Servlet-3 Async Context, how to do asynchronous writes?

I’ve found the Servlet 3.0 Asynchronous API tricky to implement correctly and helpful documentation to be sparse. After a lot of trial and error and trying many different approaches, I was able to find a robust solution that I’ve been very happy with. When I look at my code and compare it to yours, I notice one major difference that may help you with your particular problem. I use a ServletResponse to write the data and not a ServletOutputStream.

Here my go-to Asynchronous Servlet class adapted slightly for your some_big_data case:

import java.io.IOException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

import javax.servlet.AsyncContext;
import javax.servlet.AsyncEvent;
import javax.servlet.AsyncListener;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebInitParam;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import org.apache.log4j.Logger;

@javax.servlet.annotation.WebServlet(urlPatterns = { "/async" }, asyncSupported = true, initParams = { @WebInitParam(name = "threadpoolsize", value = "100") })
public class AsyncServlet extends HttpServlet {

  private static final Logger logger = Logger.getLogger(AsyncServlet.class);

  public static final int CALLBACK_TIMEOUT = 10000; // ms

  /** executor service */
  private ExecutorService exec;

  @Override
  public void init(ServletConfig config) throws ServletException {

    super.init(config);
    int size = Integer.parseInt(getInitParameter("threadpoolsize"));
    exec = Executors.newFixedThreadPool(size);
  }

  @Override
  public void service(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {

    final AsyncContext ctx = req.startAsync();
    final HttpSession session = req.getSession();

    // set the timeout
    ctx.setTimeout(CALLBACK_TIMEOUT);

    // attach listener to respond to lifecycle events of this AsyncContext
    ctx.addListener(new AsyncListener() {

      @Override
      public void onComplete(AsyncEvent event) throws IOException {

        logger.info("onComplete called");
      }

      @Override
      public void onTimeout(AsyncEvent event) throws IOException {

        logger.info("onTimeout called");
      }

      @Override
      public void onError(AsyncEvent event) throws IOException {

        logger.info("onError called: " + event.toString());
      }

      @Override
      public void onStartAsync(AsyncEvent event) throws IOException {

        logger.info("onStartAsync called");
      }
    });

    enqueLongRunningTask(ctx, session);
  }

  /**
   * if something goes wrong in the task, it simply causes timeout condition that causes the async context listener to be invoked (after the fact)
   * <p/>
   * if the {@link AsyncContext#getResponse()} is null, that means this context has already timed out (and context listener has been invoked).
   */
  private void enqueLongRunningTask(final AsyncContext ctx, final HttpSession session) {

    exec.execute(new Runnable() {

      @Override
      public void run() {

        String some_big_data = getSomeBigData();

        try {

          ServletResponse response = ctx.getResponse();
          if (response != null) {
            response.getWriter().write(some_big_data);
            ctx.complete();
          } else {
            throw new IllegalStateException(); // this is caught below
          }
        } catch (IllegalStateException ex) {
          logger.error("Request object from context is null! (nothing to worry about.)"); // just means the context was already timeout, timeout listener already called.
        } catch (Exception e) {
          logger.error("ERROR IN AsyncServlet", e);
        }
      }
    });
  }

  /** destroy the executor */
  @Override
  public void destroy() {

    exec.shutdown();
  }
}