尝试将JSON字符串发送到客户端android端时500内部服务器错误GAE？

I am working on Google App Engine to store data.My application is a android application and I want to receive datas from GAE datastore side.However when I want to receive ArrayList as a JSON text , I am getting "Error : 500 Internal Server Error" message and I could not find any solution about that I put my client and server side codes here ;I am working on Google App Engine to store data