fetch请求，No ‘Access-Control-Allow-Origin‘ header的解决方法，SpringBoot支持跨域

Failed to load http://localhost:8080/a/b: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost:3000' is therefore not allowed access. The response had HTTP status code 403. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

原因：服务器端不支持跨域访问

方法1：使服务器端支持跨域访问，前端不用配置。

添加maven:

       <!-- https://mvnrepository.com/artifact/com.thetransactioncompany/cors-filter -->
        <dependency>
            <groupId>com.thetransactioncompany</groupId>
            <artifactId>cors-filter</artifactId>
            <version>2.5</version>
        </dependency>

在controller