How to make a servlet respond to multiple requests?

Question

I have a task that I want to wrap in a servlet to provide the ability to run the task remotely, by http request.

I know I can achieve this with REST API, but currently I assume (and please correct me if I'm wrong) that a simple servlet will do.

One of the things I want to achieve is that if a request to the servlet is made while another request is still processed, I'll get an appropriate response - "Task is already running".

I've built a simple servlet, using servlet-3.0, that calls the jar I want to run, but when I make 2 requests, the second one is not processed until the first one is finished.

EDIT:

My servlet is a simple http serlvet. service method overriden. I have a system.out.println("a") in the start. when I call the servlet in debug mode and then (while stopped at breakpoint) call it again, the message is printed only one time and printed the second time when I release the breakpoint and the first run finishes.

You wrote that you don't want to use REST API. Can you tell us why? It's really easy to create REST using JAX-RS/Jersey. — pepuch, Mar 20 '13 at 07:09
You want to make your servlet singlethreaded - why? The servlet container will by default handle more requests for you. — Darwind, Mar 20 '13 at 07:09
Related question: [Difference between each instance of servlet and each thread of servlet in servlets?](http://stackoverflow.com/q/2183974/1065197) — Luiggi Mendoza, Mar 20 '13 at 07:33
A possible solution would be having another component in your server that handles these jobs in parallel and you can ask about the state of current work i.e. a Resource Adapter Module (rar) that process the jobs and you communicate both the web and the rar modules using JMS. — Luiggi Mendoza, Mar 20 '13 at 07:45

score 0 · Answer 1 · answered Mar 20 '13 at 07:15

0

First of all, this does not seems like REST at all. If you really just want to spawn a (single) background task, make sure you do it in a separate worker thread, not the request thread.

answered Mar 20 '13 at 07:15

NilsH

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In a servlet, I think it's safe to have blocking doGet() etc. handlers but scalability could be an issue. – seand Mar 20 '13 at 07:24

lichengwu · Answer 2 · 2013-03-20T08:41:05.137

0

Maybe you need a lock:

public class Task extends HttpServlet {
    // for singleton
    //private volatile boolean running = false;

    // or try this:

    public static boolean running = false;

    public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException
    {
        if(running){
             PrintWriter out = response.getWriter();
             out.println("running");
             return;
        }
        synchronized(Task.class){
            if(!running){
                running = true;
                // run the task
                running = false;
            }
        }
    }
}

edited Mar 20 '13 at 08:41

answered Mar 20 '13 at 07:17

lichengwu

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1

If he's using multiple http servers you'll need to lock on something external such as a database. – seand Mar 20 '13 at 07:26
1

Synchronized(this) won't work since the servlet container normally instantiates one servlet per request. You need a static run variable and a static lock. Syncin on the class would do. – Alexander Torstling Mar 20 '13 at 07:33
It's not always correct that servlet instance is created for every servlet. – Ramesh PVK Mar 20 '13 at 08:35

How to make a servlet respond to multiple requests?

2 Answers2