If you want to run and control a long-running process, better let it run in its own Thread instead of the request's Thread. Store a reference to this Thread in the session scope so that the client can use ajaxical requests (using the same session!) to request the server side for the current progress (and automagically also to keep the session alive so that it doesn't timeout).
Here's a basic example of such a servlet:
package mypackage;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class RunLongProcessServlet extends HttpServlet {
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
{
if ("XMLHttpRequest".equals(request.getHeader("x-requested-with"))) {
LongProcess longProcess = (LongProcess) request.getSession().getAttribute("longProcess");
response.setContentType("application/json");
response.getWriter().write(String.valueOf(longProcess.getProgress()));
} else {
request.getRequestDispatcher("runLongProcess.jsp").forward(request, response);
}
}
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
{
LongProcess longProcess = new LongProcess();
longProcess.setDaemon(true);
longProcess.start();
request.getSession().setAttribute("longProcess", longProcess);
request.getRequestDispatcher("runLongProcess.jsp").forward(request, response);
}
}
class LongProcess extends Thread {
private int progress;
public void run() {
while (progress < 100) {
try { sleep(1000); } catch (InterruptedException ignore) {}
progress++;
}
}
public int getProgress() {
return progress;
}
}
..which is mapped as follows:
<servlet>
<servlet-name>runLongProcess</servlet-name>
<servlet-class>mypackage.RunLongProcessServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>runLongProcess</servlet-name>
<url-pattern>/runLongProcess</url-pattern>
</servlet-mapping>
And here's a basic example of the JSP (with a little shot jQuery, an ajaxical JS framework which I by the way greatly recommend):
<!doctype html>
<html lang="en">
<head>
<title>Show progress of long running process with help of Thread and Ajax.</title>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"></script>
<script type="text/javascript">
$(document).ready(init);
function init() {
if (${not empty longProcess}) {
$.progress = 0;
checkProgress();
}
}
function checkProgress() {
$.getJSON('runLongProcess', function(progress) {
$('#progress').text(progress);
$.progress = parseInt(progress);
});
if ($.progress < 100) {
setTimeout(checkProgress, 1000);
}
}
</script>
</head>
<body>
<form action="runLongProcess" method="post">
<p>Run long process: <input type="submit"></p>
<p>Current status: <span id="progress">0</span>%</p>
</form>
</body>
</html>
Open it at http://localhost:8080/yourcontext/runLongProcess and click the button.
If this is a really, really long running process, you may improve "efficiency" by increasing the ajax request intervals in the setTimeout() to 5 seconds (5000 ms) or so, so that the server doesn't feel getting DDOS'ed ;)
Hope this helps.