when does response.getWriter (). Write write data to the browser?

    @RequestMapping("/testOutStream")
    public void testOutStream(HttpServletResponse response) throws IOException, InterruptedException {
        response.getWriter().println("test1");
        Thread.sleep(5000);
    }

doesn"t response.getWriter (). Write write data to the browser after being called? When the method is finished? I don"t know how it works inside.
ask the boss to help me with the answer.