android中Http访问时 connection.getResponseCode()不被执行，且报错 org.json.JSONException: End of input at character 0 of .

问题：用 android 4.4 写android访问http时，到connection.getResponseCode() 就不被执行，也不报错：但是抛出org.json.JSONException: End of input at character 0 of .异常：

连接代码：

public static String getJsonContent(String url_path){
    try {
        System.out.println("url_path---->>:"+url_path);
        URL url=new URL(url_path);
        HttpURLConnection connetion=(HttpURLConnection)url.openConnection();
        connetion.setConnectTimeout(3000);
        connetion.setRequestMethod("GET");
        connetion.setDoInput(true);
        System.out.println("connetion---->>:"+connetion);
        int code=connetion.getResponseCode();
        System.out.println("code---->>>:"+code);
        if (code==200){
            
            return changeInputStream(connetion.getInputStream());
        }else{
            System.out.println("---->>请求码不正确！！！");
            return "";
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
    return "";
}

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