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param是json字符串，将json字符串转换为json对象，获取迭代器
JSONObject backParamJson = JSONObject.parseObject(param);
Iterator keys = backParamJson.keySet().iterator();
while (keys.hasNext()) {
String key = (String)keys.next();
String val = backParamJson.getString(key);
returnUrl += "&" + key +"=" + val;
}