用户登录时启动服务

Intent i= new Intent(context,YourService.class);
i.putExtra("KEY1","Value to be used by the service");
context.startService(i);

并在用户注销时停止服务

stopService(new Intent(this,YourService.class));

要了解有关服务的更多信息,您可以参考此处

Service : Android developers

Service : Vogella

要了解有关asynctask vs service的更多信息,请参阅此内容

Android: AsyncTask vs Service

When to use a Service or AsyncTask or Handler?