大致代码如下：
HttpServletResponse response;
response.reset();//note 这里要添加reset，否则报异常 getWriter() has already been called for this response.
response.setCharacterEncoding("UTF-8");//note 这里要设置编码，否则会乱码
response.setContentType("application/json;charset=UTF-8");//note 这里要设置返回值的类型，因为全部是json接口。
PrintWriter out = response.getWriter();
out.print(jsonString)out.flush();
out.close();//这里要关闭
