Question

I have developed a simple server using Tomcat which runs a servlet.

The servlet calls a command line program – which takes about 20 seconds to execute then returns the result to the user via JSON. The problem is – if i make above 2 simultaneous requests, the servlet blocks until one of the previous requests is completed.

An example of this can be seen below – “Im in” is the top of the servlet, and the list of results is after the servlet is executed. All requests were made at the same time – but you can clearly see they are not dealt with simultaneously. What setting do I need to change in tomcat in order to have all requests handeled at the same time?

Im in 

Im in 

FVFNT01 STOP_IDLE FVFNT03 STOP_IDLE FVFNT16 STOP_IDLE FVFNT17 STOP_IDLE
FVFNT01 STOP_IDLE FVFNT03 STOP_IDLE FVFNT16 STOP_IDLE FVFNT17 STOP_IDLE
Im in

FVFNT01 STOP_IDLE FVFNT03 STOP_IDLE FVFNT16 STOP_IDLE FVFNT17 STOP_IDLE
Im in

FVFNT01 STOP_IDLE FVFNT03 STOP_IDLE FVFNT16 STOP_IDLE FVFNT17 STOP_IDLE
Im in

FVFNT01 STOP_IDLE FVFNT03 STOP_IDLE FVFNT16 STOP_IDLE FVFNT17 STOP_IDLE
Im in

FVFNT01 STOP_IDLE FVFNT03 STOP_IDLE FVFNT16 STOP_IDLE FVFNT17 STOP_IDLE
Im in

FVFNT01 STOP_IDLE FVFNT03 STOP_IDLE FVFNT16 STOP_IDLE FVFNT17 STOP_IDLE
Im in

FVFNT01 STOP_IDLE FVFNT03 STOP_IDLE FVFNT16 STOP_IDLE FVFNT17 STOP_IDLE

Answer 1

Requests to servlets are handled concurrently by default. There is no setting that enables/disables this behaviour. This is confirmed by the JavaDoc for HttpServlet:

Servlets typically run on
multithreaded servers, so be aware
that a servlet must handle concurrent
requests and be careful to synchronize
access to shared resources.

However, if your servlet implements the marker interface SingleThreadModel, the servlet will only handle one request at-a-time. However, use of this interface is generally considered a bad practice, and because you didn’t mention it, I assume you’re not using it.

Of course even if you’re not implementing SingleThreadModel you can make any servlet single-threaded using (probably inappropriate) synchronisation, e.g.

class MyServlet extends HttpServlet {

  private void Object sharedObject = new Object()

  protected synchronized void doGet(HttpServletRequest req, HttpServletResponse resp) {
    // method logic goes here
  }

  protected void doPost(HttpServletRequest req, HttpServletResponse resp) {

    synchronized(sharedObject) {
      // method logic goes here
    }

  }

  protected void doPut(HttpServletRequest req, HttpServletResponse resp) {

    synchronized(this) {
      // method logic goes here
    }
  }
}

In the example above, only one thread may execute the same request method at-a-time, though it is possible (for example) for one thread to execute doPost() while another is executing doGet().

If you don’t understand why this is, then I recommend you do some reading on concurrent programming in Java before investigating your problem further.