访问位于“”的XMLHttpRequesthttp://localhost:8084/jia“from origin”null“已被CORS策略阻止：对飞行前请求的响应未通过访问控制检查：请求的资源上不存在”access control Allow origin“标头。

这个错误是由于浏览器的CORS（跨域资源共享）策略导致的。当网页尝试从一个域名（比如 http://localhost:8084）请求数据时，浏览器会向目标服务器发送一个预检请求（OPTIONS请求），以确定是否允许跨域请求。如果目标服务器没有正确配置CORS，就会出现你提到的错误。
要解决这个问题，有几种方法：

在目标服务器的响应头中添加Access-Control-Allow-Origin头部，允许指定的域名或所有域名访问该资源。例如，在Java Web应用程序中，可以在响应头中添加以下代码：

response.setHeader("Access-Control-Allow-Origin", "*");
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在目标服务器上使用代理，将请求转发到目标服务器上，并在代理服务器上设置CORS策略。

如果是开发环境下的问题，可以在浏览器中安装CORS插件，禁用浏览器的CORS策略。

需要注意的是，为了保障网站的安全性，不建议在生产环境下使用第三种方法。

相关问题

访问位于“”的XMLHttpRequesthttp://localhost:8081/login?name=1&password=1'来自原点'http://localhost:8080'已被CORS策略阻止：请求的资源上不存在“Access Control Allow Origin”标头。

这是因为浏览器的同源策略（Same-Origin Policy）限制了在一个源（协议、域名、端口号）下的 JavaScript 脚本只能访问同源下的资源，而不能访问其他源下的资源。这是一种安全限制，防止恶意脚本窃取用户的数据。
为了解决这个问题，可以在服务器端设置 Access-Control-Allow-Origin 头，允许指定的源访问该资源。例如，在响应头中加上以下内容：
Access-Control-Allow-Origin: http://localhost:8080
这样就允许来自 http://localhost:8080 的请求访问该资源了。如果需要允许多个源访问，可以使用逗号分隔，例如：
Access-Control-Allow-Origin: http://localhost:8080, http://localhost:8081
当然，这只是解决跨域访问的一种方式，还有其他更复杂的方法，比如 JSONP、CORS 等。

from origin 'http://localhost:8081' has been blocked by CORS policy

The CORS (Cross-Origin Resource Sharing) policy is a security measure implemented by web browsers to restrict access to resources (such as APIs) on a different origin (domain, protocol, or port) than the one the web page is served from.
In your case, it seems that you are making a request from 'http://localhost:8081' to a different origin, and the server is blocking that request due to CORS policy restrictions. To resolve this issue, you can try the following solutions:

Enable CORS on the server-side: If you have control over the server-side code, you can configure the server to include the appropriate CORS headers in the response. These headers allow the browser to determine if it should allow the request. The minimal required headers are 'Access-Control-Allow-Origin' and 'Access-Control-Allow-Methods'. You can set the value of 'Access-Control-Allow-Origin' to '*' to allow requests from any origin during development. However, in production, it's recommended to set it to the specific origin of your application.

Use a proxy server: If you don't have control over the server-side code or enabling CORS is not feasible, you can set up a proxy server that acts as an intermediary between your client-side code and the server. Your client-side code will make requests to the proxy server, and the proxy server will forward those requests to the actual server. Since the proxy server will be on the same origin as your client-side code, you won't face any CORS issues.

Disable CORS in the browser: Although not recommended for production environments, during development, you can disable CORS checks in your browser. Keep in mind that this is a temporary solution and should not be used in a production environment. The method to disable CORS varies between browsers, so you need to search for instructions specific to your browser.

Remember to consider security implications when implementing any solution and ensure that proper CORS configurations are applied in a production environment.