post：

# Json格式

RequestBody requestBody = RequestBody.create(MediaType.parse("application/json;charset=utf-8"), String.valueOf(json));

# key-value格式

RequestBody requestBody = new FormBody.Builder().add("key1", value1).add("key2", value2).build();

String url = "";
OkHttpClient okHttpClient = new OkHttpClient();

Request request = new Request.Builder().url(url).post(requestBody).build();
return okHttpClient.newCall(request).execute().body().string();