一.问题背景

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。

本文解决常见的CORS跨域问题，以及，集成CAS 5.3单点登录内嵌页面时，发送复杂请求产生的跨域问题。

二.解决方案

1.Nginx或Tomcat配置

通过Nginx或Tomcat配置，返回相应的请求头，本文采用Tomcat配置方式。

（1）修改conf/web.xml，添加过滤器，若存在其他过滤器添加在最前面。

<filter>
  <filter-name>CorsFilter</filter-name>
  <filter-class>org.apache.catalina.filters.CorsFilter</filter-class>
  <init-param>
    <param-name>cors.allowed.origins</param-name>
    <param-value>*</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>10</param-value>
  </init-param>
</filter>
<filter-mapping>
  <filter-name>CorsFilter</filter-name>
  <url-pattern>/*</url-pattern>
</filter-mapping>

（2）此时tomcat8.5以上版本可能会报错javax.servlet.ServletException: 当allowedOrigins=[*]时，不允许配置supportsCredentials=[true]

去掉20~23行配置，或将cors.allowed.origins配置为允许跨域访问当前服务器资源的地址，多个地址用逗号拼接。

示例：

<filter>
  <filter-name>CorsFilter</filter-name>
  <filter-class>org.apache.catalina.filters.CorsFilter</filter-class>
  <init-param>
    <param-name>cors.allowed.origins</param-name>
    <param-value>http://cpmp.fulongai.cn,http://192.168.2.154:8888</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>10</param-value>
  </init-param>
</filter>
<filter-mapping>
  <filter-name>CorsFilter</filter-name>
  <url-pattern>/*</url-pattern>
</filter-mapping>

至此，大多数CORS的问题都可以解决了，但对于复杂请求仍需进一步分析。

2.复杂请求

对于复杂请求，浏览器会先发送一个OPTIONS类型的请求进行预检，服务端会根据请求标头判断是否允许访问请求的资源。

举个例子：

截图的中的预检请求包含请求表头如下：

Access-Control-Request-Headers: authorization
Access-Control-Request-Method: GET
Origin: http://192.168.2.154:8888

服务端应允许相应的访问，才能预检成功。看看我们之前的配置，GET请求和http://192.168.2.154:8888源都满足，缺少Access-Control-Request-Headers的authorization支持，因此OPTIONS预检返回403，无权限访问。

控制台：

仍然报错No 'Access-Control-Allow-Origin' header is present on the requested resource.

解决方案：

在web.xml中添加相应的配置（14行末尾authorization），问题解决：

<filter>
  <filter-name>CorsFilter</filter-name>
  <filter-class>org.apache.catalina.filters.CorsFilter</filter-class>
  <init-param>
    <param-name>cors.allowed.origins</param-name>
    <param-value>http://cpmp.fulongai.cn,http://192.168.2.154:8888</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,authorization</param-value>
  </init-param>
  <init-param>
    <param-name>cors.exposed.headers</param-name>
    <param-value>Access-Control-Allow-Origin,Access-Control-Allow-Credentials,Access-Control-Allow-Headers,Access-Control-Allow-Method,Set-Cookie</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>1000</param-value>
  </init-param>
</filter>
<filter-mapping>
  <filter-name>CorsFilter</filter-name>
  <url-pattern>/*</url-pattern>
</filter-mapping>

重启tomcat，再次请求，请求标头和响应头相对应，预检通过，可正常访问。

至此，问题解决。

参考：

https://tomcat.apache.org/tomcat-8.5-doc/config/filter.html#CORS_Filter

https://developer.mozilla.org/zh-CN/docs/Web/HTTP/CORS