Sync0.java

package async;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet(urlPatterns = "/sync0")
public class Sync0 extends HttpServlet {

  private static final long serialVersionUID = 1L;

  @Override
  protected void service(final HttpServletRequest request, final HttpServletResponse response) throws ServletException, IOException {
    
    Boolean status = (Boolean) request.getAttribute("dispatch");
    
    println("'status' attr: "+status);
    
    if (Boolean.TRUE == status) {
      //this condition will never run
      println("Received dispatch, completing on the worker thread.");
      response.getWriter().write("Dispatch received. Worker thread completed \n"); 
      response.getWriter().flush();
      
    } else {
      
      response.getWriter().write("Before starting job \n");
      response.getWriter().flush();
        
      Thread thread = new Thread(new HeavyTask(request, response));
      thread.start();
      request.setAttribute("dispatch", Boolean.TRUE);
      
      response.getWriter().write("After starting job \n");
      response.getWriter().flush();
    }
  }
  
  class HeavyTask implements Runnable {
    HttpServletRequest req;
    HttpServletResponse res;
    
    HeavyTask(HttpServletRequest req, HttpServletResponse res) {
      this.req = req;
      this.res = res;
    }
    
    @Override
    public void run() {
      try {
        Thread.currentThread().setName("Job-Thread-" + req.getParameter("job"));
        for(int i=0; i<7; i++) {
          res.getWriter().write("Doing work ("+i+")\n");
          Thread.sleep(1000);
          println("work "+i+" completed");
        }
        println("Job finished now dispatching...");
      } catch (InterruptedException e) {
        e.printStackTrace();
      } catch (IllegalStateException e) {
        e.printStackTrace();
      } catch (IOException e) {
        e.printStackTrace();
      }
    }
  }
  
  public static void println(String output){
    System.out.println("["+Thread.currentThread().getName()+"]"+output);
  }
}