doGet方法完成后，Servlet是否返回响应？

Question

Obviously, the doGet method has a return type of void, so, it doesn't return anything. In this sense, I'm using the word "return" to mean send the response back to the client that requested it.

I'm trying to implement a long-polling Servlet. It would be beneficial for it not to send a response until I have something that I would like to send back. So, in the doGet method I add the connected user's ID and AsyncContext to a map:

private ConcurrentMap<String, AsyncContext> contexts = new ConcurrentHashMap<>();
//...in the doGet method when I need to add the context...
contexts.put(userId, context);

Then, when I have something to send back, I can retrieve the appropriate context and write to it's responses output stream:

AsyncContext context = contexts.get(userId);
PrintWriter writer = context.getResponse().getWriter();
writer.write("something to send to the client");

But, the client never seems to receive the response. Looking at the Network tab in the developer console of the browser, I can see the GET request is sent and then returns (with a status of 200). This occurs before I actually send something back. Which is leading me to believe that after the doGet method is finished the response is returned. And perhaps because of this, after this point, nothing can be sent to the client because the connection is not opened.

Does the doGet method send the response to the client once the method is finished executing? If this is the case, how can I keep the connection open for a long-polling effect?

Answer 1

To answer my own questions: Does the doGet method send the response to the client once the method is finished executing?

Yes, when the doGet (or any HttpServlet method, ex: doGet, doPost, etc.) method finishes executing it sends the response back to the client.

If this is the case, how can I keep the connection open for a long-polling effect?

Using asynchronous Servlets (which I was using, however, I found my particular problem must be elsewhere, yet these answers are still relevant to the questions asked). On the ServletRequest object call the startAsync method, like so:

AsyncContext context = request.startAsync(request, response);

"This will notify the Web Container that at the end of the request call it should free the handling thread and leave the connection open so that other thread writes the response and end the connection." Reference Link.

Also, I will add the solution to my particular problem (the client wasn't receiving the response) was because in my Servlet, I wasn't calling the complete method on the AsyncContext object:

asyncContext.complete();

Answer 2

Yes, the response stream is flushed and closed when doGet() finishes executing.

Keeping UI threads occupied for extended periods of time violates Java Enterprise best practice.

Recommend you rather return immediately if nothing to respond, and implement a timer on the client (browser) side to poll the server for results every so often.