实际上application/json，您不是在发布实体，而是在发布具有application/x-www-form- urlencoded单个值对json =（encoded json）的HTTP表单实体（）。
代替
List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(2);
nameValuePairs.add(new BasicNameValuePair("json", j.toString()));
httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
尝试
httppost.setEntity(new StringEntity(j.toString(),"application/json","UTF-8"));
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