java後臺設置可跨域訪問，前端ajax獲取json數據，解決瀏覽器限制跨域訪問的問題

  在本地用ajax跨域訪問請求時報錯：javascript XMLHttpRequest cannot loadhttp://xxx/. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'null' is therefore not allowed access.html  

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