Some time maven complains can not find the jar in .m2/repository, while it seems the jar is actually there. now should delete the folder let maven download the jar again. could fix the issue. because there's mistakes in other files, which could cause maven think there's no jar in the place.
urlConnection.getResponseCode()-------------------when and only when this line excuted, server side rest API will be reached.

-- Sam 17:41 29/11/2019

& 在javascript中是&，但是还是必须分行来写，因为不然的话，即使第一个条件不满足，第二个条件也会被解释，从而造成在content==null时的第二个条件报错。

if(content != null){

if(content.startsWith("\"")){

-- Sam 10:23 26/02/2018