5.从请求头获取请求的方法HttpMethod requestMethod = httpHeaders.getAccessControlRequestMethod();//6.设置预检请求响应头的值HttpHeaders responseHeaders = response.getHeaders();responseHeaders.add(HttpHeaders.ACCESS_CONTROL_ALLOW_ORIGIN,httpHeaders.getOrigin());//6.1 只允许www.baidu.com访问你的资源的话
//         responseHeaders.add(HttpHeaders.ACCESS_CONTROL_ALLOW_ORIGIN,"https://www.baidu.com");
//         response.setHeader("Access-Control-Allow-Origin", "*");
//         response.setHeader("Access-Control-Allow-Methods", "POST,OPTIONS,GET");
//         response.setHeader("Access-Control-Max-Age", "3600");
//         response.setHeader("Access-Control-Allow-Headers", "accept,x-requested-with,Content-Type");
//         response.setHeader("Access-Control-Allow-Credentials", "true");
//         response.setHeader("Access-Control-Allow-Origin", "http://192.168.10.118:8070");responseHeaders.addAll(HttpHeaders.ACCESS_CONTROL_ALLOW_HEADERS,httpHeaders.getAccessControlRequestHeaders());if(requestMethod != null){responseHeaders.add(HttpHeaders.ACCESS_CONTROL_ALLOW_METHODS,requestMethod.name());}responseHeaders.add(HttpHeaders.ACCESS_CONTROL_ALLOW_CREDENTIALS, "true");responseHeaders.add(HttpHeaders.ACCESS_CONTROL_EXPOSE_HEADERS, "*");responseHeaders.add(HttpHeaders.ACCESS_CONTROL_MAX_AGE, MAX_AGE);//7. 预检请求通常是一种OPTIONS请求，判断是不是预检请求if(request.getMethod() == HttpMethod.OPTIONS){//7.1响应状态码设置为 200 OKresponse.setStatusCode(HttpStatus.OK);return Mono.empty();}//最后return回webFilterChainreturn webFilterChain.filter(serverWebExchange);};}
}