后台操作：
1.先在myeclipse里面建立如下文件；
2.在MyServlet.java里面找到doGet{}在里面输入，如下：
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
//解决中文乱码
request.setCharacterEncoding("utf-8");
response.setCharacterEncoding("utf-8");
//在服务器端设置允许在其他域名下访问，及响应类型、响应头设置
response.setHeader("Access-Control-Allow-Origin", "*");
response.setHeader("Access-Control-Allow-Methods","POST");
response.setHeader("Access-Control-Allow-Headers","x-requested-with,content-type");
System.out.println("good");//后台打印
PrintWriter out = response.getWriter();//创建相应前端的对象类
out.print("hello");//转出后端数据到前端
out.flush();//把数据转化成数据流的形式发送到前端
out.close();//out是随便取的名字，关闭数据流
}