前后端分离项目跨域问题，浏览器控制台报错：No 'Access-Control-Allow-Origin' header is present on the requested resource. 请求方法为OPTIONS，状态值为302或403。

报错信息如下：

我的JS请求代码如下：

function doPost() {
            let params = {
                userName:"朱",
                pwd : "bestv001",
                birthday : "2022-12-06"
            };
            const xmlHttp = new XMLHttpRequest();
            xmlHttp.open("POST", "http://localhost:8180/demo_war_exploded/login2?accessToken=dfgddf");
            xmlHttp.setRequestHeader('Content-Type', 'application/json; charset=utf-8');
            xmlHttp.send(JSON.stringify(params));
            xmlHttp.onreadystatechange = function () {
                // 为了保证 数据 完整返回，我们一般会判断 两个值
                if (xmlHttp.readyState == 4 && xmlHttp.status == 200) {
                    //（5）服务端相应：如果能够进入这个判断，说明数据请求成功了
                    console.log('数据返回成功：' + JSON.stringify(xmlHttp.responseText));
                    alert(JSON.stringify(xmlHttp.responseText))
                }
            };

        }

是 POST 请求，且 Content-Type = 'application/json; charset=utf-8'，其实Content-Type字段的类型为application/json 的 POST 请求,CORS规定，Content-Type不属于以下类型的，都属于预检请求 （preflight request）：

application/x-www-form-urlencoded
multipart/form-data
text/plain

发送"预检请求"时，需要用到方法 OPTIONS 。

服务端的代码很简单，如下：

    @PostMapping(value = "/login2")
    @ResponseBody
    public LoginReq login2(@RequestBody LoginReq req, HttpServletRequest servletRequest, HttpServletResponse response) {
        System.out.println("---- login2 ----- userName = " + req.getUserName());
//        // 设置js请求跨域解决
//        response.setHeader("Access-Control-Allow-Origin", "*");
//        response.setHeader("Access-Control-Allow-Methods", "POST, GET, PUT, OPTIONS, DELETE");

        String accessToken = servletRequest.getParameter("accessToken");
        System.out.println("---- login2 --------------- accessToken = " + accessToken);

        String user = JSONObject.toJSONString(req);
        System.out.println("---- login2 ----- user = " + user);

        System.out.println("---- login2 -----  birthday = " + req.getBirthday());

        SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm");


        Date date = null;
        try {
            date = simpleDateFormat.parse(simpleDateFormat.format(req.getBirthday()));
        } catch (ParseException e) {
            e.printStackTrace();
        }

        System.out.println("date = " + simpleDateFormat.format(date));
        req.setBirthday(new Date());    
        if (req.getUserName().isEmpty() || req.getPwd().isEmpty()) {
            throw new RuntimeException("userName or pwd is empty");
        }
        return req;
    }

解决办法：通过修改 Tomcat 配置

修改本地服务端项目中的 web.xml 文件，如下图：

如果项目中有其他的 filter，那么把即将添加的跨域 filter 放在其他 filter 的前面，如下：

<filter>
        <filter-name>MyCorsFilter</filter-name>
        <filter-class>org.apache.catalina.filters.CorsFilter</filter-class>
        <init-param>
            <param-name>cors.allowed.origins</param-name>
            <param-value>http://localhost:63342</param-value>
        </init-param>
        <init-param>
            <param-name>cors.allowed.methods</param-name>
            <param-value>GET,POST,HEAD,OPTIONS,PUT</param-value>
        </init-param>
        <init-param>
            <param-name>cors.allowed.headers</param-name>
            <param-value>Content-Type,X-Requested-With,accept,Origin,Access-Control-Request-Method,Access-Control-Request-Headers</param-value>
        </init-param>
        <init-param>
            <param-name>cors.exposed.headers</param-name>
            <param-value>Access-Control-Allow-Origin,Access-Control-Allow-Credentials</param-value>
        </init-param>
        <init-param>
            <param-name>cors.support.credentials</param-name>
            <param-value>true</param-value>
        </init-param>
        <init-param>
            <param-name>cors.preflight.maxage</param-name>
            <param-value>18</param-value>
        </init-param>

    </filter>

    <filter-mapping>
        <filter-name>MyCorsFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

其中 http://localhost:63342 就是我的网页地址，改成你自己的地址就好，重启 Tomcat ,再次运行就 OK了。